Abstract: Vein-Filling Calcites (Petrography and Geochemistry): Balcones Fault Zone, Upper Cretaceous Outcrops, North-Central Texas

William C. Dawson, H. M. Szymczyk, D. F. Reaser

The Austin Chalk and the underlying Eagle Ford Shale are transected by en echelon faults and fractures, associated with the Balcones fault zone, along an outcrop trend that extends from Dallas to Austin, Texas. These faults and fractures strike northeastward (N35° to 40°E) and are cemented by multiple generations of sparry calcite. Vein widths range from 0.1 to 45 cm. Slickensides, preserved on outer surfaces of calcite cements, record fault movement. Inclusions of country rock are present within calcite-filled veins, and rarely, open fracture porosity is observed in outcrop. Early calcite cements are nonferroan and later cements are ferroan calcite. Petrographic analyses indicate the occurrence of "fir-tree" zoning and fluorescent inclusions within calcite cements.

Vein-filling cements have stable isotopic signatures (^dgr¹⁸O -6.3 to -9.8^pmil PDB; ^dgr¹³C +1.6 to +2.5^pmil PDB) that are markedly depleted in ^dgr¹³O relative to the chalk matrix, inoceramid shells, and the estimated value for Cretaceous seawater. Trace element analyses indicate the presence of strontium (980-1600 ppm), manganese (750-1975 ppm), zinc (10-20 ppm), and copper (5-10 ppm) in the calcite cements. The negative ^dgr¹⁸O signatures of the vein-filling calcites are suggestive of precipitation from warm fluids. ^dgr¹³C analyses reveal that the Austin Chalk buffered the carbon incorporated into the calcite cements. These veins probably formed by a "crack-seal mechanism" whereby episodic increases in hydropressure caused f acturing. Precipitation of calcite cements within fractures is induced by subsequent decreases in pore pressure. Tectonic and diagenetic features in these outcrops provide analogs for fractured Austin Chalk reservoirs.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994