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Coastal and Marine Geology Program
Surficial Geology of the Inner Shelf off Sarasota, Florida

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Chief Scientist
David C. Twichell, U.S. Geological Survey, Woods Hole, MA
Guy Gelfenbaum, U.S. Geological Survey, St. Petersburg, FL
Gregg Brooks, Department of Marine Science, Eckerd College, St. Petersburg, FL

shaded-relief image of bathymetry

Figure 1: Shaded-relief image of bathymetry of Sarasota, FL survey. Illumination is from the northwest. Coastline and the profiles shown in figure 2 are white lines and ridge crests are highlighted by gray lines.

A sidescan sonar, bathymetric, and high-resolution seismic-reflection survey was conducted off Sarasota, Florida to describe the relationship between the inner shelf and adjacent shoreface. The survey area covers 15 km of the coast along Longboat Key and Lido Key and is centered around New Pass. It extends offshore from the 3.5-m contour to approximately the 12-m contour, a distance of 11 km. Data collected along tracklines, spaced approximately 150 m apart were used to construct a digital sidescan sonar mosaic, bathymetric map, and preliminary subsurface maps of the study area.

The bathymetry reveals a relatively steep shoreface that is interrupted by Big Sarasota Pass and New Pass and several discontinuous, low-relief ridges on the flatter offshore part of the study area. These ridges are slightly oblique to the shoreline, 1.5-3 m high, 1-2 km wide, and are broken into discontinuous segments that are 1-4 km long (Figure 1).

Subbottom profiles show that the ridges consist mostly of an acoustically transparent facies that lies on a nearly flat surface that outcrops in the troughs between the ridges (Figure 2). This surface is interpreted to be an erosional unconformity that was cut during the Holocene transgression. Subbottom profiles show several
subbottom profiles

Figure 2: 3.5-kHz subbottom profiles across one of the low-relief ridges off Sarasota, FL. Profile locations are shown in figure 1. Top profile is shoreward of this ridge. The second profile crosses the ridge where it has about 1 m relief. The third profile crosses where the ridge has about 3 m relief. Note the horizonatal reflector underlying the ridge and the transparent nature of the ridge deposits. The fourth profile is the farthest offshore, and crosses where the ridge is decreasing in relief again.

older depressions and intervening highs below the unconformity that do not coincide with the locations of the younger ridges which suggests that the older topography is not controlling the locations of the ridges that have formed since the Holocene a preliminary interpretation of the mosaic; two associated with the ridges and one with the intervening troughs. The two ridge facies include a low-backscatter (dark tones) facies on their southeastern sides and a high-backscatter (light tones) facies on their northwestern sides. The boundary between these two facies is abrupt and coincides with the crests of the ridges (Figure 3). A low-backscatter signature also represents the seaward edge of the beach face and the ebb tidal deltas, and some of the low-backscatter areas on the ridges are connected to the nearshore deposits. Analysis of sediment samples indicates that the low-backscatter shoreface and ridge material is fine-grained siliciclastic sand. Samples from the high-backscatter northwestern sides of the ridges recovered coarse shell hash. This dramatic facies difference on the seafloor is not resolved in the subbottom profiles which show the whole ridge to be acoustically transparent (Figure 2). The third facies on the sidescan mosaic occurs in troughs and consists of a discontinuous high backscatter return. Diver observations show that these areas commonly are hardgrounds that contain cemented shell hash and live Octocorals.

This detailed survey of a 11 by 15 km section of the inner shelf shows a fairly consistent relationship between the sedimentary facies and the very subdued shelf bathymetry. Additionally, the fine-sand facies that occurs along the beach face and on the ebb-tidal deltas is texturally similar to and in some cases connected to the offshore ridges. The relationship between these inner shelf ridges and the beach face and ebb tidal deltas needs to be established.
sidescan sonar mosaic

Figure 3: Sidescan sonar mosaic of the inner shelf off Sarasota, FL. Low-backscatter areas are dark tones and high-backscatter areas are light tones. Ridge crests are identified by white lines.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Framework > Surficial Geology of the Inner Shelf off Sarasota, Florida U. S. Department of the Interior | U.S. Geological Survey
Center for Coastal & Regional Marine Studies

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Updated May 06, 2013 @ 09:24 AM  (JSS)