Burial History of Central Texas Cretaceous Carbonates

Shawn Fullmer¹ and F. Jerry Lucia^2
1Jackson School of Geosciences, The University of Texas at Austin
²Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin

Little is published on the burial history of Cretaceous carbonates of the Austin–San Antonio corridor, and there is considerable disagreement among experts regarding the maximum depth of burial. Conventional wisdom suggests that the Cretaceous section in Central Texas has undergone burial of no more than 2,000 ft (610 m). The critical observation supporting this notion is that 2,000 ft (610 m) is the maximum vertical offset observed on the Balcones fault. Assuming that this offset is a proxy for total burial requires that the movement on the Balcones be the only structural element controlling the regional tectonics of Central Texas. No documented effort, however, has yet been presented to either refute or defend the shallow-burial hypothesis. We suggest that a maximum burial of 2,000 ft (610 m) is insufficient to account for the thickness of the preserved Cretaceous section observed in the area plus the thickness of the Tertiary section that once covered the area of the Balcones fault. On the basis of Paleogene thicknesses of 4,500 to 5,000 ft (1,370 to 1,520 m) logged in wells 50 mi (80 km) to the east of the Austin, Texas, and 3,200 ft (975 m) of Cretaceous section mapped in the area, we estimate that the base of the Cretaceous in Central Texas has been buried to a minimum depth of approximately 6,000 ft (1,830 m) and a maximum depth of 8,000 ft (2,440 m). Exact overburden thickness depends on the amount of Paleogene thinning that occurred between the Balcones fault and the well control points to the east. This range of burial depths is supported by depth estimates obtained from porosity-depth curves and porosity values of 18.8 percent for the Austin Chalk and 15 percent for the upper Glen Rose. Vitrinite reflectance values from the Austin Chalk also suggest a depth within this range.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana