Stratigraphy and petrology of the Antigua Formation (Oligocene) : Antigua, British West Indies

Includes bibliographical references.Includes maps.The purpose of this study is a detailed stratigraphic and petrologic analysis of the Antigua Formation, and an interpretation of its depositional environment. The island of Antigua has three main physiographic divisions: the southwestern volcanic hills, the Central Plain, a lowland that includes both terrestrial and marine deposits (the Central Plain Group), and the moderately hilly northeastern third of the island, the Antigua Formation. The rocks of the Antigua Formation dip northeasterly from horizontal to 20°, averaging about 8°. Local reversals in the attitudes along the north coast were caused by small olivine basalt intrusions. Past authors have said the Antigua Formation is 450 meters thick, but recent work gives a new figure of 500-550 meters. The Antigua Formation consists mainly of closely compacted marls either white or pale yellow, with numerous shells, corals and large foraminifera. There are also interbedded tuff units which have been shown to be reworked and water-laid. The limestone units can be divided petrographically into three major lithofacies: a biomicrite facies, a poritid biomicrite facies, and a foram biomicrite facies. The measured sections are distributed along depositional strike; apparently there was little migration of lithofacies in this direction, with the result that the sections do not correlate readily. 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 sediments of the formation. A petrographic analysis of the samples shows there is no dolomite in the Antigua Formation and this has been reconfirmed on 20 percent of the samples using the Gensmer (1977) method. X-ray analyses of acid-resistant materials show the terrigenous sediment to consist of clays, quartz, feldspar and cristobalite (in one location). Because the Antigua Formation was deposited by a transgressive sea, the depositional environment was changing with time. It can be shown that at various times the Antigua Formation has transgressed through reef, forereef and upper island-slope environments.M.S. (Master of Science