SUBSURFACE GEOLOGY OF THE GEORGIA COASTAL PLAIN 



Stephen M. Herrick and Robert C. Vorhis 



ABSTRACT 



The subsurface geology of the Coastal Plain of Georgia has been restudied using data from 354 litholog- 

 ic-paleontologic logs. Two contrasting areas of deposition are described: an updip area of elastics and a 

 downdip area of limestones. Because faunas of the elastics are found to be different from those of the lime- 

 stones, foraminiferal lists for each type are included as well as for each geologic unit. 



In the Coastal Plain the sediments are wedge-shaped, being in general thinnest inland and thickest near 

 the present shoreline. This wedge is modified in some of the units by the presence of depocenters where 

 the thickness is greater than in surrounding areas. Locally, overlap is important in the northern part 

 of the Coastal Plain with middle Eocene sediments overlapping those of the Paleocene and lower Eocene 

 and being overlapped in turn by upper Eocene sediments. 



An outgrowth of the study has been some reinterpretations as well as some reinforcing of the stratigraphy. 

 The Charlton Formation is regarded by the authors as being late Miocene in age and is tentatively corre- 

 lated with the Duplin Marl of the Carolinas and eastern Georgia. The Cooper Marl and the underlying 

 Barnwell Formation of late Eocene age are the updip clastic equivalents of the upper member of the Ocala 

 Limestone. The lower member of the Ocala Limestone is the part of the formation that crops out in 

 Georgia, the upper member not extending far enough updip to crop out. The Lisbon and Tallahatta Forma- 

 tions of middle Eocene age extend through much of the subsurface of Georgia and are the updip equivalents' 

 of the Avon Park and Lake City Limestones of Florida. The lower Eocene clastic deposits correlate with the 

 Wilcox Group of Alabama and their downdip limestone equivalent is the Oldsmar Limestone of Florida. The 

 Paleocene deposits consist of theClayton Formation. overlying, in southwest Georgia and in Chatham County, 

 fossiliferous marls equivalent in age totheTamesi(Velasco) of Mexico. The surface updip post-Tuscaloosa 

 deposits correlate with their downdip marine equivalents of Navarro, Taylor, and Austin age. 



The geologic structure is outlined on maps showing the top of the Oligocene, upper Eocene, middle 

 Eocene, lower Eocene, Paleocene, Cretaceous, Tuscaloosa Formation, Lower Cretaceous(?), and pre- 

 Cretaceous. Other maps show the thickness and distribution of sediments of the Recent to Miocene, Oligo- 

 cene, upper Eocene, middle Eocene, lower Eocene, Paleocene, post-Tuscaloosa Cretaceous, Tuscaloosa 

 Formation, and Lower Cretaceous(?) sediments. Additional interpretation of the structure is shown on 8 

 geologic sections. The major structural basins in Georgia are the Atlantic Embayment in the southeast 

 and the Gulf Trough in the southwest. 



INTRODUCTION 



The most important mineral resource of the Georgia Coastal Plain is its ground-water supply. In order 

 to ascertain the magnitude and distribution of this supply in the sediments of the Coastal Plain, a good 

 understanding of the geological framework that contains the ground water and directs its flow is needed. 

 The purpose of this paper is to present the interpretation of the subsurface geology so that the ground-water 

 hydrology of the 35,000 square miles that comprise the Coastal Plain of Georgia will be better understood. 

 This report does not deal with ground water directly but is a basis for detailed ground-water studies in the 

 Georgia Coastal Plain counties. 



This report is based largely upon the records of wells reported by Herrick (1961) in a report frequently 

 ferred to herein as "the well-log report." In that report the lithology and fauna from numerous samples 

 e described in detail. For more information on individual wells the reader is referred to the well-log 

 port. 



Cuttings from water wells have been a source of much of the geologic knowledge of the Coastal Plain. 

 However, the vast magnitude of the ground-water resource in Coastal Georgia has caused most wells to 

 be drilled to relatively shallow depths, thereby limiting the data available on the geology of underlying for- 

 mations and aquifers. 



So far about a hundred wildcat oil-test wells have been drilled in the Coastal Plain of Georgia and these 



