Mini-Shelves in Taylor Shale: Stratigraphy of Late Cretaceous and Early Tertiary Units, Bastrop County, Texas

Delos R. Tucker, Daniel P. Hencey

Several well-log cross sections and isopach maps demonstrate the spatial relationships among the Late Cretaceous and early Tertiary stratigraphic units in Bastrop County, Texas. Each of the major structural features that are present partially within the county--the San Marcos arch, Round Rock syncline, East Texas basin, Mexia-Talco and Luling fault systems, and Travis volcanic field--has influenced the units investigated. Most of the stratigraphic units follow normal stratigraphic patterns (i.e., they thin toward the outcrop and toward the San Marcos arch). There are some notable exceptions, however.

The Atco Formation, the lowest unit of the Austin Group, exhibits a gently narrowing channel. The Atco is about 180 ft thick on the sides of the channel and thins to a minimum of 11 ft in the center of the inferred channel. Black, pyritic shale in the Atco Formation suggests euxinic conditions in the area of the channel. Stagnant, nearly stationary, probably acid water precluded much deposition, and the channel was filled by the lower units of the overlying Vinson Formation.

The volcanic episode that resulted in the Travis volcanic field took place during the deposition of the upper part of the Austin Group and the lower part of the Taylor Group. Pyroclastic mounds (serpentine plugs) are reflected by thickness and/or facies changes in both the overlying and contemporaneous strata. Hydrocarbon production has been obtained from the serpentine itself, and from a genetically related high-energy beach facies, the "Dale lime." The "Dale" occurs both downthrown and upthrown from faults, and may be east, west, or north of the plug.

The Sprinkle Formation, the lowest unit of the Taylor Group, is a shelf sequence of shale and chalk that can be easily traced across the county. The overlying Bergstrom Formation, predominantly shale, is a prograding, downlapping shelf sequence, the lower part of which has bed forms that are very different from those in any other rock body investigated. The lower part of the Bergstrom has four "mini-shelf" bed packages that progressively downlap onto the Sprinkle. Each "mini-shelf" package has its own distinct shelf (undaform), slope (clinoform), and basin (fondoform) segments. As one "mini-shelf" package thins and pinches out, the overlying "mini-shelf" thickens, resulting in an overall geometry similar to cross-bedding in a bar deposit. The part of the Bergstrom above the "mini-shel es" has bedding features that suggest that they, too, will downlap somewhere to the east.

The upper contact of the Taylor Group with the basal sandstones of the Navarro Group is a regional, angular, toplap-type unconformity. The Navarro is a shelf sequence, also predominantly shale, with silty sands in the lower 100 ft and erratically distributed dirty sands in the upper 150 ft. More sands are present in the western half of the county than in the eastern half. However, downdip to the east, in Fayette County, there are cleaner sands in the Navarro than any seen in Bastrop County.

The paleontologically defined Cretaceous-Tertiary boundary apparently conformable is 30-100 ft below the E-log marker bounded mapping horizon of this study. The Paleocene Midway shale grades upward into the shelf sands, barrier-bar sands, and shelf and lagoonal shales of the regressive lower Wilcox Group.

Some faulting in the Mexia-Talco fault system in Bastrop County (Paige fault) began in Early Cretaceous time, perhaps as a consequence or cause of salt movement. Displacement on the Smithville and associated faults, however, is clearly post-early Eocene. Faults of the Luling fault zone had little, if any, vertical displacement during the Late Cretaceous or early Tertiary; however, some acted as fissures that served as conduits for the Late Cretaceous volcanoes. The major episode of vertical separation is post-Wilcox.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.