TheFlorida Platform is a flat geological feature that underlies all of Florida and southernAlabama andGeorgia and their adjacentcontinental shelves. Thebasement of the platform, composed ofigneous,metamorphic, andsedimentary rocks, was originally part of the African continent that became attached to the North American continent in theJurassic geological period. The basement rocks are overlain by up to 5 kilometres (3.1 mi) ofevaporite,carbonate, andsiliciclastic sedimentary deposits that are primarily of marine origin.
The Florida Platform is a large, level, and relatively stableplatform of sedimentarystrata 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.[1] Most of the basement of the platform was originally part of Africa that became attached to North America during theOrdivician Period as the supercontinent ofPangaea formed. The platform separated from Africa as Pangaea broke up in the Jurassic, remaining attached to North America.[2] The Florida Platform remained submerged for most of theCenozoic Era, and the sedimentary strata of the platform are primarily of marine origin.[3]
Thebasement of the Florida Platform comprises igneous, metamorphic, and sedimentary rocks ofProterozoic,Paleozoic, andTriassic age. The top of the basement of the platform is marked by anunconformal surface of pre-Middle Jurassic age overlain by evaporite, carbonate, and siliciclastic deposits of Middle Jurassic toHolocene ages. The top of the basement is approximately 915 metres (3,002 ft) belowmean sea level (MSL) 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 5,180 metres (16,990 ft) below MSL in southern Florida.[4]
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 theFlorida 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 theSuwannee terrane, a crustal fragment that originated on the coast of what is now West Africa when it was part ofGondwana.[5][6]
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 viaseismic 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 underApalachee Bay, The Tampa Basin, the Sarasota Arch, and the South Florida Basin.[7]
The Florida Platform originated inGondwana in the early Paleozoic. Data frompaleontology,isotope geochemistry, andpaleomagnetism studies indicate that the platform was part of thewestern African continental margin nearSenegal. Boreholes in northern Florida that reached sandstones and shales of Paleozoic age have recovered fossils ofgraptolites,brachiopods,trilobites,crinoids,molluscs,conodonts,palynomorphs, andchitinozoans. Thefaunal 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 (northwest of the Bahama Fracture Zone) containsrhyolitic rocks that correlate geochemically with rocks found in locations along the western margin of Africa. The St. Lucie metamorphic complex matches rocks of the RokelideOrogen inGuinea. The Osceola Granite resembles the Coya Granite in Senegal. The Suwannee Basin sedimentary rocks have been correlated with those of theBove Basin inGuinea-Bissau; the two features are likely remnants of an extensive basin.[8]
In the late Paleozoic Era,Laurentia (the core of what is now North America) converged on Gondwana, closing theIapetus Ocean and creating the Pangaea supercontinent. The part of Gondwana facing Laurentia included what are now western Africa and northern South America. The collision createdstrike-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.[9] 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 theNorth American Plate wassubducted under Gondwana as the Iapetus Ocean closed, and that the anomaly marks the lower end of the subducted North Americanlithosphere under the Tallahassee-Suwannee terrane (the northern part of the Florida Platform), the northern boundary of which may be as much as 100 kilometres (62 mi) 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 fromVolusia County toHernando County, marks thesuture between North America and the Florida Platform.[10]
Reconstructions of Pangaea place the Florida Platform at the junction of the African, North American, and South American continents. A system ofrifts 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 byhot 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 (along the pre-Pangaean margin between Africa and North America), but stopped and became anaulacogen, a failed arm of atriple junction. The rifting shifted, separating the Florida Platform and the Bahamas Banks from Africa and South America.[11]
North America began separating from South America in the early Jurassic, with the Gulf of Mexico opening in the early Jurassic. Grabens andhorsts 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 andMississippi. 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 ofepeirogenic movement caused by density changes resulting from dissolution oflimestone.[11]
Clastic 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 (related to theLouann Salt). Clastic sediments (Norphlet Sand) developed on top of the evaporates in the northern part of the platform, and with a later rise in sea level, theSmackover Limestone was laid down.[12]
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 theLate Jurassic through theCretaceous into thePaleogene (early Cenozoic), 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 clasticdeltaic 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 ofgypsum andanhydrite developed.[13]
During the late Jurassic and earlyCretaceous, the Florida Platform was part of a very largecarbonate platform complex known as the Bahamas - Grand Banks Gigaplatform, which stretched nearly 7,000 kilometres (4,300 mi) from the Bahamas Banks through the Florida Platform and up the coast of North America to Canada, three times longer than theGreat Barrier Reef. Carbonate sedimentation north of Florida ceased during the Cretaceous, likely due to some combination of the migration of much of theNorth 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.[14]
During most of the Cretaceous the Florida-Bahamas megaplatform extended from the West Florida Escarpment (the edge of the continental shelf west of Florida in the Gulf of Mexico) to the Blake-Bahamas Escarpment, which borders theBlake 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 floorsubsided 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 14 kilometres (8.7 mi) 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.[15]
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. TheCaribbean plate collided with the North American plate in the late-Cretaceous through the Middle Eocene, causing the platform to buckle and flex, opening up theStraits of Florida, which separated the Florida Platform from the Bahama Banks and provided a passage for theFlorida Current.[15]
Late in the Cretaceous atrough, the Suwannee Channel or Strait, developed across southern Georgia and the Florida panhandle connecting the Tallahasse Embayment and the Southest Georgia Embayment. In the middleEocene the Suwannee Channel was replaced by theGulf Trough, which ran from the Chattahoochee Embayment in eastern Florida panhandle northwestward into Georgia, but did not connect to an embayment on the Atlantic coast. (The Tallahassee and Chattahoochee embayments occupied essentially the same position, and are collectively called the Apalachicola Embayment.)[16]
The Suwannee Channel and Gulf Trough effectively separated the primarily carbonate sedimentation of the southern platform from the clastic sedimentation of the northern and western platform. The southward movement of clastic sedimentation suppressed carbonate sedimentation. When the sea level was higher the channel moved north and carbonate sediments formed over more of the platform. The channel filled in with clastic sediments during theOligocene, as a result of lower sea levels and slower currents in the channel caused by a shift in theGulf Stream. The Appalachian Mountains wereuplifted in the Paleogene, increasing the siliciclastic sediments carried by streams towards the channels.[17]
Aseismic profile feature in Florida called the Mid-Cretaceous Sequence Boundary has been interpreted to represent a fall in sea level followed by a large rise leading to the deposit of deep-water carbonate sediments in the later Cretaceous over earlier shallow-water carbonate deposits. The sea level relative to the Florida Platform rose and fell repeatedly in the Cenozoic, alternatively uncovering and flooding parts of the platform. Evaporite deposits in the Paleogene were thinner and covered smaller areas than those in the late Cretaceous. Carbonate sedimentation was higher in theEocene and Oligocene, while clastic sediments have predominated since the Miocene.[18] The oldest sediments that are exposed in Florida are Eocene limestones of theAvon Park Formation at the crest of theOcala Platform inCitrus andLevy counties.[19]
The Florida Platform is almost completely covered by Cenozoic deposits, which vary in thickness from less than 1 metre (3 ft 3 in) in the central Florida panhandle and parts of the west-central peninsula to more than 300 metres (980 ft) in southern Florida. The Suwannee Channel/Gulf Trough, which had largely blocked southward movement of siliciclastics across the platform in the Paleogene, apparently had filled in by the end of the Oligocene and siliciclastics began accumulating over the southern platform, suppressing the formation of carbonate deposits over almost all of the platform.[20]
The change in dominant deposition means thatPaleogene (Paleocene–Eocene–Oligocene) carbonate deposits are widely overlain byNeogene (Miocene–Pliocene) siliciclastic deposits. The upper surface of the Paleogene deposits in the central and northern Florida peninsula and the Florida panhandle had eroded and developed akarst topography. Thenon-conformable surface between the Paleogene and Neogene deposits has a number of highs and basins. The highs include theOcala Platform, Chattahoochee Arch, Sanford High, Saint Johns Platform, and Brevard Platform. Basins include the Gulf of Mexico Basin, Georgia Channel System (Appalacicola Embayment, Gulf Trough, Southeast Georgia Embayment, and Jacksonville Basin), Osceola Low, and Okeechobee Basin. ThePeninsular Arch is an earlier feature that affected deposition in the Paleogene, but not in the Neogene. Neogene sediments are generally thick over the basins and thin on the highs.[21]
The sea level relative to the Florida Platform fell and rose at least three times during the Miocene. TheChattahoochee andSt. Marks formations were laid down when much of the platform was submerged in theEarly Miocene. TheChipola,Oak Grove, andShoal River formations, and theHawthorn Group were laid down in theMiddle Miocene. TheJackson Bluff, lowerTamiami, and other poorly defined formations were laid down in theLate Miocene.[22]
Sea levels rose during thePliocene, covering most of Florida. Pliocene siliciclastic sediments covered more of the platform than during any prior period, including under the Pleistocene formations of the Florida Keys. Low areas on the platform accumulated thick deposits, more than 100 metres (330 ft) in the Apalachicola Embayment, and up to 30 metres (98 ft) in the Okeechobee Basin. The Pliocene sediments in much of the panhandle, including theCitronelle andMiccosukee formations, were part of adelta complex, now covered by the Northern and Western Highlands of Florida. In the Apalachicola Embayment and Gulf Trough theJackson Bluff and Intracoastal formations were laid down. The Cypresshead,Nashua, andCaloosahatchee formations form a gradient from north to south on the peninsula, complicated by the erosion and southward transportation of Cypresshead sediments. Florida south ofLake Okeechobee was a broad lagoon during the Pliocene, probable rimmed by reefs andbarrier islands. ThePeace River and upperTamiami formations were laid down over southern Florida during the Pliocene.[23]
The sea level in relation to the Florida Platform rose and fell duringPleistocene in response to cycles of glaciation. Geologists have identifiedseveral marine terraces from the Pleistocene, believed to have been created when sea level stood at a higher level for a prolonged period. TheAnastasia,Caloosahatchee, andFort Thompson formations, and theKey Largo andMiami limestones were laid down in the Pleistocene.[22]
The platform's western edge, orFlorida Escarpment, is normally defined where water depths at 300 feet (91 m) drop dramatically and in a short distance to 10,000 feet (3,000 m). The Florida peninsula is located on the eastern side of the platform, where in places it lies only 3 to 4 miles (4.8 to 6.4 km) from the platform's edge. On the gulf side the platform ends over 100 miles (160 km) to the west of the modern shoreline, where a massive cliff rises over 6,000 feet (1,800 m) from the 10,600 feet (3,200 m) depth of the gulf floor. The western reaches of the platform just offTampa were explored by the submersibleDSVAlvin in 1984 and theNOAASOkeanos Explorer in 2018.[24][25] Examination has placed the depth ofcarbonate rocks at greater than 20,000 feet (6,100 m).
27°39′44″N84°01′44″W / 27.6622°N 84.0289°W /27.6622; -84.0289