SEA-LEVEL CHANGE AND STORM-SURGE DEPOSITION IN A LATE HOLOCENE FLORIDA SALT-MARSH

A 300-km length of west Florida's coastline is dominated by an open-ma rine coastal marsh system, Located along the central part of this sedi ment poor region, Waccasassa Bay is presently a broad, shallow embayme nt rimmed by an expansive Juncus roemerianus salt marsh, In this syste m, sediments were first deposited in a forested swamp from similar to 4400 to 1800 cal yr BP, Contemporaneously, large oyster reefs grew in the embayment near a paleo-river mouth. These deposits indicate a peri od of slow sea-level rise during the Late Holocene that correlates wit h the establishment or regression of other west Florida coastal system s. Circa 1800 cal yr BP, a rapid transgression of 2-4 km led to salt m arsh growth over the former swamp, and a brackish marsh developed over a previously unflooded upland surface. Calculated rates of shoreline retreat are 10 to > 20 m/yr at this time and compare to < 3.0 m/yr dur ing most of the Late Holocene. Timing of the event in Waccasassa Bay c orresponds with transgressive sequences in other Atlantic and Gulf coa st systems and supports a relative sea-level fluctuation ca. 1800 cal yr BP. Following this event, sediments overstepped during the transgre ssion were reworked onto the low-lying tidal marsh, Rapid accretion al ong the shore edge outpaced sea-level rise and led to the local develo pment of high intertidal and supratidal levees, Evidence indicates tha t storm-driven surge was the principal mechanism of sediment transport for these deposits, In Waccasassa Bay, storm surge deposition has pla yed a significant role in marsh surface accretion, distinguishing the system from more typically tide-dominated wetlands, Within the past si milar to 150 years, most of the supratidal environment became tidally inundated and there was a widespread transition into the modern salt m arsh. This change and evidence from adjacent portions of the Florida c oast implicate a rise in sea level within the past several hundred yea rs.