Quaternary and Tertiary Systems 



RECENT TO MIOCENE SERIES 



Deposits of Recent to Miocene age have been identified throughout about three-fifths of the Coastal 

 Plain of Georgia in more than 300 wells. (See fig. 2.) The uppermost unit is composed mainly of sand and 

 is restricted in general to the coastal counties of southeast Georgia. The sand of post-Miocene age, 

 is not discussed further in this report for it is of little importance in the subsurface, is remarkably 

 barren of microfossils, and is the subject of another paper currently being prepared by the senior author. 



The Miocene sediments compose the major portion of the deposits as mapped in figure 2 and the northern 

 limit as shown is the general boundary of the occurrence of Miocene sediments. This inner limit of the 

 outcrop trends from the southwest corner of Decatur County northeastward through the counties of Grady, 

 Mitchell, Crisp, Bleckley, to Laurens County and thence southeasterly to the Savannah River along the 

 southeast corner of Burke County. 



Lithologically the upper and middle members of the Miocene in Georgia are composed of elastics, 

 while the lower member consists of a series of limestones. The elastics are continuous throughout the 

 entire area covered by this unit. If they grade downdip into limestones, such rocks have not yet been found 

 anywhere in the subsurface of Georgia. It is possible, however, that such a downdip limestone facies does 

 exist somewhere off the coast of Georgia. In the six coastal counties and eastern Wayne County the 

 upper unit of the Miocene consists of dark-brownish-green, granular, rather loosely consolidated, abund- 

 antly micaceous, locally phosphatic and fossiliferous clays which rest either on beds of dolomitic lime- 

 stone also of Miocene age as in Chatham County, or directly upon the underlying clays of the Hawthorn 

 Formation, as for example in Glynn County. This upper member rapidly pinches out up the dip, coming 

 to the surface as isolated outcrops along the major river valleys. Examples are exposures along the south 

 bank of the Savannah River, particularly at Ebenezer Landing, Effingham County, along the south bank 

 of the Altamaha River at Doctortown, Wayne County, and along the St. Mary's River south and southwest 

 of Folkston, Charlton County. These strata represent the Charlton Formation (Veatch and Stephenson 

 1911, p. 392); they are tentatively correlated by the authors with the Duplin Marl of late Miocene age 

 in the Carolinas and eastern Georgia, whereas the U. S. Geological Survey considers them to be of Plio- 

 cene age. 



The Hawthorn Formation, the middle unit of the Miocene Series, consists of pale to dark-green (mottled 

 at the surface), phosphatic (at depth), very sandy, locally fossiliferous and cherty, micaceous clays that 

 are interbedded with scattered tongues of fine to coarse-grained, arkosic, phosphatic sand; both the 

 clays and sands gradually thicken and become fossiliferous in a downdip direction. Beneath these elastics 

 but to some extent interfingering with them is a series of limestones considered to be Tampa equivalent 

 of early Miocene age. These limestones are white to cream, sandy, phosphatic, locally cherty, and sparing- 

 ly fossiliferous. In southwest Georgia, particularly in Mitchell and Colquitt Counties as well as along 

 the Georgia- Florida border from Decatur County eastward through Camden County, these basal Miocene 

 limestones have been locally altered, becoming light to dark-brown, recrystallized, saccharoidal, sandy, 

 phosphatic, dolomitic limestones. In areas where dolomitization has not taken place the lower Miocene 

 limestones are distinguished from the underlying but older limestones of Oligocene age through the pre- 

 sence of quartz grains and phosphatic pebbles, and by the fossils where present. 



The Recent to Miocene thickens gradually from a few feet in its updip outcrop area to over 600 feet 

 in two depocenters (see fig. 2). One of these depocenters is long and linear extending diagonally across 

 Grady County in a northeasterly direction as far as northeastern Toombs and northwestern Tattnall Coun- 

 ties. The other area of greatest thickening appears to center in Brantley, Pierce, and Glynn Counties. 



Some of the publications in which Miocene microfossils are described and illustrated include several 

 articles by Cole (1931 and 1941) and Cushman (1918 and 1930). Fossils that are diagnostic of the sub- 

 surface Miocene of Georgia include molluscan shells, occasional vertebrate remains such as fish teeth, 

 vertebrae(?), etc.; ostracods; and the Foraminifera Archaias floridanus (Conrad') and Rotalia beccarii 

 (Linne) var. Small Foraminifera* were noted in two recently drilled test holes in updip Chatham County, 

 Ga., and Beaufort County, S. C. Subsequent analysis of this microfauna by the senior author indicated 

 these Foraminifera to be late Miocene (Duplin) in age. 



*M. J. McCollum U. S. Geological Survey geologist in Savannah, Ga., first called the authors' attention 

 to the presence of these fossils in these test holes. This microfauna is being studied and processed for 

 future publication by the senior author. 



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