Paleohydrology and Paleogeomorphology of Dockum Formation (Triassic), West Texas

Andrew P. Frelier

Dockum Formation (Upper Triassic) sediments are exposed along the eastern High Plains escarpment of west Texas and consist of fluvial-channel sandstone and overbank deposits. Channel sands are classified into five orders that represent fluvial deposits of differing stream types. The largest channel sands (first and second order) contain thalweg facies, lower point-bar facies, and upper point-bar facies that exhibit grain size and bed-form structure trends corresponding to channel facies in fully developed meander bend flow. Channel sands of first, second, and third order represent deposits of high sinuosity (P > 1.65), freely meandering streams. Paleocurrent and petrographic analyses indicate trunk streams (analogous to first-order sands) flowed from south to north wit headwaters reaching into the Ouachita metamorphic core complex approximately 400 km to the south. Fourth- and fifth-order streams were low-sinuosity, mixed-load to bed-load channels that derived sediment exclusively through floodplain erosion. Paleohydrological reconstruction of these channel orders is aided by abandoned channel outcrops and well-defined lateral accretion bedding.

Quantitative estimates of paleochannel flow and morphology are based on relevant modern stream data bases. Empirical estimates of drainage basin area and stream length are larger than those inferred from outcrop sedimentological evidence for Dockum first-order streams. This discrepancy is possibly due to a bias in the drainage area regression relation caused by the inclusion of very small drainage basins (< 7 km²) and the absence of large basins (< 5,000 km²) used in the regression analysis.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.