| Hawthorn Group | |
| Type: | Group |
| Age: | Miocene |
| Namedby: | L.C. Johnson, 1887 |
| Region: | Southeastern United States |
| Country: |  United States |
| Subunits: | (See text) |
| Overlies: | Ocala Limestone |
| Thickness: | > 330 feet |
The Hawthorn Group (also Hawthorne Group, previously called Hawthorn(e) Formation) is a stratigraphic unit of Miocene age in South Carolina, Georgia, and Florida, in the United States. It is known for its phosphate rock resources, and for its rich assemblages of Neogene vertebrate fossils.
The Waldo Formation was described by L.C. Johnson of the United States Geological Survey in 1887. It was later included in the Hawthorne Beds, named for Hawthorne, Florida, where its phosphate-rich rock was quarried and processed for use as fertilizer. The Hawthrone Beds were later renamed the Hawthorne Formation. Late in the 20th century the Hawthorn Formation was redesignated as the Hawthorn Group consisting of several formations.[1]
The Hawthorn Group is the widest spread stratigraphic unit of Miocene age in the southeastern United States, making up almost the entire thickness of Miocene strata over the area in which it occurs. The Hawthorn group has complex bedding, primarily consisting of clay, silt and sand. Stratigraphy varies, but the group usually consist of three main zones, a lower calcareous zone, a middle clastic zone, and an upper mixed zone of clastic and carbonate rocks. Phosphate deposits are found throughout the Hawthorn group, but particularly in the lower zone, where beds of dolomite and dolomitic limestone are found. Hawthorn Group deposits are mined for phosphate in central Florida.[2]
The Hawthorn group was deposited on a continental shelf. It is predominantly siliciclastic, consisting primarily of fine-to medium-grained quartz sand. Clay is also a component, interstitially and in beds. Other common components of Hawthorn units include phosphate, palygorskite, sepiolite, chert, and dolomite or dolostone.
Period
Neogene
Epoch
Earliest Miocene (Aquitanian) to early Miocene (Burdigalian) in the eastern Florida panhandle, to middle Miocene (Serravallian) in northern Florida, Georgia, and South Carolina, and to earliest Pliocene (Zanclean) in southern Florida, ~23 to ~3.6 mya a period of
North American land mammal age
Late Arikareean through early Blancan
The Hawthorn Group includes several geologic formations found in southeastern South Carolina, the coastal plain of Georgia, and much of the Florida Peninsula.The Hawthorn Group extends from southeastern South Carolina through the coastal plain of Georgia and the Florida Peninsula to south Florida.
In Florida, the Hawthorn Group encompasses in part the counties of Gilchrist, Levy, Dixie, Citrus, Sumter, Alachua and Marion County. The Hawthorn is also present below undifferentiated sediments (TQu) as well as the Tamiami Formation from Polk County south through Highlands, Glades, Hendry, Dade, Collier, and Monroe County at depths ranging from mean sea level near Polk to below 600 meters in Monroe Co.[3] The Hawthorn overlies Ocala Limestone[4]
The Alachua Formation may have resulted from the weathering of Hawthorn Group sediments, intermixed with Pliocene deposits. Scott does not consider it to be part of the Hawthorn Group.
Reptiles
Birds
Mammals