Abstract

The Upper Triassic Dockum Group accumulated in relict Paleozoic basins defined by the Amarillo and Sierra Grande uplifts on the north, the Pedernal uplift on the west, and the ancestral Glass Mountains on the south. These basin were reactivated during the late Paleozoic or early Mesozoic by tectonic activity that was probably related to the opening of the Gulf of Mexico. As basins subsided and some relict positive elements were uplifted, sedimentation rates increased.

More than 610 m (2,000 ft) of terrigenous clastic sediments, derived chiefly from Paleozoic sedimentary rocks, accumulated within the basin. Sediment transport was from the south, east, north, and west from source areas in Texas, Oklahoma, and New Mexico. The Dockum Group accumulated in a variety of depositional environments including (1) braided and meandering streams; (2) alluvial fans and fan deltas; (3) distributary-type lacustrine deltas (high-contructive elongate deltas); (4) ephemeral and relatively long-lived lakes; and (5) mud flats.

Alternation of wet and dry climate caused cyclic sedimentation in the Dockum. The main control on climate was most likely tectonism. During wet periods, lake levels were relatively stable. Meandering streams supplied sediment to high-constructive elongate deltas in the central basin area of Texas and New Mexico, whereas braided streams and fan deltas were dominant depositional elements along southern and northern basin margins. Lake areas and depths decreased when dry conditions prevailed. Under these conditions, base level was lowered, valleys were cut into older Dockum deposits, and small fan deltas were built into ephemeral lakes; evaporites, calcretes, silcretes, and soils developed across emergent surfaces ranging from floors of ephemeral lakes to delta platforms.