Middle Eocene rocks - - Sediments of middle Eocene* age have been observed from over 100 

 wells in 44 counties of the Coastal Plain of Georgia. This stage uncomformably overlies the lower Eocene, 

 is overlain unconformably by the upper Eocene, and is composed of the Lisbon and Tallahatta Formations. 



The Lisbon Formation is the subsurface equivalent of the McBean Formation (Veatch and Stephenson, 

 1911, p. 237) which crops out at McBean and on McBean Creek in Richmond County in eastern Georgia. 

 The Lisbon Formation includes the rocks in the subsurface between the underlying Tallahatta Formation 

 and the overlying Gosport Sand. Both the Lisbon and McBean as now used by the Federal Geological Sur- 

 vey include only the equivalent of the Cook Mountain Formation or the Ostrea sellaeformis zone. As 

 originally defined the McBean included some beds of late Eocene (Jackson) age and as used by Cooke 

 (1943) it included beds of the Tallahatta west of the Flint River. In order to eliminate such inconsistencies 

 in stratigraphic terminology when applied to the subsurface, Counts and Donsky (in press) and Herrick 

 (1961) have extended the use of the formational names Lisbon and Tallahatta throughout eastern Georgia. 

 Based on known occurrences as given in the well-log report, the Lisbon Formation has been found east 

 of the Flint River in the subsurface of the following Georgia counties: Appling, Atkinson, Bleckley, Burke, 

 Chatham, Coffee, Crisp, Dooly, Emanuel, Jenkins, Liberty, Montgomery, Pulaski, Screven, Toombs, and 

 Turner. 



The Tallahatta Formation as reported by Herrick (1961) is found in the following Georgia counties 

 east of the Flint River: Appling, Atkinson, Chatham , Coffee, Crisp, Dooly, Emanuel, Liberty, Pulaski 

 and Toombs. 



The Lisbon and Tallahatta Formations compose the updip, clastic facies of the middle Eocene in Georgia 

 and are correlated with the same formations in Alabama. They also correlate with their downdip limestone 

 equivalents, the Avon Park and Lake City Limestones of peninsular Florida. 



In the subsurface the areal extent of the middle Eocene (see figure 8) is somewhat less than that of 

 the upper Eocene, although solution of the Ocala Limestone of southwestern Georgia tends to cause the 



maps to fail to show this. In southeastern Twiggs County the middle Eocene is overlapped by beds of late 

 Eocene age (Barnwell Foramtion), the middle Eocene appearing along the major stream valleys as erosional 

 "windows". Furthermore, the Tallahatta Formation of early middle Eocene age is overlapped by the geologi- 

 cally younger Lisbon Formation of late middle Eocene age — such overlap taking place in eastern Sumter 

 County. From this point the line of overlap continues in a northeasterly direction across the Coastal 

 Plain through southern Houston County and through the middle of Bleckley, Laurens, Emanuel, and Screven 

 Counties. 



The updip, clastic facies of the Lisbon Formation consists of interbedded, fine to coarse, subangular, 

 sparsely phosphatic, locally fossiliferous sand; cream to gray to pale-bluish-green to dark green, sandy, 

 finely glauconitic, cherty, fossiliferous clay or marl; and white to light-gray, rather dense, massive, 

 sandy, coarsely but sparsely glauconitic, fossiliferous limestone. These sediments interfinger with their 

 downdip limestone equivalents along a line that runs approximately through northern Seminole County 

 east northeastward through the centers of Mitchell, Tift, Telfair, Treutlen, and Emanuel Counties, thence 

 easterly through northeastern Effingham County to the Savannah River. White to gray, coarsely glauconitic 

 limestone is prominent in deep wells in Toombs and Emanuel Counties, thus proving the existence of this 

 facies of the Lisbon Formation this far north in the Coastal Plain. In updip areas the base of the Lisbon 

 Formation is often composed of white to cream, rather massive, sparsely glauconitic, shelly, coquina- 

 like limestones, which show up on an electric log as prominent resistivity "kicks". Downdip from Mont- 

 gomery and Toombs Counties these white to gray, coarsely glauconitic, rather massive limestones are 

 replaced by cream, somewhat chalky, much calcitized, granular, gypsiferous, sparingly fossiliferous, 

 locally dolomitized limestones. In Echols, Clinch, Camden, and Glynn Counties the Lisbon con- 

 sists entirely of alternating beds of cream, chalky, and brown, dolomitic limestones, a type of lithology 

 that is similar to that of the Avon Park Limestone of northeastern Florida. Where present, the bulk of 

 the limestone facies of the Lisbon is composed of these cream, chalky, granular limestones. The chalky 

 limestones are for the most part lacking in microfossils, except for certain horizons where such fossils 

 occur abundantly. 



The updip or clastic facies of the Tallahatta Formation consists of interbedded, fine to coarse, sparsely 

 phosphatic, fossiliferous sand; thin, dark-green to dark-brownish-gray, silty, micaceous, glauconitic, 



♦Owing to a lack of fossils the Gosport Sand is not always differentiated from the underlying and geologi- 

 cally older Lisbon Formation. For this reason it has been thought best to include this formation as part 

 of the Lisbon in the discussion that follows. 



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