St. Croix, U.S., Virgin Islands
Frost, Stanley H., 1939-
M.S. (Master of Science)
Department of Geology
Reefs--Virgin Islands of the United States--Saint Croix||Geology--Virgin Islands of the United States--Saint Croix
During recent road-cut excavation and quarrying operations on south-central St. Croix, a sequence of carbonate sediments was discovered to unconformably overlie the Middle Miocene Kingshill Marl. Initial investigation of this unit by Curth, Gerhard, Multer, and Frost (1974) suggested a reef-tract depositional model and a Pliocene age for these deposits. The purpose of this thesis is to reconstruct the stratigraphic, ecological and sedimentological relationships of this reef tract. Detailed stratigraphic sections were measured and intensively sampled in the field. The principal species of reef biota were then identified by comparison with the literature and reference collections at Northern Illinois University. Thin-section analysis and scanning electron microscopy were used to study the petrology and sedimentology of the sediments generated by these reefs, while the environmental relationships were inferred by analogy with modern reef communities of St. Croix. The Pliocene rocks in the study area have a total thickness of approximately one hundred feet. These are divided into lower and upper formations, the Annaberg and Blessing, respectively. Three cycles of deposition are distinguished in these two formations. Each depositional cycle is separated by slight angular unconformities and/or thin zones of montmorillonitic clay. Sporadic movement of a graben-like fault system, possibly coupled with an eustatic change of sea level, can explain these cyclic periods of deposition. Each formation contains a gradation from, lagoonal sediments into coarser reefal sediments. Lagoonal deposits of both formations contain a highly diversified mixed assemblage of foraminifera and a diverse mollusk and solitary hermatypic coral assemblage. The reefal deposits are characterized by a high-turbulence assemblage of foraminifera, mollusks, colonial hermatypic corals, and boring organisms. Infilling of borings by micritic mud and differential solution of the aragonite coral skeletons offer a rare opportunity to study the bioerosional activities of a variety of organisms. Intensity of bioerosion decreased toward the lagoonal environment. Petrographically, the rocks in the Pliocene formations are mostly biosparites and recrystallized biomicrites which contain varying amounts of micritic pellets. A shallow lagoon and fringing reef depositional model can be inferred from the petrology of these sediments. The paleoecological reconstruction resulting from this study has an excellent modern analog at Great Pond Bay, on the southeastern coast of St. Croix. A fringing coral reef spans a lagoonal embayment in both examples. Back-reef coarse carbonate sands grade into lagoonal carbonate sands and muds. Seagrass banks and mangrove swamps dominate the near-shore environment.
Behrens, Gene K., "Stratigraphy, sedimentology and paleoecology of a pliocene reef tract : St. Croix, U. S., Virgin Islands" (1976). Graduate Research Theses & Dissertations. 5068.
vi, 93 pages
Northern Illinois University
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