University of Massachusetts, United States

Lithologic units transitional from the San Sebastian Formation to Lares Limestone were examined in northern Puerto Rico. As in previous studies, stratigraphic, sedimentologic and paleontologic data allowed for interpretation of marginal marine to reef depositional environments as part of a marine transgression, but detailed data further define changes in relative water depth within the Lares Limestone. Fossils in the lower transitional units are identified as abundant plant material, bivalves of the families Pectinidae, Osteridae and Arcidae, gastropods of the families Naticidea, Cypraeidae and Volutidae, and partial specimens of foraminifera of the Miliolidae. Massive corals, robust erect bryozoa and abundant foraminifera of the Miliolidae dominate the upper units. Common coral genera include Porites, Goniopora, Cladocora, Alveopora, Antiguastrea, Acropora, Montastrea, Colpophyllia, Favites and Leptoseris among others. Calcified siphon sheaths of Kuphus bivalves extracted from stratigraphic units containing reef-building corals yielded (super 87) Sr/ (super 86) Sr isotope values ranging from .708087 to 708108 within the Lares Limestone and identify the age of reef building at approximately 27my within the Chattian stage of the Oligocene. The San Sebastian and Lares formations of Puerto Rico have been identified in earlier publications as Eocene to Oligocene based predominantly on foraminifera. The strontium chemostratigraphic marker correlates to Oi2b via the 27my date obtained from tephra in ODP site 999, thereby allowing for correlation of deep water tropical events with shallow, reef-building episodes. Isotope data extracted from foram shells allow for determination of ice volume and sea surface temperatures for the tropical Atlantic for this time period. Abstract 113587 modified by 156.56.66.129 on 7-11-2006