As part of the U.S. Geological Survey's Regional Aquifer-System Analysis (RASA) program, the Gulf Coast RASA was initiated to investigate all Tertiary and Quaternary aquifers underlying the Coastal Plain in the south-central United States. Geohydrologic units that make up two of the three regional aquifer systems Mississippi embayment and Texas coastal uplands in the area are described in this report. The gulfward boundary of the outcrop of the two aquifer systems is the southernmost outcrop or subcrop of the Vicksburg-Jackson confining unit, and the updip boundary is the contact between Cretaceous and Tertiary deposits, extending northward to the southern tip of Illinois. The uppermost Cretaceous aquifer, the McNairy-Nacatoch aquifer in the northern part of the Mississippi embayment, is also included where it may be hydraulically connected to the younger sediments.
Major regional geohydrologic units generally are coincident with previously defined geologic units. Most of the geohydrologic units consist of alternating sand and clay; however, the entire sequence becomes a clay and carbonate facies gulfward.
The regional geohydrologic units delineated in this study, from youngest to oldest, are (1) Mississippi River Valley alluvial aquifer, (2) Vicksburg-Jackson confining unit, (3) upper Claiborne aquifer, (4) middle Claiborne confining unit, (5) middle Claiborne aquifer, (6) lower Claiborne confining unit, (7) lower Claiborne-upper Wilcox aquifer, (8) middle Wilcox aquifer, (9) lower Wilcox aquifer, (10) Midway confining unit, and (11) McNairy-Nacatoch aquifer. The Mississippi embayment aquifer system contains all of these units and has a maximum thickness of about 5,000 feet. The Texas coastal uplands aquifer system, which is contiguous with the Mississippi embayment aquifer system and extends westward and southwestward from the Sabinc uplift, contains all of the foregoing geohydrologic units except the Mississippi River Valley alluvial aquifer, the lower Wilcox aquifer, and the McNairy-Nacatoch aquifer. The Texas coastal uplands aquifer system has a maximum thickness of about 7,000 feet.
