Geologic units in Hamilton county, Florida

Hawthorn Group, Statenville Formation (Miocene) at surface, covers 81 % of this area

The Statenville Formation occurs at or near the surface in a limited area of Hamilton, Columbia and Baker Counties on the northeastern flank of the Ocala Platform. The formation consists of interbedded sands, clays and dolostones with common to very abundant phosphate grains. The sands predominate and are light gray to light olive gray, poorly indurated, phosphatic, fine to coarse grained with scattered gravel and with minor occurrences of fossils. Clays are yellowish gray to olive gray, poorly consolidated, variably sandy and phosphatic, and variably dolomitic. The dolostones, which occur as thin beds, are yellowish gray to light orange, poorly to well indurated, sandy, clayey and phosphatic with scattered mollusk molds and casts. Phosphate occurs in the Statenville Formation in economically important amounts. Silicified fossils and opalized claystones are found in the Statenville Formation. Permeability of these sediments is generally low, forming part of the intermediate confining unit/aquifer system.

Undifferentiated sediments (Pleistocene/Holocene) at surface, covers 12 % of this area

Undifferentiated Quaternary Sediments - Much of Florida's surface is covered by a varying thickness of undifferentiated sediments consisting of siliciclastics, organics and freshwater carbonates. Where these sediments exceed 20 feet (6.1 meters) thick, they were mapped as discrete units. In an effort to subdivide the undifferentiated sediments, those sediments occurring in flood plains were mapped as alluvial and flood plain deposits (Qal). Sediments showing surficial expression of beach ridges and dunes were mapped separately (Qbd) as were the sediments composing Trail Ridge (Qtr). Terrace sands were not mapped (refer to Healy [1975] for a discussion of the terraces in Florida). The subdivisions of the Undifferentiated Quaternary Sediments (Qu) are not lithostratigraphic units but are utilized in order to facilitate a better understanding of the State's geology. The siliciclastics are light gray, tan, brown to black, unconsolidated to poorly consolidated, clean to clayey, silty, unfossiliferous, variably organic-bearing sands to blue green to olive green, poorly to moderately consolidated, sandy, silty clays. Gravel is occasionally present in the panhandle. Organics occur as plant debris, roots, disseminated organic matrix and beds of peat. Freshwater carbonates, often referred to as marls in the literature, are scattered over much of the State. In southern Florida, freshwater carbonates are nearly ubiquitous in the Everglades. These sediments are buff colored to tan, unconsolidated to poorly consolidated, fossiliferous carbonate muds. Sand, silt and clay may be present in limited quantities. These carbonates often contain organics. The dominant fossils in the freshwater carbonates are mollusks.

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

Peninsular Lower Oligocene carbonates crop out on the northwestern, northeastern and southwestern flanks of the Ocala Platform. The Suwannee Limestone is absent from the eastern side of the Ocala Platform due to erosion, nondeposition or both, an area referred to as Orange Island (Bryan, 1991). The Suwannee Limestone, originally named by Cooke and Mansfield (1936), consists of a white to cream, poorly to well indurated, fossiliferous, vuggy to moldic limestone (grainstone and packstone). The dolomitized parts of the Suwannee Limestone are gray, tan, light brown to moderate brown, moderately to well indurated, finely to coarsely crystalline, dolostone with limited occurrences of fossiliferous (molds and casts) beds. Silicified limestone is common in Suwannee Limestone. Fossils present in the Suwannee Limestone include mollusks, foraminifers, corals and echinoids.

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

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.