129-34 Sequence-stratigraphic architecture of the Claiborne aquifer, southwest Georgia: aquifer quality, extent, and connectivity

Abstract:

Increasing drought frequency and intensity in southwest Georgia warrant research into secondary sources of freshwater below the often overused Upper Floridan Aquifer. One of these alternative sources is the underlying Claiborne Aquifer, separated from the Upper Floridan by the Lisbon Confining Zone. The GA-FIT project aims to evaluate the properties of the Claiborne Aquifer and its potential connectivity to the Upper Floridan Aquifer. To characterize the sequence-stratigraphic architecture of the Claiborne and Lisbon, we use gamma, resistivity, and lithology logs from 42 wells and cuttings from 13 wells distributed along a north–south dip line, three updip east–west clusters, and one strike line. Lithologically, the Claiborne consists of variably shelly, silty to sandy, shallow-marine siliciclastics. There is substantial lithologic heterogeneity among wells, even those less than a kilometer apart, complicating correlation. The Claiborne contains at least five genetic sequences, bounded by flooding surfaces (following Galloway, 1989). Sharp-based sands in the gamma logs suggest the additional presence of multiple sandy valley fills within the aquifer, ranging from 15 to over 50 feet thick. The exact facies of these valley fills are unknown, as are their lateral extent and traceability, due to considerable variability among wells. The Claiborne–Lisbon boundary is conformable and consists of a retrogradational sequence set that passes upwards into a uniform shale. The shaly Lisbon is over 100 feet thick in updip counties (Webster, Terrell, Dougherty) but thickens and transitions to carbonate in downdip counties (Decatur, Mitchell, Grady). This heterogeneity within the Claiborne, notably its stacking pattern, valley fills, and updip–downdip variation, suggests laterally and vertically variable reservoir properties as well as connectivity between the Claiborne and Upper Floridan Aquifers. Updip, we find no evidence of connectivity between the Claiborne and Upper Floridan. However, updip pump tests indicate highly localized connectivity, which may suggest relief on the Lisbon–Upper Floridan unconformity (Hubert, 2026). Downdip, the shale-to-carbonate transition in the Lisbon may permit connectivity to the Upper Floridan. This possibility must be considered in groundwater management decisions in the context of high demand for freshwater in this agriculturally dominant region.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.