GEOLOGIC HISTORY AND HYDROGEOLOGIC SETTING OF THE EDWARDS-TRINITY AQUIFER SYSTEM, WEST- CENTRAL TEXAS By Rene A. Barker, Peter W. Bush, and E.T. Baker, Jr. U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 94-4039 Austin, Texas 1994 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: U.S. Geological Survey Earth Science Information Center District Chief Open-File Reports Section U.S. Geological Survey Box 25286, Mail Stop 517 8011 Cameron Rd. Denver Federal Center Austin, TX 78754-3898 Denver, CO 80225-0046 CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .......................................................................................................................................................................... 2 Purpose and Scope .................................................................................................................................................... 2 Acknowledgment...................................................................................................................................................... 2 Aquifer-System Boundaries...................................................................................................................................... 2 Geographic Setting ................................................................................................................................................... 5 Geologic History .................................................................................................................................................................. 8 Paleozoic Era ............................................................................................................................................................ 10 MesozoicEra ............................................................................................................................................................ 10 Triassic and Jurassic Periods ........................................................................................................................ 10 Cretaceous Period ........................................................................................................................................ 12 Comanchean Epoch: Trinitian Age .................................................................................................. 12 Comanchean Epoch: Fredericksburgian and Washitan Ages ........................................................... 18 Gulfian Epoch: Eaglefordian Through Navarroan Ages .................................................................. 23 CenozoicEra............................................................................................................................................................. 26 Hydrogeologic Setting ......................................................................................................................................................... 29 Stratigraphy .............................................................................................................................................................. 29 Trinity Group................................................................................................................................................. 29 Fredericksburg and Washita Groups ............................................................................................................. 31 Regional Aquifers and Confining Units ................................................................................................................... 33 Ground-Water Hydrology ......................................................................................................................................... 34 Balcones Fault Zone ...................................................................................................................................... 34 Hill Country .................................................................................................................................................. 36 Edwards Plateau ............................................................................................................................................ 38 Trans-Pecos ................................................................................................................................................... 41 Summary .............................................................................................................................................................................. 43 References Cited .................................................................................................................................................................. 46 FIGURES 1-3. Maps showing: 1. Location of the study area of the Edwards-Trinity Regional Aquifer-System Analysis ........................... 3 2. Location of the aquifers and confining units in the Edwards-Trinity aquifer system and of selected adjacent aquifers .......................................................................................................................... 4 3. Location of geographic subareas of the Edwards-Trinity aquifer system and the major springs and streams..................................................................................................................................... 6 4. Schematic section showing generalized relation between the Edwards-Trinity aquifer system and the pre-Cretaceous rocks and hydrogeologic units that form the base of the system ........................................... 9 5. Map showing paleogeographic and structural features in west-central Texas and parts of adjacent States and northern Mexico .................................................................................................................................. 11 6. Map showing lateral distribution of Trinity rock units in west-central Texas ...................................................... 13 7. Schematic diagrams showing vertical distribution of Trinity rock units in west-central Texas and their relation to geographic subareas..................................................................................................................... 14 8. Schematic diagram showing structural controls on the deposition of Fredericksburg and Washita strata of the Edwards-Trinity aquifer system........................................................................................................ 19 9. Map showing lateral distribution of Fredericksburg and lower Washita rock units in west-central Texas ..................................................................................................................................................................... 20 10. Schematic diagrams showing vertical distribution of Fredericksburg and lower Washita rock units in west-central Texas and their relation to depositional environments ........................................................ 21 11. Schematic diagram showing progression of major depositional, tectonic, and diagenetic events affecting the development of the Edwards-Trinity aquifer system....................................................................... 24 CONTENTS III TABLE 1. Correlation chart showing the chronostratigraphic, lithostratigraphic, and regional hydrogeologic units in the Edwards-Trinity aquifer system, west-central Texas ......................................................................... 16 2. Approximate maximum thickness of lithostratigraphic units that compose the Edwards-Trinity aquifer system, west-central Texas ....................................................................................................................... 26 CONVERSION FACTORS, VERTICAL DATUM, AND WATER-QUALITY UNIT ____________Multiply_____________________By__________________To obtain__________ acre-foot (acre-ft) 0.001233 cubic hectometer acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year cubic foot per second (fi?/s) 0.02832 cubic meter per second foot (ft) 0.3048 meter foot per mile (ft/mi) 0.1894 meter per kilometer foot squared per day (fAd) 0.09290 meter squared per day gallon per minute (gal/min) 0.06308 liter per second inch (in.) 25.4 millimeter inch per year (in/yr) 25.4 millimeter per year mile (mi) 1.609 kilometer ______mile squared (mi2)____________________2.590__________________kilometer squared_______ Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. Water-quality unit: mg/L, milligram per liter Geologic History and Hydrogeologic Setting of the Edwards-Trinity Aquifer System, West- Central Texas By Rene' A. Barker, Peter W. Bush, and E.T. Baker, Jr. Abstract rocks in the fault zone and extending the depth of freshwater diagenesis, the faulting vertically The Edwards-Trinity aquifer system under­ displaced the terrain, which steepened hydraulic lies about 42,000 square miles