Vein-filling calcites (petrography and geochemistry): Balcones fault zone, Upper Cretaceous outcrops, north-central Texas
- Texaco, Inc., Houston, TX (United States)
- Univ. of Texas, Arlington, TX (United States)
The Austin Chalk and the underlying Eagle Ford Shale are transacted 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{degrees} to 40{degrees}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 {open_quotes}fir-tree{close_quotes} zoning and fluorescent inclusions within calcite cements. Vein-filling cements have stable isotopic signatures ({delta}{sup 18}O-6.3 to -9.8{per_thousand} PDB; {delta}{sup 13}C +1.6 to +2.5{per_thousand} PDB) that are markedly depleted in {delta}{sup 13}O 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 {delta}{sup 18}O signatures of the vein-filling calcites are suggestive of precipitation from warm fluids. {delta}{sup 13}C analyses reveal that the Austin Chalk buffered the carbon incorporated into the calcite cements. These veins probably formed by a {open_quotes}crack-seat mechanism{close_quotes} 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.
- OSTI ID:
- 96218
- Report Number(s):
- CONF-941065--
- Journal Information:
- AAPG Bulletin, Journal Name: AAPG Bulletin Journal Issue: 9 Vol. 78; ISSN 0149-1423; ISSN AABUD2
- Country of Publication:
- United States
- Language:
- English
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