Abstract: Depositional Environment of Grayson Formation (Upper Cretaceous) of Texas

Ernest A. Mancini

The Grayson Formation (Upper Cretaceous) of Texas in the area between the Central Texas platform and the Stuart City reef trend is characterized by a geochemical composition like that of present nearshore shales, by a microfauna predominated by shelf taxa, and by macrofaunal populations of moderate diversity predominated by epifaunal suspension feeders with adaptations for living on a soft substrate. These sedimentary and faunal characteristics suggest that the Grayson was deposited in a normal-marine inner-shelf to middle-shelf environment.

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 material from the northwest during lower Grayson deposition and from the northeast during middle Grayson deposition. A marine transgressive phase was at maximum during middle and upper Grayson deposition.

Faunal composition reflects changes in carbonate and clastic materials, 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.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas