Depositional Architecture of the Quaternary BlackWater Draw and Tertiary Ogallala Formations, Texas Panhandle and Eastern New Mexico

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Deposition of basal fluvial sediments of the Ogallala Formation was controlled by topography on the middle Tertiary erosional surface. Paleovalley-fill sequences consist of gravelly and sandy-braided stream deposits overlain by eolian sediments deposited as sand sheets and loess. The change from fluvial to eolian sedimentation may have resulted from diversion of Ogallala streams to form the Pecos and Canadian Rivers. Paleostream divides on the middle Tertiary erosional surface are overlain primarily by eolian sediments. Source areas for eolian sediments may initially have been Ogallala braided streams, and later the floodplains of the newly formed Pecos and Canadian Rivers.

Ground-water calcretes are extensively developed in the fluvial portion of Ogallala sediments. Pedogenic calcretes, consisting of nodular, laminated, brecciated and recemented, or pisolitic calcium carbonate occur primarily in the eolian portions of the Ogallala Formation.

The geomorphic processes of eolian deposition, deflation, and pedogenesis have operated on the Southern High Plains from Ogallala time to the present, as evidenced by the distribution of coarse eolian deposits, which make up both the upper part of the Ogallala Formation and all of the Blackwater Draw Formation.

The distinctive reddish sediments of the Blackwater Draw Formation contain as many as six well-developed buried soils that resemble each other, as well as the surface soils, in lithology and morphology. Although the Blackwater Draw Formation was originally assigned an Illinoian age, pedologic similarities of soils beneath ash deposits dated as 1.4 million years (Guaje ash) and 0.6 million years (Lava Creek "B") in Crosby and Swisher Counties, respectively, to paleosols at a thick section in Lubbock County, suggest that pedogenic processes have operated throughout the Quaternary. It is hypothesized that, at least from near the end of the Pleistocene, eolian sediments aggraded contemporaneously with lacustrine facies in a mosaic of laterally restricted lenses of eolian and playa sediments. Pulses of deposition were separated by relatively long periods of either landscape stability, during which soil development occurred, or deflation, which stripped surface horizons from newly formed soils. Modern processes appear to be analogous to those that operated much earlier, suggesting that on the Southern High Plains, the present is indeed the key to the past.


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