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| Texas Water Science Center |
By Rene' A. Barker and Ann F. Ardis
U.S. Geological Survey
Water-Resources Investigations Report 91–4071
Regional Aquifer-Systems Analysis (RASA) program
The Edwards-Trinity aquifer system is underlain by an extensive complex of rocks, ranging from Late Cambrian through Late Triassic in age, that are typically about 10 to perhaps 1,000 times less permeable than those composing the aquifer system. The Cretaceous rocks of the aquifer system are separated from the pre-Cretaceous rocks by an unconformity that spans about 60 million years of erosion during the Jurassic Period. The upper surface of the pre-Cretaceous rock complex forms the base of the Edwards-Trinity aquifer system. The configuration of the base reflects the original topography of the eroded pre-Cretaceous land surface plus the effects of subsequent deformation.
The most permeable pre-Cretaceous rocks are in the eastern half of the study area where they compose the Hickory aquifer (in Upper Cambrian rocks), Ellenburger-San Saba aquifer (Upper Cambrian-Lower Ordovician), and Marble Falls aquifer (Lower Pennsylvanian). These aquifers are hydraulically connected to the northeastern fringe of the Edwards-Trinity aquifer system, as their up-turned margins crop out around the flanks of the breached Llano uplift. The Rustler aquifer in rocks of Late Permian age underlies parts of the Trans-Pecos region, where it yields small amounts of greatly mineralized water for industrial and agricultural purposes. The Dockum aquifer in rocks of Late Triassic age directly underlies the Edwards-Trinity aquifer system in western parts of the study area, and locally increases the saturated thickness of the ground-water-flow system by an average of about 200 feet. Despite these notable exceptions, the collective effect of the pre-Cretaceous rocks is that of a barrier to ground-water flow, which limits the exchange of water across the base of the Edwards-Trinity aquifer system.
Abstract
Introduction
Purpose and scope
Location of study area
Previous investigations and methods of present study
Edwards-Trinity aquifer system
Geologic history of the pre-Cretaceous rocks
Paleozoic Era
Mesozoic Era
Configuration of the base of the Edwards-Trinity aquifer system
Balcones fault zone
Hill Country and southern Edwards Plateau
Northern Edwards Plateau
Trans-Pecos
Hydrogeology of the pre-Cretaceous rocks
Cambrian through Pennsylvanian rocks
Permian and Triassic rocks
Hydraulic characteristics
Summary
Selected references
| Plate 1. | Map showing configuration and age of the rocks that constitute the base of the Edwards-Trinity aquifer system and selected contiguous hydraulically connected units, west-central Texas |
| Figure 1. | Map showing study area and geographic subareas of the Edwards-Trinity aquifer system |
| 2. | Map showing paleogeographic and structural features in west-central Texas and parts of adjacent states and northern Mexico |
| 3. | Generalized hydrogeologic section through the study area showing relation between the Edwards-Trinity aquifer system and the pre-Cretaceous rocks that form the base of the system |
| Table 1. | Generalized correlation chart of the rocks that compose the Edwards-Trinity aquifer system |
| 2. | Generalized correlation chart of the pre-Cretaceous rocks that form the base of the Edwards-Trinity aquifer system |
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Persistent URL: https://pubs.water.usgs.gov/wri914071 Page Contact Information: USGS Publishing Network Last modified: Friday, October 06 2006, 04:21:10 PM |
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