THE MIO-PLIOCENE FLORRIDOR RIVER DELTA OF APPALACHIAN SAND IS THE FOUNDATION FOR THE FLORIDA REEF TRACT

The Mio-Pliocene, Florridor Fluvial System brought large volumes of coarse-grained siliceous sand and gravel 1000 km from the Appalachians down the Florida Peninsula to build a deltaic embankment off the Keys, the Foundation for the Florida Reef Tract. The sand and granules, surprisingly rich in feldspar, are up to 4mm. in diameter; the gravel includes discoid quartzite pebbles up to 4 cm. A multi-cycle complex of braided streams fueled by monsoonal rains is the most likely mode of this long distance transport. These siliceous deposits are only well preserved in southern Florida from the latitude of Lake Okeechobee to the Florida Keys. There, they are localized in the subsurface along a north-south corridor, which extends from the Peninsula across Florida Bay and then curves eastward across the present Keys. There, these sands with gravel accumulated as a large subaqueous “delta” up to 145 m thick seen in seismic profiles as a wedge of eastward-prograding clinothems some 15 km wide. Seismic profiles show that locally this deltaic embankment had down-slope erosional channels and hints of contour current deposition. When sea level flooded the top of the Florridor deltaic embankment beginning in the late Pliocene, accumulation of skeletal carbonate began. Later in the Plio-Pleistocene, corals became established and reefal limestone developed during high stands. During the 5e high stand some 125 thousand years ago, when sea level was some 7 meters above its present position, a complex of patch reefs accumulated that now are the Upper Florida Keys (Key Largo Ls.). Holocene flooding allowed a second and final episode of reefal and skeletal carbonates to develop as a seaward-thickening wedge reaching 17 m, the deposits of the present Florida Reef Tract. Comparable successions of Quaternary carbonates on silicic foundations occur in the Central Great Barrier Reef, in the Belize Barrier Reef Complex and in the Carboniferous and Devonian of North America.