Depositional Gradient Analysis in Transgressive Systems Tracts and Highstand Systems Tracts, Eocene of the Brazos River Valley Section, Texas

Abstract

Middle-late Eocene third-order sequences in the Brazos River section are composed of transgressive and highstand systems tracts of shelfal marine and marginal marine sediments. Diagenesis has destroyed most fossils (except palynomorphs, arenaceous forams, and traces), but sedimentary structures and fabric provide evidence for determination of depositional gradient. Tidalite deposits dominate shallower parts of the gradient and proximal and distal storm deposits dominate deeper parts of the gradient. As seen in the Weches, Crockett, Yegua, Caddell, and Manning formations, a characteristic shoaling gradient consists of a coarsening-upward section of deeper water mudstones with thin distal storm deposits overlain by sediment with proximal storm deposits (with upward-increasing storm bed content), by planar cross-bedded sands and by intensely bioturbated shore-zone sands. A characteristic deepening gradient is a fining-upward section of cross-bedded sands overlain by flaser bedded (and plant-fragment rich) tidalites, by proximal storm deposits (usually fossil and glauconite rich), and by shales with thin distal storm deposits. Maximum flooding surfaces (associated with sediment starvation) are indicated by submarine exposure surfaces and zones of amalgamated storm beds. The lithologic character of storm deposits varies with availability of grains (sand, shell, glauconite pellets, silt) and distance of transport. Gradient analysis was used to locate themore » maximum flood of the Jackson Stage in the Brazos River section. It lies in a parasequence within the lower lignite-bearing portion of the Manning Formation, as verified by the last appearance datum of Textularia hockleyensis.« less