Eolian sediments have accumulated as non-glacigenic loess and thin sand sheets on the Central and Southern High Plains grasslands of Texas and New Mexico since the late Miocene. Locally as much as HO m of eolian sediments with numerous paleosols are preserved in the Quaternary Blackwater Draw Formation and the upper part of the Miocene-Pliocene Ogallala Formation. These sediments and paleosols, which cover more than 130,000 km2, are similar to recent surface sediments and soils and record a long period of episodic eolian transport and sedimentation, and pedogenesis on a stable low-relief grasscovered landscape. Eolian sections, which comprise the fine sand to coarse silt lithofacies of the Ogallala Formation, and the very fine to fine sand and sandy mud lithofacies of the Blackwater Draw Formation, generally lack primary sedimentary structures. Grain size of Ogallala sediments decreases from west to east, and grain size of Blackwater Draw sediments decreases from southwest to northeast. Soil horizonation is well developed in most sections, and buried calcic and argillic horizons are common. Calcic horizons are characterized by sharply increased CaC03 content in the form of filaments, nodules, and petrocalcic horizons (calcretes). Argillic horizons are characterized by increased illuvial clay, pedogenic structure, and darker reddish hues. Rhizocretions are common locally. Open root tubules, which are typically less than 1 mm in diameter and characteristic of small plants like grasses, are present in all Ogallala and Blackwater Draw eolian sediments. Paleosols preserved in eolian sediments of the High Plains reflect periods of sedimentation followed by episodes of landscape stability and pedogenesis, and negligible sedimentation. Episodes of sedimentation and soil development likely resulted from cyclic decreases and increases in available moisture and vegetative cover. Eolian sediments were eroded and transported eastward during dry periods when vegetation was sparse in source areas, such as the western High Plains and the Pecos Valley. During humid periods more abundant vegetation probably protected source areas from deflation, and resulted in landscape stability across the High Plains.
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