Florida Platform
The Florida Platform is a flat geological feature that underlies all of Florida and southern Alabama and Georgia and their adjacent continental shelves. The basement of the platform, composed of igneous, metamorphic, and sedimentary rocks, was originally part of the African continent that became attached to the North American continent in the Jurassic geological period. The basement rocks are overlain by up to of evaporite, carbonate, and siliciclastic sedimentary deposits that are primarily of marine origin.
Description
The Florida Platform is a large, level, and relatively stable platform of sedimentary strata on the trailing or eastern edge of the North American continent, extending between the basins of the Gulf of Mexico and the Atlantic Ocean. It underlies all of Florida and its adjacent continental shelves, as well as the southern parts of Alabama and Georgia. Most of the basement of the platform was originally part of Africa that became attached to North America during the Ordivician Period as the supercontinent of Pangaea formed. The platform separated from Africa as Pangaea broke up in the Jurassic, remaining attached to North America. The Florida Platform remained submerged for most of the Cenozoic Era, and the sedimentary strata of the platform are primarily of marine origin.Basement
The basement of the Florida Platform comprises igneous, metamorphic, and sedimentary rocks of Proterozoic, Paleozoic, and Triassic age. The top of the basement of the platform is marked by an unconformal surface of pre-Middle Jurassic age overlain by evaporite, carbonate, and siliciclastic deposits of Middle Jurassic to Holocene ages. The top of the basement is approximately below mean sea level in the north-central Florida peninsula, and slopes downward on the east towards the Atlantic Ocean basin, on the west towards the Gulf of Mexico basin, and on the south towards the South Florida Basin, reaching below MSL in southern Florida.The basement rocks of the Florida Platform are divided into two regions by the Bahama Fracture Zone, which runs northwestward from the Bahamas across southern Florida under the Gulf of Mexico and the extreme western end of the Florida panhandle in to Alabama. The fracture zone is also known as the Jay Fault in Florida and the Pickens-Gilberton Fault in Alabama. Northeast of the Bahamas Fracture Zone is the Suwannee terrane, a crustal fragment that originated on the coast of what is now West Africa when it was part of Gondwana.
Southwest of the Bahama Fracture Zone in the southern part of the Florida peninsula the basement is primarily composed of the Mesozoic-age South Florida Volcanic Rocks. Across the West Florida Continental shelf and southern Florida southwest of the Bahama Fracture Zone there are several basins and arches at the top of the basement rocks discernible via seismic imaging. The Apalachicola Basin on the continental shelf is offshore of the Apalachicola Embayment or South Georgia Rift. It is bordered on the southeast in turn by the Middle Ground Arch, which extends across the Bahama Fracture Zone under Apalachee Bay, The Tampa Basin, the Sarasota Arch, and the South Florida Basin.
Origin
Collision
The Florida Platform originated in Gondwana in the early Paleozoic. Data from paleontology, isotope geochemistry, and paleomagnetism studies indicate that the platform was part of the western African continental margin near Senegal. Boreholes in northern Florida that reached sandstones and shales of Paleozoic age have recovered fossils of graptolites, brachiopods, trilobites, crinoids, molluscs, conodonts, palynomorphs, and chitinozoans. The faunal assemblages resemble African or South American assemblages, but none have resembled any North American assemblage. Bore holes have also penetrated granite and other volcanic rocks. The Tallahassee-Suwannee terrane that forms the basement under northern Florida and southern Georgia contains rhyolitic rocks that correlate geochemically with rocks found in locations along the western margin of Africa. The St. Lucie metamorphic complex matches rocks of the Rokelide Orogen in Guinea. The Osceola Granite resembles the Coya Granite in Senegal. The Suwannee Basin sedimentary rocks have been correlated with those of the Bove Basin in Guinea-Bissau; the two features are likely remnants of an extensive basin.In the late Paleozoic Era, Laurentia converged on Gondwana, closing the Iapetus Ocean and creating the Pangaea supercontinent. The part of Gondwana facing Laurentia included what are now western Africa and northern South America. The collision created strike-slip faults in Gondwana as blocks of crust moved in response to irregularities in continental margins. There are several possible faults from this period in the basement of the Florida Platform, the most prominent being the Bahama Fracture Zone. The Brunswick Magnetic Anomaly, consisting of a northern negative component and a southern positive component, runs east-west across southern Georgia and Alabama. The anomaly is commonly believed to mark the boundary between the Florida Platform and the rest of North America. An alternative explanation for the anomaly is that the North American Plate was subducted under Gondwana as the Iapetus Ocean closed, and that the anomaly marks the lower end of the subducted North American lithosphere under the Tallahassee-Suwannee terrane, the northern boundary of which may be as much as north of the anomaly. Another proposal is that the Brunswick Magnetic Anomaly was a Mesozoic rift basin. It has also been proposed that the Florida Magnetic Anomaly, crossing the Florida peninsula approximately from Volusia County to Hernando County, marks the suture between North America and the Florida Platform.
Rifting
Reconstructions of Pangaea place the Florida Platform at the junction of the African, North American, and South American continents. A system of rifts that would eventually produce the Atlantic Ocean began opening along the pre-Pangaea continental margins during the Triassic. The rifting is believed to have been initiated by hot spots, including one near the southern end of the Florida Platform. Isotopic signatures of the South Florida Volcanic Rocks indicate they were produced by a hot spot. The South Georgia Rift, a graben, started to form across what is now southern Georgia, but stopped and became an aulacogen, a failed arm of a triple junction. The rifting shifted, separating the Florida Platform and the Bahamas Banks from Africa and South America.North America began separating from South America in the early Jurassic, with the Gulf of Mexico opening in the early Jurassic. Grabens and horsts in the basement of the western part of the Florida Platform may have developed due to stresses on the crust caused by the opening of the Gulf of Mexico, or may be horsts that broke off of North America and reattached to the Florida Platform. Similarly, horsts that broke off of South America may underlie southern Alabama and Mississippi. The Florida Platform has been tectonically quiet since the end of the rifting. There has been regional uplift in the sedimentary layers that have accumulated on the Florida Platform, but they are the product of epeirogenic movement caused by density changes resulting from dissolution of limestone.
Post-rifting sediments
Jurassic and early Cretaceous sediments
sediments began accumulating in incipient rifts associated with the Florida Platform in the late Triassic and early Jurassic. As the Gulf of Mexico opened up in the middle Jurassic, carbonate, evaporite and siliciclastic sediments accumulated on the Florida Platform. The Apalachicola Embayment and Tampa Basin developed thick sediments of evaporite. Clastic sediments developed on top of the evaporates in the northern part of the platform, and with a later rise in sea level, the Smackover Limestone was laid down.From the late Triasssic into the middle Jurassic, most of the Florida Platform was above sea level and no sedimentary strata have survived from that period. From the Late Jurassic through the Cretaceous into the Paleogene, sea levels were higher than at present, and most of the platform was below sea level most of the time. The platform was a rimmed shelf during the Jurassic and Cretaceous. As sea levels rose and fell during glacial cycles, layers of sediments were laid down on the submerged platform, primarily carbonate and evaporite sediments on the southern portions of the platform, while clastic deltaic and marine sediments were more common on the western and northern portions of the platform. Clastic sediments reached further south when sea levels were relatively low. Reefs formed along the western margin of the platform during the early Cretaceous, and around the southern end of the platform in the late Cretaceous, creating silled basins where thick beds of gypsum and anhydrite developed.
Cretaceous carbonate platforms
During the late Jurassic and early Cretaceous, the Florida Platform was part of a very large carbonate platform complex known as the Bahamas - Grand Banks Gigaplatform, which stretched nearly from the Bahamas Banks through the Florida Platform and up the coast of North America to Canada, three times longer than the Great Barrier Reef. Carbonate sedimentation north of Florida ceased during the Cretaceous, likely due to some combination of the migration of much of the North American plate into colder climates, eutrophication from excessive nutrients from runoff from land and from ocean upwelling, burial by siliciclastic sediments eroding from the Appalachian mountains, and fluctuations in sea level. Carbonate sediment formation continued into the present on the Florida-Bahamas megabank because it remained in a tropical or sub-tropical climate, siliciclastic sediments did not reach the area, and it was generally not subject to environmental stress.During most of the Cretaceous the Florida-Bahamas megaplatform extended from the West Florida Escarpment to the Blake-Bahamas Escarpment, which borders the Blake Plateau and Bahama Banks to east, and south to the middle of present-day Cuba. The Bahama Banks originated on shallow seafloor in the late Jurassic, possibly on a submerged terrane. The ocean floor subsided as the Atlantic Ocean widened, but carbonate sedimentation was able to keep pace, keeping the water over the top of the platform shallow enough to allow carbonate formation to continue. The Bahamas Banks are composed of shallow-water carbonate sediments up to thick. The opening of the Suwannee Channel across southern Georgia in the mid- to late-Cretaceous carried siliciclastic sediments onto the Blake Plateau, ending carbonate formation.
The western and southern parts of the Florida-Bahamas megaplatform were subject to segmentation and drowning during the mid-Cretaceous. The southern margin of the megaplatform retreated north, but then grew southward again during the late Cretaceous and Cenozoic. The Caribbean plate collided with the North American plate in the late-Cretaceous through the Middle Eocene, causing the platform to buckle and flex, opening up the Straits of Florida, which separated the Florida Platform from the Bahama Banks and provided a passage for the Florida Current.