Copyright fey Ricardo Jose Padilla y Sanchez 1982

GEOLOGIC EVOLUTION OF THE SIERRA MADRE ORIENTAL BETWEEN LINARES, CONCEPCION DEL ORO, SALTILLO, AND MONTERREY, MEXICO

by

Ricardo Jose Padilla y Sanchez, INGENIERO GEOLOGO

DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY

THE UNIVERSITY OF TEXAS AT AUSTIN May 1982

AGRADECIMIENTOS

El autor de esta disertacion agradece ante todo, a todos los profesores de la Facultad de Ingenieria de la Universidad Nacional Autdnoma de Mexico, por el esfuerzo y dedicacion con que transmitieron sus conocimientos a la Generacion 1971 de Ingenieros Geologos, de la cual me siento orgulloso de formar parte.

Agradezco tambien, muy especialmente al Sr. M. en A. Diego A. Cordoba Mendez, por su amistad y apoyo. Igualmente al Dr. Jose 7 C. Guerrero Garcia, no solamente por su amistad y apoyo, sino por haber formado parte del Jurado Examinador.

A todos los profesores del Departamento de Ciencias Geologicas de la Universidad de Texas en Austin y muy especialmente al Dr. William R. Muehlberger, quien fungi 7 como supervisor de este estudio, y al Dr. Stephen E. Clabaugh quien mas que profesor es un amigo. Tambien a los Doctores, John C. Maxwell, Keith Young y Amos Salvador, no solo por haber servido en el Jurado Examinador, sino tambien por su amistad.

El agradecimiento final, pero muy especial, es para mis queridas Geles y Mariana, mi esposa e hija, sin cuyo apoyo durante estos anos no hubiera salido adelante.

ACKNOWLEDGMENTS

My sincere thanks to Dr. William R. Muehlberger, who as dissertation supervisor, spent many hours discussing the geology of southern United States and northern Mexico. Dr. John C. Maxwell, Dr. Keith Young, Dr. Amos Salvador, and Dr. Jose 7 C. Guerrero Garcia, served as a members of the supervisory committee and critically reviewed the text.

Special thanks to the M.A. Diego A. Cordoba M., former Director, and to Dr. Jose C. Guerrero Garcia, Director, both of the Institute de Geologia of the Universidad Nacional Autonoma de Mexico, for their support, not only economic but moral.

A very special note of appreciation to my wife Geles, and my daughter Mariana, who provided invaluable encouragement and support.

GEOLOGIC EVOLUTION OF THE SIERRA MADRE ORIENTAL BETWEEN LINARES, CONCEPCION DEL ORO, SALTILLO, AND MONTERREY, MEXICO

Publication No.

Ricardo Jose' Padilla y Sanchez, Ph.D. The University of Texas at Austin, 1982

Supervising Professor: William R. Muehlberger

The Sierra Madre Oriental between Saltillo, Monterrey, and Linares shows a bend in structures that strike from approximately N 35 0 E to about N 35° W. Most of the rocks involved in the Curvature of Monterrey are Mesozoic in age and range from Late Triassic to Late Cretaceous. Large amplitude folds and thrust faults contribute to the structural complexity of this region. The structural trends present in northeast Mexico are the result of the Late Paleocene-Early Eocene Laramide Orogeny, and their different styles of folding are intimately related to the fundamental landforms of Early Mesozoic paleogeography. Relatively mild deformation is shown in the Mesozoic sedimentary cover that overlies the stable paleocontinental basement highs of the Coahuila, La Mula and Monclova Islands, Tamaulipas Archipelago, and El Burro-Peyotes Peninsula. The tight folding observed in the Sierra Madre Oriental is the result of regional northeastward decollement blocked by the basement

highs. Two prominent west-northwest-trending lineaments transect the region: herein named the’'Boquillas-Sabinas I 'and "Sierra Mojada-China" Lineaments. Prominent paleogeographic highs lie on the outside of the area bounded by these lineaments, with the area between occupied by major basins and small "islands". Recurrent motion along these lineaments seems likely and movement along them in Early Mesozoic time blocked out the paleogeographic elements discussed in this dissertation. It is proposed in this study that the structural features of northeast Mexico are the result of a sinistral relative movement of southern United States (westward) with respect to northern Mexico (eastward) during the Laramide Orogeny, contemporaneously with a regional decollement event produced by the tilting toward the northeast of the so-called "Unnamed Occidental Continent" . Thus from the detailed study of the mapped structures and stratigraphic sequences at the Curvature of Monterrey, and from detailed interpretations of satellite photographs, the model presented here for the mechanism of deformation of northeast Mexico explains not only the bend in structures at the Curvature of Monterrey, but also most of the structural trends in northeast Mexico, includong the en echelon folds in the Sabinas Gulf.

TABLE OF CONTENTS

I.- INTRODUCTION 1

A. - Location 1

B.- Methods of Study 1

C.- Previous Works 8

D.- Geologic and Tectonic Setting 9

11.- STRATIGRAPHY 12

A.- Pre-Mesozoic Rocks 18

B.- Triassic System 26

1.- Huizachal Group 26

C.- Jurassic System 33

1.- Zuloaga Limestone 37

2.- Olvido Formation

3-- La Caja Formation

La Casita Formation

D.- Cretaceous System 50

1.- Taraises Formation 50

2.- Cupido Limestone 52

3-- La Pena Formation 57

4.- Tamaulipas Superior Limestone 59

5-- Cuesta del Cura Limestone 63

6.- Indidura Formation 65

7.- Agua Nueva Formation 67

8.- Caracol Formation 69

9.- San Felipe Formation 71

10.- M6ndez Shale 7 2

11.- Parras Shale 73

12.- Difunta Group 78

E.- Tertiary System 80

1.- Ahuichila Formation 80

2.- Intrusive Rocks 81

F.- Quaternary System 83

1.- Alluvium, Alluvial Fans, Playa

Deposits, and Travertine 83

111.- PALEO GEOGRAPHY 85

A.- Late Triassic-Middle Jurassic 86

B.- Late Jurassic 89

C. - Cretaceous 93

D.- Tertiary 103

IV.- STRUCTURES AT THE ’’CURVATURE OF MONTERREY” 105

A.- Northern Area 11l

B. - Southern Area 119

C.- Shallow Basement Area 133

D. - Area of Opposite Vergence 137

E.- La Silla Anticline. An Exception

F.- Origin and Age of Folding and Faulting ...

V.- TECTONICS AND STYLES OF FOLDING IN NORTHEAST

MEXICO 157

A.- Areas of Basement Highs (Horsts) 166

1.- El Burro-Peyotes Peninsula 167

2.- La Mula Island 171

3«- Monclova Island (?) 172

Coahuila Island 173

5.- Tamaulipas Archipelago 175

B.- Areas of Basement Lows (Grabens) 17?

1.- Sabinas Gulf 178

2.- Sierra Madre Oriental 180

3.- Parras Basin 182

C.- Lineaments 183

1.- The Boquillas-Sabinas Lineament ....

2.- The Sierra Mojada-China Lineament .. 188

D.- Model for the Mechanism of Deformation of

Northeast Mexico 189

VI.- SUMMARY AND CONCLUSIONS 201

REFERENCES CITED 205

LIST OF ILLUSTRATIONS

Figure 1-1.- Map showing the location of the area of this study 2

Figure 1-2.- Map showing the physiography of the studied area

Figure 1-3• - DETENAL topographic sheets used as base maps for this study 6

Figure Major paleogeographic elements of Northeast Mexico 11

Figure 11-l.- Map showing the location of known pre-Mesozoic rocks 19

Figure 11-2.- Schematic representation of the rocks of the Huizachal Group 27

Figure 11-3.- Schematic representation of Late Jurassic facies changes

Figure Areal distribution of reef breccias, reefs, and basinal facies 53

Figure 11-5«- Generalized map showing areas of exposures of Late Cretaceous 75

Figure 111-l.- Late Triassic-Middle Jurassic paleogeography of northeast Mexico ... 8?

Figure 111-2.- Oxfordian paleogeography of northeast Mexico 91

Figure 111-3-- Kimmeridgian-Tithonian paleogeography of northeast Mexico

Figure 111-L.- Berriasian-Valanginian paleogeography of northeast Mexico

Figure 111-5»- Late Neocomian paleogeography of northeast Mexico 95

Figure 111-6.- Albian-Cenomanian paleogeography of northeast Mexico 98

Figure 111-7-- Turonian paleogeography of northeast Mexico 99

Figure 111-8.- Coniacian-Santonian paleogeography of northeast Mexico 101

Figure 111-9.- Campanian-Maastrichtian paleogeography of northeast Mexico 102

Figure 111-10.- Tertiary paleogeography of northeast Mexico lOL

Figure IV-1.- Sketch map showing the major anticlines, thrust faults and 106

Figure IV-2.- Map showing the five major areas of different styles of folding at the Curvature of Monterrey 109

Figure IV-3.- Map showing the northern area at the Curvature of Monterrey 115

Figure IV-L.- Sketch map view and profiles of

Las Comitas Anticline 118

Figure IV-5-- Map showing the southern area at the Curvature of Monterrey 120

Figure IV-6.- Map showing the area of shallow basement at the Curvature of Monterrey 135

Figure IV-7-- Map showing the area of opposite vergence at the Curvature of Monterrey 138

Figure IV-8.- Sketch map of the Curvature of Monterrey and surrounding, areas 143

Figure IV-9.- Structural map of the Curvature of Monterrey 145

Figure IV-10.- Schematical cross-sections at the Curvature of Monterrey 147

Figure IV-11.- Sketch map showing the influence basement highs on fold shape 150

Figure IV-12.- Diagram showing the general structure of La Silla Anticline 151

Figure V-l.- Physiography of northeastern Mexico 158

Figure V-2.- Structural map of northeast Mexico 161

Figure V-3-- Generalized geologic map of southern

United States and northeastern Mexico.. 163

Figure V.- Schematic cross-sections showing the different styles of folding 169

Figure Dynamically impossible set of faults proposed by Alfonso-Z 186

Figure V-6.- Diagrammatic distribution of Early Mesozoic horsts and grabens 192

Figure V-7. - Idealized block-diagram showing the major Early Mesozoic grabens and.. 193

Figure V-8.- Idealized distribution of structures if 196

Table 11-l.- Generalized stratigraphic table of northeastern Mexico 13

Table 11-2.- Measured sections of Mesozoic formations at the Curvature of Monterrey 16

Table IV-1.- Structural data of the anticlines in the northern of the Curvature of Monterrey 112

Table IV-2.- Structural data of the anticlines in the southern area of the Curva- ture of Monterrey 123

Plate 1.- Geologic map of the Curvature of Monterrey (in pocket)

Plate 2.- Cross-sections of the Curvature of Monterrey (in pocket)

I.- INTRODUCTION

A.- Location

The area of this study is located in northeastern Mexico, between the cities of Saltillo, Monterrey, Linares, and the village of Concepcion del Oro. It includes parts of the States of Coahuila, Nuevo Leo'n, San Luis Potosi, and Zacatecas, and its areal extent is approximately 23,000 km (Figures 1-1 and 1-2).

B.- Methods of Study

This study was done using vertical aerial photographs at an approximate scale 1:50,000, released by DETENAL (Direccidn de Estudios del Territorio Nacional), and plotting the geology on topographic sheets, scale 1:50,000, also published by DETENAL (Figure 1-3).

Stratigraphic and structural data were collected during numerous field seasons, from January, 1975 until October, 1981. Compilation scale was 1:50,000 for the geologic map presented here at a scale 1:200,000.

The fossils collected for this study were identified by the Departamento de Paleontologia of the Instituto de Geologia of the Universidad Nacional Autonoma de Me'xico.

Figure 1-1.- Map showing the location of the area of this study.

Figure 1-2.- Map showing location of named mountain ranges and measured sections of the study area. My-Monterrey; SA-San Antonio de las Alazanas; A-Allende; MS-Montemorelos; G-Galeana; L-Linares; EC-E1 Carmen; ES-E 1 Salvador; LV-La Ventura; CO-Concepcion del Oro; S-Saltillo.

Figure 1-3.- DETENAL topographic sheets used as base maps for this study.



Figure 1-1



Figure 1-2



Figure 1-3

C.- Previous Studies

The previous studies involving different parts of the area approached stratigraphic and structural problems in small areas mainly in the western and northern parts of this study. The first workers made preliminary recon - naissance surveys ["Bose, 1923; Baker (1920-21 and 25), published in 1971; Haarman, 1917], with later studies more detailed (Humphrey, Acevedo and Marquez, 1950; Diaz-G., 1951), and/or regional interpretations (De Cserna, 1955, 1955; Rogers et al. , 1955, 1951; Tavera, 1950; Wall et al., 1951). More recent studies have been directed to specific problems (Cantu-Chapa, 1975; Wilson and Pialli, 1977; Padilla y Sanchez, 1978 a, 1978 b; Moor, I98O; and Kleist et al., I98O). Numerous additional studies have been done in this area in the last five years by the Facultad de Ingenieria of the Universidad Nacional Auto'noma de Mexico, and by geologists of PEMEX (Petroleos Mexicanos), but unfortunately they remain unpublished.

D.- Geologic and Tectonic Setting

The Sierra Madre Oriental of Mexico is considered to be that cordillera which strikes NW-SE between Cd. Juarez, Chihuahua, and Torreon, Coahuila; about E-W between Torreon and Saltillo, Coahuila; about SE-NE and E-W again, between Saltillo, Coahuila and Monterrey, Nuevo LeJn; and about NW-SE southeast' from Monterrey, until it is covered by the Trans-Mexican Vocanic Belt. It reappears further southeast where it is called Sierra Madre del Sur.

The Sierra Madre Oriental is composed mainly of a thick sequence of Mesozoic carbonate and terrigenous rocks that were folded and thrust faulted during the Late Paleocene-Early Eocene Laramide Orogeny.

This Cordillera has two main bends in structure, one at the vicinity of Torreon, Coahuila, and another at the vicinity of Monterrey, Nuevo Leo'n. The latter is herein named the ’’Curvature of Monterrey" (Figure I-M •

The Curvature of Monterrey is located at the southern end of the Sabinas Gulf, southward of the horst block of the Coahuila Island, and westward of the horst block of the San Carlos Island (Figure I-M . The arcuate forms of the folds were generated during the Laramide Orogeny, when the Mesozoic rocks slid northeastward on

a decollement over Early Mesozoic evaporites, and were folded and thrust-faulted against the mentioned basement highs, producing a general vergence of these structures toward the north and northeast.



Figure I-J.- Major Late Jurassic Paleogeogfaphic elements of northeastern Mexico. The Sierra Madre Oriental is a younger feature developed during the Early Tertiary Laramide Orogeny. After Humphrey (19^9), Gonzalez (1976), and Lopez-Ramos (I98O).

II.- STRATIGRAPHY

The Mesozoic and Cenozoic rocks of northeastern Mexico have been studied since the late 1800’s and as a consequence, a considerable number of stratigraphic studies are available today in the literature. In most of these works the principal objective was to determine the stratigraphy of this area through the study of fossils, largely ammonites, and lithologic descriptions. The results of these studies set the basis for many formational names. A great number were defined for very local exposures which produced a very complicated region al stratigraphic nomenclature. However, some attempts were made to relate regional facies to paleogeographic reconstructions (Imlay, 1938; Humphrey, 1956; Gonzalez, 1976).

Because of the complexity of the stratigraphic nomenclature in northeastern Mexico, the use of formational names here is restricted to the minimum, with the exception of the area of detailed study, which comprises the highlands between the cities of Concepcion del Oro, Saltillo, Monterrey, and Linares (see Figure 1-1). Table 11-l shows the generalized stratigraphy between the Rio Grande to the north, the Gulf of Mexico to the east,

parallel 30’ Lat. N to the south, and meridian 104 W to the west.

Most of the rocks present in the mountains of the Curvature of Monterrey are Mesozoic and vary in age from Late Triassic to Late Cretaceous, but in some places Tertiary igneous bodies intrude them. The valleys between the mountains are generally filled by alluvium, alluvial fans, and playa-deposits (Table 11-l, column 1).

The Mesozoic formations in this area have large differences in thickness from place to place. A summary of these variations is shown in Table 11-2.

HUMPHREY, 1956; RAMIREZ-C., 1966. HUMPHREY, 1956; MARQUEZ at al., 1976. HUMPHREY, 1956; AGUAYO, 1978. HUMPHREY, 1956; RAMIREZ-C,I966;RODRIGU;TZ, E ‘ R A SERIES SUBSTAGES AGE in m. y. 1 AREA OF THIS STUDY 2 COAHUILA ISLAND 3 SABINAS GULF 4 TAMAULIPAS ARCHIPELAGO 5 ANCESTRAL GULF OF MEXICO ithology WEST J EAST lithology S 0 UTH NORTH SOUTEAST NORTHWEST WEST EAST WEST EAST c a A T riOLOCENE - f - ALLUVIAL FANS, ALLUVIUM, PLAYA DEPOSITS, LACUSTRINE DEPOSITS, AND RESIDUAL SOILS 02 recent(marine) PL E IITO c in t (marine) E N O Z o 1 ? Bi / G 1SALTS T E R T 1 A R Y n-EJO 1 Wurt PLIOCENE -2-3 -- -5—7 -- - 26 - -37-38- -53-54- BASALTS — ? — RANITES 0 0 L 1 A D FORMATION - p L.A 6 AR TO FORMATION MIOCENE OLIGOCENE X X i < < ■ —_____ < X rf x t X X * * .o:o~ Q; (ACID INTRUSIVESP ? ' G RANI IhS / 2 OAKVILLE FORMATION ANAHUAC FORMATION ! F H 1 0 FOP HATIOU - VICKSBUR9 0 R 0 U P 0 M J A C X B 0 N 6 R 0 U P EOCENE 7> CLAIBORNE CLAIBORNE 9 R 0 U P WILCOX WILCOX 9 R 0 U P MIDWAY 9 R 0 U P MENDEZ SHALE c M C R PALEOCENE — - 65 - - ' ■ 9 DIFUNTA GROUP — p DIFUNTA 2. CESCOND/DO FM 2 —— MEN DEZ SHALE MAASTRICHT! AN .... ....A u r w nr? cu A i r z —-~—■ OLMOS FM. SANMIGUEL FM. S E N 0 CAMPANIAN SANTONIAN RR C. U L. Z O fl pi L. £L Q _ _ _ _ PARRAS SHALE C PARRAL ft U A I F MENDEZT UPSON FM. •. — — — - _ S A—,—‘ SHALE 5 S HA L E( E * t - - - . Xf 1 - TL • J* f — — — — — — • i M AUSTIN CHALK AUSTIN CHALK p E T UPPER FFl IPF FM C 1 1 — CONIACIAN 7i^C^ CARACOL FM - £ Si - r -T y TURONIAN INDI DURA FM. > AGUA NUEVA 1 N D 1 D U R A FM. FORD Iwashita group INDIDURA BEAGLE FORD F M. EAGLE FORD F M. EAGLE FORD F M. S O z o A C E O u s LOWER - 136 - CENOMANIAN ALBIAN r |CUESTA DEL AMAULIPAS ) CURA RIO R j— r } stoneZ^ l i me stone AURORA > r L! ME STONE AT IT A F M~ t ~^ L CUESTA DEL ORA LI M ESTON E . . r-.. > n r- . n r. . . . r-.x ** S " ,TA yp T AMAULIPAS K •-* U P E R 1 OR AURORA LIMESTONE LIMEST 0NE TA MAU LIPA S SUPERIOR LIMESTONE p T| An k N GARGASIAN BEDOULIAN BARREMIAN HAUTERIVIAN VALANGINIAN BERRIASIAN 1 -A hr± LA F t N A r M. ) i ® ■; CUPIDO LIMESTONE 7 \ L P L A PENA F M . i CUPIDO LIMESTONE lm r c. n r m CUPIDO L 1 M E S TO TAUAULI PA LA V1 P 6 E N INFERIOR L A M U L A FM. c. M HOMLL r PR. N E L A » L. FiA • RR • Vz I aa i L. O i IW • CUPIDO INFERIOR LIMESTONE*-* FORMATION TAMAULIPAS INFE F 0 R M AT 1 0 — ? RI OR N 2 a TARAISES FORMATION j^r 1 - 1 SAN MARCOS ARKOSE > P A D / L L A F TA RA ISES TARAISES FORMATION TARAISES p FM. J u R A S s 1 c UPPER TITHON1AN KIMMERIDGIAN OXFORDIAN LA CAJA FM. LA CASITA FM. \0 O D 0 0© DO 1 LA CASITA FM. LA CASITA FM. ill T? PIMIENTA LA CASITA FM. FM AZULOAGA LIMESTONE FM. / > p — 5L p LIMESTONE — ZULOAGA LIMESTONE MINAS VIEJAS FM. FM. P ZULOAGA UM ESTON ’ LIMESTONE — MIDDLE CALLOVIAN L —- r - — v — r A JOYA' FM. ? FM. BATHONIAN BAJOCIAN I 2 LO CATION MAP ' X E X 1 C o\\ U.S.A. xCaW'-A AALENIAN LOWER 1 c 190-195- z*“• 1 I1GN MOI EOUS-META- RPHIC COMPLEX LA GR <MJIZACHAL BOCA {.GROUP (2) — — ? V. R 1 A S S 1 c UPPER MIDDLE RHAETIAN NORIAN CARN IAN LADIN IAN ANISIAN v —- — ** — T GROUP p . ~s - X 1 1 \ -■ 2 .1 PAR / In ./ / ✓ Z\ \ ' \ V ' \ )\\ LOWER SCYTHIAN — — — — — _ — — — — — —- — — — ANITES B GRANODIORn 2 —— L 'A k E O z o 1 r PERMIAN -230 — /las DELICIAS-ACATITA 1 1 PERMIAN FLYSCH 1 ES METASEDIMENTS a <7T CARBONIFEROUS DEVONIAN -235 - -395 - -43O-44< - - 500 — ) SILURIAN ORDOVICIAN ' c,X ' >oro »' \ . —» \ • L » I ( CD. VICTORIA S • CAMBRIAN ? PRECAMBRIAN 3 fiOO + 2

Table 11-l.- Generalized stratigraphic table of northeastern Mexico. For detailed descriptions of the stratigraphic units of this study see text.

s. s. s. s. s. s. s. s. s. s. s. s. s. s. s. s. s. s. s. LOCALITY de los Muertos de la Silla de San Lucas de San Cristobal del Chorro de Jame Hermosa El Muerto Las Hormigas El Jabali San Juan El Pedregoso de Rocamonte de las Mazmorras de las Valias Pinal Alto El Gateado El Oregano Labrador THICKNESSES Jz Jloj Ties Kta 200* 800 300 60* 300 265 122* 421 200 100 195 8* 550 240 250* 604 350 75* 92 305 37* 90 150 270* 90 160 117* 103 150 190* 100 139 600* 160 100 270* 150 150 60* 95 14-7 90 250*380 150* 85 350 OF MESOZOIC FORMATIONS Kcu Kip Kts Kcc Ki 645 85 380 102 800 90 350 690* 72 348* 500 67 357 750 7 280* 490 25 150* 620 110 575 80 420 95 103* 290 65 106 30 230 50 110 42 299 61 117 50 300 65 130 30* 500 125 270* 412 120 250 79 345 98 175 1,550*100 150 700* 75 4oo 490 70 180 520 55 200 80 1 (in meters) Kan Kca Ksf 293 150* 60* 75* 100 150* 150* 350 250* 110 193* 100 173* Kpa Km 200* 70 25* section

Table II-2.- Measured sections of Mesozoic formations at the Curvature of Monterrey. For location of sections see Figure 1-2 and Plate 1. Jz-Zuloaga Limestone; Jlcj-La Caja Formation; Jlcs-La Casita Formation; Kta-Taraises Formation; Kcu-Cupido Limestone; Klp-La Pena Formation; Kts-Tamaulipas Superior Limestone; Kcc-Cuesta del Cura Limestone; Ki-Indidura Formation; Kan-Agua Nueva Formation; Kca-Caracol Formation; Ksf-San Felipe Formation; Kpa-Parras Shale; Km-Mendez Shale.

A.- Pre-Mesozoic Rocks

Little is known about the pre-Mesozoic rocks in this area, because there are no outcrops, and because of the absence of subsurface information. Even when some information can be used to try to understand the basement rocks in this part of Mexico, any conclusion must be regarded as preliminary until direct evidence (i.£. core samples) becomes available.

There are only two outcrops of pre-Mesozoic rocks, near the studied area; Acatita-Las Delicias and Apizolaya (Caopas-Rodeo) to the west, and four wells that have penetrated basement, the Benemerito-1, the Teran-1, the Trincheras-1, and the Linares-1 to the northeast (Figure 11-l).

The Paleozoic rocks that crop out at Las Delicias- Acatita area are composed mainly of fossiliferous, Permian, marine sandstones and shales (Acatita), intrud ed by granodioritic bodies (Late Triassic) cut by aplitic dikes (post-Late Triassic-pre-Late Aptian).

The sedimentary rocks of Las Delicias-Acatita, known as the ’’Permian Flysch”, were first studied by Haarmann (1913) and Bose (1923), and more recently by King (193 M King, et al. , Humphrey (1955), Newell (1957), and Wardlaw, et al., (1979)- This sedi-

Figure 11-l.- Map showing the location of known pre- Mesozoic rocks, surface and subsurface, in the areas surrounding this study.

mentary sequence is formed mainly by more than 3,000 m (King, et al., of greywackes, shales, and conglomerates with gravels of limestones and quartz. Some synsedimentary volcanic flows (andesites and basalts) are interbedded in the sequence. There are also large blocks of reefal limestones (Leonardian-Guadalupian), which have been considered by Newell (1957) as "exotic blocks" originally deposited in a neritic zone westward of this area. The age of this sequence has been determined by means of its abundant marine fauna, which consists mainly of Permian fusulinids, brachiopods, and ammonites, as Leonardian-Amarassian, according to Wardlaw, et al., (1979).

This "flysch-liKe” sequence is intruded by biotite and hornblende granodiorite, and by a few bodies of hornblende granodiorite. Both plutonic types are also intruded by aplitic dikes. The age of these igneous rocks was considered for a long time to be post-Permianpre-Late Aptian, until Denisson, et al., (1969) obtained ages of and m.y. (Late Triassic) in biotite taken from the granodiorite, and ages of and m.y. in whole rock samples of a biotite argillite, by the K/Ar method.

The pre-Mesozoic rocks that crop out in the Apizo- laya area (Caopas-Rodeo) are less well known than those of Acatita-Las Delicias. They are present in the core of the San Julian Anticline, which trends about N 30° W and include Caopas Schist, and the overlying Rodeo and Taray Formations.

The term Caopas Schist was first used by De Cserna (1956), but was formally proposed by Rogers, et al. (1961), who designated the village of Caopas as the type locality of this unit. Co'rdoba described this unit as a hard, slabby, siliceous, green metarhyolite which consists of an aphanitic groundmass with small phenocrysts. Locally it does not show well developed schistosity, but has pronounced lineations and foliations. Its thickness was estimated to be more than 500 m by Cordoba (op. cit.).

The age of the Caopas Schist is uncertain, Rogers, et al. (op. cit) considered it to be Late Paleozoic because of its degree of metamorphism, the complexity of the structure, and its similarity with the Carrizo Mountain Schist of the Van Horn area. This age was not confirmed by later geochronologic determinations, which mostly suggested an age that varies from Lower Cretaceous to Pemo-Triassic. Fries and Rincon-Orta (1965) obtained ages of 156- 195 - 20, 200- 60,

and 220i30 m.y. by the Rb/Sr method using rock samples Dennison, et al. (1969) obtained on age of 141140 m.y. by the Rb/Sr method using one whole rock sample. A more recent geochronologic determination was made by Halpern, et al. (1974) who obtained an age between 125 and 225 m.y. by the Rb/Sr method using whole rock samples. As can be seen, these ages are not significant and their differences have not been explained.

Unconformably overlying the Caopas Schist is the Rodeo Formation which consists, from bottom to top, of crenulated schists, chloritic schist, conglomeratic phyllites, phyllites, and a conglomerate with pebbles of chert and metamorphic rocks (Co'rdoba, 1964) The thickness of this unit in the type locality is 996 m. The age of this formation is also unknown. Cordoba (op. cit. ) assigned it tentatively to the Precambrian because of its similarity with the green schists of Zacatecas. Rogers, et al. (1961), previously had suggested abate Paleozoic age (probably Permian) for the Rodeo Formation but, again, no definite age has been stated, and no further studies have been made of this unit.

Overlying unconformably the Rodeo Formation is the Taray Formation, informally defined by Cordoba

(op. cit. ). It consists mainly of (from bottom to top) light brown phyllite, steel-gray phyllite interbedded with conglomeratic phyllite, light and dark gray phyllite interbedded with greywacke, dark gray dolomite interbedded with gray phyllite and novaculite, brown greywacke and gray phyllite, novaculite with interbedded light-gray phyllite, and yellow-brown greywacke with a few conglomeratic beds. Cordoba (op. cit.) estimated the thickness of this unit at more than 690 m, and assigned an age of Late Paleozoic, probably Early Devonian to Early Pennsylvanian (?), because of its resemblance with the Tesnus Formation and the Caballos Novaculite of the Marathon region of West Texas. There have been no later studies of this formation.

The subsurface information about pre-Mesozoic rocks near the studied area is restricted to four wells which penetrated basement rocks (Figure 11-l). All of these four wells penetrated granitic rocks, of which three were dated by geochronologic methods (Rb/Sr ?) (Rivera- J. , 1976). The results obtained were as follows:

WELL LITHOLOGY AGE (inm.y.) Benemerito-1 Granite Teran-1 Granite Trincheras-1 Granite ? Linares-1 Granodiorite

It can be seen that with these scattered data available in the literature about the pre-Mesozoic rocks of northeastern Mexico, it is very difficult to make a non-speculative interpretation. At least, we can conclude that the western part of the San Carlos Island is, apparently, formed by granites that vary in age from Late Permian to Late Jurassic.



Figure 11-l

B.- Triassic Svstem

1.- Huizachal Group.

The oldest Mesozoic rocks in the studied area are unfossiliferous, non-marine redbeds, that have been assigned to the Huizachal Group, a term proposed by Mixon, et al. (1959), tut first used by Imlay, et al. as Huizachal Formation, to describe a sequence of redbeds that crops out in the Valle de Huizachal, located about 22 km south-southwestward from Ciudad Victoria, Tamaulipas.

The rocks of the Huizachal Group in the Galeana area had been described by Diaz (1951), Tavera (i 960 and Padilla y Sanchez (1978 b Later studies by Belcher (1979), a nd Moor (I98O) have added some detail to the knowlwdge of this sequence, but until today, no detailed sedimentological nor stratigraphic studies have been made in this area.

The redbeds which crop out at the base of the Mesozoic sequence in the Galeana area can be roughly divided into a Lower and Upper unit (Figure 11-2). They have been correlated to La Boca and La Joya, respectively, of the Huizachal Group of Mixon, et al. (op. cit.), but in this study they were mapped together as a single unit.

Figure 11-2.- Composite schemetic representation of the rocks of the Huizachal Group that crop out in the Galeana area. The lower part is about 130 m thick, whereas the Upper part is approximately 20 m thick. The succession is not shown at scale.

The lower part of the Huizachal Group (its base is not exposed) consists, from the bottom to the top, of approximately IjO m of thick beds of red, purple and green siltstones with abundant volcaniclastics, and btownish, crossbedded, immature sandstone that contain numerous cut-and-fill structures, trough-tabular crossbedding, parallel laminations, and isolating climbing ripples. These rocks are folded, faulted, intruded by several aplitic (?) dikes and sills (1 mt thick), and have an incipient schistosity. In general, the tendency of these beds is to coarsen upward, which suggests that they could be the result of fluvial deposition, perhaps braided streams.

The upper part of the Huizachal Group is apparently unconformable over the lower part, but the contact is mostly covered by soil. The upper part consists mainly of about 20 m of reddish conglomerate interbedded with brownish conglomeratic sandstone. The conglomerates consist mainly of angular to subangular cobbles and pebbles of metamorphic (schists and quatzites) and igneous rocks (granites and andesites ?), that are poorly sorted and contained in a matrix of fine sand and silt. The conglomeratic sandstones are immature, poorly sorted, and almost structureless, but sometimes show graded bedding.

These coarser deposits are not laterally continuous, which suggests that they can be the result of debrisflows that descended from nearby highlands and were deposited over the finer clastics of the lower part.

The Huizachal Group in this area definitely represent a continental deposit. The lower part resembles more closely a fluvial environment with braided streams on an alluvial plain or on distal parts of alluvial fans, whereas the upper conglomerates and conglomeratic sandstones could represent proximal facies (debris flows) on alluvial fans as Belcher (1979) suggested.

The unconformity between the upper and lower part of the Huizachal Group rocks is particularly significant because it could represent an indication of tectonic activity or just a period of uplift and erosion. This point is important if we compare these rocks with those of the Cd. Victoria area, where La Boca (Late Triassic ?) and La Joya (Late Jurassic ?) are separated by a sharp angular unconformity (Mixon, et al., 1959)• This unconformity has been interpreted by Carrillo-Bravo (1971, p. 93) as the result of the stresses of the "Orogenia Permo-Triasica” which folded and uplifted the redbeds of the La Boca Formation.

In the Galeana area the contact between both parts of the Huizachal Group is not well exposed as in the Huizachal Valley and the upper conglomerates occur only at two localities (Cerro El Cuatro and Cerro La Nieve). The Olvido Formation unconformably overlies both the lower and the upper parts of the Huizachal. Because the angular unconformity between the lower and upper parts of the Huizachal Group is not very significant (8° to 12°), it seems more reasonable to interpret it as ’’sedimentary” and not tectonic as Belcher (1979) suggested .

The age of the redbeds in the Galeana area has been highly controversial, because their base is not exposed, there are no fossils, and because it is unconformably overlain by the Olvido Formation, which also has not been properly dated. The nearest, reliably dated fossils are from the Kimmeridgian La Casita Formation, which conformably overlies the Olvido Formation and is separated from the redbeds by several hundred meters of interbedded gypsum and limestone. Belcher (op. cit.) speculated that the upper part of the Huizachal Group in the Galeana area is interbedded with the Olvido Formation and he assigned a Kimmeridgian- Tithonian age to these conglomerates. I do not agree with his interpretation because he did not point out

the exact locality where he observed the mentioned interbedding, and also because after several years of field work in this area I always have found the Olvido Formation overlying unconformably, although indistinctly, both the lower and the upper parts of the Huizachal Group, but never an interbedding.

In this study the redbeds of the Huizachal Group are considered to be post-Early Triassic-pre-Late Oxfordian, based on stratigraphic relationships between the redbeds and the overlying Olvido Formation, which laterally interfingers with the Zuloaga Limestone in the San Roberto Area (see Table 11-l).



Figure 11-2

C.- Jurassic System

In northeastern Mexico the early Upper Jurassic seas allowed the deposition of thick sequences of terrigenous, carbonate, and evaporitic rocks, which extend ed all over the so-called ’’Mexican Geosyncline” , the Sabinas Gulf, and the Ancestral Gulf of Mexico. The conglomerates and sandstones of the La Gloria Formation are characteristically restricted to the borders of paleopositive landmasses; the limestones of the Zuloaga and Novillo Formations are carbonates deposited on shallow and extensive platforms; and the evaporites of the "Metate” (anhydrite), Olvido (gypsum), and Minas Viejas (halite and anhydrite), are deposits restricted to grabens. The regional relationships between these lithostratigraphic units are still poorly understood, but some attempts have been made to relate their facies (Humphrey and Diaz, 1956; Humphrey, 1956; Madrid, 1976; Aguayo, 19?8). Figure 11-3 shows the lateral facies relationships and the formational names used today for Late Oxfordian-Early Kimmeridgian rocks in northeastern Mexico.

In the area of this study the Zuloaga Limestone crops out mainly in the cores of breached anticlines in the northwestern part, and the Olvido Formation

Figure 11-3-- Schematic representation! of Late Jurassic facies changes and. stratigraphic nomenclature in northeastern Mexico. The cross-section is not shown at scale.

crops out mainly in the southeastern part. Both formations conformably underly fossiliferous beds of La Caja and La Casita (Late Kimmeridgian-Late Tithonian) Formations. The base of the Zuloaga does not crop out, whereas the Olvido unconformably overlies older redbeds of the Huizachal Group in the southern part of the studied area.



Figure 11-3

1.- Zuloaga Limestone

This formation was formally defined by Imlay (1938 b in the Sierra de Sombreretillo, located north of the village of Melchor Ocampo, Zacatecas. The Zuloaga Limestone crops out in the studied area mostly in the west and northwest portions. It is fairly homogeneous lithologically throughout the mapped area, but its base is never exposed. In most of the area, with the exception of the southern part, the Zuloaga Limestone consists (from bottom to top) of medium bedded (0.30-0.80 m) gray mudstones and wackestones with some thin intercalations of reddish, limy shales, which at some localities (Sierra de Rocamonte) can be as thick as m and contain some nodular black chert. Upward, the formation become more calcareous and the shale beds become thinner and fewer. The upper carbonates are formed mostly by thick beds (up to m thick) of dark gray to black mudstones and wackestones, mostly unfossiliferous, which at some localities have horizons containing gastropods identified as Nerinea sp., abundant fragments of corals, and considerable unidentifiable bioclastic material. Random dolomitization also impedes identification of organic remains. The uppermost part of the Zuloaga Limestone is characterized in the northern and northwestern parts of the mapped area by some poorly exposed oolitic horizons. In the Sierra de Rocamonte it was possible to distinguish three different lithologies for the Zuloaga. The lower unit consist of about 300 ni of mostly . mudstones with intercalated limy shales; the middle unit (10 to 15 m thick) consists of thin beds of gypsum, shale, and siltstone; and the upper unit of about 290 m of dense black mudstones with some zones containing Nerinea sp.

The Zuloaga Limestone was probably deposited in warm and shallow water, as is suggested by fragments of corals and by oolites, and with a very low input of clastic and terrigenous material, which also suggest low relief in the positive areas.

The thickness of the Zuloaga Limestone is unknown in the studied area because its base is not exposed in any outcrop. Incomplete sections measured by this author vary from 8 m in an outcrop south of El Chorro Canyon, to more than 600 m in the Sierra de Rocamonte. For these reasons it is very difficult to try to estimate the thickness of this formation. Although in a general manner, it can be said that the Zuloaga Limestone thins ’’outward” from the studied area, that is, to the north against the Coahuila Island, to the south against the Valles-San Luis Potosi Platform, to the west against the "Unnamed Occidental Continent" and to the east against the San Carlos Island (see Figure 11-3 Table 11-l, and Chapter 111 refering to paleogeography)

The age of the Zuloaga Limestone has been, and is, a controversial point because of its lack of characteristic fossils. Some authors have assigned it to the Late Oxfordian (Rogers et al., I96I; Moor, I98O), whereas others to the Late Oxfordian-Early Kimmeridgian (Humphrey, 1955; Padilla y Sanchez, 1978 a). In this study the Zuloaga Limestone is considered to be pre- Late Kimmeridgian, because it conformably underlies the La Caja and La Casita Formations, which have fossils that belong to the Late Kimmeridgian.

The Zuloaga Limestone is correlated to the north, west, and northwest with La Gloria Formation, to the south with the Olvido Formation, and to the east, northeast, and southeast with the Novillo Formation. Figure 11-3 and Table 11-l.

2.- Olvido Formation

This formation was first described by Heim in the eastern part of the Huizachal Valley, in the canyon of the abandoned Olvido Ranch, where it consists mostly of yellow to orange-weathering limestone and dolomite with interbedded green, red and violet clay some sandstone, and blocks of gypsum. Heim (op. cit., p. 320) named this formation ’’Gypsum Beds”, and stated

... in the wild canyon of the abandoned Olvido Ranch, the variegated clays show fresh exposures with big blocks of gypsum.

The writer nowhere found exposure of the contact with the overlying Tamabra Limestone. In Peregrina Canyon, the lowest exposures of Tamaulipas Limestone seem to overlie the Gypsum Beds directly. The age is still problematical...”

On the other hand, Lopez-Ramos (1980, p. 313) states that in the region of Ciudad Victoria the Olvido Formation lies conformably on the Zuloaga (Novillo) Formation and, also conformably underlies the La Casita Formation. There it consists of the lower unit of thin gypsum intercalated with red, yellow and green

shales; a middle unit of medium-bedded, fine-grained limestones alternating with green and yellow marls, and some thin beds of gypsum; and an upper unit of mediumbedded limestones with nodules of white chert and abundant recrystallized foraminifera.

The mentioned lithologic sequences are somewhat similar to the rocks of the Olvido Formation in the Galeana area, with the exception that in the latter the gypsum beds are much more abundant than in the type locality. Because of its different lithology and its different stratigraphic position this formation has been the cause of many discussions in the Galeana area. Diaz (1951) considered this unit to be Late Oxfordian, and consequently, belonging to the Olvido Formation. Later on, De Cserna (1956) assigned it to the Minas Viejas Formation (Early Oxfordian), and Tavera assigned it to the Olvido Formation.

The reason for these discrepancies is mainly that the Olvido Formation at the type locality overlies the Novillo Limestone (Zuloaga equivalent),whereas in the Galeana area it overlies the redbeds of the Huizachal Group. However, the fossiliferous La Casita Formation conformably overlies (at the type locality and at Galeana) the Olvido Formation, this being the reason why Diaz (op. cit. ), Tavera (op. cit. ), and Padilla y Sanchez (1978 b assigned it to the pre-Late Kimmeridgian. Further more in the Galeana-San Roberto area the Olvido Formation passes laterally into the Zuloaga Limestone (Table 11-l, Figure 11-3, and Plate 1).

The Olvido Formation in the Galeana area consists, from bottom to top, of a thin basal unit m) of reddish silts and clays, interbedded with thin layers of gypsum. Thick beds of wackestone and gypsum overlie the basal unit, the wackestone contains abundant unidentifiable fragments of gastropods and microfossils in its lowest part, while the gypsum beds are characterized by the presence of thin layers (3-9 m) of darker gypsum which contains traces of clay. Toward the upper part of the formation the beds of limestone become unfossiliferous, thinner, and less abundant, whereas clay and gypsum (thinly bedded) predominate. The upper contact of this formation with the overlying La Casita Formation is transitional but well defined.

The thickness of this formation was not measured due to the fact that it is highly folded and distorted. However, Diaz (1951) estimated the thickness of this unit to be more than 600 m in the Galeana area.

The presence of silt in the base of the Olvido

Formation suggests that it was possibly derived from the last vestiges of the positive lands which produced the underlying conglomerates (Huizachal). The transition from a silty base to thick masses of gypsum and limestone suggests that they were deposited in a closed basin, where circulation was poor and evaporation kept the shallow waters hypersaturated. During periods of flooding a new supply of normal sea-water produced shallow, submarine carbonate muds, where marine organisms which had not been able to live in earlier saline waters, temporarily flourished in laterally extensive mounds. Later, and longer, periods of hypersalinity possibly caused the deposition of thick masses of gypsum, with a sporadic supply of marine water, but not long enough to permit the development of life. Toward the end of the Early Kimmeridgian the region underwent greater subsidence, which allowed the deposition of the large amounts of silts and clays of the overlying La Casita Formation.

The Olvido Formation is considered here to be pre Late Kimmeridgian, and correlatable with the Zuloaga Limestone. Table 11-l and Figure 11-3.

3.- La Caja Formation

La Caja Formation was defined by Imlay (1938) at the Vereda del Quemado, in the southern flank of the Sierra de la Caja, north of Mazapil, Zacatecas. There, it consists mainly of thin bedded limestone and marl, and occurs in narrow strips around the outcrops of the Zuloaga Limestone. Its deposits become finer and more calcareous toward its top. In a detailed study of this formation Rogers, et al. (1961) distinguished four dif ferent lithologic units.

La Caja Formation in the area of this study conformably overlies the Zuloaga Limestone, and also conformably underlies the Taraises Formation. It consists of 25 to 150 m of thin beds of wackestones rhythmically intercalated with siltstones and shales. Toward its upper part it becomes more calcareous, and contains thin layers of gypsum (Sierra del Fraile, Sierra del Jabal!) mostly in the western part of the mapped area. It is highly fossiliferous throughout its entire section, but mainly in the parts where large phosphatic concretions (sometimes 1.3 m in diameter) occur. The lower part of the formation contains well-preserved shells of Trigonia sp. and Idoceras sp. Several fossils were collected in the middle and upper part of this unit and

they were identified as Metahaploceras boesi (Burckhardt), Idoceras zacatecanum Burckhardt, Idoceras balderun (Oppel), Aspidoceras sp., Lytoceras sp., Involuticeras mazapilense (Burckhardt), Glochiceras fialar (Oppel), Glochiceras diaboli Imlay, Perisphinctes sp., Haploceras sp., several Aptychus sp., bones of reptiles (dinosaurs ?)., and pieces of petrified wood.

La Caja Formation was probably deposited in shallow marine water, and over a very extensive platform that covered most of the studied area. This is suggested by lithology and fossils. It passes laterally into La Casita Formation, which is considered to be its nearshore equivalent (Imlay, 1938).

The age of La Caja Formation is Late-Kimmeridgian- Late Tithonian as shown by its fossil content (Table II 1).

4.- La Casita Formation

La Casita Formation was named by Imlay (1936) for a sequence of fossiliferous black shale, sandstone, conglomeratic sandstone, and intercalated limestone, which are overlain by the Taraises Formation and underlain by the Zuloaga Limestone. Its type locality was designated at the Canon de la Casita, about 16 km south of General Cepeda, Coahuila, and about 50 km southwest of Saltillo.

La Casita Formation outcrops extensively in the cores of most anticlines, in the eastern part of the area of this study. It conformably overlies both the Zuloaga and Olvido Formations, and also conformably underlies the Taraises Formation. Its thickness varies from about 100 m, south of Saltillo, to nearly 800 m south of Monterrey. Its lithology is also variable. In the vicinity of Saltillo, in El Chorro Canyon (the base is not exposed), it consists, from bottom to top, of a lower unit of coarse conglomerates with pebbles of quartz, schist, and granite, in a coarse-grained sandy matrix, which becomes finer upward into a quartz-rich sandstone intercalated with thin layers of siltstone with remains of plants (?); a middle unit of mostly fine-grained, yellowish, quartz-rich sandstone, intercalated with dark brown silty shale; and a upper unit

of coarse-grained, sometimes conglomeratic, sandstone intercalated with sandy shale and siltstone, and occasional, thin bedded, gray, laminated, sandy (rounded quartz grains) limestone.

In this locality Ms. Kathy Fortunato*, a graduate student from the University of New Orleans, collected several samples bearing agglutinated foraminifera and ostracoda. These microfossils were studied and identified by C.C. Albera from AMOCO Production Company of Houston, who assigned them a probable Upper Cotton

Valley (Kimmeridgian-Portlandian) age. The species collected were the following:

Foraminifera Ostracoda Citharina entypomatus Cyprideis pfannensteili Citharina lepida Cytherelloidea sp. Eoguttulina oolithica Hutsonia sp. Lenticulina dilecta Schuleridea triangularis Lenticulina gaultina Lenticulina guttata Lenticulina nodosa Marginulinopsis phragmites

Marsonella trochus Planularia cordiformis Saracenaria cornucopia Tristix oolithica Trocholina sp. Vaginulina kochil Vaginulinopsis loeblichorum

East of El Chorro Canyon, west of El Tunal, in the same anticline, La Casita Formation shows a sudden change in lithology. There, it consists of mostly black shale with abundant fossils (Gryphaea sp.), intercalated with fossiliferous sandy limestone. This lithology occurs in most of the outcrops between Saltillo and San Rafael (Sierra El Pinal Alto), to the south, and between Saltillo and San Jose de Boquillas, to the east. In the Cano'n del Viejo, east of San Rafael, this formation is intruded by thin (20-30 cm) aplitic dikes. Eastward of a line conecting the towns of Santa Catarina-Carbonera-San Jose 7 de Boquillas-Cienega del Toro, La Casita Formation becomes more shalier and calcareous (in some cases with gypsum interbedded), and it consists of a lower member of black, carbonaceous shale that contains abundant oysters (Gryphaea sp.) and undeterminable bivalves; and an upper member of packstones

and wackestones with abundant ammonites (Idoceras and Haploceras), intercalated with calcareous shales which contain a large number of black, phosphatic, fossiliferous concretions. Some of the fossils collected in this formation were determined as Aspidoceras sp., Haploceras zacatecanum Burckhardt, Haploceras transatlanticum Burckhardt, Perisphinctes sp., Durangites sp., Idoceras complanatum Burckhardt, Exogyra sp., and Aptychus sp.

Generally, La Casita Formation has coarser terrigenous material toward the north-northwest part of the studied area, and becomes shalier and more calcareous eastward. Its lithology suggests that the deposition of this unit took place along the shallow waters surrounding the Late Jurassic emerged lands (Coahuila and San Carlos Islands), which supplied the clastic sediment. La Casita Formation is assigned here to the Late Kimmeridgian-Late Tithonian, but it is possible that in some places (i.e. Sierra del Pinal Alto), it would be as young as Berriasian, because in the Canon del Viejo (Sierra del Pinal Alto) it interdigitates with, and overlies the La Caja Formation. Table 11-l.

* Written communication of profesor W.C. Ward, Univ, of New Orleans, to M. Sandstrom, Univ, of Texas at Austin, June 5, 1981*

D.- Cretaceous System

1.- Taraises Formation

The Taraises Formation was defined by Imlay (1935) in the Canon de Taraises, western part of the Sierra de Parras, where it consists of about 7° m of thin-bedded (30-60 cm) limestone with intercalations of shale.

In the studied area the Taraises Formation conform ably overlies both La Caja and La Casita Formations. It has a thickness that varies from about 150 m in the western part to 350 m in the eastern part. It consists of thin to medium bedded (0.30-1.0 m), laminated mudstone interbedded with thin layers of silt and shale. Its lithology is very homogeneous throughout the entire area, with the exception of the northern and northeastern part of Galeana, where it shows a basal member of about 15 m thick of reddish brown sandstone named the Galeana Member (Diaz, 1951)• The Taraises has more macrofossils in the western part of the area than in the eastern part, where it does contain abundant microfossils. The macrofossils collected in the western part were determined as Olcostephanus symonensis (Bose), Olcostephanus discoideus Imlay, Olcostephanus coahuilensis Imlay, and Belemnites sp. Some of the microfossils collected in the eastern part were determined as

Stenosemellopsis hispanica (Colom), Calpionellopsis oblonga (Cadisch), Lorenziella hungarica Knauer and Nagy, Calpionella alpina Lorenz, Tintinapsella longa

(Colom), Nannoconus steinmani Kamptner, Nannoconus globulus Bronnimann, Pithonella ovalis Kaufmann, and Amphrorellina subacuta Colom.

Parallel and wispy laminations are the only sedimentary structures observed in the Taraises Formation, which together with its lithology and its fauna suggest a shallow-water depositional environment.

The origin of the sandstones found at its base in the Galeana area, is difficult to explain because the sandstones do not extend laterally, and in nearby outcrops the carbonates and shales characteristic of the Taraises are exposed. It is possible that these sandstones could be part of a submarine channel, because they show cross-bedding and cut-and-fill structures, but the outcrops are poor and neither their lateral stratigraphic relationships, nor their base, nor their top are exposed. The age of the Taraises Formations is Berriasian-Early Hauterivian, although in some places (Sierra del Pinal Alto) it base could be younger (Late Valanginian) than Early Berriasian.(Table 11-l).

2.- Cupido Limestone

The Cupido Limestone was defined by Imlay (1937) in the north wall of the Canon del Mimbre, in the middle part of the Sierra de Parras, where it consists of about 300 m of unfossiliferous, thick to medium-bedded, gray limestones above the Taraises Formation and below La Pena Formation.

In the area of this study the Cupido Limestone conformably overlies the Taraises Formation. It varies greatly in both thickness and lithology. Its thickness varies from about 3°o m in the southern and southwestern parts, to about 1,000 m in the northern and eastern portions of the mapped area. This change in thickness is correlatable to lithologic differences, being thicker in the reefal facies and thinner in the basinal facies. Three different facies are recognizable (Figure 11-M :

-Reefal Facies.- These rocks are characterized by their abundance of rudists, and commonly form the crests of most anticlines in the northern part of the area (Figure 11-M • Monopleurids, requienids, and caprinids occur everywhere, but in some areas oysters, fragments of corals, and codiacians (?) are observed as accessory organisms.

Figure Areal distribution of reef breccias, reefs, and basinal facies of the Cupido Limestone in the area of this study.

These rocks form the thickest sequences of the Cupido Limestones.

-Reef Breccias.- These rocks form a narrow belt which borders the north-northeastern part of the reefal facies (Figure 11-M« They are formed mainly by fragments of rudists derived from the reefal areas.

-Basinal Facies.- These rocks crop out mostly in the southern part of the area, and they constitute the thinnest part of the Cupido Limestone. They are formed mainly by medium to thick-bedded mudstone, with abundant chert nodules (gray and white), large stylolites, and pyritic and hematitic concretions, and are characterized by macrofossils (mainly ammonites) toward the west of San Rafael-El Carmen. Eastward of San Rafael- E 1 Carmen, The Cupido Limestone becomes shalier and contains only a few microfossils. The fossils collected in the basinal facies of the Cupido Limestone were:

MACROFAUNA: Pseudohaploceras sp. Hemicrioceras sp. Saynoceras mexicanum Imlay Ancyloceras sp. Procheloniceras sp.

MICROFAUNA: Nannoconus bermudezi Bronnimann Nannoconus steinmanni Kamptner

Nannoconus elongatus Bronnimann

The age of the Cupido Limestone is Late Hauterivian- Early Aptian.



Figure 11-4

3.- La Peña Formation

La Pena Formation was defined by Imlay (1936) in the western part of the Sierra de Parras. The type locality was assigned at the north wall of the Canon del Mimbre, in the Sierra de Taraises, near the Hacienda de la Pena. This formation was redefined later by Humphrey in the Sierra de los Muertos area, where he proposed that the term La Pena Formation be used only for the upper shaly part of the unit previously defined by Imlay.

La Pena Formation in the studied area is an excellent stratigraphic marker, due to its very distinctive lithology, its great number of fossils, its restricted thickness, and its wide distribution. It conformably overlies the Cupido Limestone, and also conformably underlies both the Tamaulipas Superior Formation and its lateral equivalent the Cuesta del Cura Limestone. La Pena Formation crops out in all the area of this study, and consists of thin bedded, gray wackestones with abundant lenses of black chert, intercalated with brown shale and siltstone, which at some localities contain thin bentonitic layers. It contains abundant fossils, mainly Dufrenoyia sp., Dufrenoyia justinae (Hill), and Colombiceras sp. Its thickness varies from 7 m (Canon

de El Charro) to about 100 m (Sierra de Rocamonte), and it is Late Aptian. (Table 11-l).

The lithology and fossils of La Pena Formation suggests that it was deposited in basinal areas, while vulcanism was active somewhere in the western and southwestern parts of Mexico.

4.- Tamaulipas Superior Limestone

The name Tamaulipas Limestone was first used by L.W. Stephenson in 1921, during private work for the Mexican Gulf Oil Co. (in Muir, 1936, P« 23, footnote) for those carbonates of Albian-Cenomanian age which outcrop in the Sierra de Tamaulipas. This term was first published by Belt (1925) in a very general description and without designating a type locality. In the late 20’s and early 30’s the term Tamaulipas Limestone was used indiscriminately to describe Neocomian, as well as Albian-Cenomanian limestones in eastern Mexico. Muir (1936) considered the Tamaulipas Limestone as formed by two members, upper and lower, separated by a cherty, black, fossiliferous shale which he called the Otates Horizon (La Pena equivalent). Thus, Muir (op. cit., p. 31) first introduced in the literature the names "Lower Tamaulipas” (Late Neocomian-Aptian) and "Upper Tamaulipas" (Albian-Cenomanian), following an oral suggestion by C. Burckhardt. The type locality for the "Upper Tamaulipas" was designated at the Cano'n de la Borrega, on the western flank of the Sierra de Tamaulipas, about 133 km northwest of Tampico, Tamaulipas, where this unit consists, from bottom to top, of 93 m of massive, whitish, stylolitic limestone with

Kingena wacoensis Romer, which passes upward through Al m of irregularly thin bedded limestone with chert, that conformably underlies beds of Turonian age (Agua Nueva Formation).

In the area of this study the term Aurora Limestone has been used by some authors to describe the Albian- Cenomanian limestone that crops out in the southern parts of Coahuila and Nuevo Leo'n (De Cserna, 1956; Humphrey, 19A9, 1956; Tavera, I96O; Rogers, et al., 1961).

The writer believes that it is not correct to use the term Aurora Limestone in this area, because this name was assigned by Burrows (1910) to a sequence of about 1,500 ni of medium to thick-bedded, dark-gray to black limestone that contains Requienia sp., Radiolites sp., Pecten (Neithea) sp., Aretostrea sp.cf. carinata Lamark, Enallaster sp.cf. E. bravoensis Bose, Exogyra texana Romer, and Orbitolina texana (Romer), which crops out in the Sierra de Cuchillo Parado (Sierra de la Aldea), Mina de la Aurora, about A. 8 km northeast of the village of Cuchillo Parado, eastern Chihuahua. Because the Aurora Limestone represents, mainly reefal (or shoal) accumulation, its lateral continuity is restricted to the edges of platforms contemporaneous to

it, which is a limiting constraint for using this formational name throughout all northeast Mexico. For this reason the term Tamaulipas Superior Limestone, instead of Aurora Limestone, will be used here to describe the carbonate strata that conformably overlies La Pena Formation, and also conformably underlies, both, the Indidura and Agua Nueva Formations (Table 11-l).

In the area of this study the Tamaulipas Superior Limestone crops out in the eastern and southeastern parts, where it consists, from bottom to top, of medium to thick bedded, gray, mudstones with minor intercalations of calcareous shales, and numerous stylolites parallel to bedding. Upward, they grade from mediumbedded mudstones with nodules of dark-gray chert and pyryte concretions, to thin, wavy-bedded mudstones with abundant layers of chert. The thickness of this formation varies from about 200 m in the Galeana area to 35° m in the Rayones area.

The Tamaulipas Superior Limestone is not very fossiliferous. The macrofossils collected from it were identified as Turrilites sp., Ancyloceras zacatecanum Bose, and Dyptychoceras mazapilense Burckhardt; whereas the microfossils, relatively more abundant, were identified as Saccocoma sp. , Mi crocalamo ides sp. , Colomiella mexicana Bonet, Colomiella recta Bonet, and Globochaete alpina (Lombard). The age of this formation is Albian- Cenomanian, and it correlates to the west and north with the Cuesta del Cura Limestone.

Immediately south of Saltillo there is an outcrop of Albian-Cenomanian limestones that contains fragments of requienids, radiolitids, and oysters, which has been assigned tentatively in this study (see Plate 1) to the Aurora Limestone.

The lithology and the fossils of the Tamaulipas Superior Limestone suggests that it was deposited in a basinal environment.

5.- Cuesta del Cura Limestone

The Cuesta del Cura Limestone was defined by Imlay (1936) in the western part of the Sierra de Parras, about km westward of the village of Parras, Coahuila where it consists of about 80 m of thin-bedded, undulating, dark-gray to black, dense limestone with lenses, layers, pseudobeds, and nodules of black chert; it overlies the Aurora Limestone (Tamaulipas Superior ?), and underlies the Indidura Formation.

In the studied area the Cuesta del Cura Limestone crops out extensively, and it consists of thin, wavybedded (boudinage-like), laminated beds of mudstone that contain layers, lenses, and nodules of black chert. It conformably overlies La Pena Formation and laterally passes to the Tamaulipas Superior Limestone. It underlies, also conformably, both, the Indidura (west) and Agua Nueva (east) Formations (Table 11-l). The Cuesta del Cura Limestone does not contain abundant fossils, but three ammonites were collected by the writer in the Sierra de las Mazmorras; Ptychoceras sp., Ancyloceras sp., andd Diptychoceras sp., which suggests an Albian- Cenomanian age for this unit. The thickness of this formation varies in the studied area from 65 m in the Sierra de la Silla, to 250 m in the Sierra de las Maz-

morras.

The lithology and fossils of the Cuesta del Cura Limestone suggests that it was deposited in an offshore, basinal environment.

6.- Indidura Formation

The name Indidura Formation was first used by Kelly (1936) in the region of Las Delicias, Coahuila, to describe a sequence of about 3$ m of thinly bedded, platy beds of limestone intercalated with shale, that conformably overlies the Cuesta del Cura Limestone. Imlay (1936) redefined it in the western part of the Sierra de Parras, where it overlies the Cuesta del Cura Limestone and underlies the Caracol Formation.

In the area of this study the Indidura Formation crops out extensively in the western and northern parts, whereas eastward it gradually changes to the Agua Nueva Formation (Padilla y Sanchez, 1978 a). The zone of

transition of one unit to the other extends from San Antonio de Gonzalez toward the north-northeast to the vicinity of the city of Monterrey (Table 11-l).

In this area the Indidura Formation consists of a sequence of 3° to 80 m of thin bedded, laminated, wackestone interbedded with shale. This formation is not very fossiliferous and only contains well preserved shells of Inoceramus labiatus Schlotheim 5.1., which indicates Early Turonian. It conformably overlies the Cuesta del Cura Limestone, and, also conformably, it underlies both, the Caracol (in the west) and the San

Felipe (in the north and east) Formations. The lithology of the Indidura Formation is fairly uniform throughout the studied area, but in the Sierra del Fraile, west of Go'mez Farias, it has thin horizons (1 cm thick) of gypsum and sandstone. This suggests that this formation was probably deposited on a shallow water extensive platform that was gently sloping toward the east, to the deeper water in which the Agua Nueva Formation was deposited.

7.- Agua Nueva Formation

The Agua Nueva Formation was introduced to the literature by Muir (193 M although Stephenson (1921, unpublished report in Muir, op. cit., p. 382) first separated these beds from the overlying San Felipe and gave them formational rank. The type locality was designated at the Canon de la Borrega, in the western foothills of the Sierra de Tamaulipas, about 25 km east of Forlon railroad station, where the Tamaulipas Superior Limestone is overlain by about 100 m of platy chert-bearing limestones interbedded with black carbonaceous shales.

In the area of this study the Agua Nueva Formation crops out extensively in the east, and conformably overlies the Tamaulipas Superior Limestone. It consists of about 100 to 250 m of laminated, thin-bedded, darkgray to black wackestone with a moderate to abundant number of nodules of black chert, interbedded with black, sometimes carbonaceous, shale. Its lithology is fairly homogeneous and changes slightly toward the west, where it becomes thinner, shalier, and the chert nodules and the laminations decrease notably. This formation contains only a few macrofossils mainly Inoceramus labiatus Schlotheim 5.1., and fewer microfossils,

Pithonella ovalis_ Kaufmann and Globotruncana sigali Reichel, which indicates that the age of this unit is Turonian.

8.- Caracol Formation

The Caracol Formation was defined by Imlay (1937) in the western side of the Arroyo Caracol, near the western end of the Sierra de San Angel, in the middle part of the Sierra de Parras, where it conformably overlies the Indidura Formation and consists of about 300 m of devitrified tuff, sandstone, and shale with subordinate amounts of limestone.

The Caracol Formation crops out extensively in the western part of the studied area where it conformably overlies the Indidura Formation. It consists of a rhytmic sequence of thin-bedded, fine-grained, limy sandstone and siltstone that become finer ans shalier toward the top. The sandstone is mainly composed of rounded grains of quartz, feldspar, and minor amounts of metamorphic rock (mainly schists), in a matrix of calcareous silt, and at some places shows low-angle cross bedding and graded bedding. Toward its upper contact with the Parras Shale, the Caracol Formation shows lesser amounts of terrigenous, and it becomes more calcareous and shalier. Laterally, it passes to the east into the San Felipe Formation (Padilla y Sanchez, 1978 a), where it becomes more calcareous, and contains more bentonitic horizons. No fossils were

found in the Caracol Formation in this area, but because of its stratigraphic position it has been tentatively assigned to the Coniacian-Santonian (Rogers, et al., 1961, p. 105).

The lithology of the Caracol Formation suggests that it was possibly deposited on a deltaic, fluvialdominated, complex shoreline that was prograding to the east-northeast.

9.- San Felipe Formation

The San Felipe Formation was first introduced in 1910 by Jeffreys in an unpublished report in Tampico (in Muir, 1936, P« 58), tut it was formally defined by Muir (Ibid, p. 59) to describe an incomplete section of Coniacian and Santonian limestone and shale occurring in various exposures west of San Felipe, about km east of Ciudad Valles.

In the area of this study the San Felipe Formation conformably overlies the Indidura or the Agua Nueva Formations, and also conformably underlies the Parras or the Mendez Shales. It crops out mostly in the eastern half of the mapped area, where it consists of a sequence of 110 to 350 m of thin-bedded, laminated, light gray wackestone, interbedded with siltstone, shale and benton ite. Its lithology is very characteristic and uniform throughout the entire area of exposure, but it becomes coarser to the west, where it passes laterally into the Caracol Formation. The San Felipe Formation is poorly fossiliferous and only some fragments of undeterminable globigerinids, and a specimen of Heterohelix sp. were collected by the writer. The age of the San Felipe is uncertain, but it is here tentatively assigned to the Coniacian-Santonian.

10.- Mendez Shale

The beds of this formation were first described in 1910> in an unpublished report, by Jeffreys (Muir, 1938, p. 68), in an outcrop 3°° m east of the Mendez railroad station, where they consists of mainly darkgray, blue, black, and dark reddish-brown limy shale.

In the area of this study the Mendez crops out extensively in the eastern part and conformably overlies the San Felipe Formation. It consists of dark-gray to black, fissile shale, with some minor intercalations of dark-olive, calcareous shale. Its lithology is very uniform throughout the area and it passes laterally to the west and northwest to the Parras Shale (Padilla y Sanchez, 1978 a). The Mendez Shale is highly folded in this area, and its top is eroded everywhere. For these reasons, it was not possible to measure its thickness, but Diaz (I 951 estimated a thickness of more than 550 m. No fossils were found by the writer in this formation, but it is considered here tentatively as Campanian-Maastrichtian.

11.- Parras Shale

The Parras Shale was defined by Imlay (1936) in the Lomas de San Pablo, about 6 km east of Parras, Coahuila, where it consists of a sequence of about 1,200 m of thin-bedded, fissile, black shale, that overlies the Indidura Formation and underlies the Difunta Group.

In the area of this study the Parras Shale conformably overlies the Caracol Formation, and it mainly consists of black, fissile shale. It crops out only in the northwestern part of the area. The Parras Shale is poorly fossiliferous, and for this reason its age is uncertain and has been correlated with the Caracol and San Felipe Formations by several authors (Imlay, 1936; De Cserna, 1956; Rogers, et al., 1961). They assigned it to the Coniacian-Santonian based on the fact that this formation conformably overlies the Indidura and Caracol Formations in southern Coahuila (S. de La Pena). Tardy (1962, p. 63) found at the base of the Parras Shale Early Campanian microfossils, which indicates that this formation is equivalent with the Mendez Shale, as has been suggested before by Forde (1959) (in Weidie, 1961, p. 3°), and shown by Padilla y Sanchez (1978 a Correlation of the Parras Shale has been made mainly on the

basis of its stratigraphic position between two fossil bearing formations, the Indidura below and the Difunta above, but it has not been explained the remarkable similarity of lithologies that exixt between the Parras and Mendez. According to Weidie (op. cit. , p. 32-33), near Monterrey, Exogyra costata occurs in the upper part of the Parras, which indicates a Campanian-Early Maastrichtian (?) age. This suggests that the Parras is not as old (Coniacian-Santonian) as that proposed by Weidie (Ibid, p. 22)

Figure 11-5 shows why in this study the Parras Shale is considered to be Campanian-Maastrichtian, and laterally equivalent to the Mendez Shale.

It is generally accepted that the age of the Mendez is Campanian-Maastrichtian. On the other hand, the Difunta Group is assigned to the Campanian-Paleocene, because it contains fossils of Campanian age in the western part of the Sierra de Parras (Imlay, 1936), Maastrichtian fossils in the Saltillo area (Rogers, et al., 1961), and Midwayan (Early Paleocen) fossils in the vicinity of San Miguel (Wall, 1961). Figure 11-5 (b) shows how the variations in age of the Difunta Group are not incompatible with the correlation of the Parras and Mendez Shales, and that the fact that the Parras

Figure 11-5«- (a) Generalized map showing areas of exposures of Late Cretaceous sediments between Torreon, Saltillo, Monterrey, and Linares, (b) Diagrammatic stratigraphic relationships of Post-Turonian sediments in northeast Mexico, (c) Idealized diagram showing the time-space relationships in a prograding deltaic system. Ages are ideal and are included in order to show the places where hiatus could be present; points and 5 •

Shale is overlying the Caracol and Indidura Formations and underlying the Difunta, is not sufficient evidence to correlate it with the San Felipe, as has been done before (De Cserna, 1956). Figure 11-5 diagrammatically shows that in a prograding delta the presence of one or more hiatuses can become more and more evident toward its direction of progradation, which could be a compara ble case to explain the stratigraphic relationships between the Parras, Me'ndez and Difunta sediments.







Figure 11-5

12.- Difunta Group

Imlay (1936) first named the Difunta Formation for the beds exposed in El Pozo-Boquillas hills, north of the western end of the Sierra de Parras, where they have a composite thickness of about 4,000 m and contain Exogyra ponderosa. Murray, et al. (1962) raised this formation to the rank of Group, and divided it into seven formations, from older to younger, Cerro del Pueblo, Cerro Huerta, Canon del Tule, Las Imagenes, Cerro Grande, Las Encinas, and Rancho Nuevo, previously defined informally by Weidie (1961). Later, Me Bride, et al. (1974) formally introduced five new formational names, from older to younger, Muerto, Potrerillos, Adjuntas, Viento, and Carroza Formations in La Popa Basin, and outlined a deltaic complex as the depositional environment for the Difunta Group in both the Parras and La Popa Basins.

In the present study the Difunta Group (Plate 1) was mapped undifferentiated in order to show its outcrops and structures in the areas inmediately north of the ranges of the Sierra Madre Oriental. It crops out only in the northwestern end of the studied area where it consists of mostly medium to thick-bedded fine-grain ed sandstones (Cerro del Pueblo and/or Muerto Forma-

tions), redbeds (Cerro Huerta and Las Imagenes Formations), and conglomeratic sandstones and shales with some carbonate lenses (Canon del Tule and Cerro Grande, or Potrerillos and Viento Formations).

E.- Tertiary System

1.- Ahuichila Formation

This formation was formally defined by Rogers, et al. (1961), in the Fronton de Ahuichila, along the southern flank of the Sierra de Jimulco, where it consists of a lower member, 30 m thick, of sandstone interbedded with tuff and an upper member, 275 m thick, of a massive-bedded conglomerate with subangular cobbles and pebbles of limestone, sandstone, and volcanic rocks in a sandy matrix.

In the area of this study the Ahuichila Formation crops out mainly in the western half of the area. It occurs in scattered, erosional remnants and consists mainly of a massive-bedded conglomerate with cobbles and pebbles of Mesozoic, marine limestones in a matrix of conglomeratic sandstones.

The age of the Ahuichila Formation is uncertain, but Rogers, et al. (1961) have correlated it with the Conglomerado Rojo de Guanajuato, in which Fries, et al. (1955), and Edwards (1955) found remains of Late Eocene to Early Oligocene vertebrates.

2.- Intrusive Bodies

In the area of this study intrusive bodies are present in the western part (Plate 1) in the Sierra de Rocamonte and Cerro El Penuelo, where they intrude the Mesozoic rocks and the Laramide folds. These plutons show well developed joints as well as numerous aplitic dikes and produced some mineralization (Au, Ag, Pb, Zn, and Cu) of economic significance. In the Sierra de Roca monte these igneous rocks occur in the middle and southeastern parts and have been called Rocamonte Stock and Matehuapil Stock, respectively, by Rogers, et al. (1961). Both, the Rocamonte and Matehuapil plutons, are monzonite which microscopically shows phenocrysts of K-feldspar, oligoclase, hornblende, clinopyroxene, and minor amounts of quartz, magnetite, biotite, apatite, and zircon. The intrusive of El Penuelo is a quartzmonzonite that microscopically show phenocrysts of Kfeldspars, oligoclase, biotite, hornblende, and quartz, and minor amounts of apatite, magnetite, and zircon.

The age of these intrusive rocks is uncertain because no radiometric ages are available, but they are assigned here tentatively to the Miocene (?) primarily because they cut Laramide (Late Paleocene-Early Eocene structures, and secondarily, because no fragments of

them have been found in the Ahuichila Formation (Late Eocene-Early Oligocene), whereas fragments derived from the intrusions are abundant in Pliocene-Pleistocene conglomerates (Table 11-l).

F.- Quaternary System

1.- Alluvium, Alluvial Fans, Playa Deposits, and Travertine.

The valleys and "bolsones” of the studied area are covered by alluvial deposits that consist generally of unconsolidated, fine gravel, sand, silt and clay, locally cemented by caliche. These deposits, laterally interdigitate with coarser deposits of alluvial fans, and they are covered, locally, by playa sediments deposited in closed basins (Plate 1).

Well-developed alluvial fans occur throughout the area, and they consist of angular fragments of Mesozoic rocks. In the Sierra de Rocamonte they contain abundant angular fragments of monzonite.

Playa deposits occur in closed basins as those south and north of the Sierra de Rocamonte and northwest of the Cerro del Potosi (Plate 1). At most localities they overlie and interdigitate with alluvium. Their lithology is very uniform and consists mainly of fine silt and clay interbedded with thin layers of evaporite (anhydrite and gypsum), which make them whitish. They form giant blocks (dessication features) separated by ’’mud-cracks'’ , which can be easily observed in aerial photographs. Some of the deposits could be of economic

significance due to their content of calcium and sodium sulphate.

Travertine deposits in the Canon del Chorro and in the Cola de Caballo waterfall, west of El Cercado, are closely associated with cold-water springs. These travertines are used locally in the construction industry.

III.- PALEOGEOGRAPHY

The Mesozoic and Cenozoic paleogeographic history of northeastern Mexico is intimately related to the origin of the Gulf of Mexico, which began opening in the Late Triassic, when the North American plate began to separate from the South American and African plates. The break-up and separation of these plates allowed the formation of grabens and horsts that would control the distribution of lands and seas and, consequently, the sedimentary patterns of northeastern Mexico for the remainder of the Mesozoic and Cenozoic.

Because of the regional scope of this part of this study, and because of the complexity of the stratigraphic nomenclature in northeast Mexico (Table 11-l), the use of formational names is here restricted to a minimum in order to make more clear the presentation of the paleogeographic reconstruction. To do so, a large number of lithologic descriptions and paleontologic data have been considered and compiled mainly from Imlay (1936, 1937, 1938), Kellum, et al. (1936), Humphrey (1956), Gonzalez (1976), Lopez-Ramos (I98O), and unpublished information by the writer.

A.- Late Triassic-Middle Jurassic

Lower Mesozoic rocks of northeastern Mexico rest unconformably over older marine sediments, metasediments, and igneous complexes of uncertain age. Localities where the pre-Mesozoic basement is exposed are limited, and little has been deduced from them about the nature and extent of the Early Mesozoic lands. Thus, the configuration of these lands must be largely inferred from the sedimentary patterns and tectonics of the later Mesozoic.

The oldest Mesozoic rocks known in northeast Mexico are redbeds of uncertain age that are believed to be Late Triassic (Lopez-Ramos, I98O) of Early to Middle Jurassic (Gonzalez, 1976) (Figure 111-l). The best surface exposure of these rocks is located in the Galeana area (Padilla y Sanchez, 1978 a) as mentioned before, whereas several subsurface locations are well known in the eastern part of the Tamaulipas Archipelago (Aguayo, 1978). Redbeds in the subsurface westward of the mentioned archipelago are less well defined, but their presence could be expected to the north and west. Farther west, in the vicinity of Caopas and Rodeo, Zacatecas, a series of volcaniclastic rocks and conglomerates have been placed as correlatives to the mentioned

eastern redbeds (De Cserna, 1955) and to those near Torreon (Me Leroy and Clemons, 1965).

The information available about the pre-Late Jurassic redbeds is scanty, as well as the areas of exposure. Little is known about this part of the Mesozoic, except that extensive areas were exposed to conditions of aridity, which could explain the absence of fossils. Some poorly preserved fossil plants were found in the Huizachal Valley by Mixon, et al. (1959), which permitted them to separate the Huizachal Group into two formations, La Boca and La Joya, that are considered to be Late Triassic and Early to Middle Jurassic, respectively.

All of these rocks had been considered Late- Triassic-Early Jurassic on basis of their stratigraphic position and with no other evidence for assign them to this age.



Figure 111-l.- Late Triassic-Middle Jurassic paleogeogra phy of northeast Mexico.

B.- Late Jurassic

At the begining of the Late Jurassic an extensive transgression started in northeast Mexico and continued until the Late Cretaceous. During this time, sea-water flowed toward the graben areas and defined the limits of islands and peninsulas that would controlled the sedimentary patterns of this region during most of the Mesozoic (Figure UI-2).

Through the Late Oxfordian-Early Kimmeridgian terrigenous sediments (La Gloria Fm.) were deposited at the western part of the Sabinas Gulf and along the borders of El Burro-Picachos Peninsula, La Mula and Coahuila Islands, and the Tamaulipas Archipelago (Gonzalez, 1976). Shallow-water carbonate (Zuloaga and Novillo Limestones) and evaporites (Olvido and Minas Viejas Formations) were deposited over extensive platforms southward of El Burro-Peyotes Peninsula and the Coahuila Island, and on the eastern part of the Tamaulipas Archipelago. A belt of oolitic banks restricted the water circulation and allowed the deposition of evaporites along the eastern part of the Tamaulipas Archipelago (Aguayo, 1978). It is possible that the oolitic banks between Saltillo and Monterrey caused the restricted circulation of the waters at the Sabinas

Gulf (Figure 111-2) allowing the deposition of evaporites in this area.

By the Late Kimmeridgian-Tithonian only El Burro- Peyotes Peninsula and the Coahuila and La Mula Islands were emergent, whereas the Tamaulipas Archipelago was completely covered by the seas by Late Tithonian (Figure 111-3). Conglomerate, conglomeratic sandstone, and sandstone bordered the emergent lands, whereas shalier sediments were deposited on the eastern part of the Sabinas Gulf and over the former Tamaulipas Archipelago. All these terrigenous sediments were included by Humphrey (1956) into La Casita Group. Shallow-water carbonate and shale (La Caja Formation) were deposited southward of the Coahuila Island, whereas deeper water shale and carbonates (Pimienta Formation) were deposited on the eastern part of the Sabinas Gulf and eastward of the former Tamaulipas Archipelago (Figure 111-3).



Figure 111-2.- Late Oxfordian-Early Kimmeridgian paleogeography of Northeast Mexico.



Figure 111-3.- Late Kimmeridgian-Tithorian paleogeography of northeast Mexico.

C.- Cretaceous

At the begining of the Cretaceous a monotonously thick sequence of carbonates started to develop, while the subsidence of this area continued and the seas con tinned their advance over the land. The Burro-Peyotes Peninsula and the Coahuila and La Mula Islands were still emergent, bordered by clastic sedimentation (San Marcos and Hosston Formations), while in the area of the Sabinas Gulf, shale and carbonate (Menchaca Formation), and limy shale (Barril Viejo Formation) were deposited. Contemporaneously, a sequence of carbonates with intercalated shale (Taraises Formation) was depos. ited toward the south and east, and farther east-south, east, a sequence of carbonates with chert nodules (Tamaulipas Inferior Formation) (Humphrey, 1956) was also deposited during this time (Berriasian to Valanginian) (Figure 111-M •

During the Hauterivian to Early Aptian, the emergent El Burro-Peyotes Peninsula and the Coahuila Island continued their influence on the sedimentation while La Mula Island was covered by shales (La Mula Formation), and later on, by carbonates (Padilla Formation) (Figure 111-5). The El Burro - Peyotes Peninsula and the Coahuila Island were still partially emerged, and were bordered by terrigenous (Patula For-

mation), and shale and silt (La Mula Formation), while in most areas of the Sabinas Gulf and southern Coahuila Island, an extensive deposit of limestone, shale, and evaporites (La Virgen Formation) took place behind an almost continuous trend of barrier and patch reefs (Cupido ’’Reefal Member”), that extended from Nuevo Laredo, Tamaulipas, southward to Monterrey, Nuevo Leon, and westward to Torreon, Coahuila (Marquez, et al., 1976). Most of these carbonate builups developed over the site of the buried islands of the Tamaulipas Archipelago (Lampazos, Sabinas, and Picachos Islands), that even when they were covered since the Late Tithonian by marine deposits, still continued their influence over the sedimentary patterns to the Early Aptian. A deeper water carbonate sequence (Tamaulipas Inferior Formation) continued to be deposited east and southeastward of the mentioned reef trend. It is possible that salt anticlines and diapirs on the eastern part of the Sabinas Gulf allowed the development of reefs during the Early Cretaceous (Humphrey, 1955)• However, Stabler and Marquez (1977) have suggested that upwelling shale of the thick shale sequence at the top of the Jurassic (La Casita Group) and the base of the Cretaceous (Menchaca Formation) could be the cause for local uplifts,

where reefs could have developed. By the Late Aptian, the entire area was covered by the seas and a thin horizon of limestones and shales was deposited (La Pena and Otates Formations).

During the Early Albian-Late Cenomanian (Figure 111-6), the development of reefs was restricted to the borders of the submerged Coahuila Island (Aurora Formation), and a reef trend was developed westward of Cd. Acuna and Nuevo Laredo, over the former El Burro-Peyotes Peninsula. Behind the reef trends, the restricted circulation of the waters allowed the deposit of evaporites (Acatita Formation) on the former Coahuila Island (Gonzalez, 1976), while in the El Burro-Peyotes Peninsula similar conditions probably existed, but this has not been demonstrated. The rest of northeastern Mexico was covered by platformal carbonates in the Sabinas Gulf and by deeper water limestones with chert nodules (Tamaulipas Superior Formation) and limestones with chert layers (Cuesta del Cura Formation).

By the Turonian (Figure 111-7) the entire area was covered by marine water, but the development of the characteristic thick sequences of carbonates decreased sharply. In general, a shalier sequence with minor amounts of carbonate was deposited in the northern part of this area covering almost the entire Coahuila State. Lopez-Ramos (I98O) has considered these rocks to be the southward extension of the equivalent Eagle Ford Formation of Texas. Southward, a thin sequence of carbonate and shale (Indidura Formation) was deposited, while eastward, a carbonate sequence (Agua Nueva Formation) was also deposited.

During the Coniacian to Santonian the sedimentary pattern had changed slightly (Figure 111-8). Most of the northern part of this area was covered by shallow water carbonate and shale, that have been considered to be equivalent to the Austin Group (Lopez-Ramos, I98O), while the southern part was covered by sandstone and shale (Caracol Formation), and between Torreon and Monterrey, the deposition of shalier sediment (lower Parras Formation ?) took place. Eastwardly, the deposition of a thicker sequence of thinly-bedded carbonate and shale, with bentonite horizons (San Felipe Formation) covered an extensive area.

By the Late Cretaceous, Campanian to Maastrichtian, the complete area was covered by terrigenous sediment originated by fluvial streams that flowed from the west (Figure 111-9) and small basins were formed (Sabinas Coal Basin and Parras Basin).

■-'Graduate student. University of Texas at Austin



Figure 111-4.- Berriasian-Valanginian paleogeography of northeast Mexico.



Figure 111-5.- Late Neocomian paleogeography of northeast Mexico.



Figure 111-6.- Albian-Cenomanian paleogeography of northeast Mexico. El Burro-Peyotes paleogeography from Barcelo- D.* (personal communication).



Figure UI-?.- Turonian paleogeography of northeastern Mexico.



Figure Coniacian-Santonian paleogeography of northeastern Mexico, r



Figure 111-9.- Campanian-Maastrichtian paleogeography of northeastern Mexico.

D.- Tertiary

Early in the Tertiary the Mesozoic sequence deposit ed in northeast Mexico was deformed by the Laramide Orogeny, and the Cenozoic seas retreated eastwardly, leaving behind them a thick sequence of sandstone and shale as shown in Figure 111-10.

The Tertiary Burgos Basin was formed after the Laramide Orogeny and is considered to be the southern continuation of the Rio Grande Embayment. The Burgos Basin then started to be filled with terrigenous sediments of the Midway and Wilcox Groups (Paleocene-Early Eocene), deposited in a complex system of barrier bars and deltas which allowed the development of thick sequences of shale and sand. This bar-deltaic complex prograded eastward throughout the entire Cenozoic era, allowing the development of numerous growth faults that formed favorable traps for the accumulation of gas (Echanove, 1976).



Figure UI-10.- Tertiary paleogeography of northeastern Mexico.

IV.- STRUCTURES AT THE "CURVATURE OF MONTERREY”

The name "Curvature of Monterrey"is here used to describe the region where the Sierra Madre Oriental changes its structural trend from an east-west direction near Saltillo to a northeast direction (northeast of Saltillo), and then from an east-west direction near Monterrey to a southeast direction (west of Linares). The Curvature of Monterrey, as it is defined here, includes all the folds in the area between the cities of Saltillo, Monterrey, Linares, and Concepcion del Oro (Figure IV-1).

The folds and faults at the Curvature of Monterrey vary greatly in shape and size. They constitute the highlands of the Sierra Madre Oriental and contrast sharply with the folds of the southern end of the Sabinas Gulf, the folds of the eastern part of the Parras Basin, and the lowlands of the western Gulf Coastal Plain (see Figures V-2 and V-4).

The major anticlines shown in Figure IV-1 do not entirely correspond with the mountain ranges because many of the anticlines are breached; thus, at some localities two or three mountain ranges are the surface exposure of a single fold. On the other hand, when an anti-

cline is overturned, it is commonly seen as a single ridge, which generally is the overturned flank (compare Figure IV-1 with Plates 1 and 2).

The folds at the Curvature of Monterrey can be divided in five major areas of different styles of folding (Figure IV-2): A) a northern area of tight, mostly arcuate, symmetric folds with nearly vertical axial planes; B) a southern area of tight, arcuate and elongated, symmetric and asymmetric, mostly overturned folds the axial planes of which dip from 75° to 45° to the south and southwest; C) a shallow basement area of long, broad, almost symmetric folds (i.e. El Potosi Anticline) with numerous tight, symmetric and asymmetric, secondary folds; D) an area of opposite vergence of tight, elongated, asymmetric, overturned folds the axial planes of which dip nearly 50° toward the northeast; and E) the area of La Silla Anticline, which is an entirely different structure in the area of this study.



Figure IV-1.- Map showing the major anticlines, thrust faults, and strike-slip faults at the Curvature of Monterrey. CO-Concepcidn del Oro; LV- La Ventura; ES-E 1 Sal vador; EC-E1 Carmen; G-Galeana; L-Linares; S-Saltillo; SA-San Antonio de las Alazanas; MS-Montemorelos; A-Allen de; MY-Monterrey. For a more detailed location of structural features see Plate 1.



Figure IV-2.- Map showing the five major areas of different styles of folding at the Curvature of Monterrey. For identification of major cities see Figure IV-1.

A.- Northern Area

The northern area of the Curvature of Monterrey is typified by tight, symmetric anticlines and synclines with nearly vertical axial planes, that vary in length from 15 to about 60 km (Table IV-1, Figure IV-3, and Plates 1 and 2). This area comprises, from north to south, the anticlines of Las Mitras, Los Muertos, Los Nuncios, Las Comitas, Agua del Toro, San Lucas, San Cris tobal, Arteaga-San Juan Bautista, and El Chorro (Figure IV-3). This group of folds is what De Cserna (1956) called the ’’Arteaga Anticlinorium” . Upper Jurassic formations (Zuloaga and La Casita) and gypsum, are exposed in the cores of the breached anticlines, whereas when the anticlines are unbreached the Cupido Limestone crops out at their crests (i.e_. Las Mitras and Las Comitas Anticlines) (Plate 1).

The northern area is limited to the west and north by the smaller, and tighter, folds of the Parras Basin. Its eastern limit is the Frontal Thrust Fault, whereas its southern boundary is located along the syncline south of El Chorro Anticline, along the thrust fault north of the Sierra Hermosa Anticline, and along the thrust fault north of the Potrero de Abrego Thrust Fault (Figure IV-3).

Figure IV-3.- Map showing the northern area (stippled) at the Curvature of Monterrey. For identification of major cities see Figure IV-1.

Variations in shape, wavelength, and amplitude of the folds are closely related to lithologic variations of the formations contained in the fold system of the northern area. This stratigraphic influence is more important where reefal bodies (Lower Cretaceous Cupido Limestone) are involved, as in Las Comitas Anticline, where the central part of the fold has a larger wavelength km) than the wavelength (or 3 km) of its northwestern end (Figure and Plate 1). However, the periodicity and symmetry of the fold system in this area is remarkably constant (Plate 2, section E-E’). Also remarkable, is the absence of thrust faults or strikeslip faults in this area, with the exception of those at its limits, and some small normal faults.

The direction in which this de'collement glide sheet moved can be deduced from the arcuate shapes of the fold system (Figure IV-3), that is, toward the concave side of the Curvature of Monterrey, or northward.



Figure IV-3



Figure IV-4.- Sketch map view (a) and profiles (b) of Las Comitas Anticline showing the differences in wavelength produced by the thickening of a reefal body.

ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) AXIAL PLANE DIP REMARKS Las Mitras 15 5 ~ 2.4 nearly vertical Symmetric. Unbreached. Elongate. Both ends plunge steeply. Northwestern end apparently offset by fault (off the nap). Southeast end bifurcates into two small anticlines. Los Muertos 62 . 5-2 ~ 2.6 Southwest partz75°to the SE Central partz 90* Southeast part — 80* to the SW Symmetric. Breached. Arcuated. South- western endi overturned. Southeastern endi overturned and truncated by fault. Los Nuncios 31 5 r 2.5 nearly vertical Symmetric. Breached, slightly arcuated. Western end is a bifurcation of Los Muertos Anticline (see Plate 1). Eastern end plunges gently (S’25*) to the east. Las Comitas 22 — 5 ~ 2.4 nearly vertical Symmetric. Unbreached. Elongate. Ends tighter than central part. Northwestern end plunges gently (=20*) to the NW. Southeastern end truncated by fault. Agua del Toro 5 ~2.6 nearly vertical Symmetric. Breached. Slightly arcuate. Western end plunges gently (s2 5*) to the W. Southeastern end plunges (=40°) to the SE. San Lucas 5 -2.5 Western partz80*to the S Nearly vertical at its central and southeastern parts Symmetric. Breached. Arcuate. Western end truncated by fault and bifurcated. Southeastern end plunges about 40° to the SE. San Cristobal 23 — 2.3 nearly vertical Symmetric. Breached. Elongate. North- western end plunges gently («30*) to the NW. Southeastern end plunges gently (z30* ) to the SE. Arteaga-San Juan Bautista 56 5 -2.5 Nearly vertical Western part slightly inclined to the south Symmetric. Breached. Arcuate. Western end plunges about 35* to the W-SW. Southeastern end plunges gently (?25°) to the SE. ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) AXIAL PLANE DIP REMARKS El Chorro 5A 5-3 - 2.7 nearly vertical Symmetric. Breached. Arcuate. South- western end plunges about 40° to the SW, and its northwestern limb is truncated by a thrust fault (Plate 1). Southeast- ern end truncated by thrust fault.

Table IV-1.- Structural data of the anticlines in the northern of the Curvature of Monterrey.

B.- Southern Area

The southern area of the Curvature of Monterrey consists of mostly asymmetric, overturned, tight anticlines and synclines with a general vergence toward the northeast (Figure IV-5). It is also typified by numerous thrust and strike-slip faults, which are more common in the eastern part of this area (Plate 1). This part of the Curvature of Monterrey comprises 32 major anticlines (Table IV-2) and several thrust faults from which the Frontal Thrust Fault and the Secondary Thrust are the longest.

The northern limit of the southern area is placed along the northern flanks of the Sierra Hermosa and Jame Anticlines, and along the Potrero de Abrego thrust fault Its eastern limit is the Frontal Thrust Fault, and its western and southern limits are the meridian 101° 20’ and the parallel 104° 30’, respectively (Figure IV-5). The two areas of different styles of folding within the southern area (blank areas in Figure IV-5) will be described and discussed later.

Normal faults in the southern area are only a few, and they are commonly associated with Quaternary or very recent extensional events. They are local features that in most cases are not more than km long, and their

Figure IV-5.- Map showing the southern area (stippled) at the Curvature of Monterrey. For identification of major cities see Figure IV-1.

offset ranges from a few meters to about four hundred meters. They are sparse throughout the entire area and do not follow any regional trend (Plate 1).

Joints are also scattered throughout the southern area, but largely restricted to intrusive bodies as well as to the thick beds of the Cupido Limestone (Plate 1). The largest joint sets are in the intrusives of Rocamonte and El Penuelo. Those at Rocamonte pluton strike randomly 12° NE, 25° NE, and 42 NW, whereas the joints at El Penuelo intrusive form more definite sets that generally strike either 53° NE or 42° NW. Apparently these joint sets at both intrusives are the result of cooling, but some small offsets (10-30 cm) were observed at thin dikes in El Penuelo intrusive, which could indicate the possibility of sets of micro strike-slip faults (probably conjugate) due to compressive stresses.

The other set of ’’mappable” joints is located at the Potrero de Abrego Anticline, in the thick-bedded limestones of the Cupido Formation. Their strikes range from N 17° W to N 25° E, thus being approximately perpendicular to the fold axis (Plate 1). This set of joints was probably formed as the result of folding, and therefore would be comparable to the set number 1 of Stearns (1968).

As in the northern area, lithologic variations in the Mesozoic formations appear to have controlled variations in shape, wavelength, and amplitude of the folds of the southern area. The thick sequence of Upper Jurassic gypsum (Olvido Formation) and shale (La Casita Formation), the massive Lower Cretaceous reefal bodies of the Cupido Formation, and the thick shale sequence of the Mendez Shale are the formations that had the largest influence in fold variations.

This stratigraphic influence on the structures is more clear in the northeastern part of the southern area, where in most localities reefal bodies of the Cupido Limestone are thrust over the Mendez Shale (i.e_. anticlines of Potrero de Abrego, La Marta, Sierra Borrada, Labrador, La Ventana, Mediodia, Mauricio, and La Muralla) (Plates 1 and 2, cross-sections C-C r and D-D’). However, not only lithologic variations influenced the fold system but also the changes in thikness (over short distances) of the Cupido Limestone, which cause the reefal bodies of this formation to overthrust younger formations in many places.

The larger thrust faults in the eastern part of the southern area (Figure IV-5), are not as closely related to stratigraphic variations as in the northeastern part of this area. The largest of this type of fault is the Frontal Thrust Fault, which delineates the frontal ranges of the Sierra Madre Oriental from Monterrey, Nuevo Leon, for approximately 130 km to the vicinity of Ciudad Victoria, Tamaulipas, with an average dip of 30° SW. Dip flattens with depth until it reaches the level of regional decollement (Plate 2, cross-section A-A’). Its net displacement is difficult to determine, but it is estimated to be 8 to 10 km at the southeastern end of Las Comitas Anticline, and to increase considerably toward the southeast (perhaps »3° km). One of the problems that impede estimating the minimum net displacement is the absence of tectonic windows (fensters) and/or klippen associated with this fault.

Another important thrust fault in the southern area is the Secondary Thrust, located immediately westward of the Frontal Thrust. It strikes approximately parallel to the Frontal Thrust, and it dip is sligtly steeper (about 35° SW). The length of this fault is about km, and its minimum displacement is estimated to be more than km, which is about half the wavelength of the syncline between this fault and the Santa Rosa Anticline (Plates 1 and 2, cross-section B-B’).

Westward of the northwestern part of the Secondary Thrust there are several thrusts which appear to be related to each other (see Figure IV-5 and Plate 1). For example, the thrust faults at the southeastern end of El Coahuilon Anticline, those at the northeastern flanks of La Ventana and Mediodia Anticlines, and two klippen westward of these faults, all seem to be parts of one thrust fault. The average stike of this thrust is about S 45° E, and it dips approximately 20° SW (Plate 2, cross-section C-C’). The minimum length of this thrust is about 30 km, and its minimum displacement is about 5 km, the approximate width of the El Coahuilon Anticline.

The eastern part of La Marta Anticline, and the northeastern flank of the Sierra Borrada Anticline are limited by a low-angle thrust fault (Figure IV-5). The average strike of this fault is about N 40° E, its fault plane is almost horizontal at its eastern end, and becomes progressively steeper westward to a dip of 30° SW (see Plate 2, cross-section C-C* ). This thrust is at least 44 km long, and it has a minimum displacement of about 4 km.

The northern part of the Potrero de Abrego Anticline is bordered by two thrust faults (Figure V-l). The southern of these two faults strikes east and dips approximately 60° to the south. This fault is about 21 km long and its minimum displacement is estimated to be approximately 4 km (Plates 1 and 2, cross-section D-D*). The second fault strikes almost parallel to the first one but dips approximately 30° S. Its length is at least 19 km, and its minimum displacement is estimated to be at least 3 km, half the width of El Chorro Anticline.

Immediately southeast of Saltillo the northeastern part of the Sierra Hermosa Anticline and the western part of El Chorro Anticline have overriden a small unnamed reefal body (Figure IV-5 and Plate 1). These two thrust faults strike about east and N 50° E and dip approximately 60° to the south and southeast respectively. Minimum displacement is difficult to estimate, but it is at least about 1 km, the approximate thickness of the missing stratigraphic sequence that includes the Indidura, Caracol, and Parras Formations (Plates 1 and 2, crosssection F-F’ ).

Strike-slip faults in the southern area are commonly associated with thrust faults. One of the largest strikeslip faults in this area is located at the western end of the Potrero de Abrego Anticline, where it strikes about N 85° E and has a dextral displacement of approximately 4 km (Plate 1). Several other strike-slip faults are scattered throughout the southern area, with lateral

displacements that range from 200 m to about A km, and they commonly offsett both folds and/or thrust faults (Plate 1 and Figure IV-5).

The direction in which the southern area moved is deduced from both the vergence of the folds and the direction of slip of the faults. Local de'collement sliding directions can be stablished (Figure IV-5). Thus, it can be seen that most of the eastern part of the southern area moved east-northeastward; that the northern part of this area moved north-northwestward (Anticlines of Potrero de Abrego, Jame, Sierra Hermosa, Cuauhtemoc, Chapultepec, Huachichil, El Muerto, La Leona, Catana, El Duraznero, and El Fraile)} and that the southern part of the southern area moved northeastward (Anticlines El Pedregoso, Rocamonte, Las Mazmorras, La Tomita, and Las Valias. The direction is about N 30° E (Figure IV-5). This direction of movement for the southern area is different from the direction obtained for the northern area (Figure IV-3). This difference will be discussed later.



Figure IV-5

ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) AXIAL PLANE DIP REMARKS Jame 32 ~ 5 ~ 2.6 about 75° to the south Asymmetric. Overturned. Breached. Arcuate. Southwestern end plunges (=45®) to the SW. Southeastern end truncated by strike-slip fault. Potrero de Abrego ? 24 .. ? 9 9 Only the southern flank is exposed be- cause of a thrust fault along the fold. Ends truncated by faults. (Figure IV-5 and Plate 1). San Antonio 30 ~5.8 3 about 80° to the southwest Asymmetric. Overturned. Breached. Elon- gate. Overturned flank exposed. Only one third of its southwestern flank is ex- posed. Southeastern end plunges (=30®) to the SE. Northwestern end apparently truncated by fault (Plate 1). El Coahuilon 23 ~ 5 about 80° to the southwest Asymmetric. Unbreached. Elongate. North- western end plunges (=25®) to the NW. Southeastern end truncated by fault (Plate 1) La Marta - 6.5 -3.2 • Northwestern and central parts i about 80® to 85* to the SW. Southeastern partt about 77® to the SW Asymmetric. Overturned. Breached. Elon- gate. Only overturned flank is exposed. Northwestern end plunges (cr20°) to the NW. Southeastern end truncated by fault (Plate 1). Pinal Alto 30 ~ 7.8 qr 3-7 about 65° to the southwest Asymmetric. Overturned. Breached. Elon- gate. Only overturned flank is exposed. Northwestern end plunges (=25®) to the NW. Southeastern end bifurcates into two smaller anticlines. Cienega del Toro 21 cr 6 - 3 about 85° to the southwest Asymmetric. Breached. Arcuate. South- eastern end joins Sierra Borrada Anti- cline. Northwestern end plunges (=40®) to the NW. Sierra Borrada 26 cr5 — 2. 6 about 75° to the west-south- west Asymmetric. Overturned. Breached. Elon- gate. Thrust fault is along its eastern flank. Both ends truncated by faults (Plate 1). ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) Axial plane dip REMARKS La Ventana ? Ill- 9 ? 9 Only southwestern flank is exposed. Thrust fault along the fold axis. Ends truncated by faults (Plate 1). Mediodia ? ? ? Only southwestern flank is exposed. Thrust fault along the fold axis. Ends truncated by faults (Plate 1). Labrador > ? ? ? Only northeastern flank is exposed. Northwestern end truncated by fault. Southeastern and off the map (Plate 1). Mauricio A9 ~ 5 ~ 2.6 about 60° to the west-south west Asymmetric. Overturned. Breached. Elon- gate. Northwestern end truncated by fault (Plate 1). Southeastern end plunges (-35°) to the SE. Santa Maria 22 a , 5 er 2.1 about 80° to the southwest Asymmetric. Breached. Arcuate. Northern end joins La Muralla Anticline. South- eastern end joins the Iturbide Anticlir (Plate 1). La Muralla 20 c4.3 er 2.1 about 75° to the southwest Asymmetric. Overturned. Breached. Arcu- ate. Northwestern end joins the Santa Maria Anticline. Southeastern end joins the Iturbide Anticline. Iturbide 17 cr 2 about 6o° to the southwest Asymmetric. Overturned. Breached. Elon- gate. Northwestern end bifurcates into the Santa Maria and La Muralla Anti- clines. Southeastern end plunges {-35 a to the SE. Los Nogales- Santa Rosa >98 9 ? ? Only southwestern flank is exposed. Thrust fault along the fold axis. Nortl western end plunges (=35°) to the NW. Southeastern end off the Map (Plate 1) ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) AXIAL PLANE DIP REMARKS Las Valias >21 ~ 4.1 - 2 about 65° to the southwest Asymmetric. Overturned. Breached. Elon- gate. Northwestern end truncated by faults. Southeastern end off the Map (Plate 1). La Tomita > 6 .. ~3-2 ~1.6 about 85° to the southwest Asymmetric. Breached. Elongate. Only its southeastern part is exposed and plunges fc:30*) to the SE. Las Mazmorras -15 4 ~1.9 central part: about 85° to the southwest Asymmetric. Breached. Elongate. North- western end plunges (=35* ) to the NW. Southeastern end plunges (=35°) to the SE. Rocamonte 18.5 ~5 ~2.4 Northwestern parti about 85* to the SW Southeastern parti about 70° to the SW Asymmetric. Overturned. Breached. Elon- gate. Central part intruded by pluton (Plate 1!. Northwestern end plunges (-40°) to the NW. Southeastern end plunges (=35*) to the SE. Pedregoso 16 4 Of 2 nearly vertical Symmetric. Breached. Elongate. Western end plunges (=45*) to the W-NW. Eastern end plunges (=45* ) to the E-SE. El Mezquite- La Cuchilla >20 - 5 =-2.4 about 60° 'to the south-south- west Asymmetric. Overturned. Breached. Elon- gate. Offset at eastern part by strike- slip fault (Plate 1). Western half off the Map. Eastern end plunges (=30*) to the E-SE. San Juan >14 ~ 2.5 about 65° to the southwest Asymmetric. Overturned. Breached. Arcuate. Only overturned flank is ex- posed. Ends not exposed. El Fraile- E1 Jabal i 66 — 6 ~ 2.8 about 65° to the southwest Asymmetric. Overturned. Breached. Arcuate. At central part only portions of the overturned flank are exposed. Western end plunges (=40°) to the W. Southeastern end plunges (=35*) to the SE. ANTICLINE LENGTH (in km) WAVELENGTH (in km) AMPLITUDE (in km) AXIAL PLANE DIP REMARKS El Duraznero- E1 Venado 57 ~ 5 ~ 2.5 about 65° to the southwest Asymmetric. Overturned. Breached. Arcuate. Only portions of the overturned flank are exposed at western and central parts. Eastern end plunges (=35°) to the SE. Catana-El Toro Las Hormigas 61. -2.5 about 60° to the southwest Asymmetric. Overturned. Breached. Arcuate. At central part only portions of the overturned flank are exposed. Middle part offset by strike-slip fault. Western end truncated by thrust fault. Southeastern end plunges (=35°) to the SE. La Leona >16 ~5*5 ~2.6 about 75° to the south Asymmetric. Overturned. Breached. Elon- gate. Only its eastern part is exposed. Eastern end plunges (a-45°) to the E. El Muerto >32 ? about ?0° to the south Asymmetric. Overturned. Breached. Arcuate. Only its overturned flank is exposed. Western end plunges (=35°) to the W. Huachichil 5 2.4 about 85 ° to the', south Asymmetric. Unbreached. Arcuate. South- western part well exposed. Only portions of southeastern part are exposed. South- western end plunges (=30*) to the SW. Chapultepec 26 5 2.5 about 85° to the south Asymmetric. Unbreached. Arcuate. South- western part well exposed. Only portions of southeastern part are exposed. South- western end plunges (=30°) to the SW. Cuauhtemoc 20 5 2.5 nearly vertical Asymmetric. Unbreached. Arcuate. South- western part well exposed. Only portions of southeastern part are exposed. South- western end plunges (=35°) to the SW. Sierra Hermosa 19 6 2.8 about 65° to the south Asymmetric. Overturned. Breached. Arcuate. Southwestern end plunges (= ) to the SW. Southeastern end plunges (c^S 0 ) to the SE.

Table IV-2.- Structural data of the anticlines in the southern area of the Curvature of Monterrey.

C.- Shallow Basement Area

Another very distinctive area inside the Curvature of Monterrey is the area of El Potosi Anticline, which is the only area where the basement is shallow (Figure IV-6). The Potosi Anticline is perhaps the most outstanding structure in the Curvature of Monterrey, because of its size (more than 50 km long), wavelength of about 10 km, and an estimated amplitude of 4 km. It is a broad, breached, asymmetric anticline that contrasts notably with the tighter folds beside it (Plates 1 and 2, crosssection B-B’). Its northwestern end plunges gently (~30°) toward the northwest, whereas to the south it seems to grade into several smaller folds which lie beyond the map area (Plate 1). Its axial plane is nearly vertical, but it has a slight hade of probably a few degrees to the southwest. The rocks exposed at its breached core are the oldest known in the Curvature of Monterrey (Huizachal Group). Numerous secondary folds, randomly oriented, are present in rocks of the Olvido Formation in the Sierra de los Rincones, which is a part of the core of this major fold.

Only a few normal faults are present in the Sierra de las Mazmorras (Plate 1), but neither thrust faults nor strike-slip faults were observed in this area.

The eastern limit of this shallow basement area has been placed southwest of the Cie'nega del Toro and Sierra Borrada Anticlines, and west of the Labrador Anticline, whereas its western limit was placed along the last outcrops of the smaller, secondary folds of the Potosi Anticline (Figure IV-6, Plate 1, and Plate 2, cross-sections A-A’ and B-B’). These boundaries approximately coincide with those proposed by Belcher (1979) for this horst block.

The development of folds in this area was influenced, primarily, by the shallow depth of the basement and secondly by lithologic variations in the Mesozoic formations. The thick, Upper Jurassic, sequence of evaporites (Olvido Formation) and shale (La Casita Formation), are the formations that allowed the de'collement of the Mesozoic rocks (Plate 2, cross-sections A-A’ and B-B*).

The direction of local decollement sliding, northeastward, is deduced from the arcuate form of El Potosi Anticline at its northwestern end (Figure IV-6).

Figure IV-6.- Map showing the area of shallow basement (stippled) at the Curvature of Monterrey. For identification of major cities see Figure IV-1.



Figure IV-6

D.- Area of Opposite Vergence

A very distinctive area of folds overturned to the southwest is located immediately westward of the area of shallow basement (Figure IV-?). El Zorrillo, El Oregano, and El Gateado Anticlines are the folds included in these area. They trend about N 65° W, which is more or less parallel to the general trend of the surrounding structures, but their axial planes dip an average of 60° toward the northeast, which is opposite to the general vergence of the folds in the Curvature of Monterrey (Plates 1 and 2, cross-section B-B’).

In this area there is no evidence of major litholog ic variations that could have controlled the development of folding. But it seems reasonable to assume that the basement high beneath El Potosi Anticline influenced and was the cause for the anomalous vergence of these structures. The local direction in which these structures moved during the decollement sliding is shown in Figure IV-7, and their possible origin will be discussed later in this chapter.

Figure IV-?.- Map showing the area of opposite vergence (stippled) at the Curvature of Monterrey. For identification of major cities see Figure IV-1.



Figure IV-7

E.- La Silla Anticline. An Exception

La Silla Anticline is an entirely different fold from the rest at the Curvature of Monterrey. The main difference is that it is a symmetric structure at its extremes, while its middle part is overturned, asymmetric, and complicated by a thrust fault (Figure IV-1). Further, it is located outside the frontal ranges of the Sierra Madre Oriental (as is Las Mitras Anticline); it trends obliquely with respect to the major folds of the Sierra Madre Oriental; and its trend is almost parallel to the Frontal Thrust Fault of the Sierra Madre Oriental (Plate 1).

La Silla Anticline is an elongate, tight, breached, symmetric to asymmetric, partly overturned, anticlinal fold which trends N 30° W, and is km long and has a wavelength of about 5 km. It is characterized by steeply dipping flanks, and it also plunges steeply at its ends, where its axial plane is about vertical, whereas in its middle part the axial plane dips some ?0° to the southwest. Its middle part is also complicated by a thrust fault, which is responsible for the absence of its northeastern flank (Plate 1). At its southeastern end the structure bifurcates into two small anticlines that plunge gently southeastward.

There is no evidence of stratigraphic control over this structure, and most probably the horst of the San Carlos Island influenced the shape of this fold. The origin of La Silla Anticline will be discussed later in this chapter.

F.- Origin and Age of Folding and Faulting

The Late Paleocene-Early Eocene Laramide Orogeny is responsible for the structural features at the Curvature of Monterrey. The arcuate shapes of the folds of this area were produced when the Mesozoic rocks slid northeastward over Late Jurassic evaporites and shale, and were directed (practically ’’injected”) between the more stable blocks of the paleo-islands of Coahuila and San Carlos (Figure IV-8). Both horsts acted as barriers against which the Mesozoic sedimentary rocks were folded and thrust.

The general vergence of the folds (overturned toward the north and northeast) indicate that the direction of movement of the entire folded belt was northeastward (Figure IV-9), but local directions indicated by different vergences, suggests that into the Curvature of Monterrey independent decollement ’’glide sheets” moved relative to each other in different directions (Figures IV-8 and IV-9). These groups of folds with different directions of movement are those described in the first part of this chapter (Figure IV-2), and they correspond to the northern area, southern area, shallow basement area, opposite vergence area, and La Silla Anticline area (Figures IV-8, IV-9, and IV-10).

Figure IV-8.- Sketch map of the Curvature of Monterrey and surrounding areas, showing the location of basement highs, former basins, and the relative movements of the detached Mesozoic sedimentary sequence. MY-Monterrey; L-Linares; G-Galeana; CO-Concepcion del Oro; S-Saltillo.

Figure IV-9.- Structural map of the Curvature of Monterrey showing local (solid arrows) and regional (blank arrow) directions of de'collement sliding. Also shows location of schematic cross-sections shown in Figure IV-10.

Figure IV-10.- Schematic cross-sections at the Curvature of Monterrey. See text for discussion. For approximate location of sections see Figure IV-9.

The average direction of movement of each one of these ’’glide sheets” was as follows:

Northern area - northward Southern area - 30° northeastward Shallow basement area - northeastward Opposite vergence area - southwestward La Silla Anticline - 60° northeastward

Thus, the average direction of movement of the entire Curvature of Monterrey is estimated to be about 30° northeastward (Figure IV-9).

It has been established that the folds at the Curvature of Monterrey are the result of a northeastward regional de'collement. Let’s now explain the possible cause, or causes, that originated the distinctive fold shapes on each one of the five areas previously described.

The remarkable periodicity of the fold system of the northern area (Plates 1 and 2, cross-section E-E’) is probably the result of the ’’free” gliding of this decollement sheet northeastwardly, in between the horst blocks of the Coahuila and San Carlos Islands, allowed by the thick sequence of Upper Jurassic evaporites and shale (Figures 111-2 and 111-3). These ductile rocks were transported northward, between the autochthon and the allochthon, and produced diapiric structures at the

Figure IV-12.- (a) Diagram showing the general structure of La Silla Anticline, (b) Idealized map view and crosssections of La Silla Anticline showing the possible relative position of the western edge of the San Carlos Island (labeled ’’basement” ) and its role in the origin of this fold.

southeastern end of the Sabinas Gulf (Figure IV-10, crosssection 3-3’, and Figure IV-11). The arcuate shape of the folds of this area was probably the result of a ’’delay” in its northward movement, caused by the Coahuila and San Carlos Islands, which allowed the overturning of the southwestern ends of the El Chorro and San Lucas Anticlines, and the overturning of both ends in Los Muertos Anticline (Figure IV-9).

After the northern area stopped its movement, the southern area continued to move northeastwardly, as shown by the overriding of the southern area over the southern edge of the northern area. This is represented by the two thrust faults located immediately northward of the Potrero de Abrego Anticline, which truncate the southeastern parts of the Jame and El Chorro Anticlines (Figures IV-9 and IV-10, cross-sections 3-3’ and ; and Plates 1 and 2, cross-section D-D’).

In the southern area is where variations in the lithology of the Mesozoic sedimentary sequence clearly show a control on the development of folds and faults. This is more noticeable in the northeastern part of the southern area, where most of the thrust faults have Lower Cretaceous Cupido Limestone in the allochthon, and Upper Cretaceous Me'ndez Shale in the autochthon (Plates 1 and 2, cross-sections C-C’ and D-D’). In all of these local ities reefal Cupido facies are present, suggesting that because of the rigidity of the reefal bodies, they slid over less rigid rocks instead of being folded.

The San Carlos Island impeded the northeastward movement of the southern area producing the Frontal Thrust and the Secondary Thrust faults, and probably a dextral oblique thrust at the Frontal Thrust (Figures IV-8 and IV-9). On the other hand, the Coahuila Island played the same role impeding the movement of the detached sedimentary sequence, with the exception that perhaps its southern edge was less steep than the western edge of the San Carlos Island, and consequently, the resulting folds and faults were less prominent, but were arcuated slightly northeastwardly (Anticlines of Sierra Hermosa, Cuauhtemoc, Chapultepec, Huachichil, and El Muerto) (Figure IV-9).

The folds at the western part of the southern area were arcuated approximately in the same direction that the regional movement of decollement, perhaps because most of them have almost no lithologic variations in the stratigraphic sequence that constitutes them (Figure IV-9 and Plate 1).

In the southern area strike-slip and thrust faults appear to be contemporaneous with folding. However it is possible that faulting was initiated in the early stages of folding, but field evidence throughout the area show that most thrusts are located along the overturned flanks of folds, which suggests that faulting took place immediately after initial folding occurred.

Irregularities on the basement topography also influenced the shapes of the folds inside the Curvature of Monterrey and on its northeastern part. Thus, the area of opposite vergence (Figures IV-8 and IV-9) is interpreted here as the result of the influence of the northwestern part of the shallow basement area, which partially impeded the northeastward movement of the detached Mesozoic sequence, allowing the overturning toward the southwest of the Anticlines of El Ore'gano, El Zorrillo and El Gateado (Figures IV-9 and IV-10, cross-section 6-6’ ), as well as the broad, and large El Potosi Anticline over the site of this area of shallow basement (Plates 1 and 2, cross-section B-B’).

The origin of La Silla Anticline is controversial, but subsurface data (Figure 11-l, wells Benemerito-1 and Teran-1), as well as paleogeographic reconstructions (Figures 111-l and 111-2), suggest that a basement high (San Carlos Island) acted as a barrier, against which,

only the central part of this anticline was overturned and thrusted onto the high (Figure IV-12, and Plates 1 and 2, cross-section D'-D”).

The time when the maximum stresses of the Laramide Orogeny deformed the Mesozoic rocks in the Curvature of Monterrey is considered here to be post-Early Paleocene and pre-Late Eocene. The reasons to assign it these boundaries are that the youngest Mesozoic deformed formations, Mendez Shale and Difunta Group, contain fossils that are Early Paleocene; and that the Late Eocene-Early Oligocene (?) Ahuichila Formation (continental molasse) rests unconformably over the deformed Mesozoic and probably Early Paleocene rocks.

However, the first evidence of the beginning of the Laramide could be represented (westward of the Curvature of Monterrey) by the uplift of most of western and southwestern Mexico, which produced the sediments of the Caracol and Parras Formations, and perhaps also initiated the northeastward decollement of the Mesozoic sedimentary sequence, until it stopped its movement sometime between the Late Paleocene-Early Eocene.



Figure IV-8



Figure IV-9



Figure IV-10



Figure IV-11.- Sketch map (a) showing how the presence of two basement highs could have influenced the fold shapes at the northern area of the Curvature of Monterrey, and the possible migration of evaporites, between the autochthon and allochthon (b), in the same direction of the decollement (northward).



Figure IV-12

V.- TECTONICS AND STYLES OF FOLDING IN NORTHEAST MEXICO

The structural trends present in northeastern Mexico are the result of the Early Tertiary (Late Paleocene- Early Eocene) Laramide Orogeny. Their different styles of folding permits one to infer where ancient stable or basinal areas were located. Also based on structural styles, one can distinguish areas of structures due to gravity gliding (decollements), diapirism, structures associated with compression and basement shortening, and linear features (lineaments) that often have been interpreted either as crustal fracture zones, megashears, strike-slip faults, or normal faults.

In this chapter an attempt is made to put together the information available in the literature, unpublished studies by the writer, and information derived from the interpretation of an ERTS satellite photographic composite map (scale l;2,000,000, unpublished), in order to correlate the different structural styles of folding with known paleogeographic continental and marine areas.

The area of this chapter includes the states of Coahuila, Nuevo Leo'n, and parts of the states of Tamaulipas, San Luis Potosi and Zacatecas. Its limits are the Rio Grande to the north, the Gulf of Mexico to the

east, the parallel N to the south, and the meridian 104° W to the west (Figure V-l).

Present day geomorphology of northeastern Mexico represents the distribution of different structural patterns that are intimately related to the fundamental landforms of early Late Jurassic paleogeography. Thus, from the observation of satellite photographs one can distinguish several groups of mountains that directly correlate with areas of different styles of folding and with ancient landmasses (Figures V-l and V-2).

Since early in this century Bose (1923) suggested that an ancient continental mass, or masses, was present in northeast Mexico. He based his ideas primarily on different structural styles and stratigraphic observations. Later workers continued to determine other paleocontinental areas in northeastern Mexico, also based on distinct structural styles (e.g., Kellum, et al., 1936). Today, using satellite photos it is possible to distinguish not only different styles of folding, but intriguing linear features that have a close correlation with the boundaries of Late Jurassic continental areas (Figure V-2).

In the following pages the correlation between different styles of folding and paleogeographic elements

is anal:/zed. In order to explain the structural and tectonic evolution of northeastern Mexico since the begining of the Mesozoic, the area of this study has been divided according to structural styles into two major groups; structures over ancient continental areas, and structures developed on ancient basinal areas.



Figure V-l.- Physiography of northeastern Mexico.



Figure V-2.- Structural map of northeastern Mexico showing the approximate location of Early Jurassic continental areas. The location of wells is approximate. The lines of sections shown in this map correspond to those of Figure V-4.



Figure V-3.- Generalized geologic map of southern United States and northeastern Mexico showing the surface and subsurface occurrences of pre-Mesozoic rocks. Modified from Garrison et al.(1980 ), after Flawn et al. (1961), Lopez-Ramos (1972), and unpublished sources.

A.- Areas of Basement Highs (Horsts)

Today in northeast Mexico five blocks of ancient continental areas have been recognized (Figure V-2) which are named here, El Burro-Peyotes Peninsula; La Mula, Monclova (?), and Coahuila Islands, and the Tamau lipas Archipelago, that is formed by the Lampazos-Sabinas-Picachos and San Carlos Islands. These areas were covered by marine waters at different times during the Mesozoic, but they continued to influence sedimentary patterns and structural styles until the Tertiary.

The structures over the described areas are broad, often breached, and symmetric anticlines, that are called by some Mexican geologists "tortugones” (turtle structures), that contrast sharply with the tighter folds of basinal areas (Figure V-2).

1.- El Burro-Peyotes Peninsula

This peninsula is limited to the south by the Boquillas-Sabinas Lineament, which will be described later, and may be the southern continuation of the Solitario and Marathon areas (Gonzalez, 1976). Its eastern and southeastern limits are less well defined, but subsurface information has shown that its southeastern end is located somewhere near the well Reforma-1 (Figure V-2), and it is separated from the Lampazos Island by what has been called the ’’Portal de Anahuac” (Alfonso, 1976).

The oldest rocks known in this area (Sierra del Carmen) are Paleozoic (Flawn, et al., 1961), and possibly Precambrian (Garrison, et al., I98O) (Figures V-3 and V-M ; they are overlain by Lower Cretaceous (Albian) rocks of the Glen Rose Limestone, whereas in the well Peyotes-2A, Berriasian rocks are unconformably overlying gneisses and quartzites (Gonza'lez, 1976). Redbeds (Triassic ?) and Upper Jurassic limestone and terrigenous rocks, have been reported in the Potrero de Oballos, located in the Sierra de las Hermanas (Humphrey, 1956) (see Figure V-l).

El Burro-Peyotes Peninsula was an emergent land since the Early Jurassic, and was progressively covered by Cretaceous seas, and entirely submerged by the Maas-

trichtian. This peninsula was considered by Humphrey (1956) as the northwestern part of what he termed the ’’Tamaulipas Peninsula” .

The most important structures over this paleopeninsula are the anticlines of El Burro, Peyotes, and El Carmen (Figures V-l and V-2). These structures are broad anticlines, of which, El Carmen is the smallest. El Burro and Peyotes Anticlines are large folds that appear to be the northwestern continuation of the Salado Arch of Murray (1959)* These anticlines contrast sharply with the tighter structures of thr Sabinas Gulf to the south, as is shown in section I-I of the Figure V-A.



Figure V-4.- Schematic cross-sections showing the different styles of folding in northeast Mexico. All four crosssections have been slightly modified by the writer, but their vertical and horizontal scales are the same used in the original sources. For location of sections see Figure V-2.

2.- La Mula Island

This basement high was first considered by Kellum, et al. (1936) as the western limit of the Sabinas Gulf, and as part of the so-called ’’Coahuila Peninsula”. It is uncertain who was the first to postulate this island, but it is probable that the original definition is in some unpublished report in the files of Petrdleos Mexicanos. However, Alfonso (1976) claims to be the one who first postulated this island in the unpublished report (1968) NE-M-1089 of PEMEX.

Nevertheless, it is very interesting to speculate if the boundaries of this island could be deduced from the shape of the folds that cover it, because there is no accurate information in the literature about its limits. From the observation of satellite photographs it can be seen that there are three large amplitude anticlines, apparently symmetric, that correspond to the Sierra de la Mula and to the Sierra del Fuste (Figures V-l and V-2). On the basis of these data, and knowing that in the Potrero de la Mula, granitic Early-Mesozoic rocks underlie the Padilla Limestone (Humphrey, 1956), the boundaries of La Mula Island were sketched (Figure V-2). This island was emergent during the Late Jurassic, but was covered by marine water during the Hauterivian (Alfonso, 1976).

3.- Monclova Island (?)

It is unknown who first described this island and when, but it is mentioned frequently in recent publications (Marquez, et al., 1976; Gonzalez, 1976; Alfonso, 1976). Its boundaries are unknown, and they were sketched in Figure V-2, only on the basis of the information presented by the above authors. According to them this island was emergent from the Berriasian until the very beginning of the Hauterivian, when it was covered by the Padilla Limestone. No other information is available, but it is possible that this ’’island” (?) could be related to diapiric mobilization (piercement folds) of Jurassic evaporites in the Sabinas Gulf (Humphrey, 1956) . If that is the case, then it should not be interpreted as a basement high, as has been done by Alfonso (1976, p. fig. 17).

4.- Coahuila Island

This paleogeographic unit was first described by Kellum, et al. (1936) as the ’’Coahuila Peninsula”, a Late Jurassic continental area, that was believed to extend northward from the Acatita-Las Delicias-Paila- San Marcos area to connect with El Burro-Peyotes Peninsula, including La Mula Island (Figure V-2).

But recent subsurface data and studies done by PEMEX geologists (Ma'rquez, et al. , 1976) have shown that this area was emergent since the Late Jurassic, and that it was an island, at least, since the very beginning of the Cretaceous (Berriasian), but possibly also during the Late Jurassic, until it was finally covered by marine water during the Late Aptian. It is still unclear whether this landmass was a peninsula or an island during the Late Jurassic (Gonzalez, 1976), but it seems probable that it was an island (Alfonso, 1976), and that the Sabinas Gulf was a sea connected with the Chihuahua Mesozoic Trough. Smith (1970) first suggested the possibility of an island in the ’’Coahuila Peninsula” for the Late Aptian, but stated that this interpretation was only a guess, because no subsurface data were available in the literature by that time.

Northward of the cities of Torreon and Saltillo there is a group of large amplitude structures very different from the tighter structures that form the ranges of the Sierra Madre Oriental (Figure V-2). This is where the approximate southern limit of the Coahuila Island has been placed (Gonzalez, 1976). The northern limit of this island is determined by a very linear feature that seems to continue southeastwardly, passing between the San Carlos and the Picachos islands (Figure V-2), and disappearing in the surroundings of China, Nuevo Leon. This linear feature has been named herein as the Sierra Mojada-China Lineament, and will be discussed later in this chapter. The western limit of this island is uncertain but it possibly is located somewhere west of Tlahualillo and Sierra Mojada.

The structures over the Coahuila Island are broad anticlines that trend generally northwestward. The most important are La Paila, Alamitos, El Venado, Los Remedios, El Zapatero, and Tlahualillo (Figures V-l and V-2), which are relatively simple and symmetric folds (Figure sections I-I” and 11-II" ).

5.- Tamaulipas Archipelago

The Tamaulipas Archipelago was first postulated, as a paleogeographic element by Alvarez (1958), but it was not until the last decade that its existence was demonstrated by wells that penetrated Paleozoic rocks, as for example the Lampazos-1, Barreta-2, Cerralvo-1, Carbajal-1, Herreras-1, Benemerito-1, Suarez-1, Las Blancas-2A, Trincheras-1, Linares-1, Chaneque-1, Lantrisco-1, and several others (Figure V-2). All wells that penetrated Paleozoic metasediments are located on the eastern side of the Tamaulipas Archipelago (see Figure V-2), and one wonders whether these metasediments are similar to the interior zone of the Ouachita system. All the wells drilled in the western part of these islands, penetrated granitic rocks that had radiometric ages that vary from to m.y. (Rivera J., 1978) (Figures 11-l and V-2).

This archipelago was considered by Humphrey (1956) to form a continuous continental mass during the Late Jurassic which bordered the western part of the ancestral Gulf of Mexico. He named this land ’’Tamaulipas Peninsula” , whereas other authors called it ’’Tamaulipas Platform” (Lopez-Ramos, I98O), as well as ’’Tamaulipas Arch” in its southern end (Murray, 1959)•

The Tamaulipas Archipelago includes, from northwest to southeast, the Lampazos Island*, the Sabinas Island*, the Picachos Island*, and the San Carlos Island (Figure V-2). It is bounded to the north by the ’’Portal de Anahuac” (a depression between the El Burro-Peyotes Peninsula and the Lampazos Islands), to the east by the Gulf Coastal Plain, to the west by the Sierra Madre Oriental and the Sabinas Gulf, and to the south by the Soto la Marina River. However, the southern and of this archipelago is located in the Tampico region (Tamaulipas Island), south of the area of this study.

Each island of the Tamaulipas Archipelago is characterized by broad anticlines, that at the surface, form low relief, elongate mountains (Figure section 11-II’), but the San Carlos Island has been complicated by Miocene granitic intrusions at its southern end.

Marine water covered this archipelago during the Late Jurassic (Aguayo, 1978), and it continued to subside through the entire Cretaceous until it was deformed and uplifted by the Laramide Orogeny.

* In this study the Lampazos, Sabinas, and Picachos Islands are considered to be isolated positive areas, but they possibly once formed (in the Late Triassic) a continuous block.

B.- Areas of Basement Lows

Since the beginning of the Mesozoic Era the continental blocks described before, controlled the deposition of redbeds (Upper Triassic ?); evaporites, carbonates, and terrigenous rocks (Upper Jurassic); thick sequences of carbonates, clastics, and evaporites (Early Cretaceous); and mainly clastics and shales (Late Cretaceous), over extensive shallow-water platforms, and in some localities deeper basins. All these sedimentary rocks were folded later during the Laramide Orogeny (Late Paleocene-Early Eocene), and are characterized by very tight folds, sometimes overturned and thrust faulted, and sometimes diapiric. One can distinguish three areas of different styles of folding; the Sabinas Gulf, the Sierra Madre Oriental, and the Parras Basin (Figures V-2 and V-3).

1.- Sabinas Gulf

This region of northeastern Mexico was defined by Humphrey (1956) as the Sabinas Gulf, which he considered to coincide with the geomorphologic province of the Coahuila Ridges and Basins. In this paper the name Sabinas Gulf is assignated to the same region, but its area and boundaries have been modified. The Sabinas Gulf is limited to the north by El Burro-Peyotes Peninsula; to the east by the Tamaulipas Archipelago; to the south by the front ranges of the Sierra Madre Oriental, and the Coahuila Island; and to the west by the Chihuahua Trough Its western limit is uncertain because of the presence of Middle Tertiary plutonic rocks, and Late Tertiary- Early Quaternary volcanic rocks (Figure V-3).

The marine rocks in the Sabinas Gulf vary in age from Late Jurassic to Late Cretaceous, and they are generally represented by terrigenous and carbonate rocks in the areas bordering the paleopositive elements, and evaporites and carbonates in the deeper regions (Gonzalez, 1976) (Figure V-4, sections I-I’ and 11-II’).

The structures of the Sabinas Gulf are large, elongate, tigh folds, often breached, that can be easily differentiated from the structures over the more stable psleocontinental areas. However, toward the boundaries

with these stable areas, the folds are often overturned and thrust faulted with a vergence toward the more stable paleoelements, as is the case of the Sierra Hermosa, Sierra Mojada, Sierra de la Fragua, and Sierra de San Marcos (Figures V-l, V-2, and V, section I-I’). Toward its southern end there is a distinct area where the folds are dome-like, an area that have been considered by Wall, et al. (1961) to be the result of diapirism of Upper Jurassic evaporites in the Sierra de Minas Viejas and Sierra del Fraile (Figure V, section 11-II’).

2.- Sierra Madre Oriental

The Sierra Madre Oriental is that area of northeastern Mexico that has the highest relief of this region. Relative relief between low hills and peaks is on the order of 800 to 1,9°0 m. It occupies the southern part of the area discussed in this chapter and is limited to the east by the Tamaulipas Archipelago and to the north by the Parras Basin (Figure V-3). It was considered by Humphrey (1956) as the ’’Mexican Geosyncline”, but this term is no longer used.

The oldest rocks known in this area are Paleozoic rocks (at Caopas-Rodeo and Aramberri); Upper Triassic (?) redbeds (at Galeana and Aramberri), and metavolcanic sediments (at Caopas-Rodeo) (Figure V-2). Covering these rocks are evaporitic terrigenous, clastic, and carbonate rocks (Upper Jurassic); carbonate (Lower Cretaceous); and terrigenous and clastic rocks (Upper Cretaceous).

The structures of this region are very distinct from those of the rest of the area. They are characterized by very tight overturned folds, sometimes isoclinal, which often are thrust faulted. Its eastern limit is a large thrust fault that extends from Aramberri to Monterrey, where the mountainous belt curves sharply westward and southwestward to the vicinity of Saltillo.

I There, the ranges trend westward to the city of Torreon, where they curve again northwestward into the State of Chihuahua (Figure V-2). An example of the style of folding in the Sierra Madre Oriental is shown in Figure V-4, section IV-IV’.

3.- Parras Basin

This Basin is located between the cities of Torreon, Saltillo, and Monterrey. It is limited to the north by the Coahuila Peninsula, to the west, south, and southeast, by the front ranges of the Sierra Madre Oriental, and to the northeast by the Sabinas Gulf (Figure V-3). The structures in this basin are easily differentiated from those of other areas (Figure V-2), and have been divided by Weidie (1961) into three zones of different structural styles; an area of mild deformation to the northeast, a zone of greater deformation in the southeast, and an area of intensely deformed strata in the narrow western part of the basin.

The structures in this basin are the result of Laramide stresses on Upper Cretaceous deltaic sediments of the Difunta Group, which has a thickness that varies from 1,000 to 3,500 m (Gonzalez, 1976). Subsurface data obtained from the wells Paila-IA and Mayran-1, have shown that the Difunta sandstones and shales are underlain by a thin sequence of Lower Cretaceous carbonates and terrigenous rocks, that have a combined thickness of no more than s°o m (see Figure V-A, section I-I’).

C.- Lineaments

Lineaments have been defined as: * - Significant lines of landscapes which reveal the hidden architecture of the rock basement. They are character lines of the Earth’s physiognomy. ** _ An essentially rectilinear topographic feature resulting from a fault. ** - A topographic line that is structurally controlled.

In the area under consideration for this chapter, and with the aid of satellite photographs, it was possible to interpret several linear features that coincide with the boundaries of different styles of folding. Two of these lineaments are easily identified: the Boquillas- Sabinas, and the Sierra Mojada-China (Figure V-2),

* A dictionary of Geology, by J. Challinor; New York: Oxford University Press, 1978.

Dictionary of Geological terms, prepared under the direction of the American Geological Institute: Doubleday & Company, Inc., New York, 1962.

1.- The Boquillas-Sabinas Lineament

The Boquillas-Sabinas Lineament is defined here as that linear feature observable on satellite photographs in northeastern Mexico that trends N 55° W from the . vicinity of the town of Boquillas, Coahuila, passing near Sabinas, Coahuila, and becoming unrecognizable southeastward, in the vicinity of Lampazos, Nuevo Leon. It has a width of about 10 km, and it coincides with the sharp discontinuity present between the tight folds of the Sabinas Gulf and the broad, gentle folds of the El Burro- Peyotes Peninsula (Figure V-2).

The Boquillas-Sabinas Lineament has been previously considered by Tardy (I98O) as the northern limit of what he called the ”de'collement du le bassin de Sabinas"*, and parallel to the Texas Lineament and to the ”Caltam Lineament”*.

Alfonso (1976) proposed five NW trending strikeslip faults in northeastern Mexico which he named from north to south, Texas, Sabinas, Monclova, San Carlos, and Padilla. He stated that the lateral movement of these faults segmented the Marathon-Ouachita system into six blocks during the last stresses of the ’’Orogenia Apala-

Figure V-5«- (a) Dynamically impossible set of faults proposed by Alfonso-Z. (1976, p. 147, Fig. 16) to explain the position of the Marathon-Ouachita system in northeastern Mexico during the pre-Late Jurassic. It is not possible primarily because the postulated relative displacement between blocks is inconsistent; secondly, because the shape of blocks 1,2, 3,4, 5» and is not even similar to their known shapes, previously shown by the same author (Ibid, p. 136, Fig. 2, and p. 147, Fig. 17) and by Gonzalez (1976, Figs. 4 and 5); and thirdly because the lithology indicated for blocks 3,4, and 5, is incorrect as shown. Diagrams (b), (c), and (d), show the possible senses of slip between blocks when they are subject to shear (sense of slip can be opposite when a shear couple is applied, but it depends on where the forces act) or compressional stresses.

* These terms are used by the cited author in his thesis but he doesn’t explain the source where they were first described.



Figure V-5

2.- The Sierra Mojada-China Lineament

The Sierra Mojada-China Lineament is here defined as that linear feature observable on satellite photographs in northeast Mexico that trends N- 67° W from the village of Sierra Mojada, Coahuila, southeastward to the vicinity of the town of China, Nuevo Leon. This lineament approximately coincides with the northern boundary of the Coahuila Peninsula and the southern limit of the Sabinas Gulf, more or less following the axes of the Sierra Mojada, Sierra de la Madera, Sierra de San Marcos, and Sierra de la Gavia, passing near the northern end of the Sierra de Minas Viejas, and finally through the depression between the Picachos and San Carlos islands toward the site of the town of China, Nuevo Leon (Figures V-l and V- 2 ).

The Sierra Mojada-China Lineament has not been described before, but it appears to continue northwestward passing near the town of Villaldama, Chihuahua, and beyond that town. For this reason, and because of its trend., it could be possibly related with the so-called .... ’’Mojave-Sonora Megashear’’ proposed by Silver and Anderson (197 M • Whether the Sierra Mojada-China Lineament is a strike-slip fault or not will be discussed later in this chapter.

D.- Model for the Mechanism of Deformation of NE Mexico

The origin of the structures of the northeastern region of Mexico has been a matter of conjecture since the early years of this century, and today it is still an open question. Several authors have suggested that the structures in northeast Mexico are the result of coaxial compressive forces that acted from the southwest and ’’squeezed” the Mesozoic sediments against pre-Mesozoic stable continental areas during the Laramide Orogeny (Imlay, 1938 b; Humphrey, 1956; Gonzalez, 1976; and several others). The relatively ’’weak” deformation shown by the Mesozoic sedimentary cover that overlies paleocontinental areas was considered to be the result of the major resistence of these basement highs, which absorbed the Laramide stresses. On the other hand, another group of authors (Haarman, 191?; De Cserna, 1956; Tardy, I98O) have also supported the idea of coaxial compressive forces acting from the southwest toward the northeast and postulated that the folds of the Sierra Madre Oriental were the result of a ” de’collement” in the Torreon- Monterrey area. But only a few authors have suggested that the structures of northeastern Mexico could be the result of sinistral shear (east-west) stress (Murray,

1961; Krutak, 196?; Mullan, 1978).

In this study it is proposed that the structures in northeast Mexico are the result of a sinistral relative movement of southern United States (westward) with respect to northern Mexico (eastward) during the Late Paleocene-Early Eocene, contemporaneously with a regional decollement that was produced by the tilting toward the northeast of the so-called "Geanticlinal Occidental” (Alfonso, 19?6), or ’’unnamed western continent” of Humphrey (1956). Thus, the following pages are dedicated to explain what is the evidence which supports this idea.

In the model proposed here, no attempt is made to explain the pre-Mesozoic tectonics of northeast Mexico.

Little is known about the Early Mesozoic tectonics in northeastern Mexico, with the exception that by that time most of this region was emergent while the North American, South American and African continents were still together. During the Late Triassic northeastern Mexico was subjected to tensional stresses related to the early opening og the Gulf of Mexico. This allowed the development of a complex system of grabens and horsts that influenced Jurassic and Cretaceous sedimentation (Figure V-6). The grabens were filled with continental redbeds and fluvial sediments (Salvador and Green I98O). Differential subsidence, faster eastward, during the early Late Jurassic allowed the eventual entrance of sea waters from the south forming extensive areas of restrict ed circulation which favored the deposition of evaporites into the grabens as terrigenous sedimentation continued along the flanks of the horsts. Later on, carbonate deposition was predominant, while deposition of terrigenous sediment continued in minor amounts, and a series of peninsulas and islands started to form (Figure V-7). The same conditions prevailed during the Early Cretaceous until the end of the Aptian, by which time most of the region was covered by the ocean and marine deposits. From the Turonian until the Early Paleocene the sea retreated to the east leaving behind a predominantly terrigenous sedimentary sequence. During the Late Paleocene to Early Eocene the stresses of the Laramide Orogeny folded and faulted the rocks deposited throughout the Mesozoic.

To analyze the mechanism, or mechanisms, that deformed the Mesozoic rocks in northeastern Mexico, two models are presented and compared, assuming that in both the genesis of basins, platforms, and other positive areas was by block-faulting tectonics (Figures V-6 and V-7). two different mechanism are proposed for the de-

Figure V-?.- Idealized block-diagram showing the major Early Mesozoic grabens and horsts in northeastern Mexico, and the inferred transgression of the sea over continental areas during the Late Jurassic (dotted line), Early Cretaceous (dashed line), and Late Cretaceous (solid lane).

formation of the Mesozoic sedimentary cover in northeast Mexico; first, compressive forces acting from southwest to northeast; and second, a sinistral shear couple of forces acting in an east-west direction.

With the distribution of grabens and horsts in north east Mexico during the Late Triassic is that shown in Figure V-6, compressive forces acting in a southwestnortheast direction (Figure V-8a), would develop folds essentially parallel to the more stable areas. Overturned folds and thrust faults would show a vergence toward the stable areas. Besides, if a simultaneous, regional, northeastward tilting of the western ’’unnamed continent” is also assumed, then a ’’decollement zone” would develop causing the sliding downward (to the northeast) of the sediments of the Sierra Madre Oriental, and tight overturned folds and thrust faults would be parallel to the edges of the more stable areas (i.e. horsts). Also, it would be reasonable to expect that the evaporites in the lower part of the Mesozoic sedimentary sequence would not only allow the sliding of the rocks over them, but also to be squeezed and carried-out toward the front of the '’decollement zone” producing some diapiric structures along the outer edge of the front ranges of the Sierra Madre Oriental, in the southeastern end of the Sabinas

Figure V-8.- Idealized distribution of structures if (a) SW-NE coaxial compressive forces, or (b) a sinistral E-W shear couple of forces were applied to a sedimentary post-Late Triassic-Late Cretaceous cover. See text for discussion.

Gulf. Potential strike-slip faults could develop at narrow areas of the pre-Mesozoic basement, while low amplitude folds would be develop over the relatively more stable basement highs (Figure V-8a).

On the other hand, if a sinistral shear couple of forces (Figure V-9b) is assumed instead of coaxial compressive forces, and exactly the same contemporaneous tectonic conditions are also assumed (i.e_. regional tilting of the western "unnamed continent", decollement of the Mesozoic sedimentary cover, and diapiric structures), then different folding patterns and potential strike-slip faults would be expected. In general, the structures over the relatively stable basement highs, the structures associated with the "decollement zone", and the diapirs (i.e_. S. del Fraile, S. de Minas Viejas), would be essentially the same as those in the model of compressive forces. However, two significantly different features would be present; "en echelon" folds in the Sabinas Gulf, and the strikes of the potential strikeslip faults across the narrow areas of basement highs, which would follow approximately the same strike of older normal faults, which could also produce significant lineaments (Figure V-8b).

When the comparison is made between the two alter- native models (Figure V-8), and the structures in northeast Mexico (Figure V-2), it can be concluded that a relative sinistral movement of southern United States (westward) and northern Mexico (eastward) during the Early Tertiary (Laramide Orogeny is a better explanation for the structural patterns and lineaments).

If it is assumed that the mechanism proposed in Figure V-8b is responsible for the structures in northeast Mexico, then it has to be assumed that the Boquillas Sabinas and the Sierra Mojada-China lineaments are, most probably, left-lateral strike-slip faults that in turn were older faults (normal ?).

The evidence that supports the idea of a left-lateral, strike-slip movement along the Boquillas-Sabinas Lineament is the following:

The termination along this lineament of the anticlines of Sierra de los Guajardo, Sierra de la Babia, Sierra Atravesada, and Sierra Hermosa (Figures V-l and V-2), the ”en echelon” left relationship of the Sabinas Gulf (Figures V-2 and V-8b). the apparent offset between the horsts of El Burro-Peyotes Peninsula and the Tamaulipas Archipelago (Figure V-2), and

the discontinuity of the Albian-Cenomanian reef trends across this lineament (i.e,. Serrania del Burro-Sierra Hermosa-Sierra de la Bahia) .

On the other hand, only the apparent offset between the Picachos and San Carlos Islands, and the notable differences of styles of folding between the Coahuila Island and the Sabinas Gulf (Figure V-2), support the idea of a sinistral strike-slip movement along the Sierra Mojada-China Lineament.

Finally, it is assumed here that the movement along these faults occurred during the Early Tertiary (Laramide Orogeny), contemporaneously with the folding of the Mesozoic sedimentary rocks. However, if these lineaments truly represent major sinistral strike-slip faults that acted throughout the entire Mesozoic, they could explain the puzzling rotations of 130° counterclockwise of Mexico relative to North America during the Early Mesozoic proposed by Gose et al. (1982).



Figure V-6.- Diagrammatic distribution of Early Mesozoic horsts and grabens in northeast Mexico.



Figure V-7



Figure V-8

VI.- SUMMARY AND CONCLUSIONS

The Early Mesozoic evolution of the Curvature of Monterrey area is intimately related to the origin of the Gulf of Mexico, which began opening in the Late Trias sic when the North American, South American, and African plates were still together. The break-up and separation of these plates allowed the formation of grabens and horsts, that later on would control the geographic distribution of land and sea, the sedimentary patterns, and finally the structural features at northeast Mexico.

The Coahuila and San Carlos Islands were the horst blocks that influenced the most the sedimentary patterns and the development of structures at the Curvature of Monterrey. The first to be covered by sea water was the San Carlos Island (Late Jurassic), whereas the Coahuila Island was not covered until the Late Aptian. Terrigenous sediments bordered these positive elements, while evaporites and shales were deposited in the graben areas during the Late Jurassic. At the beginning of the Cretaceous terrigenous sedimentation continued along the edges of the Coahuila Island, while carbonates and shales started to be deposited in the area of the Curvature of Monterrey. By Middle Neocomian numerous reefs started to grow at the edges of the horsts, causing the deposition of lagoonal shale, limestone, and evaporites in the backreefal areas, while deep water carbonates continued to be deposited on slapes and in basinal areas. A rapid change in sedimentation, from carbonates to terrigenous, took place by the beginning of the Turonian in the west and continued until the end of the Mesozoic. By the beginning of the Tertiary the previously deposited rocks were folded and faulted by the maximum stresses of the Laramide Orogeny, which produced the main structures at the Curvature of Monterrey.

The lithologic variations in the Mesozoic stratigraphic sequence deposited in the Curvature of Monterrey had a strong influence in the development of structures, together with the more regional control performed by the horst blocks of the Coahuila and San Carlos Islands. Thus, the evaporites and shale at the base of the sequence allowed the northeastward de'collement of the thick sequence of carbonates and terrigenous overlying them, until its movement was stopped by the stable Coahuila and San Carlos blocks.

Five distinct fold systems were caused by the de'collement in the Curvature of Monterrey: 1) a system of symmetric folds in the north, named here northern area;

2) a system of asymmetric, mostly northeastwardly overturned folds, named here southern area, commonly associat ed with thrust faults along their flanks, in most of which reefal bodies of the Cupido Limestone (Lower Cretaceous) overthrust the Mendez Shale (Upper Cretaceous); 3) a group of asymmetric, southwestwardly overturned folds, named here area of opposi te vergence; M an area of broad, asymmetric folds, named here area of shallow basement; and 5) the area of La Silla Anticline, which is a symmetric fold at its ends, and asymmetric at its middle part, where it is also complicated by a thrust fault caused by an irregularity on the horst of the San Carlos Island.

From the detailed study of stratigraphic sequences and structural features at the Curvature of Monterrey, the following results were obtained in this study:

Paleogeographic reconstructions were done using regional facies changes between Upper Jurassic, and Lower and Upper Cretaceous formations. The development of structures in this area was primarily controlled by lithologic variations in the Mesozoic sedimentary sequence .

The arcuate shape of the Curvature of Monterrey is the result of the influence of the basement highs of the Coahuila and San Carlos Islands.

Finally, from the detailed interpretation of satellite photographs in order to explain the structures at the Curvature of Monterrey, and their relation to the regional tectonics of the surrounding areas, a new model invoking sinistral shear across northeastern Mexico during Laramide deformation was proposed.

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The vita has been removed from the digitized version of this document.

* This report was partially published in 1959 as: Mesozoic stratigraphy and structure, Saltillo-G-aleana areas, Coahuila and Nuevo Leon, Mexico: San Antonio, So. Tex. Geol. Soc., Field Trip Guidebook AI, 13 p.



25 PLATE 1 GEOLOGIC MAP OF ' THE CURVATURE OF MONTERREY, MEXICO by Ricardo J. Padilla y Sanchez 10120 IQQQ '301 I C— ,-Kdi



PLATE 2 CROSS-SECTIONS OF THE CURVATURE OF MONTERREY by Ricardo J. Padilla y Sanchez, 1982.