Geology and Hydrology of the Northern Segment of the Edwards Aquifer with An Emphasis on the Recharge Zone in the Geogetown, Texas, Area

Abstract

In March 1986, the Bureau of Economic Geology was contracted by the Texas Water Development Board to conduct geologic and hydrologic investigations of the northern segment of the Edwards aquifer along the Balcones Fault System in the Georgetown-Round Rock area, particularly to elucidate the processes and areas of recharge for the Edwards in the Georgetown area.

Geological mapping, combined with analysis of fractures in Comanche Peak, Edwards, and Georgetown limestones (Edwards aquifer strata) in the vicinity of Georgetown and Round Rock, Texas, was conducted to provide data useful in identifying potential recharge areas, assessing local groundwater flow, and enhancing our understanding of the geology of the Balcones Fault Zone.

Cretaceous Comanche Peak, Edwards, Georgetown, Del Rio, Buda, Eagle Ford, and Austin strata dip gently (1°) eastward and are overlain in some places by terrace deposits and alluvium. Several major normal faults, downthrown to the east, strike northward across the area. Gentle flexures, possibly related to faulting, parallel the faults. Minor normal faults and joints are most abundant in areas adjacent to major faults and flexures. These fractured-strata zones likely parallel the length of the faults or flexure axes and may be as wide as 1.6 km. Most minor faults strike between 340°-040°, have displacements less than 2 m, and dip from 40°-80° both eastward and westward. Most joints strike between 340°-020° and 260°-300°, and fracture densities range from 4 joints per meter to 1 joint per 5 meters in 1 to 2-meter-thick beds.

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