Hawthorn Group

The Hawthorn Group (previously called Hawthorn 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. (The "e" was eventually dropped from "Hawthorn".) Late in the 20th century the Hawthorn Formation was redesignated as the Hawthorn Group consisting of several formations.

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.