Analysis of calcite-filled faults in carbonate strata, balcones trend near Waco, Texas.
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Secrest, Stephen H.
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Five normal faults exposed in the Lehigh Quarry near Waco, Texas, were analyzed. The mechanical stratigraphy of the quarry ranges from relatively strong chalk to marly chalk to chalky marl to relatively weak marl. Higher clay content yields weaker rock. The dip angle of the normal faults varies with rock strength, with steeper dips across the stronger strata. Carbon stable isotopes in 174 samples indicate that the water from which the calcite precipitated is formation water and not meteoric water, while oxygen isotopes indicate that precipitation of secondary calcite in the fault cores occurred between 48° and 110°C. Mechanical twins in macrospar calcite indicate shear stress slightly more than 5-15 MPa, and deformation temperature of <170°C. The microspar fabric includes thin veinlets of secondary calcite and host lithons of cemented chalk wallrock whose orientation relative to the domain boundaries can be used as a shear-sense indicator.