December Luncheon - Dr. John Holbrook, Professor, TCU

Once considered practically abstractions, fluvial systems dominated by upper-flow-regime structures (UFR systems) are now considered common. Accordingly, they have gained consideration as potential reservoirs. Fundamental data and models for reservoir properties of these systems though are still lacking. Triassic strata of the Tecovas and Trujillo formations in the Palo Duro Canyon of West Texas provide an opportunity to develop models for vertical sequences and reservoir architecture of UFR systems, and lend insights into the processes that form UFR systems. Two channel-fill reservoir elements dominate the Dockum Group, a thin channel fill with low-Froude structures and a thicker channel fill with high-Froude structures. Thicker channel-fills (~2 m, aspect ratio ~24) host upper-plane-bed, symmetrical antidune, breaking-antidune, chute-and-pool, and cyclic-step structures. The thinner channel-fill population (<1 m, aspect ratio ~35) comprises upper-plane-bed and some antidune structures. Both channel-fill elements tend to record single cut-and-fill events. Bars are transverse, but include the first recorded example of a laterally accreting side-attached bar. Lamina follow accretion surfaces as these follow the bed orientation of the time. A generalized vertical sequence for large UFR channel stories has upper-plane-bed structures above the basal scour, transitioning upward into higher flow regime structures (e.g., antidunes, cyclic steps, etc.), then back to a mix of upper-plane-bed and antidunes or lower-flow-regime structures toward the top. The Tecovas Formation and overlying Trujillo Sandstone both preserve cross-cutting multivalleys and are separated by a regional sequence boundary. Calculated paleodischarges are for the flow stage where antidunes form and are ~90 m3/sec (Tecovas) and ~76 m3/sec (Trujillo). Slopes for the rivers were on the order of 10-4 – 10-5, consistent with typical continental-interior rivers. The channel fills thus record supercritical flow conditions from flashy discharge instead of excessive slopes. The abundant UFR channels in the Dockum Group provide insights into reservoir trends not previously definable from existing more isolated examples. The study provides vertical profiles and aspect ratios needed for recognition and modeling of UFR river systems from subsurface data sets. The Dockum rivers developed in a tropical to subtropical megamonsoonal environment, prone to meter-scale channel-fill events. This and related catastrophic inundations of floodplains and lakes in single floods has also made the Dockum Group a prolific preserver of large Triassic vertebrates.