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Configuration of the Base of the Edwards-Trinity Aquifer System and Hydrogeology of the Underlying Pre-Cretaceous Rocks, West-Central Texas

By Rene' A. Barker and Ann F. Ardis

U.S. Geological Survey
Water-Resources Investigations Report 91–4071

Regional Aquifer-Systems Analysis (RASA) program


Abstract

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.

Table of Contents

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

 

Illustrations

 

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

Tables

 

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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U.S. Department of the Interior, U.S. Geological Survey
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Last modified: Friday, October 06 2006, 04:21:10 PM
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