Abstract: Anatomy of a Sequence Boundary-Lower Cretaceous Glen Rose/Fredericksburg, Central Texas Platform

Clyde H. Moore

Aptian-Albian Lower Cretaceous shelf carbonates of the Central Texas Platform near Austin consist of six unconformity-bounded sequences. The Glen Rose-Fredericksburg sequence boundary is of particular interest because of excellent exposures and the presence of a Fredericksburg siliciclastic low stand systems tract (Paluxy).

The Glen Rose/Fredericksburg sequence-bounding unconformity is present across the Central Texas Platform, disappearing in the subsurface toward the shelf margin and into adjacent intrashelf basins. Near Austin, a siliciclastic wedge (Paluxy Formation) rests on the sequence boundary. South of the Paluxy pinchout, the unconformity is marked by a bored surface, dinosaur tracks and cemented oysters. Where siliciclastics are present, the unconformity surface is sharp, irregular, and exposure is marked by organ pipe caliche. Paluxy siliciclastics were generally deposited under continental conditions representing the lowstand systems tract of the Fredericksburg sequence.

The Fredericksburg transgressive systems tract is a shallow marine nodular limestone (Bull Creek Member, Walnut Formation) that onlaps both the Glen Rose unconformity as well as Paluxy sands. This contact is a transgressive surface of erosion marked by rounded and bored cobbles of the underlying sand. The top of the Paluxy often shows intense bioturbation.

The Fredericksburg transgressive systems tract consists of two phases, an early phase characterized by stacked shoaling upward grainstone cycles terminated by marine hardgrounds suggesting slow rates of relative sea level rise. These grainstones are followed by an abrupt marine flooding event marked by fossiliferous open-marine marls (Bee Cave Member, Walnut Formation) indicating an acceleration of the relative rise in sea level. The maximum flooding surface of the Fredericksburg sequence is found a short distance above the upper surface of the grainstones. This two-phase pattern is typical of Lower Cretaceous transgressive systems tracts across the Central Texas Platform.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas