Atypical ooid diversity in the Upper Cretaceous Yacoraite Formation, Argentina
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Abstract This study reports on ooid diversity from different lithotypes of the Yacoraite Formation (Salta Group basin) in the Central Andes of north‐west Argentina. The ooids display a variety of internal and external morphologies that may be deployed as proxies for seawater chemistry and hydrodynamic processes. A short review of nomenclature problems is first discussed, followed by presentation of a two‐fold quantitative and qualitative methodology. Our proposed classification addresses internal and external ooid characters in order to understand growth in response to various environmental processes at an individual particle level. This classification allows discrimination between a variety of morphologies, but also evaluation of the complexity of the processes involved in ooid formation as seen in the fossil record. This study evaluates whether the ooids present within the Yacoraite Formation share similarities with ooids formed in marine versus marine lagoon and/or lacustrine environments. A possible lacustrine interpretation finds its origin in the diverse assemblage of ooid morphotypes present, which exceeds the variations described for marine ooids. However, growth paths and occurrence of various compound morphologies point to intense marine recycling, suggesting little accommodation. Together with other sedimentological characteristics, for example, bi‐directional cross‐bedding, tidal shoals, hummocky cross‐stratification and exposure surfaces, these features suggest that marine processes had an impact on the sediments. Therefore, the ooid assemblage of the Cretaceous Yacoraite Formation was most likely formed in a shallow coastal lagoon in the framework of an epicontinental sea that at times experienced marine flooding events. A detailed evaluation of processes involved in oolite formation is needed in order to improve the stratigraphic and palaeoenvironmental understanding of ooid formation through time. This includes examining alternating constructive and destructive stages, early binding and cementation, reworking, recycling and averaging processes.Keywords:
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Shoal
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 Although the general aspects of oolitic depositional systems are well documented, seascape‐scale (≈10 3 –10 6 m 2 ) patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified. To begin to fill this basic gap in understanding, this paper describes the morphology and hydrodynamics of Lily Bank, a Modern tidally dominated Bahamian ooid shoal. In this study, integrating remote sensing imagery with quantitative, geo‐located bathymetrical, hydrological and granulometric data in a Geographic Information System documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system. The results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif, although there is considerable variation on that general theme. The seascape‐scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements, and includes the presence of mutually evasive flood and ebb channels. Sedimentologically, bars are neither homogenous nor random bodies; instead, granulometric parameters such as sorting and percentage mud vary systematically, as shaped by hydro‐geomorphic controls. The best sorted, coarsest ooids are on bar crests, whereas the finest grains are found in the lower energy, deeper interior and flanking regions. In short, results clearly document hydrodynamic‐bathymetrical influences on these ooid shoals and their granulometry, linkages akin to siliciclastic analogues. Sedimentological, hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in non‐parabolic bars are enhanced through their effect of focusing flow. Constricted flow leads to higher flow velocities, tidal flow velocity asymmetries, differential net sediment transport and growth of bathymetrical highs. This bathymetrical divergence creates separate paths for flood‐ and ebb‐tides, facilitating emergence of better‐developed parabolic forms. The resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb‐ and flood‐tide flows, or evolving toward that state. In exploring patterns and processes within carbonate shoals, this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain‐size and sorting in an oolitic carbonate system. Assuming a continuity of processes between ancient and modern, the insights from this shoal provide information on possible facies geometries and on the characteristics of grains and depositional porosity of analogous facies within ancient ooid shoals.
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Ooid shoals are present (1) in the high-energy zones of carbonate ramps, (2) across broad shelves, and (3) along rimmed carbonate shelf edges. Shoals generally form as depositional strike-oriented or dip-oriented sand bodies in these settings. Marine sand belts, locally present along the shelf edge of the Great Bahama Little Bahama Banks, are depositional strike-oriented sand shoals made of flood ramps, shields, and channels that terminate in spillover lobes. Their geometry results from an interplay between storm-generated and tide-generated currents with the sea floor. Mississippian oolitic limestones (St. Louis Limestone B-zone), whose geometries resemble modern Bahamian ooid shoals, produce oil and are current exploration targets in southwestern Kansas (Hugoton embayment). Thus, a sedimentological and stratigraphic study of a producing field (Damme field, Finney County, Kansas) and a comparison to modern ooid sand bodies were justified. The St. Louis Limestone is overlain by the Ste. Genevieve Limestone (nonproductive) in Damme field. Both formations consist of five lithofacies: (1) skeletal wackestones in an open marine shelf, (2) porous, skeletal, ooid grainstone in a marine sand belt, (3) sandy, peloid-ooid, skeletal grainstone in a mobile grain shoal, (4) quartz-arenite in a tidal inlet, and (5) fenestral lime mudstone in an intertidal mud flat. Porous grainstones of lithofacies 2 form an elongate (northwest-southeast), lenticular body 10 mi (16 km) long, 2 mi (3.2 km) wide, and up to 18 ft (5.5 m) thick. Its shape and geometry are nearly the same as the marine sand belts of the Bahamas. Lithofacies arrangements indicate two styles of shallowing-upward depositional cycles; the lower St. Louis cycle was deposited during a relative sea level rise, and the overlying Ste. Genevieve cycles were deposited after a relative drop of sea level, an event signalled by the influx of siliciclastic sediments into the carbonate shelf from the Central Kansas uplift and/or Transcontinental arch.
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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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