Abstract

A 300-km length of west Florida's coastline is dominated by an open-marine coastal marsh system. Located along the central part of this sediment-poor region, Waccasassa Bay is presently a broad, shallow embayment rimmed by an expansive Juncus roemerianus salt marsh. In this system, sediments were first deposited in a forested swamp from approximately 4400 to 1800 cal yr BP. Contemporaneously, large oyster reefs grew in the embayment near a paleo-river mouth. These deposits indicate a period of slow sea-level rise during the Late Holocene that correlates with the establishment or regression of other west Florida coastal systems. Circa 1800 cal yr BP, a rapid transgression of 2-4 km led to salt marsh 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 during most of the Late Holocene. Timing of the event in Waccasassa Bay corresponds with transgressive sequences in other Atlantic and Gulf coast systems and supports a relative sea-level fluctuation ca. 1800 cal yr BP. Following this event, sediments overstepped during the transgression were reworked onto the low-lying tidal marsh. Rapid accretion along the shore edge outpaced sea-level rise and led to the local development of high intertidal and supratidal levees. Evidence indicates that storm-driven surge was the principal mechanism of sediment transport for these deposits. In Waccasassa Bay, storm-surge deposition has played a significant role in marsh surface accretion, distinguishing the system from more typically tide-dominated wetlands. Within the past approx. 150 years, most of the supratidal environment became tidally inundated and there was a widespread transition into the modern salt marsh. This change and evidence from adjacent portions of the Florida coast implicate a rise in sea level within the past several hundred years.

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