TERTIARY STRATIGRAPHY, TECTONICS, UNCONFORMITIES AND HYDROSTRATIGRAPHY IN THE UPPER COASTAL PLAIN OF GEORGIA, USA

The Georgia Coastal Plain is unique as it provides a critical link between the Atlantic and Gulf Coastal Plains. Recharge areas of the Clayton, Claiborne, and Upper Floridan aquifers hosted by the Paleocene Clayton Formation, Eocene Tallahatta Formation, and the Eocene Ocala Group limestones and Oligocene Suwanee Limestone are exposed in Georgia's Upper Coastal Plain. Confining units include portions of the Paleocene Nanafalia Formation, the Paleocene Tuscahoma Formation and the Miocene Altamaha Formation. Recent detailed geologic mapping in the western part of the Upper Coastal Plain from the Flint River to the Chattachoochee River has defined a number of unconformities, related erosion, weathering, and ground-water alteration, and Tertiary tectonism that have adversely affected these aquifers, their confining units, and probably the related water resources.

Unconformities at the top of the Clayton Formation, within the Nanafalia Formation, the top of the Tuscahoma Formation, the residuum of the Eocene-Oligocene limestones, and the base of the Altamaha Formation are marked by weathering of limestone to clay, chert and iron oxides, kaolin-illite clays to kaolin and bauxite, multiple episodes of kaolin-iron oxide mottling and iron oxide staining of sandstones. Effects on the aquifers include significant reduction of porosity and permeability, and reduction in thickness or complete removal of limestone. Where Clayton limestone is preserved, karstification has significantly increased the secondary porosity and permeability of the aquifer. Pre-Claiborne erosion of the Tuscahoma Formation clays removed or significantly reduced large portions of this confining unit. A major unconformity at the base of the Altamaha Formation cuts down through all older Tertiary stratigraphic units and into the Upper Cretaceous Providence Formation. Erosional penetration of the confining units may allow groundwater communication between the various aquifers.

Previously unrecorded tectonism is observed as small to larger-scale folding in Upper Cretaceous through Eocene-age sediments. Larger-scale normal faulting along the Andersonville Fault was active from at least the Paleocene into the Holocene. Dip reversals and impermeable fault planes may locally alter regional ground-water flow patterns.