The Neogene Ogallala Formation is an alluvial apron that occurs east of the RockyMountains from South Dakota to the Southern High Plains of Texas. The Ogallala was deposited by coalescent, lowgradient, wetalluvial fans that headed in mountains to the west. Geometry and depositional facies of the Ogallala Formation in Texas north of the 33rd parallel have been determined from outcrop studies and drillers'log descriptions. Averagingtechniques compensated for the variability of drillers' logs by

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... mpressing all the data within a unit area (2.5--minute quadrangle) to a single data point. Three overlapping fan lobes were constructed from sediment transported east and southeast along pre-Ogallala valleysystems that are as deep as 60 m (200 ft). Each lobe displays a general downdip decrease in thickness, net sand andgravel, and percentage sand and gravel. Only medial-fan and distal-fan facies are present in Texas.Medial-fan facies, in north and northwest Texas, include a thick sand and gravel sheet that is the product of braided-stream processes. To the south and southeast, medial-fan facies grade into distal-fan facies characterized by narrow channel systems thatbifurcatedowndip.Interchannel areas, composed predominantly of mud and silt, separate distal-fan channel systems. Interfan areas are broad, low net-sand areas that separate fan lobes. Pre-Ogallala valleys and positivephysiographic elements influenced the geometry and distribution of Ogallala depositional systems. Collapse basins, formed by solution of Permian evaporites,are filled with up to 250 m (800 ft) of Ogallala sediment. Deposition shifted southward after the filling of topographic lows that existed on Permian and Triassic rocks. Sand and gravel trends bifurcate around erosional remnants capped by Cretaceous strata that were buried during the final stage of Ogallala alluviation.