An intensive multi-year field study of the modern marine stromatolites at Highborne Cay, Bahamas has identified a variety of microbial communities that colonize the stromatolite surfaces. They include both bacterial and diatom dominated communities. The "classic" microbial communities are those described by . They include Schizothrix mats, dominated by S. gebeleinii, which trap and bind ooid sand grains (Type 1 mat); biofilm mats, composed of sulfate reducing bacteria, which form thin crusts of microcrystalline carbonate (Type 2 mat); and Solentia mats, dominated by coccoid endolithic Solentia species, which create cemented layers of fused sand grains (Type 3 mat). Another bacterial mat, termed "pudding mat" due to its pudding-like texture, is dominated by thin filaments of Phormidium sp. and single filaments of S. gebeleinii, but may also be colonized by a unique species of coccoid cyanobacterium related to Cyanothece. The diatom mats include stalked diatoms and tube diatoms. The stalked diatom mats form as a thin (1-3 mm) surface pink fuzz comprised of Striatella unipunctata, or a yellow fuzz that may develop into a thick (0.5-1 cm) yellow fur with Licmophora remulus and Licmophora paradoxa. The tube diatom mats, which occur as discrete pustules that may coalesce to create uniform blankets, are formed by naviculid -like tube diatoms. These different mat types recognized based on field descriptions and light microscopy also show distinct differences based on microbial fingerprinting and carbohydrate fractionation. Denaturing gradient gel electrophoresis (DGGE) analyses show similarities between stalked diatom mat types 2 and "classic" mat types 1 and 2; these mats cluster separately from tube diatom mats and pudding mats, which each form distinct clades. In addition, the carbohydrate fractions of classic mat types are composed mostly of structural extracellular polymeric secretions (EPS), whereas stalked diatom and pudding mats contain predominantly non-structural carbohydrates. Although the pudding mats and diatom communities can contribute to the trapping of ooids, the stabilization of the unconsolidated sediment ultimately requires binding by S. gebeleinii. The combination of carbohydrate composition and ability to rapidly rebound after burial result in the high erosion resistance exhibited by the "classic" mats. Conversely, the extremely sensitive nature of diatoms to burial results in the low erosion resistance of the diatom mats. Nevertheless, all may contribute to the biogenesis of the Highborne Cay stromatolites.
To help define the habitat of modern marine stromatolites, wave-dominated flow and sediment transport were studied in the shallow subtidal region (1-2 m depth) along the slightly concave, windward face of Highborne Cay, Exuma, Bahamas - the only face of the cay that includes a population of stromatolites concentrated near the region of highest curvature of the beach. Wave energy impacting this island's most exposed beach was driven by local wind forcing which increases largely in response to the passage of atmospheric disturbances that typically affect the region for periods of a few days. Although some wave energy is almost always noted (maximum horizontal orbital speeds at the bottom are rarely <10 cm s(-1)), wave conditions remain comparatively calm until local winds increase above speeds of approximately 3-4 m s(-1) at which point maximum wave speeds rapidly increase to 50-80 cm s(-1). Stromatolites, which are largely restricted to the shoreward side of a shallow platform reef, are sheltered by the reef beyond which wave speeds are one to four times higher (depending on tidal stage). Moreover, stromatolite populations are predominantly found along a region of this wave-exposed beach that experiences comparatively reduced wave energy because of the curved morphology of the island's face. Maximum wave speeds are 1.4 to 2 times higher along more northern sections of the beach just beyond the locus of stromatolite populations. A quantitative model of sediment transport was developed that accurately predicted accumulation of suspended sediment in sediment traps deployed in the shallow subtidal zone along this beach. This model, coupled with in situ wave records, indicates that gross rates of suspended sediment deposition should be two to three times higher northward of the main stromatolite populations. Regions of the beach containing stromatolites nevertheless should experience significant rates of gross suspended sediment deposition averaging 7-10 g cm(-2) day(-1) ( approximately 4-6 cm day(-1)). Results suggest that one axis of the habitat of modern marine stromatolites may be defined by a comparatively narrow range of flow energy and sediment transport conditions.
Microporosity is very common in limestone reservoirs globally and is especially significant in many large Mesozoic reservoirs in the Middle East. Despite its common occurrence there is:Wide variation in its definition,Uncertainty around characterization, genetic controls, and distributionA rudimentary understanding of its influence on reservoir performance and hydrocarbon recovery. The results of this study, based on a global survey of microporosity and specific Middle Eastern case studies, provide clarity on each of these topics. One volumetrically significant type of microporosity occurs between micron size subhedral crystals of low magnesium calcite in matrix and within grains. This micro-pore system is very homogenous in terms of pore size distribution with 90% of pores between 1 and 3 microns in diameter. Pore throat radii range between 0.1 and 1.5 microns. Porosity, permeability, and capillarity relationships reflect this homogeneity for rocks dominated by microporosity. Rocks with less than approximately 80% microporosity exhibit a marked increase in pore system heterogeneity. A pore geometry characterization approach incorporating digital image analyses of petrographic thin-sections was developed and provides a very effective means of rapidly characterizing and quantifying the total pore system, including microporosity. The lateral and stratigraphic distribution of microporosity is systematically related to the distribution of depositional facies and the regional extent of burial diagenetic processes. Factors that inhibit burial diagenesis, such as hydrocarbon charge, also have a strong influence on the nature and distribution of microporosity. Remaining oil saturation in microporous limestone, as measured from centrifuge capillary pressure and steady state (SS) core flood experiments, is negatively correlated with the percent fraction of microporosity. Due to the homogenous nature of the micro-pore system, rocks dominated by microporosity have more favorable oil recovery than rocks with mixed pore systems. In the specific cases studied here, water provides more favorable recovery than gas. These results have implications for resource assessment, field development planning and optimization of ultimate recovery in limestone reservoirs with significant microporosity.
This is the first description of macroalgae documented from a stromatolite reef complex. Because of the sediment stress conditions that promote the development of stromatolites in this open marine environment, macroalgae are generally limited in species diversity and are sparse except for the crustose coralline alga Neogoniolithon strictum, which forms the elevated reef crest behind which the sediments and stromatolites accumulate. At the 10 sites sampled, a total of 22 species were encountered, 10 Rhodophyta, 9 Chlorophyta, 2 Phaeophyta and 1 Cyanobacteria.
Microporosity is very common in limestone reservoirs globally and is especially significant in many large Mesozoic reservoirs in the Middle East. Despite its common occurrence there is: Wide variation in its definition, Uncertainty around characterization, genetic controls, and distribution A rudimentary understanding of its influence on reservoir performance and hydrocarbon recovery. The results of this study, based on a global survey of microporosity and specific Middle Eastern case studies, provide clarity on each of these topics. One volumetrically significant type of microporosity occurs between micron size subhedral crystals of low magnesium calcite in matrix and within grains. This micro-pore system is very homogenous in terms of pore size distribution with 90% of pores between 1 and 3 microns in diameter. Pore throat radii range between 0.1 and 1.5 microns. Porosity, permeability, and capillarity relationships reflect this homogeneity for rocks dominated by microporosity. Rocks with less than approximately 80% microporosity exhibit a marked increase in pore system heterogeneity. A pore geometry characterization approach incorporating digital image analyses of petrographic thin-sections was developed and provides a very effective means of rapidly characterizing and quantifying the total pore system, including microporosity. The lateral and stratigraphic distribution of microporosity is systematically related to the distribution of depositional facies and the regional extent of burial diagenetic processes. Factors that inhibit burial diagenesis, such as hydrocarbon charge, also have a strong influence on the nature and distribution of microporosity. Remaining oil saturation in microporous limestone, as measured from centrifuge capillary pressure and steady state (SS) core flood experiments, is negatively correlated with the percent fraction of microporosity. Due to the homogenous nature of the micro-pore system, rocks dominated by microporosity have more favorable oil recovery than rocks with mixed pore systems. In the specific cases studied here, water provides more favorable recovery than gas. These results have implications for resource assessment, field development planning and optimization of ultimate recovery in limestone reservoirs with significant microporosity.
Mixed microbial-metazoan reefs, with distinct lateral zonation of microbialites, coralline algae and coral, fringe the windward-facing margin of two islands in the Exuma Cays, Bahamas: Stocking Island and Highborne Cay. These reefs offer a unique opportunity to identify and characterize controls governing the spatial distribution of microbial versus metazoan reef builders in a modern setting. Distribution patterns characterized by prolifi c stromatolite development in back-reef lagoons, prominent coralline algal growth along reef crests, and rare coral development on seaward reef edges are a direct response to intensity and frequency of sediment stress, sand abrasion and burial of reef biota. The recent discovery and dating of an outcropping coral reef underlying stromatolites in the southern Highborne Cay reef serves as a basis for interpreting the late Holocene evolution of this reef and addressing the question: what led to the demise of a metazoan and rise of a microbial reef? Comparison of growth histories of Highborne and Stocking Island reefs allows us to explore processes that have governed microbial versus metazoan reef building from mid-Holocene to present, and to speculate on future reef development at these sites. Antecedent topography and late Holocene sea-level transgression determined the timing of reef nucleation at both sites. Initial reef development was relatively unaffected by sedimentation due to a lag in sediment production relative to rising sea level. Once produced, sediment was deposited on the Pleistocene terrace, which due to rapid sea-level rise was below average wave base and sediment suspension. Slowing of sea-level rise combined with increasing sediment production decreased accommodation space. Mobile sediment progressively impinged upon the reef environment and, following the emergence of an algal ridge, sediment was trapped in the back-reef area. Although corals can tolerate some sediment stress, they cannot cope with extended periods of burial. Increasing sediment stress and more importantly increasing amplitude and frequency of sediment burial are interpreted as dominant factors leading to the demise of metazoan reef builders and the prolifi c growth of microbialites in the back-reef lagoon. Microbial buildups are surprisingly young (<1000 years) and a recent addition to the fringing reef systems along the Exuma margin. The study of modern analogues is important for our processorientated understanding, which in turn provides concepts and hypothesis to apply and validate in the rock record.
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