ABSTRACT

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 generally northeastward, and many are cemented by sparry calcite. Slickensides, preserved on outer surfaces of calcite cements, record fault movement. 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 ¹⁸O -5.7 to -9.8 PDB; ¹³C +1.6 to +2.7 PDB) that are markedly depleted in ¹⁸O relative to the chalk matrix, inoceramid shells, and the estimated value for Cretaceous seawater. The depleted ¹⁸O signatures of the vein-filling calcites are suggestive of precipitation from warm basinal fluids. ¹³C 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 fracturing. 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.