Tertiary System 



OLIGOCENE SERIES 



Beds of Oligocene age have been identified in over 300 wells in the subsurface of the Coastal Plain 

 of Georgia. In subsurface areal extent the Oligocene Series approximates that of the overlying Miocene. 

 (See fig. 3.) These strata occupy a position intermediate between the upper Eocene below and the 

 Miocene Series above. As yet, however, the authors have been unable to correlate these beds 

 of definite Oligocene age with the two outcropping formations of Oligocene age that have been map- 

 ped in Georgia: the Flint River Formation (Cooke, 1943, pi. 1) and the Suwannee Limestone (Mac- 

 Neil, 1947). Until such time as a study of the outcrop and the subsurface is successfully completed 

 it seems preferable to refer to the subsurface deposits as Oligocene Series or Oligocene undifferentiated. 



The Oligocene Series increase in thickness from a few feet in updip areas to an average of 100 feet over 

 most of the central part of the Coastal Plain of Georgia. (See fig. 4.) The maximum thickness listed by 

 Herrick (1961) is 211 feet for a well in Dodge County. 



Lithologically the Oligocene in Georgia is representative of the limestone facies, the clastic facies 

 lying much further west in Mississippi where the entire known Oligocene section of the Gulf Coast is 

 developed. The upper part of the limestone facies in Georgia is composed of light-gray to cream to 

 light-brown, dense, nodular and cherty, locally somewhat sandy, fossiliferous limestones. Locally abundant 

 chert inclusions are common particularly in the upper few feet, a characteristic that often causes diffi- 

 culty in drilling. 



The lower part of the Oligocene consists predominantly of cream, relatively soft, somewhat chalky, 

 fossiliferous limestones. At the base of this unit however, are rather dense, massive, sparingly fossili- 

 ferous limestones which on the electric log, produce a pronounced resistivity "kick." These limestones 

 contain only molds and casts of molluscan shells but no Foraminifera. 



In southwest and southern Georgia the Oligocene is dolomitized locally and is composed of light to dark- 

 brown, saccharoidal, recrystallized, unfossiliferous limestones. In Chatham County the limestones of this 

 unit become progressively sandier to the northeast, finally grading into sand in southeastern Beaufort 

 County, S. C. 



Over most of the southeastern part of the Coastal Plain these strata have been considerably eroded, and, 

 in southern Charlton and southwestern Camden Counties, are absent presumably having been completely 

 eroded subsequent to their deposition. 



Fossils are abundant in the Oligocene deposits but as yet they have not permitted the stratigraphy to 

 be worked out convincingly. The upper beds of Oligocene age in Georgia generally contain as the dominant 

 form in the smaller foraminiferal assemblages an abundance of Rotalia mexicana var. mecatepecensis . 

 Thus figure 5 which shows the occurrences in Georgia of this form presumably also indicates the areal 

 distribution of the upper limestone of Oligocene age. In many wells in which R. mexicana var. was not 

 reported another Oligocene form was reported: Rotalia byramensis var. Unfortunately, in all the wells 

 in which Oligocene Foraminifera were indentified, only one well, Mcintosh 84, has both species reported. 

 In it R_. byramensis is reported from the sample interval 445-455 feet; R. mexicana var. is reported from 

 486-505 feet. If it were not for this the authors would favor considering the large area in southern Georgia 

 where _R. mexicana var. is missing to be where the upper beds of Oligocene age were eroded or never 

 deposited. It is interesting to note that the rather large list of Foraminifera identified by Vernon (1942, p. 

 66) as being from the Suwanee Limestone does not mention R. mexicana var. but does list R. byramensis 

 var?. However, Vernon (p. 56) qualifies the stratigraphic origin by pointing out that the unit from which 

 the fossils came may "not (be) the precise equivalent of the Suwanee in its type area." He further points out 

 that correlation is rendered difficult by the lack of larger Foraminifera in the type section of the Suwannee 

 Limestone in Florida. The Foraminifera he lists accord closely with those found in the Oligocene beds in 

 wells of Lowndes and Brooks Counties. The difference in fauna could be due to difference in facies with 

 contemporaneous sedimentation or to difference in time of deposition. 



Some of the more important publications dealing with the Foraminifera of the Oligocene include papers 

 by Cushman (1922a and b), Cushman and McGlamery (1942), Cole and Ponton (1930), and Todd (1952). 

 The Foraminifera characterizing the Oligocene of Georgia are rather abundant (at least in total numbers 

 of specimens as found in well cuttings), distinctive, and varied. Some of the foraminiferal species that 

 are diagnostic of the Oligocene in Georgia include Quinqueloculina leonensis Applin and Jordan, Camerina 

 dia (Cole and Ponton), Rotalia mexicana Nuttall var., Asterigerina subacuta Cushman var. floridensis 



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