2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 3
Presentation Time: 2:05 PM


SMITH, Bruce D.1, PAINE, Jeffrey G.2, SMITH, David V.3, JOHNSON, Steven B.4, WAUGH, John5, ABRAHAM, Jared3, BLOME, Charles D.6 and SCHINDEL, Geary4, (1)U.S. Geol Survey, PO Box 25046, MS 973, Denver Federal Center, Denver, CO 80225, (2)Texas Bureau of Economic Geology, University of Texas at Austin, Austin, TX 78712, (3)U.S. Geol Survey, PO Box 25046, MS964, Denver, CO 80225, (4)Edwards Aquifer Authority, 1615 North St. Mary's Street, San Antonio, TX 78215, (5)San Antonio Water System, 1001 E. Market St, San Antonio, TX 78298, (6)USGS, MS 980, Box 25046, Denver Federal Center, Denver, CO 80225, bsmith@usgs.gov

The Valdina Farms Sinkhole is one of the most prolific recharge points within the Balcones Fault Zone of the Edwards aquifer. Aquifer recharge has been enhanced by a diversion structure across Seco Creek that directs flood stage water into the 50 m deep sinkhole. During the 1992 flood event, a record 14,631 acre-feet of water was diverted into the sinkhole from the normally dry Seco Creek. The sinkhole and associated cave system is located near the northeasterly trending Woodard Cave fault. The cave system has a mapped length of 829 m but probably extends much farther. A helicopter electromagnetic (HEM) and magnetic survey has been flown to map subsurface structure, lithology, and hydrology of the area. Ground electrical and electromagnetic surveys have been completed that provide additional subsurface information on the karstic terrain. Resistivity depth cross sections have been interpreted for each flight line of the HEM survey. The resistivity cross section through the Valdina Farms Sinkhole shows that it is associated with a shallow (few meters) zone of low resistivity that is likely caused by in-filling of surficial fractures. A distinct high resistivity area is centered at a depth of 40m. The high resistivity may be associated with the cave and fracture system. In resistivity maps, it is also associated with a linear feature extending much farther (at least 4 km) than the mapped extent of the cave. The depth of this feature is near the subsurface contact of the Edwards and Trinity Groups as interpreted from the HEM survey. This suggests that the cave/fracture system may extend into the Trinity Group. Similar features are shown elsewhere in the HEM survey and are possible additional recharge points. Ground and airborne surveys indicate much more fracturing and faulting that indicated from surface geological mapping. The three-dimensional structural fabric mapped by the HEM and ground surveys is critical in understanding the distribution of karst features and their relationship to local hydrology.