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Unusual Serpulid/Tufa Bioherms Cap Corals of a Mid-Holocene Fringing Reef, Dominican Republic: Paleoenvironmental and Climatic Controls

Meter-scale mid-Holocene serpulid/tufa bioherms in the form of mounds and terraces cap corals of the exposed fringing reef bordering much of the Enriquillo Valley, Dominican Republic. Composition of these unusual bioherms resembles that of two other localities described in the literature: Miocene mounds of the Paratethys Basin of Ukraine/Poland and Late Triassic deposits in the western Tethys of Italy. Rarity of such bioherms and their similarities indicate that their formation was the result of convergence of specific environmental conditions. In each case, restricted waters of less-than-normal marine salinity reduced ecological competition and favored opportunistic serpulid aggregation, as well as concentrated calcium carbonate for tufa precipitation. In the Enriquillo Valley, limited hard substrate, wave action, calcium carbonate-rich spring waters entering at the surface of ancient Lago Enriquillo, and periods of lake-level stability also contributed to bioherm formation. These contributing factors were a result of larger-scale climatic conditions characteristic of the mid-Holocene Caribbean, as well as local geological constraints. Around 5 ka, slowed sea-level rise, increased precipitation and resultant fluvial sedimentation owing to northward migration of the Intertropical Convergence Zone (ITCZ), and possible regional tectonic uplift restricted the mouth of the ancient Enriquillo seaway. This caused the demise of the Enriquillo fringing coral reef, and later the dead coral colonies commonly served as substrate for development of serpulid/tufa bioherms. Repeated changes in Lago Enriquillo water level and salinity created conditions that supported not only bioherm formation, but also diverse molluscan and ostracode faunas found within many bioherms. In late Holocene time, drier conditions prevailed as the ITCZ moved southward and water levels of Lago Enriquillo receded, exposing this spectacular fringing reef-serpulid/tufa mound and terrace complex.

Holocene; tufa mounds; reef; Enriquillo Valley; Dominican Republic

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