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.

Samples of Pleistocene deposits from east-central New Mexico were analyzed for clay mineral composition, and representative samples of Tertiary, Triassic, and Permian rocks were analyzed for evaluation of sediment sources. Throughout the region mont morillonite, illite, and kaolinite were found to be most widespread and abundant, and are shown to be detrital minerals from identifiable sources. Sepiolite was present only in the pond and lake deposits of the High Plains area. Attapulgite occurs in the Ogallala Formation and in middle to early Pleistocene soil profiles on alluvial deposits. Corrensite was present in three localities: a Triassic sandstone, late Wisconsinan sediment fills in basins developed in Permian rocks, and Holocene terrace deposits of the Pecos River valley.

Clay mineral compositions permit the zonation of the Ogallala Formation (Pliocene) in central-eastern New Mexico. The clay mineral assemblage of the lowest part is dominated by montmorillonite (Zone I); above is a zone of progressive upward increase of attapulgite (Zone 2); and above this, a thin zone dominated by sepiolite and attapulgite (Zone 3). Zones 1 through 3 are interpreted as representing progressive desiccation through Pliocene time. At the top of the formation, above Zone 3, are two thin zones characterized by abundant carbonate. Zone 4 is again dominated by montmorillonite with lesser amounts of illite and kaolinite, but no attapulgite or sepiolite. Zone 5 at the top, the pisolitic limestone, contains weathered montmorillonite, illite, kaolinite, and locally chlorite. These uppermost zones are interpreted as representing the modifying effects of Pleistocene events. Caliche, displaying a wide range of physical appearance, occurs on Pleistocene deposits of several ages, and in the capping zone of the Ogallala. These caliches were studied by their clay mineral assemblages, and by radiocarbon dating. Radiocarbon dates on Pleistocene caliches ranged from 11,250 to 31,700 B.P.; on the pisolitic limestone (Zone 5), at the top of the Ogallala Formation, from 27,160 to 35,000 B.P.; and on caliches from 2 to 10 ft below the top of the Ogallala (Zones 2-4), from 30,880 to 43,100 B.P. The radiocarbon dates are apparent ages and do not indicate the time of initial deposition of the caliche. The dates reflect modifications of the calcium carbonate by events during late Pleistocene and Holocene time.

A study of the Pliocene, Pleistocene, and Holocene stratigraphy, paleontology, and clay mineral assemblages was made for the northeastern part of New Mexico. The stratigraphic zonation of the Ogallala Formation, using clay-mineral assemblages developed farther south in the state, is extended to the northern border and paleontologically correlated with the floral zones identified regionally throughout the Ogallala Formation. The stratigraphic relations of several basalt flows to the Ogallala as well as to Pleistocene deposits is included. Earliest Pleistocene deposits (Nebraskan) are sparse, but deposits of Kansas to mid-Pleistocene age are extensive in the area to the north and east of the Canadian River. Wisconsinan to Holocene terrace deposits occur along virtually all of the valleys of the region; they have yielded 48 collections of fossil molluscs, and nine samples have been dated by radiocarbon analysis. The molluscan assemblages are compared through an age range from 27,000 to less than 1,000 B.P. with the living fauna and with fossil assemblages of similar age southward in New Mexico. Clay-mineral data for the Pleistocene and Holocene deposits are presented in the appendix, and the contrasting erosional histories of the several drainage systems are discussed.