The Okeechobean Sea was a Cenozoic eutropical subsea, which along with the Choctaw Sea, occupied the eastern Gulf of Mexico basin system bounding Florida.
Location The Okeechobean Sea was named for Lake Okeechobee of southern Early Oligocene Florida. It occupied southern Florida from what is now Tampa, Florida, to the Kissimmee River and to Martin County on the east and southward across the Everglades to the tip of Florida. The Okeechobean (including its subseas) survived until the Pleistocene making it the longest-lived paleosea in eastern North America.
The Okeechobean Sea was divided further into two groups of a combined 10 subseas based upon marine fauna. These subseas were named:
Dade Subsea Tampa Subsea Arcadia Subsea Murdock Subsea Tamiami Subsea Charlotte Subsea Caloosahatchee Subsea Laxahatchee Subsea Lake Worth Subsea Belle Glade Subsea
Subseas
Dade Subsea Period: Oligocene (complete). Geologic stage: Rupelian through early Chattian. ~33.9—28.4 Ma., approximately ~5.5 million years. Animal age: Orellan through Geringian. ~33.9—26.3 Ma. Post-Dade dry period: ~28.0—23 Ma. A moderate icehouse climate followed by an ice age lasting most of the Chattian ~28.4—23.03 Ma. The Dade Subsea was named for Dade County, Florida fossil reefs and is contemporary with the Bainbridge Subsea of North Florida. It is the last platform created from carbonate deposits and the predecessor of the recent Florida peninsula. It was formed out of the meteor impacts and tsunamis of the colder Eocene. By the mid Rupelian, the global climate warms and Florida consists of just Orange Island, a somewhat long island with coral reefs extending as far north as Alabama and Georgia forming the Salt Mountain formation and Flint River Formation respectively. Along its west coast lay the Pasco Reef System. separated from the mainland by the Gulf Trough or Suwannee Strait also with coral reefs. Orange Island lay just 150 km (93 mi) from Georgia. Two deep carbonate features will eventually become the substructure of the Everglades. Basilosaurus cetoides and Zygorhiza, early toothed whales, as well as Protosiren, an extinct ancestor of the manatee, inhabit these waters and are the earliest mammal fossils uncovered with specimens from Citrus County in dolomite dating to ~40—37 Ma. and Marion County, Florida dating from 37.2—33.9 Ma.
Tampa Subsea
Period: Late Oligocene—Early Miocene. Geologic stage: Chattian through early Aquitanian stage of the Miocene ~28.4—23.03 Ma., approximately 5.37 million years. Animal age: Arikareean. ~30.8—20.6 Ma. Post-Tampa dry period: Late Aquitanian through early Burdigian ~23.0—20.5 Ma. The Tampa Subsea was named for the Tampa, Florida and reef system. It is contemporary with the Chattahoochee Subsea in the Florida Panhandle. During this period, Orange Island grew in size toward the west and north while the Gulf Trough or Suwannee Strait narrowed and became shallower. On Orange Island's western side, upwelling systems were taking place with a system of lagoons and coral islands not unlike the Florida Keys evolving. In what would become the Everglades, the carbonate banks grew and the area became shallower. The Gulf Trough closed between Georgia and Florida forming the complete peninsula allowing southward movement of Florida's first land mammals such as the early canids Enhydrocyon, Osbornodon, and Phlaocyon preying upon the proto-horse Miohippus, deer-like Hypertragulidae, and the extinct camel, Nothokemas. The fossil record shows that land animals entered the newly formed peninsula with the earliest land mammals unearthed in Hernando, County dating from ~24.8 Ma.
Arcadia Subsea Period: Miocene. Geologic stage: Burdigalian through early Serravallian stages ~20.43—13.6 Ma., approximately 6.83 million years. Animal age: Hemingfordian through Barstovian ~20.3—13.6 Ma. Post-Arcadia dry period:' Early Serravallian ~13.65—12.0 Ma. The Arcadia Subsea was named after the Arcadia Formation and is contemporary with the Chipola Subsea to the north in the panhandle. During this period, the Florida peninsula forms as the Gulf Trough closes in the north and fills with mangrove forests. South of what was Orange Island and is now the peninsula's tip, was a deep water lagoon with many banks made of oyster bars and coral reefs. The largest bank was located on its southwest. Also by this time, phytoplankton blooms along the western coast poured into the Arcadia Subsea. The Middle Miocene Climatic Optimum of ~16 Ma. carbon dioxide and a climate change increases, warming the planet. The Middle Miocene disruption, an extinction of many mammal genera begins. The Arcadia begins to regress with a very short dry period lasting possibly 1 million years. During this time the Middle Miocene Disruption continues. Forests slowly draw in massive amounts of carbon dioxide, gradually lowering the level of atmospheric CO2 to approximately 100 ppmv. Metaxytherium floridanum a form of manatee, and long beaked dolphin (Pomatodelphis) swam this subsea. The fossil of Mesoplodon longirostris is found on the Miami Terrace dating to ~15.97—11.61 Ma.
Polk Subsea Period: Middle Miocene. Geologic stage: Late Serravallian through early Tortonian ~12.00—11.6 Ma., approximately 0.4 million years. Animal age: Clarendonian ~13.6—10.3 Ma through Hemphillian ~10.3—4.9 Ma. Post-Polk dry period: Early Tortonian, approximately ~11.6—9.0 Ma. The Polk Subsea was named for the rich fossil beds of Polk County, Florida and contemporary with the Walton Subsea. During this rather short period of time of 400,000 years, the peninsula's southwestern edge develops a shallow yet broad lagoon system. Near western DeSoto County, erosion creates water-filled sinkholes and rivers with sediments from this erosion enhancing the carbonate bank on the southwest and producing large oyster bioherms along the seaward side of the lagoon system as well as banks to the south. The upwelling of nutrients and phytoplankton of the Arcadia Subsea continued and increased eventually flooding the lagoon system and Okeechobee Basin and contributing to the building of the Peace River Formation. Sperm whale (Kogiopsis floridana), 9.1—9.0 Ma. Fossil specimen found in the Amalgamated Phosphate Mine; Brewster Phosphate Mine, Polk County, Florida.
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