The stratigraphic and regional distributions of paleosol morphology in latest Pennsylvanian through Early Permian strata in Colorado, Utah, Arizona, New Mexico, Texas, and Oklahoma are presented in this paper. This regional extent corresponds to a paleolatitudinal gradient spanning ~5°S to 10°N. Morphological trends from this region delineate significant and systematic temporal and spatial changes in Permian-Carboniferous paleoenvironment and paleoclimate. The inferred latest Pennsylvanian (Virgilian) through early Early Permian environmental pattern is complex, but it indicates persistently dry, semiarid to arid conditions in Colorado, Utah, and Arizona, at paleolatitudes north of ~2°N, whereas lower paleolatitude (~2°S to 2°N) tropical regions in New Mexico exhibit a stepwise shift from subhumid to semiarid and variably seasonal conditions throughout late Pennsylvanian and the first half of Early Permian (Virgilian through Wolfcampian) time, followed by a subsequent shift to more arid conditions during the latter part of the Early Permian (Leonardian). Notably, strata from the southernmost paleosites, in Texas and Oklahoma, exhibit the most significant and abrupt climate changes through this period; they show a rapid transition from nearly ever-wet latest Pennsylvanian climate (at ~5°S) to drier and seasonal climate across the Permian-Carboniferous system boundary, and finally to arid and seasonal climate by Leonardian time (at ~2–4°N). The inferred climate patterns show no robust long-term correlation with the high-latitude Gondwanan records of glaciation. Rather, the long-term record of Permian-Pennsylvanian climate indicators from the southwestern United States is most simply explained by an ~8° northward tectonic drift through (essentially) static climate zones over western tropical Pangea during the interval of study. However, the relatively rapid perturbations to climate recorded by these pedogenic archives appear to be too rapid for tectonic forces and might correspond to changes in climate drivers, such as atmospheric p CO 2 , atmospheric circulation, and glacial-interglacial cycles.

Abstract The Palomas basin in the Rio Grande rift of southern New Mexico is a Neogene half graben, whose Pliocene-lower Pleistocene fill (Palomas Formation) has been partly exhumed, providing a nearly three-dimensional view of basin-fill architecture. The distribution of Palomas Formation depositional systems and their lithofacies is strongly asymmetrical. Footwall-derived sediment was deposited on small (radial length < 2 km), steep (slopes 1-4°) fans and consists primarily of turbulent-flow, channel conglomerates and hyperconcentrated-flow, pebbly sand, whereas hanging wall-derived sediment was deposited on broader (radial length 15 km), gentler (slopes 0.7-1°) fans and displays a greater proportion of channels with lateral accretion sets and overbank mudstones, suggesting that medial and distal hanging wall-derived fan channels behaved like gravel-bed streams. Axial-fluvial sediment, deposited by the ancestral Rio Grande, occupies a narrow (< 5 km) belt near the footwall block and consists of multistory pebbly medium-coarse channel sand, mottled overbank mudstone, and fine sand deposited as crevasse splays or in small channels. One to three excursions of the axial-fluvial system toward the footwall, each lasting 10 5 -10 6 years, were accomplished by toe cutting of the footwall-derived fans and resulted in deposition of multistory channel sands, while the western edge of the floodplain was dominated by deposition of overbank mudstone and crevasse-splay/small-channel fine sands. Tectonic tilting of the half graben is the probable cause of the fluvial incursions toward the footwall, because toe cutting of the fans is largely restricted to the footwall side of the basin and the number of incursions correlates to distinct segments of the border fault system.

In the Rio Grande rift of southern New Mexico, the intrabasinal East Robledo fault in the Mesilla basin and the Jornada fault in the Jornada del Muerto basin experienced hundreds of meters to kilometers of offset during late Miocene to early Pliocene time and tens of meters of offset since middle Pleistocene. Late Pliocene and early Pleistocene activity on the faults is assessed by comparing sedimentological characteristics of the Camp Rice Formation, which is correlated by reversal magnetostratigraphy, on either side of the faults. The Jornada fault is interpreted to have been inactive from approximately 3.4 to 2.5 Ma, because Gauss-age fluvial strata at Rincon Arroyo, located on the hanging wall, have similar sediment accumulation rates, degree of development of calcic paleosols, and relative abundance of fluvial channel lithofacies as coeval footwall strata at Cedar Hill and Lucero Arroyo. In contrast, syndepositional movement on the Jornada fault along the northern flank of the Dona Ana Mountains from 2.5 to 0.7 Ma is suggested by a condensed Matuyama interval characterized by mature stage III and IV calcic paleosols at Lucero Arroyo. The northern segment of the East Robledo fault was active during Gauss time, based on an abundance of relatively thick (59 m), fluvial-channel deposits in the hanging-wall section at Northeast Robledo. The southern segment of the East Robledo fault, however, was probably not active during most of Gauss time, because of the presence on the footwall of a thick (50 m) section of fluvial strata of Gauss age at Picacho Mountain. Major movement on the East Robledo fault near the end of Gauss time terminated sedimentation at Picacho Mountain and in the Corralitos basin, abandoning the upper La Mesa geomorphic surface.