Deposition of the basal fluvial sediments of the Miocene-Pliocene Ogallala Formation in western Texas and eastern New Mexico was controlled by topography on the underlying erosional surface. Paleovalley-fill facies consist of gravelly and sandy braided-stream deposits interbedded with and overlain by eolian sediments deposited as sand sheets and loess. Uplands on the pre-Ogallala erosional surface are overlain primarily by similar eolian sediments. Calcic paleosols, consisting mostly of glaebules and rhizoconcretions of CaCO3, occur throughout the eolian facies. Massive to laminated and locally pisolitic, brecciated, and recemented pedogenic calcretes occur primarily near or at the top of the Ogallala Formation. Eolian facies preserve numerous superposed calcretes and calcic paleosols, reflecting slow episodic aggradation on a savanna or grassland under and to subhumid climatic conditions. The change from fluvial to mostly eolian sedimentation probably resulted from diversion of streams that deposited fluvial sediments of the Ogallala Formation to form the Pecos and Canadian rivers. Source areas for eolian sediments may initially have been floodplains of Ogallala braided streams and later the floodplains of the newly formed Pecos and Canadian rivers.

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