Abstract

Sediments of the Fredericksburg Division in southcentral Texas were deposited on the slowly-subsiding west flank of the Tyler Basin. In this area, there are three stratigraphically unique sub-areas. The Southern Area has a thick Edwards limestone unit overlying a thin Walnut sequence. The Intermediate Area has a thinner Edwards sequence and Comanche Peak limestone intervening between the Edwards and an expanded Walnut formation. The Paluxy sandstone occurs at the base of the Fredericksburg in the Northern Area and there is further expansion of the Walnut formation accompanied by an attenuated Edwards biohermal limestone so characteristic of this formation in north-central Texas.

The Walnut formation has six members, from bottom to top, the Bull Creek, Bee Cave, Cedar Park, Whitestone, Keys Valley marl, and an unnamed upper marl. The Cedar Park member of the Walnut formation is emended to include only the nodular fossiliferous micrite below the oosparite and pelsparite occurring at the Cedar Park quarries in Williamson County. This oosparite and pelsparite is termed the Whitestone member of the Walnut formation. A similar development in the vicinity of Moffat, Bell County, is referred to as the Moffat mound of the Edwards formation.

The Edwards, Comanche Peak and Walnut are intergradational one into the other. The Walnut-Paluxy contact in southern Coryell County is unconformable, but the two units are probably time equivalents regionally. The Fredericksburg-Trinity and Fredericksburg-Washita contacts are interpreted to be unconformable.