STRATIGRAPHY 



At least four structural-depositional features characterize the Coastal Plain of Georgia in its overall 

 aspects. The first obvious feature is that the Coastal Plain is composed of a wedge-shaped block of strati- 

 fied sediments that rests upon a pre-Cretaceous basement complex ranging from Triassic(?) to Paleo- 

 zoic to Precambrian in age. In Mitchell County, Trias sic(?) rocks overlying the basement complex 

 were encountered at depth whereas black shale of Paleozoic age was penetrated at depths of 3,782 feet 

 and 6,950 feet in Echols and Early Counties, respectively (Applin, 1951, p. 25). Depth to the underlying 

 crystalline basement varies according to the position on the dip. Crystalline rocks of Precambrian age 

 were encountered in updip areas, such as Richmond and Washington Counties, at 162 and 871 feet; in 

 middip areas such rocks were penetrated at depths of 1,685 and 2,532 feet in Houston and Laurens 

 Counties; and in downdip parts of the Coastal Plain in southeastern and southern Georgia at 4,075, 4,250, 

 4,674 (Applin, 1951, p. 21), and 4,125 feet (Applin, 1951, p. 27), in Appling, Liberty, Camden, and Echols 

 Counties. In extreme southeastern Seminole County, 7,620 feet of sediments were penetrated in the 

 deepest known embayment area in the Coastal Plain of Georgia, but even so the depth was not great 

 enough to encounter pre-Cretaceous rock. 



The second outstanding feature is that most of the Coastal-Plain sediments are composed of two con- 

 trasting but stratigraphically equivalent types of sedimentary deposits, a fact first noted by the Applins 

 (1944, p. 1679). In updip parts of the Coastal Plain the deposits are distinctly clastic by nature and, in 

 their overall aspect, resemble those of the western Gulf Coastal Plain. Downdip, the lithology grad- 

 ually grades into limestone. The limestones of downdip areas are lithologically and faunally similar 

 to their stratigraphic equivalents in peninsular Florida. In Glynn County, for example, this limestone 

 facies includes all geologic formations beginning with the early Miocene down to and including the strata 

 of Navarro (Late Cretaceous) age. Beginning with the beds of Taylor age, the remainder of the sedi- 

 ments belonging to the Upper Cretaceous as well as those of the still older Lower Cretaceous(?) are 

 of the clastic type in Georgia. Still farther south, as for example the Everglades area of Florida, these 

 elastics of Cretaceous age grade laterally to limestones. Accompanying the facies change in downdip 

 parts of the Coastal Plain is a corresponding change in foraminiferal microfaunas. As pointed out by 

 the Applins (1944, p. 1680), the Foraminifera of the elastics are similar to those of the western Gulf 

 Coast whereas those of the limestone facies are similar to those of Cuba, the West Indies, and Mexico. 

 A good example of this is the foramimleral iauna characterizing the Paleocene in Georgia, a micro- 

 fauna that shows rather close relationship to that of the West Indies and Mexico. 



The third outstanding feature of the Coastal Plain in Georgia is what Murray has called "depocenters" 

 (1961, p. 5, 89). These are areas of maximum deposition. An example of such a depocenter is the east- 

 west trending belt of greatest thickness of the post-Tuscaloosa in the central part of the Coastal Plain. 

 (See fig. 15.) Murray (1961, p. 281) attributes these depocenters to: (1) major variations in locale or rate 

 of sedimentary accumulations, whatever their cause, and (2) regional warpings related to epeirogenic 

 and isostatic adjustments. 



The fourth structural-depositional feature of the Coastal Plain in Georgia requiring mention is forma- 

 tional overlap. From Lower Cretaceous(?) through upper Eocene time this was taking place in Coastal- 

 Plain Georgia. The best example of this phenomenon is that of the upper Eocene which overlaps middle 

 Eocene and Upper Cretaceous deposits in east-central Georgia, finally coming to rest directly upon 

 Precambrian rocks in the Piedmont. 



In this report a brief description of the subsurface stratigraphic section starts with the Miocene and 

 ends with the Lower Cretaceous(T), the stratigraphic units being taken up in descending order. The veneer 

 of post-Miocene strata is thin except for coastal Georgia and other more localized areas and is of such 

 minor importance in the subsurface that it is omitted from this report. 



