Birth of Orange Island

By the Early Oligocene, sea levels rose almost as much as during the late Eocene. Parts of the platform remained above water creating Orange Island, a low relief island just 150 km (93 mi) south of the Oligocene Georgia coastline and Bainbridge Subsea with a shallow channel, the Gulf Trough or Suwannee Strait, separating the two. The shallower Gulf Trough now supported a large coral reef extending to the Salt Mountain Formation and Flint River Formation of southern Alabama to Mitchell County, Georgia. The western side of the island was bounded by the Pasco Reef System, an environment now only found in the southern Pacific Ocean (Petuch).

During the middle Oligocene ~25 Ma, a worldwide cooling of the climate started the drying of the Dade Subsea, Gulf Trough, and in turn enlarging Orange Island. During the Chattian of the Late Oligocene, land mammals began their movement into what is now Florida with earliest of their fossils found in Hernando County dating to 24.8 Ma.^[4] Also from that Chattian stage through early Aquitanian age of the early Miocene, Orange Island expanded both northward and westward with carbonate lagoon system on its west side. To the south of Orange Island, the Everglades Basin remained at 250 meters (850 feet) deep. Orange Island had its own rivers carrying sediments composed of sand and clay in to the Yeehaw Strait at Orange Island's southern tip.

Orange Island appears to have been a Pleistocene animal and floral refugium for central Florida during the interglacial periods of rising water and not unlike that of the river bank of the Apalachicola River, the red hills near Marianna, Florida, and coastal flatwoods in the Florida panhandle.^[5]