Depositional Environment of the Grayson Formation (Upper Cretaceous) of Texas

ABSTRACT The Grayson Formation (Upper Cretaceous, Texas) in the area between the Central Texas platform and the Stuart City Reef Trend is characterized by sediments having a geochemical composition like that of present day nearshore shales, by a microfauna predominated by neritic taxa, and by macrofaunal populations of moderate diversity predominated by epifaunal suspension feeding organisms with adaptations for living on a soft substrate. These sedimentary and faunal characteristics suggest the Grayson Formation was deposited under normal marine inner to middle neritic conditions. Sediment accumulation was primarily controlled by the structural features of the East Texas Tyler and Maverick basins, the Central Texas and San Marcos platforms, and the Belton high. Deposition was influenced by an influx of clastic sediments from the northwest probably during early Grayson times and from the northeast probably during middle Grayson times. Deposition was during a marine transgressive phase which was at maximum probably during the middle and late Grayson times. Faunal composition reflects changes in carbonate and clastic sediments, in substrate fluidity, in water depth, and in extent of circulation with the open ocean. Pyritized specimens are probably a result of micro-reducing-environments that developed within individual shells in the soft muds.