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Memoir 30—Pliocene and Pleistocene deposits and molluscan fauna, east-central New Mexico

By A. B. Leonard and J. C. Frye, 1975, 44 pp., 3 tables, 6 figs., 3 plates, 2 appendices, 1 index.
Molluscan faunal assemblages (dated 18,000 to 5,000 B. P.) from 45 localities produces a composite fauna of 47 species. Describes paleoclimates and geology of the Cenozoic.

A reconnaissance of Pliocene and Pleistocene geology, and Pleistocene fossil molluscs was made for the region of east-central New Mexico, including the High Plains and Pecos Valley areas, northward from northern Eddy County to the south side of Canadian River valley. The Ogallala Formation (Pliocene) was deposited by streams flowing east-southeast, resulting in a broad alluviated plain that masked the earlier erosional topography. Early Pleistocene time witnessed structural tilting and warping, increased precipitation, an development of a new drainage system that flowed eastward through the now abandoned Portales Valley into Texas. An independent southward drainage system captured the drainage through Portales Valley in the vicinity of Fort Sumner, and thus produced the present Pecos River.     Molluscan faunal assemblages from 45 localities produced a composite fauna of 47 species; 2 assemblages are from deposits judged to be Kansan, 7 assemblages are from Holocene terrace deposits dated at 6,000-5,000 radiocarbon years B.P.; the remaining assemblages are Woodfordian, 7 of them radiometrically dated at ages ranging from 18,000 to 3,000 B.P. Inferences drawn from the molluscan faunas include; (1) the 2 Kansas faunas lived in a xeric environment not unlike that produced by the present-day climate; (2) a long, perhaps intermittent pluvial period existed in the region from 18,000 to 13,000 B.P. and perhaps longer; and (3) by about 5,000 B.P. the present semiarid climate was well established. While the long pluvial period produced environments suitable for branchiate gastropods, many aquatic pulmonates, and a variety of terrestrial gastropods, evidence of forest cover or a climate significantly cooler than today is not clear.

A reconnaissance study of the late Cenozoic geology and molluscan paleontology of the region of east-central NM southward from the escarpment along the south side of the Canadian River valley to the northern edge of Eddy County and westward from the Texas state line to the west side of Pecos River valley, was made during several weeks in the summers of 1971 and 1972. The region, extending more than 200 mi north-south and approximately 125 mi east-west, is in the High Plains section and the Pecos Valley section of the Great Plains Province. This segment of the High Plains, known also as Llano Estacado, is isolated on the north by a broad erosional belt, and is bounded on the east, north, and west by prominent escarpments. The Pecos River traverses the area west or the Llano Estacado, south and southeast across western Texas to the Rio Grande.

The area covered, and the locations of molluscan faunas collected, are shown. The location of described stratigraphic sections of the Ogallala Formation included with this report, and those published in 1972, are shown. The location of described stratigraphic sections of Pleistocene deposits included with this report, and the location of radiocarbon dates determined in the laboratories of the Illinois State Geological Survey, are shown. Samples collected for X-ray study of clay minerals have been reported, and a preliminary discussion of the structure of the top of the Ogallala Formation has been published. Locations of all faunas described are given. Fifteen described stratigraphic sections of the Ogallala Formation and Quaternary deposits are included.

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