Document Type

Dissertation

Date of Award

1971

Keywords

Reefs, Deltas, Oyster populations, Suwannee River (Fla.), Landscape architecture

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Don L. Kissling

Second Advisor

James E. Sorauf

Third Advisor

James R. Beerbower

Series

Science and Mathematics

Abstract

Three major oyster reefs located off the mouth of the Suwannee River on the west coast of Florida form narrow linear ridges that extend subparallel to shore in directions varying between north and northwest. Two of the reefs, Suwannee Reef and West Gap Reef, form part of an arcuate, convex-seaward reef barrier that extends southeast for 20 km along the coast. The third major reef, Lone Cabbage Reef, is located in a coastal embayment about 4.9 km east of the barrier. Lying in shoal water, these three reefs are semidiurnally exposed and covered by ebb and flood tides. The reefs are dissected by tidal channels and have a maximum relief of 3 m. No evidence of any buried reef structures dating back to the Pleistocene was found on the delta platform.

The three oyster reefs are composed of approximately 70 percent by volume shells and shell fragments of Crassostrea virginica, the common American oyster species. Quartz sand forms approximately 12 percent by volume of the reefs, and terrigenous mud approximately 7 percent. The sources of sediment for the reefs are the reef themselves, local patch reefs and cluster concentrations located principally east of the reefs, and the Suwannee River, which transports as its bedload extensive amounts of quartz sand. Morphologically, the emergent surfaces of many of the shoals that compose the three reefs can be divided into three classes: massed clusters, flatland areas, and oyster bars. Active oyster bars are always on the eastern sides of the shoals and are fed principally from local biogenous sources east of the reefs. Imbricate shell structure occurs in all three areas of the emergent reef surfaces but is most common on the oyster bars. The imbricate shell structure helps to stabilize the reef surfaces against tidal currents, which may attain velocities of 60 cm per second and more, and it is composed predominantly of the right valves of Crassostrea virginica. Segregation of left and right valves is found in the vicinity of the oyster bars.

Size analyses indicate that bulk reef and surface imbricate reef sediments are fine-skewed and have primary modes in the -4.3 ϕ to -2.7 ϕ and -1.9 ϕ to -1.1 ϕ size ranges. These modes mainly represent large and medium sized valves plus the larger fragments, and the smaller right valves and fragments, respectively. A lesser mode in the 1.5 ϕ to 3.0 ϕ size range indicates a quartz sand admixture. Sediments in the interreef areas of the delta platform are composed of 75 to 95 percent by weight quartz sand and are predominantly fine-skewed. Redox potential measurements of these sediments were consistently negative, while measurements made on the reefs themselves were positive.

The properties of salinity, temperature, and sea water pH fluctuate at the reefs according to the tides, tending to increase with flood tide and decrease with ebb tide. Bottom trends in salinity, temperature, and pH do not conform to surface trends because of the hypopycnal conditions at the mouth of the Suwannee.

Four factors judged to be of paramount importance in determining the shape and location of the Suwannee delta oyster reefs are salinity, turbidity, tidal currents, and depth. Because turbidity is low, Suwannee Reef and other smaller reefs are located close to the mouths of the distributaries. The narrow elongate shape of the reefs affords maximum utilization of the tidal currents for feeding, respiration, and spawning. The reefs forming part of the 20 km-long barrier appear to lie on a series of sand flats built up 3700 to 3100 years B.P. when the rate of postglacial sea level rise slowed.

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