The Blackwater Draw Formation (Quaternary): A 1-4-plus-m.y. record of eolian sedimentation and soil formation on the Southern High Plains

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The Southern High Plains of northwestern Texas and eastern New Mexico are mantled by a vast (>100,000 km 2 ) sheet of Quaternary eolian sediment locally as much as 27 m thick and termed the Blackwater Draw Formation. These sediments generally fine from southwest to northeast, exhibiting a linear relationship of decreasing sand content and sand size and increasing silt content with increasing distance, indicating the source area to be the Pecos River valley. As many as six well-developed buried soils (2.5YR to 5YR hues, Bt horizons 1-2 m thick, Stage II-III calcic horizons), similar to the regional Paleustalf and Paleustoll surface soils, occur in the formation. The buried soils indicate episodic sedimentation separated by long periods of landscape stability. Eolian sedimentation probably occurred during prolonged aridity, and stability and pedogenesis likely obtained during subhumid to semiarid conditions, similar to those of the late Quaternary. The presence of the 0.62-m.y. Lava Creek B Ash and the 1.4-m.y. Guaje Ash in the formation shows that the deposit accumulated throughout most of the Quaternary. Data from paleomagnetic, thermoluminescence, and radiocarbon studies suggest that each cycle of sedimentation-stability lasted for several hundred thousand years and that the last depositional event occurred at least several tens of thousands of years ago.