Geohydrologic units of the coastal lowlands aquifer system, south-central United States

The coastal lowlands aquifer system is one of the three regional aquifer systems studied as part of the Gulf Coast Regional Aquifer-System Analysis (RASA). The coastal lowlands aquifer system underlies about 160,000 square miles of the coastal areas of Texas, Louisiana, Mississippi, Alabama, and westernmost Florida, and nearby offshore areas; the aquifer system is composed of sediments of Oligocene age and younger. The sediments consist predominantly of interbedded sand, silt, and clay with minor amounts of lignite and limestone. The average thickness of the sediments is about 6,000 feet, with a maximum thickness of more than 18,000 feet occurring offshore from southern Louisiana.

The base of the coastal lowlands aquifer system is the top of the Vicksburg-Jackson confining unit, which is a massive clay that represents the last major transgression of the sea. A zone of abnormally high fluid pressure (geopressured zone) is present above the top of the Vicksburg-Jackson confining unit onshore in a narrow band along the coast of Texas and Louisiana and on the Continental Shelf. Where the geopressured zone is Present, it is considered to be the base of the coastal lowlands aquifer system.

The sediments in the coastal lowlands aquifer system are divided into five Permeable zones and two confining units. The permeable zones are not separated by intervening, regionally mappable confining units in about 64 percent of the study area. In much of the area boundaries between permeable zones were extended, as a constant proportion of the total aquifer system thickness, from areas with hydraulic-head data to areas without such data.

Average sand percentage of the permeable zones ranges from about 40 percent to more than 60 percent. However, the areal distribution of sand is variable within and among Permeable zones. A lobate pattern of greater sand percentages is typical of the Permeable zones, and all zones except one have at least one area with sand percentage greater than 80 percent.

Data that are useful for quantitative analysis of regional ground-water flow in the coastal lowlands aquifer system are presented in map format. Included for each of the five permeable zones are maps of altitude of the top, thickness, sand percentage, and aggregate thickness of sand. Included for each of the two confining units are maps showing altitude of the top, and thickness of the unit.