Pliocene and Pleistocene deposits and molluscan faunas, east-central New Mexico

Abstract

A reconnaissance of Pliocene and Pleistocene geology, and Pleistocene fossil mollusks 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, and 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 to 5,000 radiocarbon years B.P.; the remaining assemblages are Woodfordian, 7 of them radiometrically dated at ages ranging from 18,000 to 13,000 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.

Late Cenozoic sediments, molluscan faunas, and clay minerals in northeastern New Mexico

“…In this study we find that it can be traced northward to the Colorado State line with equal success. In this northeastern New Mexico area, it has been possible to correlate these clay-mineral zones, traceable north-south across New Mexico, with the floral zones described in western Texas and correlated northward to Nebraska (Leonard and Frye, 1978).…”

“…This study covers one area in a program of regional studies in the eastern third of New Mexico (Frye and Leonard, 1972;Glass, Frye, and Leonard, 1973;Frye, Glass, Leonard, and Coleman, 1974;Leonard, Frye, and Glass, 1975). It is also an extension of a long-range series of studies in the southern Great Plains, including western Texas (Frye and Leonard, 1957a, 1957bLeonard and Frye, 1962;Frye, 1971), and western Kansas and adjacent areas (Frye and Leonard, 1952;Frye, Leonard, and Swineford, 1956).…”

“…The faunas and floras that occur in the Clayton South section can be placed in the sequence of clay-mineral zones widely recognized in the Ogallala Formation of eastern New Mexico. The foregoing indicates that the molluscan fauna and associated fossil flora (Leonard and Frye, 1978)…”

“…The terms have been used in a few sections on the basis of tenuous lithologic correlation with fossiliferous sections in western Texas. Only in the northeastern part of New Mexico have fossil floras and faunas been collected (Leonard and Frye, 1978) to permit reliable correlation. Unfortunately only the two upper units, the Kimball and Ash Hollow zones, can be identified in the thin Ogallala of the region.…”

“…The tables in the appendix include 210 x-ray analyses of Ogallala samples (table 1), 85 x-ray analyses of pre-Wisconsinan Pleistocene samples, and 89 x-ray analyses of Wisconsinan and Holocene samples (table 2). Four fossil floras from the Ogallala Formation and one fossil snail fauna from the Ogallala Formation (Leonard and Frye, 1978) are included. In the appendix (table 3) there are listed 48 fossil faunas from the Wisconsinan and Holocene deposits of the region, supported by 9 radiocarbon dates (table 4).…”