DIAGENETIC ANALYSIS OF A SUBAERIAL EXPOSURE SURFACE FROM THE CRETACEOUS (ALBIAN) WALNUT FORMATION, CENTRAL TEXAS

Peritidal carbonate mudrocks at the top of the Bull Creek Member of the Walnut Formation (Middle Albian; ~107Ma) from central Texas occur at a retrogradational parasequence set boundary (Talbert & Atchley, 2000, GCAGST, L:369-377). Sedimentary evidence for subaerial exposure includes mud cracks and dinosaur footprints. The mudrocks are characterized by multiple centimeter-scale units truncated by sharp exposure surfaces. The preservation of ubiquitous fenestral fabrics provides important information on Albian stable isotope paleohydrology at 25 °N, during early cementation of the exposure surface. Carbonate components including muds, void filling spar, microspar, and marine allochems were micro-sampled from several thin slabs to analyze the covariation in their carbon and oxygen isotopic compositions, and develop interpretations of an Albian diagenetic system. Crystallotopic gypsum molds show evidence that tidal flat facies were characterized by a semi-arid paleoclimate with evaporation deficit. A restricted fauna of foraminifera, oysters, and gastropods show evidence for fluctuating salinity. Calcites from oyster shells have variable compositions, but have d¹⁸O values that range up to -1.9 ‰ and d¹³C values that range up to +3.6‰ VPDB. Blocky equant calcite spars from the Bull Creek Member have d¹⁸O values that range between -6.7 to -1.5‰ VPDB, but have d¹³C values that only range between +1.5 to +2.5‰ VPDB. These spars are interpreted to be products of early meteoric phreatic diagenesis, and produce a meteoric calcite line value of about -6‰. The lack of variable and lighter carbon isotopic values in all components suggests that the shallow groundwater system in the Bull Creek Member was not significantly influenced by pedogenic processes, possibly because of a short duration of subaerial exposure. Carbonate muds have d¹⁸O values that range between -2.5 to +0.5 along a positive linear covariant trend (PLCT), suggesting strong evaporative influences on vadose groundwater evolution.