MIOCENE REJUVENATION OF THE SOUTHERN APPALACHIAN MOUNTAINS AND FLUVIAL TRANSPORT OF COARSE SILICICLASTICS TO SOUTHERN FLORIDA

A combination of Miocene uplift of the southern Appalachian Mountains and the late Miocene drop in sea level (Messinian event) combined to allow fluvial transport of coarse siliciclastic sediments onto the Florida Platform. Siliciclastic sediments were transported over 700 km within a channelized braided stream system. The stream power was sufficient to transport discoidal quartz and quartzite pebbles with diameters up to 5 cm into southern Florida. Incised stream channels were observed in continuous seismic reflection profiles collected in the Caloosahatchee River along an east-west transect.

Fluvial transport is also suggested by the composition of the sediment which contains an average of 5% potassium feldspar within the medium sand grain size fraction and higher percentages in the fine sand size fraction. Trace amounts of plagioclase were also found in many samples. Marine transported siliciclastic sediments rarely allow preservation of feldspar, particularly within the fine sand size fraction. In addition, well-rounded metamorphic grains are commonly found in the granule and coarse fractions of the sediments. Within a few wells, feldspathic volcanic grains are a common constituent of the medium and fine sand size fraction of the sediment.

The volume of the late Miocene-age coarse siliciclastic sediment deposited in southern Florida is over 2.6 x 10³ km³ with an even greater quantity of disseminated finer-grained sand. This does not include the volume of sediments deposited north of the Caloosahatchee River within the central Florida transport corridor. Also, discharge of siliciclastic sediments occurred via a channel onto the deep water of Portales Terrace south of the Florida Keys. The volume of this sediment wedge has yet to be determined. The estimated time frame for deposition is quite shore at least than 1.5 MY.

Preservation of the feldspar in southern Florida was allowed by rapid burial of the coarse siliciclastics by a sequence of fine-grained deltaic sediments which are early Pliocene in age. Also, the channel system in northern Florida has a lower percentage of the feldspars is preserved, but large deposits of kaolin occur, which is an alteration product of the feldspar.