Petrology and depositional environment of the limestone lenses of the Central Plain Group (Oligocene): Antigua, British West Indies

Includes bibliographical references.Includes illustrations and maps.A petrographic and stratigraphic study of the calcareous lenses and associated beds of the Central Plain Group is the basis for the interpretation of the deposi- tional environment of these most-marine parts of the mixed marine and non-marine rocks of the group. Three distinct geologic regions make up the island of Antigua: the southwestern mountains, the Central Plain lowlands, and the rolling hills of the northeastern limestone region. Beds in all units dip generally toward the northeast, but range from 1°SW to 22°NE; dips decrease from southwest to northeast. Varied strike and reversals in dip occur locally near igneous intrusions or faulting. The Central Plain Group consists mainly of marine and terrigenous agglomerates and tuffs, with subordinate cherts and limestones. These tuffs are mostly water-laid, fine pyroclastics and some contain intraclasts of carbonate material. Several calcareous tuffs have been mapped by previous workers as limestones. Also, several chert beds have been shown to be silicified limestones and calcareous tuffs. Several sections were measured, but most are inadequately exposed for this purpose. These were studied by sampling the lithic succession and estimating thickness of the carbonate units. Principal faunas of the limestone lenses were collected, and identified from the literature and reference specimens. Thin-sections were used to study the petrographic and sedimentologic features of the Central Plain Group. The carbonate units are primarily composed of lime muds and skeletal sands, with corals and other reefdwelling organisms being locally abundant in the form of small patch reefs or coralline biostromes. X-ray analyses of acid-resistant materials show that the terrigenous sediment component consists mainly of quartz, oligoclase, and clay. The Central Plain Group was deposited during a period of unstable sea levels that oscillated as variations on a prevailing long-term trend of marine transgression. The best indicators of this are the transgressive patch reef sequences which contain deposits ranging from shallow-water carbonate mud flats to deeper-water, stable patch reefs dominated by large head corals.M.S. (Master of Science