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Mineral Resources > Online Spatial Data > Geology > by state > Georgia Geology

Geologic units in Decatur county, Georgia

Residuum on Oligocene sediments (Oligocene) at surface, covers < 0.1 % of this area

Residuum on Oligocene sediments - The undifferentiated Oligocene residuum, mapped on parts of the Chattahoochee "Anticline", characteristically consists of reddish brown, variably sandy clay with inclusions of variably fossiliferous, silicified limestone (Huddlestun, 1993). The residuum includes Lower and Upper Oligocene weathered sediments (Huddlestun, 1993).

Lithology: limestone; sand; clay or mud

Ocala Limestone (Eocene) at surface, covers 41 % of this area

Ocala Limestone, generally covered with Oligocene and Eocene residuum (Flint River Formation of Cooke, 1939); includes up-dip area, Tivola Limestone of Connell (1955). (*)- outcrops of Ocala Limestone on Dougherty Plain.

Lithology: limestone

Citronelle Formation (Pliocene) at surface, covers < 0.1 % of this area

Citronelle Formation - The Citronelle Formation is widespread in the Gulf Coastal Plain. The type section for the Citronelle Formation, named by Matson (1916), is near Citronelle, Alabama. The Citronelle Formation grades laterally, through a broad facies transition, into the Miccosukee Formation of the eastern Florida panhandle. Coe (1979) investigated the Citronelle Formation in portions of the western Florida panhandle. The Citronelle Formation is a siliciclastic, deltaic deposit that is lithologically similar to, and time equivalent with, the Cypresshead Formation and, at least in part, the Long Key Formation (Cunningham et al., 1998) of the peninsula. In the western panhandle, some of the sediments mapped as Citronelle Formation may be reworked Citronelle. The lithologies are the same and there are few fossils present to document a possible younger age. The Citronelle Formation consists of gray to orange, often mottled, unconsolidated to poorly consolidated, very fine to very coarse, poorly sorted, clean to clayey sands. It contains significant amounts of clay, silt and gravel which may occur as beds and lenses and may vary considerably over short distances. Limonite nodules and limonite-cemented beds are common. Marine fossils are rare but fossil pollen, plant remains and occasional vertebrates are found. Much of the Citronelle Formation is highly permeable. It forms the Sand and Gravel Aquifer of the surficial aquifer system.

Lithology: delta; sand; clay or mud; silt; gravel

Miccosukee Formation (Neogene) at surface, covers 33 % of this area

Miccosukee Formation

Lithology: clay or mud; sand; gravel

Hawthorn Group, Torreya Formation (Miocene) at surface, covers < 0.1 % of this area

Hawthorn Group, Torreya Formation - Torreya Formation - The Torreya Formation is exposed or near the surface from western Gadsden County eastward to western-most Hamilton County. It is informally subdivided into a lower carbonate unit and an upper siliciclastic unit (Scott, 1988). The majority of Torreya Formation outcrops expose the siliciclastic part of the unit. The carbonate sediments are white to light olive gray, generally poorly indurated, variably sandy and clayey, fossiliferous (molds and casts) limestone (mudstone and wackestone). The limestones often grade into calcareous-cemented sands. Phosphate is present in the carbonate sediments, particularly in the Sopchoppy Member. The siliciclastics vary from white to light olive gray, unconsolidated to poorly indurated, slightly clayey sands with minor phosphate to light gray to bluish gray, poorly consolidated, variably silty clay (Dogtown Member). The siliciclastics are sporadically fossiliferous. The Torreya Formation overlies the FAS and forms part of the intermediate confining unit/aquifer system.

Lithology: limestone; sandstone; clay or mud; silt

Floyd Shale; Hartselle Sandstone member (Mississippian) at surface, covers 7 % of this area

Hartselle Sandstone member (Mh) in Floyd County

Lithology: sandstone

Suwannee Limestone and its residuum (Oligocene) at surface, covers 7 % of this area

Suwannee Limestone and its residuum, as mapped, includes Chattahoochee Limestone in Seminole and Decatur Cos.; Flint River Formation, in part, (Cooke, 1939); Jacksonboro Limestone in Screven Co.; and Glendon and Marianna Limestones in Pulaski Co.

Lithology: limestone

Stream alluvium (Quaternary) at surface, covers 7 % of this area

Stream alluvium and undifferentiated terrace deposits

Lithology: alluvium; alluvial terrace