This article utilizes over 3000 biostratigraphic reports of fusulinid taxa in the Midland Basin to produce a series of chronostratigraphic surfaces that show shelf-to-basin profiles from the end of the Atokan to the end of the Leonardian. The position of the shelf-edge break along the Eastern Shelf was geometrically reconstructed for each of the chronostratigraphic surfaces. Comparing the location of the shelf edges produced in this study to published examples resulted in significant disagreement for some time intervals, especially the Wolfcampian. These discrepancies are inferred to be predominantly the result of lithostratigraphic-based vs. biostratigraphic-based data. Assessing shelf-edge trajectory through the Pennsylvanian into the early Permian indicates that (1) tectonic and eustatic increases in shelf accommodation resulted in retrogradation of the shelf edge during the Pennsylvanian, and (2) early Permian progradation was the likely result of cessation in tectonic subsidence that allowed bypass of the shelf and passive filling of the basin center. When placed into the context of the Ancestral Rocky Mountains, subsidence analysis of the Midland Basin agrees with tectonic models that portray a synchronous start rather than an east-to-west migration of peak subsidence. Additionally, a relatively synchronous apex of tectonic subsidence occurred in the Middle to Late Pennsylvanian.

ABSTRACT The mid-Pleistocene transition is a time interval between ca. 1.2 and 0.7 Ma during which a shift occurred from ~41 k.y. glacial-interglacial cycles to ~100 k.y. cycles. Although the mid-Pleistocene transition has been well documented in global marine records, its effects in continental environments, including North America, are incompletely understood owing to the paucity of terrestrial sediment records spanning the entire Quaternary. A notable exception is the ca. 1.4 Ma and younger Blackwater Draw Formation, an extensive eolian sequence on the Southern High Plains of the United States. Intervals of the Blackwater Draw Formation section that are inferred to span the mid-Pleistocene transition can be divided into pre–, syn–, and post–mid-Pleistocene transition parts. Weathering profiles in the pre–mid-Pleistocene transition section are dominated by weakly developed soils formed in arid environments, as evidenced by well-expressed pedogenic carbonate horizons, lack of clay formation during hydrolysis, and magnetically soft, coarse-grained magnetite/maghemite populations. Conversely, the syn– and post–mid-Pleistocene transition intervals demonstrate an increase in weathering intensity by an abrupt increase in clay content formed in part by hydrolysis of feldspars, soil profiles that demonstrate leaching and illuviation, and a fining-upward grain size of the magnetite/maghemite population. Sedimentologic, geochemical, and rock-magnetic data are consistent with a southern and coarser sediment source derived from the Pecos River drainage prior to the mid-Pleistocene transition, followed by a mixture of northern and southern sources during and after the mid-Pleistocene transition. Overall, our results indicate that pre–mid-Pleistocene transition conditions on the Southern High Plains were arid with wind energy sufficient to mobilize sand sheets out of the Pecos River and deposit them on the plateau. The syn– and post–mid-Pleistocene transition environments reflect somewhat wetter conditions and potentially an influx of silt from the north, in addition to continued sand derived from the Pecos River valley. The wetter conditions and silt influx may have resulted from longer-lived and more robust glacial activity in the Northern Hemisphere that characterized the post–mid-Pleistocene transition Earth system.

Abstract: The “Wolfcamp Shale” of the Permian Basin in west Texas is a prime source of oil reserves within the USA. Despite its growing importance, definitive ages and stratigraphic correlations across the basin remain unclear. Fusulinid studies have assigned an approximate age of Late Pennsylvanian (Missourian) to early Permian (Leonardian) for the “Wolfcamp Shale,” but exact horizons for regional stage boundaries are not well defined. Analysis of conodont specimens from three cored basinal shale intervals of the Shell Stevens L1V well in the northern part of the Midland Basin (Lynn County, Texas) improves interpretation of regional stage boundaries within the “Wolfcamp Shale” and provides an accurate means of correlation to depositional cycles on the Eastern Shelf in central Texas. Conodont biostratigraphic markers were compared to log correlations and sequence stratigraphic correlations. Associated gamma-ray (GR) and resistivity (RES) logs show distinct log markers for the Pennsylvanian–Permian boundary and base of the Leonardian Stage that are traceable to immediately adjacent wells and possibly to portions of the southern part of the Midland Basin. Third- and fourth-order sequence stratigraphic cycles were delineated using core lithology, conodont abundances, and gamma-ray and resistivity readings. These sequences are linked to those observed in the Eastern Shelf stratigraphic sections and provide a useful secondary mode of correlation.