Sequence Stratigraphy and Depositional Controls on Oligocene-Miocene Caribbean Carbonate-Dominated Systems, Puerto Rico and the Dominican Republic
University of Kansas
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The first part of this dissertation (Chapter 2) focuses on 87Sr/86Sr-derived mean ages for selected Oligocene-Miocene carbonate and siliciclastic units in Puerto Rico and the Dominican Republic. Results indicate ages of middle-late Oligocene to late Miocene (ca. 29.78–5.88 Ma) and show some significant modifications to previous chronostratigraphic studies. 87Sr/86Sr-derived mean ages allow for correlation with documented processes affecting Caribbean systems during this time (e.g., regional upwelling, warming events, coral extinctions, tectonism, and sea-level changes). The middle-early late Miocene (ca. 13-10 Ma) Ponce Limestone in southern Puerto Rico is an example of one of these systems and the focus of a detailed study presented in Chapter 3. The Ponce Limestone is composed of three carbonate sequences of shallow-water deposits that developed on a gently (<3°) dipping ramp environment in response to relative sea-level fluctuations. Facies are dominated by heterozoans and large benthic foraminifera (LBF) along with coralline red algae and cool- and turbid-water tolerant photozoan corals (Montastraea, Goniopora, Porites, and Agaricia). This facies composition is atypical of tropical shallow-water environments and indicates stressed conditions. Timing of Ponce Limestone sequence deposition correlates with a known regional upwelling event that raised nutrient levels and lowered temperatures of surface waters. Although upwelling appears to have been persistent throughout Ponce Limestone deposition, the presence of photozoan corals only in the highstand and regressive portions of sequences suggests a relationship of upwelling to relative sea level, with greater intensities during transgressions. Evidence of subaerial exposure (sequence boundaries, SB1 ~13-12 Ma and SB2 ~11-10 Ma) appear to correlate with time-equivalent unconformities identified in several other Caribbean areas, which suggests a regional (tectonism) or perhaps global control (eustacy) on Ponce Limestone sequence development. The late Miocene (ca. 6.31–5.88 Ma) Cercado Formation in northern Dominican Republic (Chapter 4) is composed of three mixed carbonate-siliciclastic prograding sequences deposited over a gently dipping (<1°) ramp during an overall relative sea-level fall interrupted by two minor relative sea-level fluctuations. Facies are characterized by exceptional high-diversity shallow-water photozoan coral assemblages (e.g., Stylophora, Pocillopora, and Dichocoenia) deposited as patch reefs. The presence of these facies in the Caribbean at that time, along with the closure of the Central American Seaway, has been used to suggest a regional decrease in upwelling intensity. Regional deposits that are time-equivalent to the Cercado Formation, however, do not show the same increase in coral diversity and instead support upwelling and nutrients still affecting the Caribbean. The unique coral development in the Cercado Formation in the Arroyo Bellaco area suggests a local control that provided a suitable environment more conducive to coral development (e.g., a local paleogeographic setting that provided protection from upwelling and excessive nutrients). Additionally, even though the late Miocene Cercado Formation photozoan coral-dominated sequences in Arroyo Bellaco show similarities to modern (Pleistocene to Recent) Caribbean tropical reef systems in coral diversity, the corals developed as patches and thickets on an overall ramp system, rather than forming a rimmed platform, indicating that the photic zone was still adversely affected.
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