Circular 145—Late Cenozoic molluscs and sediments, southeastern New Mexico

By A. B. Leonard, J. C. Frye, and H. D. Glass, 1975, 19 pp., 2 tables, 2 figs., 1 appendix.

Reconnaissance of late Cenozoic geology and correlation of data with stratigraphic, paleontologic, and clay-mineral data from adjacent parts of New Mexico and Texas. Collections of late Pleistocene fossil shells from 20 localities yielded a total of 30 species of molluscs. The faunas in the southeastern area are less abundant and somewhat less varied than the faunas farther north in New Mexico. South of the bounding escarpment of the Ogallala-capped High Plains, mineral composition and lithology of three Ogallala outliers show consistency with the Ogallala clay-mineral zones described farther north. The Wisconsinan pond and lake deposits south of the High Plains contain sepiolite, and at one locality both sepiolite and attapulgite, in contrast to the terrace deposits of the Pecos Valley. However, the molluscan faunas of the two areas do not differ significantly.

The late Cenozoic deposits of extreme southeastern New Mexico have received little attention until recently. Following several of our most recent studies in east-central New Mexico, the authors conducted a reconnaissance of the late Cenozoic geology of the southeastern part of the state to determine whether the conclusions reached in the northern work were applicable to the south. Correlating data from southeastern New Mexico with stratigraphic and paleontologic data from adjacent Texas, and with clay-mineral data from the southern High Plains of adjacent Texas were also considered. The area studied includes the dissected lowland in Lea County south of the escarpment marking the boundary of the Ogallala-capped upland plateau within the High Plains, and in Eddy County, the largely pedimented surfaces extending west to the Pecos Valley. The topography of this part of the region is the result of Pleistocene erosion and deposition, and is characterized by outliers of Ogallala Formation, buttes and mesas of Triassic and Permian rocks, pediments, colluviated surfaces, and collapse basins of various sizes as a result of solution of the underlying Permian rocks.

The Pleistocene history of Pecos Valley has been described. In eastern New Mexico, the Pecos River in its present course is post-Kansian in age. The early Pleistocene terraces north of Chaves County are related to eastward drainage through the now abandoned Portales Valley, and only the Wisconsinan terraces of Pecos Valley are related to drainage from the upper part of the basin.