Sedimentology, echinoid palaeoecology and palaeobiogeography of Oligo-Miocene Eastern Caribbean limestones

Abstract

The Upper Oligocene Antigua Formation comprises a rapidly deposited sequence of limestones developed in an oceanic-arc setting immediately before cessation of subaerial volcanism. Ten sedimentary facies are recognized within it, ranging in depositional environment from shallow sublittoral to deep bank-slope. Their present outcrop patterns largely reflect original lateral facies variation across the ancient bank margin. The latter was mainly depositional in character, with bank-top often passing rather gradually into the upper bank-slope. Although a similar range and number of sedimentary facies are recognized within the Lower Miocene Anguilla Formation, bank-edge reefs are better-developed and the upper bank-slope shows evidence of sediment bypassing. Palaeoautecological and palaeosynecological studies of echinoid faunas can provide useful information on palaeoenvironment. Twenty genera and thirty-two species of echinoids are represented in the Antigua and Anguilla Formations. Of the species, five are entirely new, seven more are described for the first time from the Lesser Antilles, and a further three have their known ranges extended within the region. Of the genera, four were previously unrecorded as fossils from the Lesser Antilles, one, Irenechinus, being described for the first time outside Australasia. Twelve genera have their diagnoses and/or synonymies significantly modified from previously accepted usage. A new subgenus is erected for the earliest-known Caribbean representative of Tripneustes. Sedimentological evidence of hurricane activity within the Antigua and Anguilla Formations indicates that mid-Cenozoic sea-surface temperatures in the equatorial Atlantic were similar to those of the present day. Cenozoic echinoid biogeography supports an arc-migration model for the origin of the Caribbean Plate, indicates that speeds of shallow marine currents have increased during the Neogene (with the most significant changes affecting eastward Undercurrents), and suggests that a proto-Gulf Stream was in operation by Eocene times.