Subsurface Sedimentology of the Miocene-Pliocene Kingshill Limestone, St. Croix, U.S.V.I.
Ivan Gill, Dennis K. Hubbard, 1985. "Subsurface Sedimentology of the Miocene-Pliocene Kingshill Limestone, St. Croix, U.S.V.I.", Deep-Water Carbonates: Buildups, Turbidites, Debris Flows and Chalks—A Core Workshop, Paul D. Crevello, Paul M. Harris
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The Kingshill Limestone (Miocene-Pliocene) was laid down in a fault-bounded basin or open seaway during a time of tectonic instability and eustatic fluctuations. Outcrops from within the seaway indicate a range of environments from deep basin to reef. The width of the seaway ranged from 8 to 16 km.
Cores taken from midway between outcrops representing basinal and reefal facies, display skeletal packstones and wackestones with no visible bedding. The allochems in the skeletal packstone facies consist of shallow-water benthic foraminifera, rounded clasts from normal-salinity reef and encrusting communities, and well-preserved globi-gerinid forams. The moldic coral wackestone facies occurs in 20 to 30 cm thick intervals and is dominated by cm-sized molds of Stylophora coral fragments.
Diagenetic textures follow a consistent pattern downcore. A highly leached interval is followed by a zone of highly cemented packstones with decreased porosity. Intervals of the moldic coral wackestone facies are included at various points throughout this low porosity zone. The low porosity zone directly overlies and grades into a more porous interval of pitted appearance with mm-size vugs. This zone is directly underlain by an abrupt transition into porous, pervasive dolomite.
The sediments were deposited in an outer shelf to slope environment open to ocean circulation. Reef growth was probably recessed several kilometers landward of these cores. Early cementation took place before complete immobilization of the lime mud matrix, since micrite layers commonly overlie early marine cement. Porosity occlusion by blocky calcite is followed by dolomitization and related dissolution.
There is not yet enough information to describe a particular model of dolomitization. This example is petrographically similar to other Caribbean dolomites of inferred mixed-water origin, and there is no evidence of hypersaline conditions. Mixed-water dolomitization is suggested as a hypothesis.
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Deep-water carbonates represent on the few frontiers remaining for carbonate exploration and research. The last decade has experienced a rapid evolution in concepts of depositional models and diagenesis which underscores the importance of these deposits as significant reservoirs and source rocks. This workshop displayed cores selected to provide subsurface geologic examples of deepwater carbonates from a variety of depositional settings. Several papers discuss depositional models, platform-to-basin reconstructions, and diagenetic sequences that are important in the development and exploration of Paleozoic carbonate debris flow and turbidite reservoirs of the Palo Duro, Delaware and Midland Basins. Many other examples are included from several different regions.