INVESTIGATING THE CAUSE(S) OF NORMAL FAULTS IN THE LEHIGH QUARRY NEAR WACO, TEXAS

The South Bosque Formation (Eagle Ford Shale) and the overlying Austin Chalk are Late Cretaceous formations that are exposed in the Lehigh Portland Cement Quarry (formerly the Atlas Cement Quarry) near Waco, Texas, ~31.478°N latitude 97.237°W longitude. Several normal faults that cut the chalk are observed in the quarry walls. These faults are located along the Balcones fault trend, which leads to the traditional inference that they are no younger than Miocene, ~15 Ma. Reports of broken water pipes along Balcones-trend faults suggest that some strands might still exhibit creep. Displacement along two of the normal faults in the quarry decreases from the base to the top of the ~8 m high quarry walls at rates of ~7 to 9 cm per meter along the slip direction. The fault cores typically indicate dilatancy, with precipitated calcite filling the voids.

The Balcones escarpment trends ~40° in this area and is located about 2 km northwest of the quarry, and the free face exposes Austin Chalk at the top as well as the upper Eagle Ford Shale. The Eagle Ford Shale is notorious for its association with slope failure and foundation damage to structures. We wondered if the faults in the quarry with strikes that are sub-parallel with the escarpment might be related at least in part to motion of the more rigid Austin Chalk caprock toward the escarpment due to creep in the shale. Physical models were constructed with viscous material (putty) or an elastic cloth representing the shale and brittle material (various fine powders) representing the chalk. While we could not uniquely identify the cause of faulting in the quarry because of poor temporal constraints for activity along these faults, gravitational sliding toward the escarpment is a viable option.