Depositional environments, sedimentology, and stratigraphy of the Dockum Group (Triassic) in the Texas Panhandle

Abstract

The Triassic Dockum Group consists of nonmarine "red beds" exposed around the Southern High Plains of Texas and New Mexico. The Dockum Group is subdivided into two formations: the lower Tecovas Formation and the upper Trujillo Formation. Both formations are comprised of conglomerate, sandstone, siltstone, and mudstone. Sandstones from the two formations are readily distinguished petrographically. Aeolian, fluvial channel, fluvial overbank, and valley-fill facies are present in the Dockum Group. Dune and interdune sub-facies are recognized in the aeolian facies. The fluvial channel facies is divided into thalweg, channel-fill, point bar, and abandonment sub-facies. Levee, crevasse splay, sheetflow, and floodplain sub-facies are identified in the fluvial overbank facies. Valley-fill facies are exposed at three localities in the study area. Extrabasinal conglomerates are associated with the valley-fill deposits. Paleocurrent data yield a grand vector mean of N 19 W for Dockum sandstones. This value compares well with previous paleocurrent analyses. The prevailing depositional model for the Dockum indicates the presence of deltaic, lacustrine, and fluvial environments. A "Dockum lake" is postulated to have formed in the relict Midland Basin. This study proposes a wholly fluvial setting for the Dockum Group. Deposits of low and high sinuosity streams dominate Dockum deposystems. Lacustrine deposits reflect only small local lakes or ponds. No deltaic deposits are present.