St. Marks Formation

St. Marks Formation
Stratigraphic range: Miocene
St. Marks Formation; Stratigraphic range: Miocene
Type	Geological formation
Unit of	None
Sub-units	None
Lithology
Primary	Limestone & sand
Location
Region	Florida Panhandle
Country	United States
Extent	Jefferson, Leon, and Wakulla counties
Type section
Named for	St. Marks River
Named by	J. Finch, 1823

Last updated February 23, 2019

The St. Marks Formation is a Miocene geologic formation in the eastern Florida Panhandle.

The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

The Florida Panhandle, an informal, unofficial term for the northwestern part of the U.S. state of Florida, is a strip of land roughly 200 miles long and 50 to 100 miles wide, lying between Alabama on the north and the west, Georgia also on the north, and the Gulf of Mexico to the south. Its eastern boundary is arbitrarily defined. The terms West Florida and Northwest Florida are today generally synonymous with the Panhandle, although historically West Florida was the name of a British colony (1763–1783), later a Spanish colony (1783–1821), both of which included modern-day Florida west of the Apalachicola River as well as portions of what are now Alabama, Mississippi, and Louisiana.

Age

Period : Neogene
Epoch : Miocene ~23.03 to 5.33 mya, calculates to a period of 17.7 million years
Faunal stage : Arikareean through Hemphillian

The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago (Mya) to the beginning of the present Quaternary Period 2.58 Mya. The Neogene is sub-divided into two epochs, the earlier Miocene and the later Pliocene. Some geologists assert that the Neogene cannot be clearly delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes (1815–1868).

In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age but shorter than a period. The current epoch is the Holocene Epoch of the Quaternary Period. Rock layers deposited during an epoch are called a series. Series are subdivisions of the stratigraphic column that, like epochs, are subdivisions of the geologic timescale. Like other geochronological divisions, epochs are normally separated by significant changes in the rock layers to which they correspond.

The Arikareean 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 30,600,000 to 20,800,000 years BP, a period of 9.8 million years. It is usually considered to overlap the Oligocene and Miocene epochs. The Arikareean is preceded by the Whitneyan and followed by the Hemingfordian NALMA stages.

Location

The St. Marks Formation is exposed in Wakulla, southern Leon and southern Jefferson County, Florida on the northwestern flank of the Ocala Platform and along with Suwannee Limestone and Ocala Limestone makes up the upper part of the Floridan Aquifer in the eastern panhandle.^[1]

Wakulla County is a county located in the U.S. state of Florida. As of the 2010 census, the population was 30,776. Its county seat is Crawfordville.

Leon County is a county located in the Panhandle of the U.S. state of Florida. It was named after the Spanish explorer Juan Ponce de León. As of 2017 Census estimates, the population was 290,292.

Jefferson County, Florida county in Florida, United States of America

Jefferson County is a county located in the U.S. state of Florida. As of the 2010 census, the population was 14,761. Its county seat is Monticello.

Composition

The St. Marks Formation consists of a white to yellowish gray, poorly to moderately hard, sandy, fossil bearing rock in molds and casts within packstone to wackestone.

Packstone class of limestone in the Dunham classification system

Under the Dunham classification system of limestones, a packstone is defined as a grain-supported carbonate rock that contains 1% or more mud-grade fraction. This definition has been clarified by Lokier and Al Junaibi (2016) as a carbonate-dominated lithology containing carbonate mud in a fabric supported by a sand grade grain-size fraction and where less than 10% of the volume consists of grains >2 mm'.

Under the Dunham classification system of limestones, a wackestone is defined as a mud-supported carbonate rock that contains greater than 10% grains. Most recently, this definition has been clarified as a carbonate-dominated rock in which the carbonate mud component supports a fabric comprising 10% or more very fine-sand grade or larger grains but where less than 10% of the rock is formed of grains larger than sand grade .

Fossils

The fossils are in molds and casts and include:

Mollusks

Related Research Articles

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References

↑ USGS Florida Geology, Open File Report 80

Florida Carbonate "Formations" and Conflicting Interpretations of Injection Well Regulations
Finch, J., Geological essay on the Tertiary formation in America: American Journal of Science, v. 7, p. 31-43, 1823.

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[1] USGS Florida Geology, Open File Report 80