|Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, FL
The barrier coast between Anclote Key on the north and Casey Key with the included tidal inlets (Figure 1), comprises the study area for this project (Pinellas, Hillsborough, Manatee and Sarasota Counties). This coastal reach includes 15 barrier islands and a like number of inlets. The islands include both wave-dominated (Anclote and Casey Keys) and drumstick (Caladesi Island and Siesta Key) morphologies, and they range in age from only decades (Bunces Key and Three-Rooker Bar) to millenia (Caladesi Island and Siesta Key). Lengths of the barriers range from only 2 km (Three-Rooker Bar) to nearly 30 km (Sand Key).
The tidal inlets also range widely in size and morphology; some have even been closed during the past decade. Both natural and human-modified inlets are included in this coastal segment. The smallest is Blind Pass and the largest is Egmont Channel at the mouth of Tampa Bay. Tidal deltas also span a wide range in both size and morphology. Some of the inlets lack flood tidal deltas such as Egmont Channel or have inactive flood deltas (Blind Pass and Midnight Pass). Other have well-developed flood deltas (Hurricane Pass and Johns Pass) which have been formed in conjunction with severe storms. The ebb-tidal deltas range through wave-dominated (Blind Pass), mixed-energy (Big Sarasota Pass) and tide-dominated (Bunces Pass and Pass-a-Grille). Most of the inlets have exhibited important changes over the past century; the extent of well-documented history.
The primary aim of this portion of the overall cooperative project is to understand the geologic history of barrier/inlet development and the more recent history (last century) of coastal change along this coastal reach. Specific objectives include: Reconstruct the history of sea level change since accumulation of the Miocene carbonate and phosphatic sequences and prior to the Holocene sea level rise. Determine and interpret the geologic history of the development of individual islands and incorporate these elements into a comprehensive history of the barrier/inlet system. Incorporate the barrier/inlet history and stratigraphy into the geologic framework history of the shoreface.Document and interpret the morphodynamics (cause and effects) of the barrier/inlet system over the past century.Formulate a comprehensive sediment budget for the barrier/inlet system.
Approach to the Investigation
Three basic approaches are being undertaken in order to achieve the objectives outlined above. The stratigraphy of the barrier system is being determined through various types of drilling and coring to recover relatively undisturbed cores of the stratigraphic sequences that comprise the study area. To date, cores ranging from less than 2 m up to 43 m have been recovered. This includes approximately 200 vibracores, about half of which have been taken as part of the cooperative project and the other half were taken from previous but related projects. Five long cores have been recovered using the drilling rigs of the Florida Geological Survey. They range from 16 m to 43 m with the longest one being complete and nearly undisturbed. These cores are described, sampled and analyzed to provide information that permits interpretation of the environments of deposition of the individual lithofacies. Selected material will be dated by various radiometric techniques to aid in the chronological reconstruction of the stratigraphic sequences preserved. In addition, the sub-barrier sequence will be incorporated into a sequence stratigraphic context.
The second approach is the determination of the morphologic changes that have taken place over the past 100 years or so during which we have good documentation. The changes in size and shape of the barriers and tidal inlets will be placed in a time-series context in order to determine cause and effects of these changes. Charts, maps and aerial photographs are available for this purpose.
The last approach is to quantify the changes that have taken place along the barrier/inlet system and establish rates for these changes. These data will be collected for individual elements of the study area. They can then be utilized to develop a sediment budget that includes the entire study area. Such a sediment budget will initially be applied in a hindcast mode to model the changes that have already taken place on this coast. The model will then be adjusted to this hindcast mode and then applied to forecast various scenarios for future coastal changes. In this manner it will provide invaluable information to aide in developing long-range coastal planning decisions.
As a result of the above approaches to studying the barrier/inlet system in the study area, we will provide both geologic, historic and future developments of this important and densely populated reach of the Florida Gulf Coast. The results of this project will provide important information for planning and managing his coast at all governmental levels from municipal through federal.
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Statement of Accomplishments During FT 95
1 - Sample and analyze beach and nearshore sediment through all barrier islands in the study area. Analysis to include textural parameters (moments), and composition to include siliciclastic and carbonate percentages.
Status - Completed in April, 1996
2 - Develop a comprehensive sediment budget for the study area.
Status - Unfunded during this FY but related project by Ping Wang has included considerable field data and modeling of nearshore sediment transport that will be incorporated into this aspect of the project. Most of this is included in Dr. Wang's dissertation which was completed in October, 1995.
3 - Morphodynamics and stratigraphy of Big Sarasota Pass and New Pass ebb-tidal deltas, Sarasota County, Florida.
Status - Completed in August, 1995; MS. thesis of Katherine Kowalski.
4 - Stratigraphy of washover deposits on Florida Gulf Coast barrier islands. Development of stratigraphic models for washover sequences ranging from active to ancient in age.
Status - Completed in August, 1995; MS. thesis of Peter Sedgwick.
5 - Stratigraphy and morphodynamics of Egmont Key, Hillsborough County, Florida. This island is not a true barrier but occupies a position in the proximal part of the ebb-tidal delta at the mouth of Tampa Bay and therefore plays an important role in the history of this important sediment sink.
Status - Unfunded from this project; being initiated in fall of 1995 and funded by Pinellas County and Florida DEP.
6 - Stratigraphic patterns and models of west-central Florida barrier islands. Several coring transects will permit stratigraphic models to be developed for wave-dominated barriers and both the prograding and retrograding ends of drumstick barriers.
Status - Project just beginning development in late FY 95.
7 - Stratigraphy and geologic history of Anna Maria Island, Manatee County, Florida. The location adjacent to the mouth of Tampa Bay and its drumstick character of this barrier make it important to investigate as a key element in the study area.
Status - Project completed in December, 1995; MS. thesis of John Pekala.
8 - Origin and distribution of beachrock at Point of Rocks, Siesta Key, Florida. As the only known outcrops of suspected Pleistocene strata along this coast possibly representing sea level stage 5e, it is important to understand its origin and history. This presumed exposure of the Anastasia Formation also may have played an important role in the geologic history of Siesta Key.
Status - Project just beginning development in late FY 95.
9 - Stratigraphy of tidal and fluvial paleochannel sequences beneath west-central Florida Gulf barrier islands. The most likely sites for preservation of post-Miocene and pre-Holocene stratigraphic sequences is in channels. This study investigated several such channels and found that only fluvial/estuarine channels contained pre-Holocene sequences.
Status - Project completed in August, 1995; MS. thesis of Megan V. FitzGerald.
10 - Stratigraphy of the inlet fill sequences at the mouth of Tampa Bay. The thickest sequences of post-Miocene and pre-Holocene are likely to be preserved in this deeply incised valley. Collection, analysis and interpretation of the stratigraphy of this area can provide the key to the sea level history of this important interval of geologic time for the eastern Gulf of Mexico.
Status - Project is now well underway with 5 thick cores, the best and thickest one being complete and penetrates Miocene bedrock at -136 ft (-41.5 m). At least one additional core will be taken from the north end of Anna Maria Island.
11 - Structure contour map on top of the Miocene bedrock beneath the barrier/inlet system. This surface is the one on which the entire coastal barrier system rests. It also represents the surface of what was a large carbonate platform which supports the entire State of Florida including the west-Florida continental shelf. This surface has important influence on the development of the present barrier/inlet coastal system.
Status - Completed May, 1996.