USE OF CHIRP SEISMIC DATA IN DETERMINING SUBTLE DIFFERENCES IN HOLOCENE INFILL OF ISOLATED BASINS WITHIN HILLSBOROUGH BAY, FLORIDA

Five small (0.5 – 2 km across) isolated basins were mapped using 40 trackline km of CHIRP seismic data in Hillsborough Bay, northeastern Tampa Bay. The basins represent local depocenters and contain a record of the transition from pre-Holocene lacustrine deposits to the present-day estuarine depositional system. Seismic facies analysis shows differences in reflection character that have been used to define a pattern of four seismic units infilling these basins. These have been correlated to sedimentary units obtained from nine vibracores, and differences in sediment characteristics such as carbonate content and abundance of shell fragments account for the seismic facies. The CHIRP is able to detect subtle differences in sediment characteristics as different reflection characters, in addition to sharp boundary contacts within the cores. The deepest unit identified is characterized by a narrow band (~ 1m thick) of strong but chaotic reflections that correlate to a tan, carbonate-rich, mud unit in the cores. Sediments matching this description have been identified as a lacustrine deposit between 11-21 ky old in central Tampa Bay. C-14 dating of wood fragments found within two of the cores should be able to constrain the age of these Hillsborough Bay ‘lake’ deposits. Above this unit the CHIRP data is nearly reflection free for 1-2 m and this correlates with a carbonate-poor mud unit that is nearly devoid of shell fragments. This unit likely represents the Holocene transgression and the switch from a freshwater to a more marine depositional environment. The next seismic facies is marked by a strong, discontinuous reflector that correlates to a shell layer in the cores. This may represent a storm deposit, but it is not continuous throughout each of the basins. In areas where this distinct reflection is absent, a darker more chaotic reflection zone occurs. The uppermost seismic facies is also a reflection free zone for the most part varying in thickness but generally < 1 m. This correlates to a shell-free, muddy deposit at the top of the vibracores. The use of CHIRP seismic facies, with ground-truth from the cores, expands its usefulness as an interpretive tool, allowing detection of subtle differences in sediment characteristics that are caused by variations in depositional environment during Holocene evolution of these basins.