STRUCTURE 



The geologic structure in the Coastal Plain of Georgia is shown on nine structure-contour maps and 

 eight geologic sections (figs. 21-28). 



Geologic data are unavailable for large areas of the Coastal Plain making the problem of structural 

 interpretation more uncertain. Even for the Oligocene, the data of which are reasonably well distributed, 

 the thickness-distribution map (fig. 4) has been one of the most difficult to contour satisfactorily. 



Knowing that as more data become available, the contours will need revision, it was believed desirable 

 to give on each map the data used. The elevations and thicknesses so given can thus be used by future 

 workers who may wish to modify the interpretations as made for this report. It is probable that additional 

 drilling may indicate greater dips, structures, and faults than have been interpreted from data now avail- 

 able. 



The name "Gulf Trough of Georgia" is herein proposed for a major structural feature of the subsurface 

 in southwest Georgia. This feature was recognized by P. L. and E. R. Applin (1944, p. 1727) as "extend- 

 ing southwestward across Georgia through the Tallahassee area of Florida to the Gulf of Mexico." This 

 trough is a linear feature extending northeastward from Grady County through northwestern Thomas and 

 Colquitt Counties (See figs. 3 and 6). The thickness of Recent to Miocene deposits (see fig. 12) suggests 

 that the trough may also continue through Tift, Irwin, and northern Coffee Counties. The fauna found in 

 the rocks of this trough is similar to that from the Gulf of Mexico. Furthermore, the presence of a tro- 

 pical Olieocene sea in central Georgia as found by Esther R. Applin (1960) suggests a connection of that 

 sea with the Gulf of Mexico which presumably could have involved this trough. The trough appears quite 

 prominently on the maps showing the top of the Oligocene (fig. 3) and the top of the upper Eocene (fig. 6). 

 The top of the middle Eocene (fig. 8) indicates that the axis of the trough parallels that of upper Eocene 

 and Miocene deposits but displaced a few miles to the southeast. Below the top of the middle Eocene nothing 

 is known regarding this feature, but the deep paralles trough in the pre-Cretaceous surface (see fig. 20) 

 suggests that it may persist in the intervening sediments. 



A major structural feature in southeastern Georgia is herein proposed to be called the "Atlantic Em- 

 bayment of Georgia" (see figs. 3, 6, 8, and 27). The deposits in the embayment contain fossils that are 

 similar to forms living in the Atlantic Ocean for which reason the name has been chosen. Although the lack 

 of data hinders an understanding of the deeper buried units, the embayment appears to have originated 

 in middle Eocene time and continued as a depositional basin intermittently through Miocene time. 



Overlap has been mentioned previously in connection with the discussions of stratigraphy. Examples 

 appear on the east-west trending geologic sections. A Cretaceous high in Wilkinson 441 (fig. 21) is over- 

 lain by upper Eocene with middle Eocene deposited down the flanks of the high. Similarly a Cretaceous 

 high is found in Pulaski 472 (fig. 27) with Paleocene and lower Eocene deposited northwest of it and Paleo- 

 cene deposited to the southeast of it but the Cretaceous high and the younger sediments on its flanks are 

 all overlain by middle Eocene. 



The sedimentary units in the northwestern half of the Georgia Coastal Plain all have a gentle dip to 

 the southeast. The tops of the Oligocene and upper Eocene both dip about 9.5 feet per mile. The dips 

 on tops of successively lower units increase to about 24 feet per mile on the Cretaceous (see table 12.) 



The dips of sedimentary units in the southeastern half of the Coastal Plain are slightly less than those 

 in the northwestern half but they are more variable in direction. The dips range from 4.0 to 23 feet per 

 mile from southward to eastward. The direction of dip can be determined from figures 3, 6, 8," 10, 12, and 

 14. 



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