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A B S T R A C TLandscapes are thought to be youthful, particularly those of active orogenic belts. Unaweep Canyon in the Colorado Rocky Mountains, a large gorge drained by two opposite-flowing creeks, is an exception. Its origin has long been enigmatic, but new data indicate that it is an exhumed late Paleozoic landform. Its survival within a region of profound late Paleozoic orogenesis demands a reassessment of tectonic models for the Ancestral Rocky Mountains, and its form and genesis have significant implications for understanding late Paleozoic equatorial climate. This discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator.

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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.

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