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Late Triassic (Dockum Group) rocks accumulated in relict Paleozoic basins bound by the Amarillo uplift on the north and the Glass Mountains on the south. Basins were reactivated by late Paleozoic or early Mesozoic tectonic activity that created the Gulf of Mexico.
More than 2,000 ft (600 m) of terrigenous clastics, derived mostly from older sedimentary rocks, accumulated within the basin. Source areas were in Texas, Oklahoma, and New Mexico. The Dockum Group accumulated in a variety of depositional systems including (1) braided and meandering streams, (2) alluvial fans and fan deltas, (3) highly constructive lobate deltas, (4) lacustrine systems including ephemeral and relatively long-lived lakes, and (5) mud flats.
Dockum sedimentation was cyclic, a reflection of alternately humid and arid climatic conditions. During humid climatic conditions lake level was relatively stable. Meandering streams supplied sediment to high-constructive lobate deltas in the central basin area; braided streams and fan deltas were dominant depositional elements within southern and northern basin areas. During arid climatic conditions base level was lowered, stream valleys evolved, and small fan deltas developed along ephemeral lake margins; evaporites, calcretes, silcretes, and soils developed on floors of ephemeral lakes and on delta platforms.
Uranium occurs within about 25 depositional facies. Highest uranium values are in lacustrine facies which developed under arid climatic conditions. Channel-lag facies of meander-belt systems generally exhibit consistently higher uranium values than other depositional facies. Crevasse-channel and crevasse-splay deposits locally contain mineralized carbonized wood. Delta-front sandstones of high-constructive lobate deltas contain uranium. Radioactive minerals are present within conglomeratic parts of the valley-fill sequence. Although a relation exists between uranium occurrence and depositional facies, prediction of uranium occurrence is difficult because of a complex groundwater history.
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