24J1: Seminole County Fossils
By Thomas Thurman
20/July/ 2024
Seminole County was established in 1920 from lands of Decatur and Early counties, so it isn’t named in the older geologic literature. USGS geologist C. Wythe Cooke mentions it in 1943 while reported Early Oligocene residuum (Flint River Formation) as occurring in Seminole County.
Seminole County.-On the Chattahoochee River the Flint River Formation is well exposed at Fairfield Landing, Seminole County, where large masses of porous and vitreous chert containing many (foraminifera) Lepidocylina favosa associated with residual gray sandy clay. It is overlain by pebble-bearing terrace sand.
The Flint River Formation has been reassigned as Undifferentiated Oligocene Residuum. It is overwhelmingly Early Oligocene in fossil content.
The Flint River Formation has been reassigned as Undifferentiated Oligocene Residuum. It is overwhelmingly Early Oligocene in fossil content.
Forming the Early Oligocene Chert
This Early Oligocene residuum is interesting (See Section 15C of this website). It began as a large, fossiliferous limestone deposit laid down in the Early Oligocene (33 million years ago). Sea levels retreated. This limestone was slowly buried in terrestrial sediments and became a freshwater aquifer. The water had a rich silica content which was absorbed by the limestone. Silica hardens limestone into chert, which can be very hard and enduring. Chert is immune to acids. But not all of the Oligocene limestone was turned to chert, or silicified. At some point the groundwater became acidic. As you know, acidic water dissolves limestone. So, the remaining limestone in the Oligocene deposit was dissolved away by acidic ground water and the silicified (chert) portions, which were immune to the acid, collapsed in place as residue of the former limestone bed. Thus; Early Oligocene residuum.
When the Oligocene residuum was dissolved and collapsed is a matter of ongoing research. That’s why Cooke mentions the gray sandy clay. In Houston County, the Oligocene residuum is found in red sandy clay.
This Early Oligocene residuum is interesting (See Section 15C of this website). It began as a large, fossiliferous limestone deposit laid down in the Early Oligocene (33 million years ago). Sea levels retreated. This limestone was slowly buried in terrestrial sediments and became a freshwater aquifer. The water had a rich silica content which was absorbed by the limestone. Silica hardens limestone into chert, which can be very hard and enduring. Chert is immune to acids. But not all of the Oligocene limestone was turned to chert, or silicified. At some point the groundwater became acidic. As you know, acidic water dissolves limestone. So, the remaining limestone in the Oligocene deposit was dissolved away by acidic ground water and the silicified (chert) portions, which were immune to the acid, collapsed in place as residue of the former limestone bed. Thus; Early Oligocene residuum.
When the Oligocene residuum was dissolved and collapsed is a matter of ongoing research. That’s why Cooke mentions the gray sandy clay. In Houston County, the Oligocene residuum is found in red sandy clay.
Additional Fossils Reported
Dr. Burt Carter (Professor Emeritus Georgia Southwestern) collected a Rhyncholampas conradi echinoid (sea urchin) from Seminole County. It is housed and cataloged in the Florida Museum of Natural History. Carter dated the sediments holding the urchin to the Late Eocene. (See Section 29 of this website)
Dr. Burt Carter (Professor Emeritus Georgia Southwestern) collected a Rhyncholampas conradi echinoid (sea urchin) from Seminole County. It is housed and cataloged in the Florida Museum of Natural History. Carter dated the sediments holding the urchin to the Late Eocene. (See Section 29 of this website)
References:
- Cooke, C. Wythe; Geology of the Coastal Plain of Georgia, United States Geological Survey (USGS), Bulletin 941, 1943
- Cooke, C. Wythe; Cenozoic Echinoids of the Eastern United States, Geologic Survey Professional Paper 321, U.S. Dept of Interior, USGS, 1959
- Huddlestun, Paul F.; The Oligocene, A Revision of the Lithostratigraphic Units of the Coastal Plain of Georgia, Georgia Geologic Survey, Bulletin 105, 1993