THE EFFECT OF LATE HOLOCENE SEA-LEVEL RISE ON THE GEOMORPHOLOGY OF SOUTHERN ESTERO BAY, FLORIDA

Holocene sea-level rise (SLR) has a direct effect on coastal systems of Southwest Florida. Rates of Holocene SLR began decelerating 5500 ybp from 23 cm / 100 years to 4 cm / 100 years in the last 3200 years. However, SLR rates in the last century measure between 20-40 cm / 100 years. If this rate were to persist, SLR would outpace sedimentation rates and prevent continued seaward progradation of the mangrove-based system. This system provides habitat for many endangered species, and forms a protective barrier for coastal development. Holocene history of southern Estero Bay was investigated to better understand future coastal response to SLR. Additionally, while the geomorphologic history of northern Estero Bay is well known, the morphology of the south is considerably different and not understood. Our work, while preliminary, permits a comparison of the two. Thirteen sediment vibracores were taken along 2 parallel on-/offshore transects. Standard practices of stratigraphy and sedimentology were used to interpret paleoenvironments. Stable carbon and nitrogen isotopic analyses of organic-rich facies, to infer floral changes, and faunal analyses, to infer salinity changes within the estuary, are pending. The relative stratigraphic position of the basal marine flooding surface is progressively higher in more proximal cores, suggesting initial Holocene transgression occurred linearly from west to east. Vermetiform gastropod- and oyster-dominated reef development is common following the transgression, the former occurring exclusively in more distal positions along the transects. Successional change from vermetiform- to oyster-domination is found in at least one core, indicating reduction in salinity through time. A progradational, regressive package follows in 4 cores with subtidal facies followed by intertidal and supratidal facies. Elsewhere in the region the progradation follows a patchwork mosaic, but the pattern in southern Estero has yet to be determined. The results of this study parallel those seen in northern Estero Bay. Unlike northern Estero, the degree of progradational occlusion of the southern bay is nearly complete, suggesting riverine sedimentation plays a more significant role here. These results demonstrate that progradational morphology of the coast is closely dependent upon rate of SLR.