6. Barrier Island Studies
The west-central Florida barrier/inlet system may be morphologically the most complicated in the world in an environment with tidal ranges of <1 m and a mean wave height of 30-40 cm. This coast contains all barrier island and inlet types in a range of sizes and ages. Barriers include both wave-dominated and mixed energy drumstick morphologies with lengths ranging from about 2 km to more than 30 km. Inlets range from tide-dominated through mixed energy to wave-dominated. Two of the latter variety closed in the 1980s.
The investigation of this barrier/inlet system was aimed at:
- determining the detailed nature of barrier stratigraphy,
- understanding our the geologic development of the system from this stratigraphic data base, and
- determining the causes and effects of the modern morphodynamics of this system.
The stratigraphic portion of the study included detailed analysis of individual barrier/inlet elements as well as the stratigraphy of entire barrier islands and of pre-barrier depositional environments. Washover deposits are a major part of all barriers along this coastal reach. The detailed stratigraphy of these sequences shows that they do have a series of signature characteristics that permit recognition of supratidal, intertidal and subtidal portions of the sequences (Sedgwick, 1995). Preserved sequences of both flood and ebb tidal deltas as multiple inlets has enabled the development of stratigraphic models for microtidal tidal deltas (Kowalski, 1995).
The stratigraphy of Anna Maria Island showed that it originated as an elongate shoal adjacent to the sediment sink at the mouth of Tampa Bay. As the island continued to develop it prograded at the updrift or north end with extension to the south (Pekala, 1996). This study provided complementary data to previous similar investigations at Anclote Key and Caladesi Island. The capstone study of barrier island stratigraphy along this coastal reach was the construction and interpretation of nine barrier-island stratigraphic sections across nine different barrier islands from Anclote Key to Casey Key (Yale, 1997). This regional investigation showed that all of the barriers have the same basic stratigraphy, and all apparently originated at or near their present location as subtidal shoals evolving to supratidal barriers. Two themes develop from this common origin; one is that of a wave-dominated barrier and the other is progradation of the barrier, typically at the updrift position of a tidal inlet.
Pre-barrier stratigraphic investigations included:
- the origin of apparent beachrock at Siesta Key,
- the stratigraphy of inlet channel -fill sequences and fluvial/estuarine channel sequences, and
- the stratigraphy of the valley-fill sequences in the deeply incised mouth of the present Tampa Bay.
The beachrock has always been considered to be Pleistocene (Stage 5e) in age because of its similarity to the Anastasia Formation of the east coast of Florida. Radiometric dating has shown these rocks to be Holocene with a maximum age of 4 ka. Petrographic studies of numerous thin sections have revealed that the cement is meteoric in nature meaning that although the strata were deposited on a beach they were lithified by fresh groundwater percolation from overlying sediments. These sediments must have been part of a pre-existing barrier island.
The inlet sequences are characterized by shell gravel bases overlain by cross-bedded sand. The fluvial/ estuarine stratigraphy shows that late Pleistocene strata of about 32-34 ka age are unconformably overlain by Holocene strata of about 7 ka (FitzGerald, 1995).
Modern morphodynamics of the barrier/inlet system are strongly influenced by anthropogenic activities such as stabilization of inlets, construction of causeways, and dredge and fill construction in the backbarrier areas. Emphasis of this investigation was on the behavior of both natural and modified tidal inlets (Barnard, 1998). Careful analysis shows that all of the inlets have been influenced directly or indirectly by human activity, which led to a reduction in the size of the tidal prism and to the general instability of the inlets.
In summary, this barrier inlet system, although quite varied in all respects, actually displays many common attributes in the development of its individual barrier islands and tidal inlets. Their stratigraphy can be viewed in a fairly simple stratigraphic model characterized by initial upward shoaling, aggradation and then, in some cases, progradation. The pre-barrier history of this area is still not well known but we now have data to show that there have been multiple incursions and excursions of sea level preserved in a wide range of estuarine to open marine sequences. The modern system has been strongly influenced by human activities associated with the extensive development of this coast.
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