|Thomas W. Ferguson, Department of Geology, University of South Florida, Tampa, FL
Richard A. Davis, Department of Geology, University of South Florida, Tampa, FL
Sea level curves applied to the Gulf coast of Florida suggest that the eustatic sea level has risen and fallen numerous times during the last 5 million years. As sea level changes, so do the coastal environments. Areas which were depositional become erosional and vice-versa. The result for the west-central Florida coast is a lack of Pliocene or Pleistocene sediment record in the barrier island complexes. The sediment which has been eroded could have been deposited in offshore basins during regressions and reworked and redeposited during transgressions. The lack of Pliocene or Pleistocene sediments in the barrier complexes, however, restricts the ability to determine local relative sea level and depositional environments.
Deep pre-Holocene channels are the most likely locations where Pliocene or Pleistocene sediments may have accumulated and subsequently preserved. During regressions, these channels may have experienced erosion, however, this erosion was likely less than that experienced by areas which were subaerially exposed. The mouth of Tampa Bay is one such channel.
As Florida's largest estuary, Tampa Bay acts as a sediment trap for sediment carried by rivers such as the Hillsborough, Alafia and Manatee. Because physical energy within the estuary is low, little sediment is transported from upper Tampa Bay to the mouth. For this reason, the sediments underlying the mouth are coastal marine in origin and are likely to have recorded transgressive and regressive sequences.
Depth to Miocene bedrock varies across the mouth from 142 feet on the north tip of Mullet Key to approximately 70 feet on Anna Maria Island. One complete and undisturbed core has been collected from Mullet Key (Fort Desoto Park core) and two additional cores will be collected from northern Anna Maria Island and near Pass-A-Grille respectively in mid-June. Each of these cores will be photographed and logged for lithology and identifiable fauna. Subsequently the cores will be sampled and analyzed for grain size distribution. Preliminary observations have indicated the upper 13 meters to be a brown to white fine grained quartz sand with varying amounts of whole and fragmented shells. These sediments are interpreted to represent shore face to nearshore deposits. Shell concentration and grain size are indicators of current energy. Zones of high shell fragment concentration (Box 8-16ft, 300-308 cm; Box 16-24ft, 487-550 cm) might indicate channel lag or storm events.
The upper 13 meters of the core visually appear relatively uniform in depositional environment with a few exceptions as noted above. From 13 meters to 28 meters below land surface, clay content in the sediment increases and shell concentration is highly variable suggesting a mixed energy environment: subtidal nearshore or shallow deltaic. From 28 meters to 32 meters the core is clay and silt dominated with little sand or bivalve shells. The dominant fauna is a bulbous gastropod, 0.5-1.0 cm in length. The gastropods are scattered and in thin beds. The bed assemblages appear to be catastrophic as many different sizes are present. The gastropod is yet to be identified, however, the environment is interpreted to be low energy, estuarine and either hypersaline or fresh water. Between 32 and 39 meters is another section of shelly sand, similar in appearance to the sediments from 13 to 28 meters.
The final 4 meters (from 39 to 43 meters below land surface) is a grey soft sandy clay that grades into a white, poorly-cemented limestone. This section, including the clay, is likely the top of the Miocene deposits; possibly either the Peace River Formation or the Arcadia Formation. The change between the shelly sand and the clay appears abrupt and may indicate a disconformity.
Interpretations based on the data will facilitate the production of lithostratigraphic and biostratigraphic columns. Sea level data and absolute dating will also be obtained in order to date high-frequency sea level fluctuations. All stratigraphic interpretations will be incorporated into a sequence stratigraphic framework.
Table 1: Project core information.