Document Type

Dissertation

Date of Award

1972

Keywords

Marine sediments, Florida, Sedimentation and deposition, Reefs

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

James R. Beerbower

Second Advisor

James E. Sorauf

Third Advisor

Paul P. Enos

Series

Science and Mathematics

Abstract

The Dry Tortugas, a horseshoe-shaped complex of carbonate banks and coral reefs, is located at the southern terminus of the Florida limestone shelf. The complex rises to the surface waters from a drowned Pleistocene surface that forms a circular platform having a general depth of 17-21 m.

Three basic biogenic buildups (facies) comprise the reef complex: 1) detrital lagoonal bank, 2) Montastrea reef bank and 3) Acropora palmata reef. These facies lie adjacent to one another and are also present in vertical succession as individual growth stages of varying thickness and lateral extent. A zone of Acropora gervicornis is developed as a transition between the Montastrea and A. palmata growth stages.

The present organic assemblages and topography bear evidence of dominantly lateral progradation and cumulative storm effects that are linked to the slow eustatic sea-level rise for the past several millenia. Long-continued storm degradation is manifested by 1) continuous removal of A. palmata and its replacement by storm-resistant coralline algae and Millepora sp. to produce truncated rocky surfaces, 2) abundant reef rubble, 3) erosion of spur-grooves, and 4) development of intertidal rubbly reef flats.

Sediments ranging from cobble-sized rubble to medium silt are composed of Halimeda, coral and mollusc grains: coralline algae and foraminifers are present in minor amounts. Variations in texture and constituent particle composition are interpreted to be mainly a result of mode of sediment transport and effect of grain shape. Broadly-defined grain size populations produced by three modes of transport have characteristic assemblages of constituent particles. The populations include a gravel-sized surface creep population, sand-sized saltation population and very fine sand-to silt-sized suspension population. Strong mixing occurs between the gravel and sand population on the storm-degraded shoals, and between the sand and silt population on the lagoon bottom. Sands flanking the reefs and reef banks show minimum mixing hence good degree of sorting. Incongruous mixtures of the in-place fraction and varying proportions of the transported populations constitute detrital lagoonal banks as a substrate stabilized by seagrass and coral growth. The gravel-sand and sand-silt mixtures are related to deposition under highly variable energy conditions. Variability in energy conditions does not cause strong population intermixing on beaches. From the same reason, beach sediments show a high degree of sorting in all size grades from cobbles to fine sand.

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