Pleistocene molluscan faunas and physiographic history of Pecos Valley in Texas

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Leonard, A. Byron (Arthur Byron), 1904-

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University of Texas at Austin. Bureau of Economic Geology


Studies of physiographic relations and of fossil molluscan faunas made in the Pecos River valley region (Val Verde County to the Texas New Mexico border) reveal extensive, well-pedimented surfaces southwest of the river extending from the Davis Mountains to near the present channel. These surfaces are described as Surface I, graded to the late Tertiary Ogallala drainageways; Surface II, graded to a lower level early Pleistocene channel of Pecos River and extensively pedimented during Illinoian time; Surface III, graded during early Wisconsinan to a level only slightly above the present channel of Pecos River; and Surface IV, developed during latest Wisconsinan and Recent as a terrace of highly fossiliferous deposits in which the Pecos River is presently incised. No molluscan fossils were observed in the sediments of Surface I; meager collections in alluvial sediments underlying Surface II confirm its Pleistocene (?) age; adequate collections of fossil mollusks from alluvial sediments beneath Surface III are characteristic of early Wisconsinan, and the uniquely rich faunas in deposits below Surface IV are characteristic of post- Twocreekan (post-Bradyan) assemblages. Physiographic features of Pecos River valley are illustrated by generalized profiles and photographs; the 33 localities from which fossil mollusks were collected are indicated on a location map, the species included in each local molluscan faunal assemblage are shown on a chart, and each species is illustrated photographically.


To obtain a print version of this publication visit: and search for: RI0045. Bibliography: p. 29.

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