HR: 10:50h
AN: H42B-03    [Abstracts]
TI: Using Geophysics to Define Hydrostratigraphic Units in the Edwards and Trinity Aquifers, Texas
AU: * Smith, B D
EM: bsmith@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964 Bldg. 20 Denver Federal Center, Denver, CO 80225, United States
AU: Blome, C D
EM: cblome@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 913 Bldg. 20 Denver Federal Center, Denver, CO 80225, United States
AU: Clark, A K
EM: akclark@usgs.gov
AF: U.S. Geological Survey, 5563 DeZavala Road Suite 290, San Antonio, TX 78249, United States
AU: Kress, W
EM: wkress@usgs.gov
AF: U.S. Geological Survey, 944 Arroyo Drive, San Angelo, TX 76903-9345, United States
AU: Smith, D V
EM: dvsmith@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964 Bldg. 20 Denver Federal Center, Denver, CO 80225, United States
AB: Airborne and ground geophysical surveys conducted in Uvalde, Medina, and northern Bexar counties, Texas, can be used to define and characterize hydrostratigraphic units of the Edwards and Trinity aquifers. Airborne magnetic surveys have defined numerous Cretaceous intrusive stocks and laccoliths, mainly in Uvalde County, that influence local hydrology and perhaps regional ground-water flow paths. Depositional environments in the aquifers can be classified as shallow water platforms (San Marcos Platform, Edwards Group), shoal and reef facies (Devils River Trend, Devils River Formation), and deeper water basins (Maverick Basin, West Nueces, McKnight, and Salmon Peak Formations). Detailed airborne and ground electromagnetic surveys have been conducted over the Edwards aquifer catchment zone (exposed Trinity aquifer rocks), recharge zone (exposed Edwards aquifer rocks), and artesian zone (confined Edwards) in the Seco Creek area (northeast Uvalde and Medina Counties; Devils River Trend). These geophysical survey data have been used to divide the Edwards exposed within the Balcones fault zone into upper and lower hydrostratigraphic units. Although both units are high electrical resistivity, the upper unit has slightly lower resistivity than the lower unit. The Georgetown Formation, at the top of the Edwards Group has a moderate resistivity. The formations that comprise the upper confining units to the Edwards aquifer rocks have varying resistivities. The Eagleford and Del Rio Groups (mainly clays) have very low resistivities and are excellent electrical marker beds in the Seco Creek area. The Buda Limestone is characterized by high resistivities. Moderate resistivities characterize the Austin Group rocks (mainly chalk). The older Trinity aquifer, underlying the Edwards aquifer rocks, is characterized by less limestone (electrically resistive or low conductivity units) and greater quantities of mudstones (electrically conductive or low resistivity units). In the western area (Devils River Trend and Maverick Basin) of the Trinity aquifer system there are well-defined collapse units and features that are marked by moderate resistivities bracketed by resistive limestone and conductive mudstone of the Glen Rose Limestone. In the central part of the aquifer (San Marcos Platform) the Trinity's lithologies are divided into upper and lower units with further subdivisions into hydrostratigraphic units. These hydrostratigraphic units are well mapped by an airborne electromagnetic survey in Bexar County. Electrical properties of the Edwards aquifer also vary across the fresh-saline water interface where ground and borehole electrical surveys have been conducted. The saline- saturated Edwards is predictably more conductive than the fresh-water saturated rocks. Similar fresh-saline water interfaces exist within the upper confining units of the Edwards aquifer (Carrizo-Wilcox aquifer) and the Trinity aquifer rocks.
UR: http:esp.cr.usgs.gov/info/edwards/
DE: 0925 Magnetic and electrical methods (5109)
DE: 1804 Catchment
DE: 1829 Groundwater hydrology
DE: 5109 Magnetic and electrical properties (0925)
DE: 9610 Cretaceous
SC: Hydrology [H]
MN: 2007 Joint Assembly