ABSTRACT: Sequence Stratigraphy and Platform Evolution of the Fredericksburg Group, Central and West Texas

BROTHERS, J. C., and S. C. ATCHLEY

Lower Cretaceous (Albian) Fredericksburg Group platform carbonates are exposed in outcrops throughout central and west Texas. These rocks were deposited as a third-order sequence consisting of normal, shallow marine shelf and marginal carbonate sediments. The Fredericksburg Group carbonate platform was terminated by subaerial exposure, which was in turn succeeded by marine facies associated with a composite second- and third-order rise in sea level.

Three unconformity-bounded fourth order sequences are identified in Fredericksburg Group outcrops in the Austin area. Each fourth order sequence represents one shallowing-upward phase of third-order platform assembly. Fourth order bounding unconformities are exposure surfaces characterized by mud cracks, bored and iron oxide-stained hardgrounds, root mottles, and occasional dinosaur tracks. Fourth order sequences are in turn comprised of progradational fifth order cycles (aka., parasequences).

Fourth order sequences progressively thin and onlap updip (north and west) from Austin area exposures. Deeper water facies thin and onlap across positive regions on the PreCretaceous unconformity. Paleotopographic highs served as nucleation points for the deposition of high-energy, shallow water facies. Analysis of the vertical and lateral distribution of cyclic stratigraphic units and component facies suggests that the Fredericksburg platform complex was deposited as a highly progradational, low-relief and gradient stratal succession.

AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas