Depositional Systems, Uranium Occurrence and Postulated Ground Water History of the Triassic Dockum Group, Texas Panhandle-Eastern New Mexico

Late Triassic (Dockum Group) rocks accumulated in a relict Paleozoic basin defined, in Texas, by the Amarillo Uplift to the north and the Glass Mountains to the south. This basin was reactivated during the Late Paleozoic or Early Mesozoic by tectonic activity that probably was related to the opening of the Gulf of Mexico. The basin subsided, some relict positive elements were uplifted, and sediments began to accumulate in the basin.

More than 2,000 feet of terrigenous clastics, derived mostly from older sedimentary rocks, accumulated within the basin. Source areas were in Texas, Oklahoma, and New Mexico; sediment transport was from the south, east, north, and west. The Dockum Group accumulated in a variety of depositional systems including: (1) braided and meandering streams; (2) alluvial fans and fan deltas; (3) distributary-type lacustrine deltas (highly constructive lobate deltas); (4) lacustrine systems including ephemeral and relatively long-lived lakes; and (5) mudflats.

Dockum sedimentation was cyclic. The underlying cause of cyclicity was an alternation of humid and arid climate; tectonism most likely was the climatic trigger. During humid climatic conditions, lake level was relatively stable (lake area and depth were maximum). Meandering streams supplied sediment to high-constructive lobate deltas in the central basin area (Texas and New Mexico), whereas braided streams and fan deltas were dominant depositional elements within southern and northern basin areas. Lake area and depth decreased during arid climatic conditions, base level was lowered, valleys were cut into older Dockum deposits, and relatively small fan deltas were constructed along margins of ephemeral lakes. Evaporites, calcretes, silcretes, and soils developed upon floors of ephemeral lakes and on delta platforms.

Occurrence of uranium in the Dockum Group has been known for years. An association between depositional facies and uranium occurrence was first documented through research by the Bureau of Economic Geology. Twenty-five distinct depositional facies have been recognized in the Dockum; each of these facies contains uranium. Highest uranium values are in lacustrine facies which developed under arid climatic conditions; however, only a few areas exhibited high values. Channel-lag facies of meanderbelt systems generally exhibit consistently higher uranium values than other depositional facies. Crevasse channel and crevasse splay deposits associated with meandering streams and delta distributaries locally contain carbonized wood, some of which contains uranium. Facies of high-constructive lobate deltas contain uranium; highest values are exhibited by delta front sandstones. Some valley-fill deposits are mineralized; radioactive minerals mostly are within conglomeratic parts of the sequence.

Uranium deposits within the Dockum Group are, for the most part, epigenetic and generally occur within sandstone bodies. Four sources of uranium for Dockum mineralization are possible: (1) Triassic volcanics; (2) igneous rocks in Oklahoma; (3) Cretaceous volcanics; and (4) Tertiary volcanics. Although there is a relationship between uranium occurrence and depositional facies, prediction of areas of uranium occurrence is difficult because of a rather complex groundwater history. Groundwater flow was for the most part basinward (down depositional slope) during deposition and shallow burial of the Dockum. Erosion during Jurassic and Early Cretaceous influenced local groundwater flow, which would have been toward erosional lows. Groundwater chemistry was probably affected by marine transgression during the Cretaceous. Following accumulation of Cretaceous sediments, erosion again dominated the area of the Dockum basin; erosion prevailed until sometime during the Pliocene. Pliocene (Ogallala Formation) wet alluvial fan deposits accumulated upon a highly dissected surface underlain in part by Cretaceous rocks, but most of the area was underlain by the Dockum Group. During and subsequent to Pliocene deposition, groundwater flow was to the east in both the Ogallala Formation and the upper part of the Dockum Group.

At present, there are two favorable areas for uranium exploration in outcrop: (1) Tule Canyon-Palo Duro Canyon area, and (2) from southern Dickens County southward through Mitchell County. Within the subsurface, a widespread radiometric anomaly occurs at the top of the lowermost progradational sequence; this anomaly is a few hundred to more than 1,000 feet below ground surface. A fourth favorable area for uranium exploration is the uppermost Dockum which has been dissected and is immediately overlain by the volcanic ash-bearing Pliocene Ogallala Formation.