Table 4. — Oligocene Foraminifera of Georgia - Continued 



REWORKED FORAMINIFERA: 

 Valvulina floridana Cole 



martii Cushman and Bermudez 

 Discorinopsis gunteri Cole 

 Coskinolina floridana Cole* 

 Dictyoconus cookei (Moberg) 

 Lepidocyclina antillea (Cushman) 



EOCENE SERIES 



Upper Eocene rocks — Upper Eocene deposits have been identified in more than 300 wells 

 that are distributed over the Coastal Plain of Georgia. This unit is uncomformably overlain by beds of 

 Oligocene age and unconformably overlies beds of middle Eocene age. The subsurface upper Eocene in 

 Georgia is correlated, in part, with the Barnwell Formation and Cooper Marl of Georgia and the Ocala 

 Lim.-stone and Inglis Limestone of Florida. The subsurface areal extent of upper Eocene sediments covers 

 a much larger part of the Coastal Plain than either of the two previously discussed stratigraphic units. (See 

 figs. 6 and 7.) 



The upper Eocene beds are composed of an updip, clastic facies, which interfingers with its middip 

 limestone equivalent, the Tivola Tongue of the Ocala Limestone (Cooke and Shearer, 1918, p. 51), along 

 a line trending northeastward through roughly the center of Houston, Bleckley, Washington, Jefferson, 

 and Burke Counties and then southeastward to the Savannah River to northeastern Screven County. More- 

 over, the Barnwell Formation progressively overlaps geologically older formations in a northeasterly 

 direction across east-central Georgia. Thus the Barnwell Formation, beginning in eastern Twiggs County, 

 successively overlies middle Eocene and Upper Cretaceous strata, finally resting directly upon crystalline 

 (basement) rocks as erosional remnants, or outliers, in southern Hancock, Warren, McDuffie, and Columbia 

 Counties. As a result of this overlap and subsequent erosion of the overlying Barnwell Formation, particu- 

 larly along the major streams, sediments of middle Eocene and Late Cretaceous age have been exposed 

 as erosional "windows" in the northeastern part of the Coastal Plain. The updip clastic facies of the upper 

 Eocene deposits in Georgia is composed of the Barnwell Formation and the Cooper Marl. Lithologically 

 the Barnwell Formation consists of fine to coarse-grained, gray to yellow to pink to red (at the surface), 

 arkosic sands interbedded with cream to bluish-gray to pale-green, blocky, glauconitic, locally fuller' s 

 earth (type), fossiliferous clay or marl, and some thin beds of rather dense light-gray, somewhat sandy, 

 sparsely glauconitic, locally fossiliferous limestone. In this report the clays or marls of the Barnwell 

 Formation are collectively called the Twiggs Clay Member, after Cooke and Shearer (1918, p. 41-81). 

 Overlying these elastics and also included in the Barnwell Formation are flat white to gray, somewhat 

 chalky, sandy, cherty, sparsely glauconitic, sparingly fossiliferous limestones which Cooke (1943, p. 65) 

 calls the Sandersville Limestone Member. This limestone occupies a small area in the subsurface of south- 

 ern Washington, Jefferson, and Burke Counties, northern Emanuel, eastern Bleckley, and probably most 

 of Johnson County. On the basis of one echinoid, Periarchus quinquefarius (Say), Cooke regards this limey 

 facies as representative of the youngest upper Eocene occurring in the Coastal Plain of Georgia. However, 

 the authors feel that this limestone may belong to the late upper Eocene, or Cooper Marl, or even to the still 

 younger Oligocene deposits. At any rate, much more subsurface data are needed in order to establish firm- 

 ly the true geologic age of the Sandersville Limestone Member of the Barnwell Formation. The remainder 

 of the updip, clastic facies of the upper Eocene sediments in Georgia belongs to the geologically 

 younger Cooper Marl which overlies the Barnwell and Ocala Formations and was named by Cooke 

 (1936, p. 73-75; 82-89) from exposures in the Coastal Plain of South Carolina. In the subsurface of 

 central-east Georgia the Cooper Marl underlies a rather extensive area that includes parts of Dooly, 



♦Considered by Douglass (1960, p. 258) as synonymous with Dictyoconus floridanus (Cole) 



18 



