3-D Geometries and Porosity Trends of Subsurface Ooid Shoal Reservoirs in the Mississippian Ste. Genevieve Formation of the Illinois Basin, USA
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An ooid sand barrier bar of Pleistocene age was deposited along the seaward side of an ooid shoal complex southwest of Miami, Florida. The bar is 35 km long, about 0.8 km wide, elongate parallel with the trend of the ooid shoal complex and perpendicular to channels between individual shoals. A depression 1.6 km wide, interpreted as a back-barrier channel, isolates the bar from the ooid shoals. During sea-level fall and subaerial exposure of the bar, the ooid sand was cemented in place, preventing migration of the barrier. No Holocene analogue of this sand body is recognized, perhaps because of the relative youthfulness of Holocene ooid shoals. This Pleistocene ooid shoal complex, with its reservoir-size barrier bar, may serve as a refined model for exploration in ancient oid sand belts.
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Abstract Oolitic strata are common through the geologic record. Ooids generally form in high-energy environments, but it remains unclear how ooids remain in such systems and form geomorphic features such as bars and shoals. Integrating remote sensing, hydrodynamic, bathymetric, granulometric, and field observations of modern tidal systems in the Bahamas provides insight into this fundamental question. Oolitic tidal sands in the northern Abacos display a geomorphic pattern in which bedrock islands restrict and focus tidal flow down a main channel. Within the channel, a shallow shoal separates an ebb-dominated subchannel from a flood-dominated subchannel. Tidal velocities in these subchannels can exceed 1 m/s, enough to transport the oolitic sediments. Hydrodynamics and bathymetry in these subchannels produce a net circular hydrodynamic pattern around the shoal (the spin cycle), allowing the sands to remain in motion without being transported out of the ooid factory. This general pattern is apparent in several other ooid shoal complexes. This concept provides integrated insights into the physical influences impacting the formation, suspension, transport, and deposition of ooids and the resulting geomorphic forms. These results represent first steps toward developing more comprehensive and predictive analogs of spatial heterogeneity in ancient tidally dominated oolitic shoals.
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Abstract High‐resolution seismic data reveal an unexpected Pleistocene topography underneath the Cat Cay shoal complex along the western margin of Great Bahama Bank, illustrating how Pleistocene topography focuses tidal flow to create different types of grainstone shoals. The 1–3 km wide and 35 km long shoal complex is composed of the Cat Cay ooid shoal that is a laterally continuous 8 m thick ooid shoal and a sequence of 300–600 m wide and less than 6 m thick skeletal‐dominated tidal deltas south of Ocean Cay. The skeletal tidal deltas overlie an irregular Pleistocene surface, while the Cat Cay ooid shoal is situated on a flat Pleistocene surface east of a Pleistocene rock ridge. This finding challenges the assumption that an antecedent high is needed for ooid shoal initiation. The base of the Cat Cay ooid shoal is an up to 4 m thick skeletal‐peloidal unit that is similar in composition to the skeletal tidal deltas south of Ocean Cay but their deposition was followed by an up to 4 m thick accumulation of ooids. The Pleistocene ridge west of the Cat Cay ooid shoal allowed accumulation of mud and peloids (the nucleus source), while to the south, muddy sediment was winnowed away and no ooids formed. The evolution of the two shoal types is ultimately the result of the presence and absence of antecedent topography adjacent to the shoal system, resulting in variations of mud accumulations and the formation of the nucleus in the ooid shoal. The coeval occurrence of ooid and skeletal shoals in the same complex implies that in the rock record, a vertical succession from oolitic to skeletal shoals does not indicate an environmental change such as climate or an anoxic event but rather a change in flow conditions created by antecedent topography.
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ABSTRACT Caicos Bank is a shallow water, sub-circular carbonate platform about 100 km across. The northern shore is fringed with islands that shelter interior tidal flats. Seaward of the tidal flats, within the bank interior is an ooid-grapestone shoal complex 20 km long and 10 km wide. Individual shoals are 300 to 2000 m wide, up to 4 m thick, and are separated by straight tidal channels up to 800 m wide and 2.5 m deep. Both shoals and tidal channels are asymmetrical in cross-section. Parts of the shoals are emergent and form small, low relief islands. Twenty-five vibracores recovered from the shoals and adjacent sediments show a general coarsening-upward sequence characterized by increasing grain size and grapestone content, and decreasing proportions of ooids, pellets and mud. The sequence is interpreted as a shallowing-upward cycle that began as a series of ooid shoals flanked by pelleted packstone and evolved into the present grapestone shoal complex. Sedimentation was initiated during Holocene flooding of a gently south sloping ramp. Sediment accumulation rates exceeded sea level rise so that the shoals lengthened and emerged with time. Shoal growth is promoted by: 1) lateral accretion of longshore derived beach sets; 2) elongation at down drift shoal terminations; 3) storm and hurricane generated surges; 4) tidal channel abandonment and shoal coalescence. Synsedimentary diagenesis plays an important role in shoal stabilization. Sediments exposed on emergent islands have been cemented by equant meniscate calcite of vadose origin. Irregular, biologically stabilized, cemented patches up to 1 m across and 15 cm thick occur on the surface and within submerged shoals. Cements are micritic, peloidal and isopachous fibrous aragonite formed in submarine phreatic environments. End_of_Record - Last_Page 159-------
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An examination of the lithology and topography of Andros Island, Bahamas, reveals it is a Pleistocene ooid sand shoal. A comparison with Joulters Cays (a modern ooid sand shoal directly to the north) shows that much of the original depositional topography is preserved through at least one cycle of sea level highstand and lowstand. Both the Pleistocene and the Holocene ooid sand bodies are a few kilometers to tens of kilometers wide. The total vertical relief of a single episode of Quaternary ooid sand deposition is more than 10 m and includes accumulation in tidal channels, shallow flat areas, and eolian dunes. Today, much of Andros Island is within 2 m of present sea level and is the site of a belt several kilometers wide consisting of muddy tidal flat sediments overlying an exposure surface. The site of ooid sand deposition and shoal complex formation is not continuous along shorelines, especially windward margins, but shifts abruptly along the margins of platforms as a result of minor fluctuations of sea level. Thus, it should be expected that ooid sand shoals (ancient and modern) should be in direct lateral and vertical contact with lagoons, tidal flats, and reefs. The Mississippian Slade Formationmore » contains many of the features of Quaternary ooid sand accumulation: abrupt vertical and lateral gradations between oolitic grainstones, packstones, and lime mudstones, vertical relief of individual oolitic sedimentary packages up to 30 m (perhaps with eolian dunes) and numerous exposure surfaces of varying intensities. These characteristics suggest that this formation represents a time of rapid fluctuations of relative sea level and abrupt shifts in the sites of ooid sand shoal complexes.« less
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