The Arcadia Formation and its sub-unit, the Tampa Member, are

Late Oligocene The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/ Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stag ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

s in

North Florida North Florida is a Regions of the United States#Florida, region of the U.S. state of Florida comprising the northernmost part of the state. Along with South Florida and Central Florida, it is one of Florida's three most common "directional" regi ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

. It is part of the Hawthorn Group.

Age

Period:

Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of ...

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago (Mya) to the beginning of the present Quaternary Period Mya. ...

Epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

: Early

through

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Faunal stage In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convent ...

Chattian The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/ Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest sta ...

through early

Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .mya, calculates to a period of

Location

The Arcadia Formation and Tampa Member is located southwestern flank of the

Ocala Platform The Ocala Platform or Ocala Uplift is a geologic feature, a structural high, and a northwest-trending uplift paralleling the Peninsular Arch along the west coast of Florida. Age Period: Neogene Epoch: Early Late Oligocene through Pliocene Fau ...

from

Pasco County Pasco County is located on the west central coast of the U.S. state of Florida. According to the 2020 census, the population was 561,691. Its county seat is Dade City, and its largest city is Zephyrhills. The county is named after Samuel Pa ...

and southward in to

Hillsborough Hillsborough may refer to: Australia *Hillsborough, New South Wales, a suburb of Lake Macquarie Canada *Hillsborough, New Brunswick *Hillsborough Parish, New Brunswick * Hillsborough, Nova Scotia, in Inverness County *Hillsborough (electoral d ...

and

Sarasota County, Florida Sarasota County is a county located in Southwest Florida. At the 2020 US census, the population was 434,006. Its county seat is Sarasota and its largest city is North Port. Sarasota County is part of the North Port–Sarasota– Bradenton, ...

. The Tampa Member and the lower part of the Arcadia Formation form the upper part of the Floridan Aquifer system in parts of southern Florida.

Composition

The Arcadia Formation is composed of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

s and dolomites which are yellowish gray to light olive gray to light brown in color. The texture is micro to finely

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

with varying sandy, clayey limestones and dolomites containing

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

. The clays are yellowish gray to light olive gray in color. They are moderately hard as well as sandy, silty, phosphatic and dolomitic. Silicified carbonates and

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

ized

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

have also been found.Scott, T. M., The lithostratigraphy of the Hawthorn Group (Miocene) of Florida: Florida Geological Survey Bulletin 59, 148 p.

Tampa Member

The Tampa Member consists predominantly of limestone with subordinate dolomite, sand and clay very similar to that of the subsurface limestone part of the Arcadia Formation. There is considerably less phosphate. The color is white to yellowish gray. It is fossil bearing and variably sandy and clayey mudstone,

wackestone Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...

, and

packstone Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limest ...

with little to no phosphate grains. Sand and clay beds are like those in the undifferentiated sediments of the Arcadia Formation.

Paleofauna

The Arcadia Formation proper contains molds and casts in dolomite containing

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. The Tampa Member contains mollusks and

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...

s in molds and casts with silicified

pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form ( crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced b ...

s and shell material.

References

{{reflist *USGS Lithostratigraphic Units of Florida
/small> *Florida Carbonate "Formations" and Conflicting Interpretations of Injection Well Regulations
/small> Paleogene Florida Paleogene Georgia (U.S. state)