ABSTRACT

Lower Cretaceous shelf carbonates of the Central Texas Platform near Austin consist of eight 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 Fm.) rests on the sequence boundary. South of the Paluxy pinch-out, the unconformity is marked by a bored surface, dinosaur tracks and cemented oysters. Where siliciclastics are present the unconformity surface is sharp and irregular, with exposure 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. The top of the Paluxy often shows intense bioturbation.

The Fredericksburg transgressive systems tract consists of two phases: (1) an early phase characterized by stacked shoaling upward grainstone cycles terminated by marine hardgrounds suggesting slow rates of relative sea level; rise, and (2) these grainstones followed abruptly by a 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.