Geology and ground-water resources of Lee and Sumter Counties, southwest Georgia

Abstract

Lee and Sumter Counties are underlain by more than 5,000 feet of Lower Cretaceous, Upper Cretaceous, and Tertiary sedimentary rocks. Water is produced from wells that tap rocks of the Upper Cretaceous series and the Tertiary system at depths ranging from 60 to 1,000 feet. The Upper Cretaceous series is composed of the Tuscaloosa formation and the overlying undifferentiated Upper Cretaceous deposits. The Teritary system is composed of, in ascending order, the Clayton formation and the undifferentiated deposits of the Midway group, the Tuscahoma formation of the Wilcox group, the Tallahatta and Lisbon formations of the Claiborne group, the Ocala limestone of the Jackson group, the Flint River formation, and the undifferentiated residuum, which may be partly of early Quaternary and Recent age. The Tuscaloosa formation consists mostly of arkosic sand and gravel and also contains lenses of clay; the undifferentiated Upper Cretaceous deposits consist mostly of marl and also contain subordinate beds of sand, sandstone, clay, and limestone. The Clayton formation consists of limestone and marl, and the undifferentiated Midway deposits consist of interbedded sand, clay, and marl. The Tuscahoma formation consists mostly of glauconitic silt underlain by glauconitic sand. The Tallahatta formation is predominantly sand, and the Lisbon formation is predominantly soft limestone or marl. The Ocala limestone consists mostly of limestone and also includes a basal sand. The Flint River formation is present only as scattered boulders of chert in the undifferentiated residuum, which is composed of sandy clay or silt. The principal structural feature of the region is the regional homocline which dips generally to the southeast. The dip increases with depth and generally ranges from 12 feet per mile on top of the Ocala limestone to 22 feet per mile on top of the Upper Cretaceous series. Dips in the homocline are greater along a southwest-northeast trending belt which crosses Sumter County than elsewhere in the region. Because data are limited, this belt is termed the "structural belt" in the present report although it may be a monocline, a fault, or a series of faults. The structural belt appears to have influenced depositional conditions and apparently marks the updip limit of the Lisbon formation and the Ocala limestone as well as that of a coquina bed at the top of the Tallahatta formation. The regional structure is also modified by the Andersonville fault in northeastern Sumter County and by similar faults in western Sumter County. The faults trend east-west and are downthrown on the north. The principal aquifers used in the region are the undifferentiated Upper Cretaceous deposits, the Clayton formation, the undifferentiated Midway deposits, the Tallahatta formation, and the Ocala limestone. The Tuscaloosa for-2 GEOLOGY AND GROUND WATER, SOUTHWEST GEORGIA mation is not used as an aquifer because of its depth; the Tuscahoma and Lisbon formations, and undifferentiated residuum act mainly as confining beds because of their relatively ...