ANALYSIS OF CALCITE-FILLED FAULTS IN CARBONATE STRATA, BALCONES TREND NEAR WACO, TEXAS

Five faults within the Balcones Fault Zone cutting Upper Cretaceous Austin Chalk are exposed in three dimensions in the Lehigh Quarry near Woodway, Texas. Fault core fabric is characterized by either a fault creep fabric that contains calcite veinlets and host rock or sparry calcite. Stratigraphic correlation across the fault plane is applicable on three of the faults allowing the measurement of stratigraphic separation. Dip-parallel shear striae along these faults and the apparent sense of stratigraphic separation indicate that these are normal faults. Stratigraphic relationships across the other two faults are more ambiguous at outcrop scale.

The macroscopic characteristics of the calcite fault-core fabric in the three unambiguous normal faults were described using field photographs, oriented hand specimens and photomicrographs of large oriented thin-sections. Thin sections of calcite from the fault cores were examined to identify whether the calcite has been mechanically twinned, and if so, whether the twins were planar or curved. Twin geometry allows for a primitive designation of the temperature of deformation. The carbon and oxygen isotopic composition of calcite samples from these faults were measured to assess whether the calcite formed near the ground surface (crystallizing at low pressure and temperature from solutions dominated by meteoric water) or deeper, under somewhat higher PT conditions.

Maps of the five faults on the quarry walls and floor were compiled, using a combination of aerial-image analysis, field photography, GPS and standard surveying techniques as appropriate. Where possible, net slip or stratigraphic separation was measured. The orientation of the fault and shear striae were measured at several locations on each fault, with spot averages computed using Fisher statistics. Fault trends in the quarry were then compared with structural trends in the surrounding area.

Observations from the three unambiguous faults were incorporated in an interpretive method that was subsequently applied to infer the sense of slip along the other two faults. The dimensional scale of these calcite-fabric observations is similar to that of core samples through faults that a geologist might need to interpret.