Abstract
WANLESS, HAROLD R. (M.S., Marine Geology)
Sediments of Biscayne Bay - Distribution and Depositional History. (September 1967) Abstract of Master's Thesis at the University of Miami. Thesis supervised by Professor A. Conrad Neumann.
Three shallow elongate bays, Biscayne Bay, Card Sound and Barnes Sound trend south from Miami along the southeast Florida coast. Hand probing and coring through Recent sedimentary sequences within and bordering these bays has established the general character of the underlying bedrock topography, has revealed the general features of the sediment bodies and the spatial relations of the sediment types and has given insight into the developmental history of the Recent sediment accumulations.
Biscayne Bay, Card Sound, and Barnes Sound are underlain by a shallow, north-south trending late Pleistocene bedrock basin 2 to 6 meters in depth. It is bordered to the east by a ridge formed by the Oolite member of the Miami limestone, and to the southwest by the low platform of the Everglades. The Basin was first invaded by the sea about 6,000 years ago during the post-glacial Holocene rise of sea level. Sedimentation that has taken place during the subsequent period of slowly rising sea level has been controlled by sediment supply and bedrock topography through its influence on wave energy, tidal currents and wind-driven circulation.
Six major Recent sediment regimes are recognized on the basis of sediment type, sediment body geometry and depositional controls:
(a) The influx of a quartz carbonate longshore sediment supply from the north during the past 3,500 years has been the dominant influence in the formation of the sedimentary barrier islands of Miami Beach, Virginia Key, and Key Biscayne and the associated lagoon and offshore shoals.
(b) Intrabay quartz sand accumulations fill depressions and channels in northwestern Biscayne Bay and form beaches-and shoals along the mainland shoreline of Biscayne Bay and Card Sound and on bedrock rises within the Bay. The quartz is derived from the late Pleistocene Pamlico formation adjacent to the northern end of the Bay.
(c) Mud and sand carbonate tidal bars are present where tidal currents are intensified and directed by shallow thresholds of the bedrock topography. Where the bedrock threshold is entirely submerged the tidal bar belt parallels the trend of the bedrock restriction (Safety Valve and Cutter Bank). Where currents are restricted to channels through the bedrock rise the tidal bars are transverse to the bedrock restriction (Featherbed Bank and Caesar Creek Shoal).
(d) Paralic peat and fresh water peat and calcitic mud swamp deposits have developed along the transgressing shorelines since marine waters first entered the bays. Except along ·the more protected shorelines in Card and Barnes Sounds, these deposits have largely been eroded.
(e) The open bay contains two district sediment types. In areas where bedrock is less than 3 to 3.5 meters below sea level a winnowed quartz and carbonate sand forms a veneer (less than 15 cm.) over bedrock. In deeper bedrock areas lime mud has accumulated in association with turtle grass, Thalassia, which may increase bottom stability.
(f) Non-tidal mud banks in Barnes Sound appear to be actively migrating shoals which have developed in response to wind-induced circulation and wave energy in the adjacent bay in a manner comparable to that of classical cuspate spits.
The dominant feature of the Holocene transgression in Biscayne Bay has not been the preservation of the transgressive history in successive sheet-like deposits, but rather the erosion and redistribution of products of either present or previous deposition in patch-like accumulations.