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Regional Upwelling as a Major Control in Development of a Miocene Heterozoan-Dominated Carbonate System in a Tropical Setting, Puerto Rico


Heterozoan carbonate systems are increasingly being recognized as important petroleum reservoirs in the rock record, yet models for such systems are lacking, especially those that developed in low latitude settings. A Middle-Late Miocene carbonate ramp system in Puerto Rico provides ideal outcrops of heterozoan-dominated carbonates that were deposited in a tropical setting. Three sequences (DS1, DS2, and DS3) developed in response to relative sea-level fluctuations. Each sequence is characterized by basal heterozoan-dominated facies (e.g. benthic foraminifera, molluscs, red algae) that grade upward to a mix of heterozoan and photozoan (e.g. cool- and turbid-water corals) facies at the top. DS1 transgressive deposits consist of in-place Kuphus incrassatus bivalves within a soritid foraminifera facies, and Amphistegina sp. foraminifera packstone interbedded with Kuphus and oyster facies. Upper DS1 consists of Montastrea sp. coral debris flows followed by in-place Goniopora sp. and Porites sp. coral reef that can be traced down paleoslope indicating deposition during highstand and relative sea-level fall. DS2 transgressive deposits consist of soritid and bivalve packstone facies that grade upward to facies containing corals (Montastrea sp., Porites sp.) deposited during highstand and relative sea-level fall. DS3 transgressive deposits consist of soritid and bivalve packstone facies, which grade upward to red algae boundstones and coral-rich facies (Montastrea sp., Porites sp., and Agariicid sp.) deposited during highstand and relative sea-level fall. Upwelling has been documented as a regional process in the Caribbean during the Middle and Late Miocene. The dominance of heterozoans and local photozoan corals tolerant of cool and turbid conditions in our study is consistent with upwelling of nutrient-rich and cooler water. Although upwelling appears to have been persistent throughout deposition, the presence of photozoans only in the highstand and regressive portions of sequences suggests a relationship of upwelling to relative sea level, with highest intensities during transgressions. Shallow water heterozoan systems that form in tropical settings require special conditions; in our study upwelling and sea level were the major controls and their interaction resulted in predictable facies partitioning. These results have application to heterozoan reservoir systems, such as those in offshore Vietnam and Venezuela.