|John G. Cargill, IV, Department of Geology, University of South Florida, Tampa, FL
John Nash, Department of Geology, University of South Florida, Tampa, FL
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, FL
Beach and nearshore sediments on the barrier islands from Anclote Key to Venice inlet display a wide range of textures and compositional variation. Sediments from the backbeach, foreshore, shallow surf zone (-1m) and the nearshore (-5m) were taken at irregular spacings from the mouth of Tampa Bay to the Venice Inlet over a few days of calm wave conditions in 1994. Samples from Anclote Key to the mouth of Tampa Bay were collected in the same manner in 1995. The short time periods and low energy conditions provide for a reasonable means for comparing these surface sediments. Overall, two hundred nine samples were taken from 49 transects on 16 barrier islands; inlets and tidal deltas were not included in the sampling plan. Mud, sand and gravel content were determined, and then mean grain size, standard deviation, skewness and kurtosis were calculated from settling tube analyses. Calcium carbonate content was also determined for the total sample and for the sand fraction only.
The objective of the study was to determine if there are any patterns to the sediment parameters along the active part of the barrier and nearshore. Because the sediment along this coast is bimodal with the modes being fine quartz sand and shell gravel, there are few, if any patterns to the sediment granulometry or composition. Previous analyses of sediments from Pinellas County have shown no patterns (Hogue, 1991). This study reinforces those findings, although a few minor trends can be seen in each of the counties sampled. The change from a limestone dominated pre-Holocene "basement" under the Pinellas County barriers to a sandy pre-Holocene "basement" of Hawthorn strata in Manatee and Sarasota counties might be a mechanism for differing trends.
As is typically the case for sediments along this coast, the only significant variable in the sediment composition and/or texture is the shell content. Mud is rare and appears only in samples at the 5 meter water depth. Only seven of sixty-two samples that contain mud contain greater than 5% by weight; most are less than 2%.
Although not common, there are some noticeable patterns in the geographic distribution of the sediment parameters in Sarasota and Manatee counties. Probably the most striking one is the distinct increase in mean grain size at about 30km in Siesta Key (Figure 1). Another unusual aspect of this change in mean grain size is that the sorting values do not show the typical decrease that coincides with increasing grain size. Sorting is about the same throughout the study area with the exception of a couple of samples. Also coincident with the change in grain size is a major increase in carbonate content (Figure 2). It would be expected that this increase in carbonate be associated with a significant increase in gravel. Such is not the case except for a modest gravel increase in the surf zone samples. There is a quite similar increase in carbonate in the backbeach and foreshore samples also. The most plausible explanation for this unusual pattern is the presence of the Point of Rocks area which is a Pleistocene beach rock complex located at the change in texture and composition. Sediments to the south are influenced by the shelly beach rock which is providing sediment source. In addition, the presence of the beach rock outcrop provides for a barrier to longshore transport of sand from the north, thus limiting the fine quartz sand fraction.
The most obvious pattern in Pinellas County occurs at Treasure Island. A significant grain size increase occurs at about 48km (Figure 3), coupled with a decrease in sorting. Further, carbonate content in the sand is over 50% greater at this point than is found anywhere along the Pinellas county coastline (Figure 4). Several renourishments of the beach over time with carbonate rich sand from Passe-A-Grille is the probable cause for a concentration of shell material at Treasure Island. A small increase in carbonate content occurs at Honeymoon Island, which is due to the increase in limestone rubble from the dredge spoil placed in 1968. There are no other significant patterns along the Pinellas county coastline. Sorting is relatively random, whereas the mean grain size is fairly constant throughout the study area. The fact that most of Pinellas county's beaches have been nourished in the recent past influences and overprints the natural distribution of sediments, and the textural and compositional trends on the beach and in the surf zone.
Textural and compositional patterns in the nearshore sediment do not reflect the trend of the beach/surf zone in either of the two study areas. Here there is no trend with the major deviations being caused by the sporadic presence of large quantities of coarse biogenic material. This may reflect shell lag concentrations on a bedrock surface. The general character of the sediment in this depth is fine and well-sorted sand with very small amounts of carbonate except where shell gravel is present.