ABSTRACT

Mid-late Eocene third order sequences in the Brazos River section are composed of transgressive and highstand systems tracts of shelfal marine and shorezone marine sediments. Diagenesis has destroyed most fossils (except palynomorphs, agglutinated forams, and traces), but sedimentary structures and fabric provide evidence for determination of depositional gradient. Tidalite deposits dominate shallower parts of the gradient while proximal and distal storm deposits dominate deeper parts of the gradient.

A characteristic coarsening-upward succession present in most parasequences in this section consists of deeper water mudstones containing thin, distal storm deposits, overlain by proximal storm deposits (with upward-increasing storm bed content), overlain by planar crossbedded sands, overlain by intensely bioturbated shorezone sands. A few parasequences contain a fining-upwards succession of deposits that is an inverse of the coarsening-upward succession.

Most transgressive systems tracts have an irregular deepening gradient that extends across several parasequences. This consists of a succession of crossbedded sands, overlain by flaser bedded (and plant fragment-rich) tidalites, overlain by proximal storm deposits (usually fossil and glauconite rich), overlain by mudstones with or without 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 has been used to locate the 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, a determination verified by the last appearance datum of Textularia hockleyensis.