Sedimentology of Dockum Group (Triassic), Texas Panhandle

Brent A. May, Thomas M. Lehman

The Triassic Dockum Group (Tecovas and Trujillo Formations) consists of nonmarine red beds exposed around the southern high plains of Texas. The base of the Tecovas Formation is unconformable with underlying Permian strata and in many areas the contact is marked by a prominent well-differentiated paleosol including a silcrete horizon. Over part of the study area, however, this paleosol is developed in the top of an eolian sand sheet of uncertain age, which rests on typical Permian strata. The base of the Trujillo Formation is unconformable with the underlying Tecovas. The relief on this erosion surface is substantial and, in places, the entire Tecovas Formation was removed prior to deposition of the Trujillo. Mature quartz-rich sandstone of the Tecovas Formation is differ nt from the less mature lithic-rich Trujillo sandstone, which contains abundant muscovite and metamorphic rock fragments. Trujillo conglomerates contain extrabasinal chert, quartzite, and vein quartz clasts, as well as clasts of Permian and Triassic sedimentary rocks.

Previous depositional models suggest that the Dockum rocks in the study area are mostly lacustrine and deltaic in origin and were derived through erosion of the Amarillo-Wichita uplift to the north. However, with the exception of minor eolian and pond deposits, Dockum rocks are of fluvial origin. Paleocurrent data indicate that Trujillo rocks were derived from the south (not north), probably from the relict Ouachita uplift. Deposition of the Trujillo may record the onset of uplift along the flanks of the proto-Gulf of Mexico rift zone, exposing the metamorphic interior zone of the Ouachita system.

AAPG Search and Discovery Article #91026©1989 AAPG Southwest Section, March 19-21, 1989, San Angelo, Texas.