Okeechobean Sea

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.

During this period, Orange Island grew in size toward the west and north while the Gulf Trough or Suwannee Strait narrowed and became shallower.

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.

Forests slowly draw in massive amounts of carbon dioxide, gradually lowering the level of atmospheric CO2 to approximately 100 ppmv.

During this rather short period of time of 400,000 years, the peninsula's southwestern edge develops a shallow yet broad lagoon system.

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.

A rather short-lived subsea, during this period the rivers and lakes of the DeSoto Plain are complete and produce vast quantities of sediment deposited in the Okeechobee Basin.

The banks on the western side of the Charlotte Subsea force material southward producing the DeSoto Delta.

The Okeechobean flooded again creating the Murdock Subsea and covering the Immokolee Delta which disbursed its sediments further south into the glades.

Upwelling of cooler, nutrient rich water halted and the Okeechobee Basin returned to evolving through carbonate sedimentation with both Immokolee and DeSoto deltas now were covered and looking similar to the Bahama Banks.

The east, west, and south were created by reefs and islands of over 100 species of coral which grew atop the Hawthornian oyster and barnacle bars.

During this period, the Tamiami Psedoatoll feature of islands and reefs became wider and the marine environment was shallower and more of a connected lagoon system.

During the late Calabrian, ~1.8 Ma.—780,000 years ago, temperatures plunged to that or possibly colder than the Messinian-Zanclean[13] with the Caloosahatchee basin rising and filling with rock and mineral deposits.

The Pre-Illinoian began as a general warming from the frigid late Calabrian stage yet contained several ice ages and interglacials of its own.

The Okeechobean Sea experienced the last deposits of sediments allowing the Loxahatchee Subsea as well as the Kissimmee Valley and Okeechobee Plain to fill and become 50% covered with mangrove forests.

On the eastern side of the new Loxahatchee Subsea, reefs became wide islands covered with pines and palmettos with deep water surrounding them.

Oolite was created behind reef sections from pounding seas originating off of Africa due to a submerged Bahamas Bank.