|Ann E. Gibbs, U.S. Geological Survey, Palo Alto, CA
Guy Gelfenbaum, U.S. Geological Survey, St. Petersburg, FL
Sean Leathem, U.S. Geological Survey, St. Petersburg, FL
Nancy DeWitt, Department of Marine Science, Eckerd College, St. Petersburg, FL
A reconnaissance study on bedrock surface geometry and overlying sediment thickness across Florida's west central gulf barrier coast was conducted between 21-30 August, 1995. Seventy four sites were jet probed along eleven cross-shore transects located between Anclote Key and Casey Key (Figure 1). Back barrier sites were also included at three locations (Belleair Beach, Indian Rocks, Indian Shores). Profile length was limited by water depth; sites were probed at various distances along the transect from as close to shore as possible to no greater than -6.5 m MLLW. Minimum depths were recorded at seven sites where refusal was not reached; five where we were limited by maximum pipe length (9.32 m) and two where the probe pipe stuck hard in the nearshore sediment.
Water depth, sediment thickness, and depth to refusal were corrected to MLLW and plotted relative to the shoreline. Preliminary data analysis reveals the greatest variation along all profiles occurs within 300m of the shoreline, however, several trends are discussed below with respect to the overall transects.
Water depth, or offshore slope, generally increases to the south, with two exceptions in the Belleair Beach and Blind Pass areas where the slope is anomalously steep. The seafloor dips gradually gulfward off Anclote, Caladesi, and Indian Rocks, whereas off Belleair Beach, Blind Pass, Longboat Key, Siesta Key, and Casey Key the surface steepens dramatically offshore. An abundance of in situ sediment around Anclote and Caladesi, and the prevalence of artificially nourished beaches in the Indian Rocks area, may partially support this trend.
Sediment thickness tends to decrease to the south and thins offshore to the west. The greatest sediment thickness (>8 m) were found along the Blind Pass and Anna Maria Island transects; the thinnest offshore Casey Key and Indian Rocks. Nearshore topographic lows off Anclote, Caladesi, Belleair, Indian Rocks, Blind Pass, and Longboat Key also support greater sediment thickness.
Considerable variation in the depth to refusal exists within the study area. Overall, depth increases to the south with the exceptions of Blind Pass and Belleair Beach transects, which both exhibit anomalously greater depths to refusal (Figures 2, 3, 4). Seven out of nine profiles (Anclote, Caladesi, Belleair, Indian Rocks north and south, Blind Pass, and Longboat) reveal a 1-3 m topographic low within 100 m of the shoreline. Gulfward of this "low", most profiles shoal gradually in an offshore direction before resuming a deepening trend in the refusal surface. In contrast, the Belleair transect shows a distinct, 1.5 m "high" at 37m, then deepens dramatically to -9 m between 550 and 716m before gradually rising back up to -6.6m at the end of the profile. Two of the six profiles, IRBnorth and Longboat Key, show a second, larger magnitude (~4m) break in slope between 115-152 m and 163-227 m, respectively. Siesta and Casey profiles display a gradual, more linear trend, with no abrupt topographic changes between probe sites. Linear regression analysis of the depth to refusal profiles shows that most transects north of Tampa Bay have similar slopes (ave. 0.0006), while the three profiles to the south are an order of magnitude greater (ave. 0.003).
Jet probing is a quick and inexpensive means of obtaining general nearshore geologic data. Additional field work is proposed for summer 1996 to fill existing gaps and improve the alongshore distribution and nearshore sample density. This additional information, coupled with the seismic and core data, should allow us to determine more clearly the continuity of and relation between features, such as the nearshore topographic lows and slope breaks, identified during this reconnaissance study.