Stratigraphy, sedimentology, and paleontology of Upper Cretaceous (Campanian-Maastrichtian) sedimentary rocks in Trans-Pecos Texas




Lehman, Thomas Mark

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Marine, paralic, and continental sedimentary rocks of Late Cretaceous (Gulfian) age are exposed in three major areas along the Rio Grande valley in Trans-Pecos Texas and adjacent Mexico. Although different stratigraphic nomenclature has been applied to strata exposed in each area (Sierra Vieja, Ojinaga, and Big Bend), the general succession and similarity of lithologies, biostratigraphic correlation, and paleocurrent data suggest that these strata are parts of the same genetic units and were formerly contiguous. In general, marine shale and limestone of the Ojinaga Formation is equivalent to that of the Boquillas and Pen formations, paralic sandstone of the San Carlos Formation is equivalent to that of the Aguja Formation, and variegated continental mudstone of the El Picacho Formation is equivalent to that of the Javelina Formation. Both Aguja and San Carlos formations are informally subdivided in this report. The entire sedimentary pile thins dramatically northeastward from the Chihuahua Trough onto the Coahuila Platform. Three major transgressive-regressive cycles are recorded in the paralic deposits, with progradational episodes occurring in Middle Turonian, Early Campanian, and Late Campanian time. The strandline migrated northeastward across the Trans-Pecos region, and sediment was derived primarily through erosion of a volcanic arc terrane in western Mexico. Progradational deposits consist of fluvial-dominated deltaic and laterally continuous sandy shoreface accumulations. Increase in thickness and continuity of shoreface deposits in the Sierra Vieja area suggests that a major deltaic headland may have existed to the north, and that much of the Trans-Pecos region occupied a broad interdeltaic embayment. An abrupt change in sedimentation occurred in Maastrichtian time with the onset of Laramide tectonism and initial development of the Tornillo Basin. Sediment transport shifted to the southeast, concurrent with an influx of coarse volcanic detritus, chert pebbles reworked from Lower Cretaceous rocks, and the widespread cannibalization of Upper Cretaceous sediments. Stream channel aggradation and incision events, and alternating periods of podzolization and calichification in floodplain paleosols, suggest that long term humid - semi arid climatic cyclicity prevailed during deposition of these sediments