Ooids Research Articles

Iron ooid beds of the Carolinefjellet Formation, Spitsbergen, Norway

Iron ooid beds are unusual deposits that have been linked to greenhouse conditions and the transgressive flooding of shallow shelves, and which were globally prevalent during certain periods. Within the marine, Aptian–Albian, Carolinefjellet Formation of Spitsbergen, chamosite ooids have been found within distinctive sandstone beds at six localities, and at a consistent stratigraphic position within the basal Dalkjegla Member. Distinctive characteristics include the iron ooids themselves, a coarser grain size, intercalation with silty siderites, grading, cross-beds indicating offshore or longshore transport, and a lack of burrowing. The enclosing sands display planar and hummocky crossstratification and abundant oscillation ripple marks, and are interpreted as lagoon-attached bar complexes. The stratigraphic position and traits of the iron ooid sands are consistent with seaward storm transport and preservation within interbar swales. Ooids vary in shape considerably, and display evidence for multiple growth events. Nuclei of quartz, opaques, carbonate clasts and laminated crusts are typically encircled by finer grained tangential chamosite and opaque laminae, sometimes with outer overgrowths of calcite and/or radial chamosite. The Dalkjegla Member is the marine portion of a large-scale transgressive tract, attached to underlying fluvio-estuarine Helvetiafjellet Formation strata. A lagoonal environment associated with the basal shales of the Dalkjegla Member represents a logical setting, where riverine iron concentration and iron silicate growth could occur. The Spitsbergen iron ooid beds extend the known occurrence of Cretaceous examples, representing a less common High-Latitude example, and one not directly associated with a transgressive flooding surface.

The end Triassic mass extinction record of Williston Lake, British Columbia

Triassic–Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spectrum of depositional environments from the western Canadian continental margin. A primarily deep-water, anoxic record of deposition, indicated by V/V + Ni, V/Cr, Th/U indices and pyrite framboid and facies data, is seen throughout the Late Triassic–Early Jurassic (Norian-Hettangian) interval in a section at Black Bear Ridge. Abundant Monotis bivalve populations in Norian strata are interpreted to have lived on the seabed during transient oxygenation events that punctuated the background anoxic conditions. Monotis went extinct around the Norian/Rhaetian boundary and the deep-water assemblages are subsequently dominated by thin-shelled pectinids. An intra-Rhaetian sea-level fall generated a major hiatus in the proximal Black Bear Ridge section whilst distinctive lowstand strata were developed in the more distal and more complete Ne Parle Pas Point and Pardonet Creek sections. These consist of shallow-water, phosphatic ooid sand facies that contain diverse bivalves. The majority of the infaunal bivalves (but not the epifaunal ones) disappeared at the acme of the regression. This is the end-Triassic mass extinction event and it occurred shortly before a terminal Triassic flooding event. This transgressive event is associated with a rapid negative shift of δ 13C org values interpreted to be the Initial Isotope Excursion of Hesselbo et al. [Hesselbo, S.P., Robinson, S.A., Surlyk, F., Piasecki, S., 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30, 251–254]. The Rhaetian record of Williston Lake thus records an extinction of shallow-water infaunal bivalves during a lowstand prior to the development of a transgressive record that is associated with a major, negative C isotope.

Disobedient sediments can feedback on their transportation, deposition and geomorphology

Most sediments are obedient to the winds, waves and currents, which direct their transportation and deposition. It has long been recognized however, that the grain size, and/or grain kind, of sediments can feedback on the processes of their own transportation, deposition and geomorphology as well as that of succeeding deposits. This note is to review three examples of marine sediments in which a single grain size or grain kind produces multiple feedbacks. Tidal bars of Holocene ooid sands on Great Bahama Bank are an example of multiple feedbacks of one grain kind on tidal currents, wave action and accumulations. These feedbacks are responsible for the distinctive pattern of elongated bars and channels, which in turn amplify tidal currents. The near constant movement of grains on the shallow bars and in the channels is where pellet nuclei are coated to form ooids; thus the development and growth of bars feeds back to produce more ooids. Regional encrinites, which consist predominantly of the disarticulated highly porous skeletons of crinoids, are common from Ordovician to Jurassic during blooms of these filter feeders. The resulting grains, which are equivalent to quartz grains a tenth of their size, can be entrained by currents as low as a knot (ca 0.5 m/s). The resulting mobile substrate deters other invertebrates (taphonomic feedback) and results in the prevalence of layering produced by traction transport of low velocity. The belt of mud extending for some 1600 km between the Amazon and Orinocco rivers is a special example of the feedback of mud on depositional processes, sedimentary structures and geomorphology of the accumulations. The clay-rich mud from the Amazon produces fluid mud which dampens and transforms wave action from the open sea to promote its own accumulation in giant bars in the inner shelf and shoreline with a variety of familiar laminations. The result is a wedge of mud-rich deposits some 24 m thick and 30 km wide capped with cheniers of sand, which has prograded seaward in some 6000 years.

Eustatic and palaeogeographic control of the western Aurès Upper Cretaceous sedimentation (Algeria)

Abstract The Upper Cretaceous sedimentation in the Aurès Mountains occurred in a subsident basin delimited to the south by the Saharan platform and by the Preatlasic high zone to the north. In these series 4 transgressive-regressive megasequences are distinguished, the first one (I) in the late Albian-Cenomanian, the second one (II) in the Turonian, the third one (III) in the Coniacian - Santonian and the fourth one (IV) in the Campanian - Maastrichtian. Each megasequence is made up of three or four sequences, which correspond to third order cycles identified in the eustatic chart of Haq et al. [1987]. In late Albian and lower Turonian periods, during the deposition of basal sequences of the megasequences I and II, the reactivation of basement faults in the Aurès basin occurs consecutively to distension phases, resulting in the formation of rotated blocks. At the same time high eustatic levels are reached according to the global eustatic curve. These processes control the drowning of the carbonate platforms pre-existing to these sequences, and deep ramps progressively form on the tilted block tops. This sedimentary setting generates in the late Albian and lower Turonian series anoxic sequences made up of calcareous and shaly transgressive pelagic intervals. The succeeding sequences lack pelagic facies and are composed of alternate marls / carbonate beds deposited on a homoclinal ramp, indicating a gradual development of shallow open marine conditions, which became progressively restricted upwards. Toward the top of these megasequences, lagunal muds and isolated rudists mounds, surrounded by bioclastic and ooid / pellet banks occur. The Coniacian-Santonian and Campanian-Maastrichtian megasequences are characterized by a shallow ramp sedimentation, essentially marly during the Coniacian, Santonian and Campanian periods, interlayered with some bioclastic / ooid carbonate banks and upwards by sequences mostly homogeneous. The Maastrichtian platform carbonates are composed of bioclastic / ooid sands and were deposited in a ramp-barrier-bank system. Some sequences in the Campanian-Maastrichtian megasequence are condensed or absent due to the accommodation reduction related to a weak subsidence rate period.

Determining the influential depth for surface reflectance of sediment by BRDF measurements

We measure the Bi-directional reflectance distribution function (BRDF) of ooid sand layers with three particle size distributions (0.5-1mm, 0.25-0.5mm and 0.125-0.25mm) and layer thicknesses on a reflecting mirror to determine the influential depth in the optical region at wavelengths of 658 nm (red), 570 nm (green) and 457 nm (blue). The hemispherical reflectance (albedo) was used as an indicator of BRDF changes between different layers. Measurements are carried out on both dry and water wetted grains. The results indicate that for both dry and wet and all size distributions, the influential depth is at most 2mm.

Análisis de facies de la Capa de Oolitos ferruginosos de Arroyofrío en la Sierra de Arcos (Jurásico, Cordillera Ibérica septentrional)

In this work we have studied the Arroyofrio Bed. This is a classic and largely studied ironstone unit in the Iberian Range. The age of this bed ranges from the Lower Callovian to Middle Oxfordian. This is a complex lithologic unit formed by irregular sedimentary events and several discontinuities associated with stratigraphic gaps of variable extent. In this work five facies containing ferruginous ooids are described and a preliminary study of the carbonate cements and Fe oolite facies is presented. This ironstone bed was formed near to an emergent high (Ejulve high). The oolites formed in a shallow marine subtidal environment. The facies and cement analysis shows that there were no long episodes of subaereal exposure at the Callovian-Oxfordian boundary. These data also demonstrate that these sediments were not deposited in deep marine conditions. The most probable hypothesis is that Fe ooids were formed in shallow subtidal and perhaps intertidal conditions and that they were eventually transported into more distal and relatively deep basinal areas.

Stromatolite-thrombolite associations in a modern environment, Lee Stocking Island, Bahamas

Columnar buildups found in a tidal channel off Lee Stocking Island, Exuma Cays, Bahamas, have been interpreted as modern giant stromatolites growing in a subtidal normal marine environment. However, these organically-formed columns reveal three discernible internal structures: (1) prokaryotic stromatolites comprised of alternating layers of coarse-grained ooids and peloids, and fine micrite that formed exclusively by microbial activity; (2) eukaryotic stromatolites comprised of microbially-induced micritic layers alternating with detrital layers accumulated, bound, and cemented by eukaryotic algae; and (3) thrombolites displaying irregular, clotted fabrics and formed by microbes, algae, and metazoans. Phanerozoic thrombolites, in contrast, have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Eukaryotic organisms overgrow all of the columns at present. Thus, the contemporaneous formation of prokaryotic stromatolites, eukaryotic stromatolites, and thrombolites under identical conditions within the present environment appears unlikely. We suggest that the prokaryotic stromatolites represent forms that began to develop in an intertidal setting with the Holocene flooding of the Great Bahama Bank. The thrombolites, however, began to form under the present, normal-marine subtidal conditions. The eukaryotic stromatolites represent intermediate forms between prokaryotic stromatolites and thrombolites. There is evidence for a gradual change from stromatolite to thrombolite reefs associated with rising sea-level. With the deepening, there would have been a decrease in salinity, an increase in energy, and possibly an increase in nutrient supply; all factors that favor thrombolite growth. We propose that the co-existing stromatolites and thrombolites found off Lee Stocking Island did not grow contemporaneously, but reflect a response to changing environmental controls with changing sea-level.

Ooid Turbidites from the Western Margin of the Bengal Fan Sediments

Abstract Sedimentology, mineralogy and geochemistry of ooid sand beds in deep sea gravity cores (water depth >2818 m) reveal that they were derived from continental shelf by turbidity currents. The presence of ooid turbidites at two different levels suggests two periods of turbidity current activity. The first activity probably took place during the Last Glacial Maximum in Late Pleistocene and the second activity prior to the stabilization of sea level in the Early Holocene.

Recent evolution of a Bahamian ooid shoal: Effects of Hurricane Andrew

Hurricane Andrew, a category 4 hurricane having wind velocities of ≈240 km/hr, passed north of Joulters Cays, Bahamas, in a westerly direction on August 23, 1992. We documented three sedimentary facies in a 2.7 km 2 study area dominated by mobile ooid sands before the hurricane, using aerial photographs, surface observations, and shallow coring. The shoal crest at this locality had aggrading and northward-prograding (parallel to depositional strike) washover bars composed of cross-bedded, well-sorted ooid sands. Burrowed, poorly sorted ooid sands were present seaward of the washover bars, whereas poorly sorted ooids and mud occupied a stabilized area bankward of the actively migrating shoal and local areas between washover bars on the crest of the shoal. The shoal was crosscut by tidal channels, and older washover bars were being dissected by tidal currents. Although Hurricane Andrew profoundly changed surface features within the study area, its effects will probably be only partly preserved. The hurricane eroded washover bars and transported sediment seaward, leaving a nearly flat shoal crest overlain by a laterally continuous, decimeter-thick lens of well-sorted ooid sand that thins seaward and bankward. Post-hurricane tidal currents deposited a centimeter-thick discontinuous layer of carbonate mud over this lens of well-sorted ooid sand and transported ooids seaward off the shoal. The well-sorted ooid sand layer will most likely be reworked when an actively migrating shoal crest is reestablished, although some of this storm deposit may be preserved on the shoal crest where the ooid sand layer was deposited in areas of normally less agitated conditions. Ooids may also be preserved in finer grained sediments seaward of the shoal, as suggested by previous studies. Mud deposits on the shoal crest may be preserved where buried beneath reestablished washover bars, although some of this mud will be removed by reworking during diurnal tides.

Peritidal Carbonate Platform Growth and Cyclicity in an Early Proterozoic Foreland Basin, Upper Pethei Group, Northwest Canada

ABSTRACT The upper Pethei Group is a 170-235 m thick, largely limestone, succession located in the east arm of Great Slave Lake, northwest Canada. Strata accumulated in a Paleoproterozoic foreland basin during the late stages of marine deposition. Rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages: (1) marginal peritidal complex; (2) back-barrier lagoon; (3) shelf margin; (4) subtidal shelf; and (5) slope and basin. Carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae, water-column precipitation of carbonate mud, and generation and transport of nonskeletal grains (stromaclasts, ooids, oncoids, peloids, and intraclasts). The upper Pethei platform is interpreted to have evolved through three basic platform geometries, peritidal rimmed shelf, subtidal rimmed shelf, and subtidal open shelf, in response to changes in the rate of creation of accommodation space. The peritidal complex and rim behaved in a manner consistent with models of Phanerozoic reefal rims, and alternatively, kept up, gave up, or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation. Three scales of cycle are recognized: (1) overall formation-scale upward-deepening of the platform; (2) laterally traceable, decameter-scale shallowing-upward cycles separated by marine flooding surfaces; and (3) laterally discontinuous, meter-scale shal owing-upward peritidal cycles. The first two cycle types are interpreted to have been allogenic, controlled by changes in eustasy and subsidence rates. Meter-scale cycles are interpreted as mainly autogenic, having formed in an aggradational tidal island model. The upper Petrel platform is similar to other Paleoproterozoic foreland-basin carbonate platforms. Important differences, however, include (1) laterally discontinuous peritidal cycles, (2) lack of widespread sand shoals inboard of the shelf rim, (3) a thick, predominantly carbonate, slope and basin succession, (4) moderate to low platformal micrite content, (5) extensive lagoonal ooid sands, and (6) low contents of terrigenous clastics.

Lime-mud layers in high-energy tidal channels: A record of hurricane deposition

Research Article| July 01, 1993 Lime-mud layers in high-energy tidal channels: A record of hurricane deposition Eugene A. Shinn; Eugene A. Shinn 1U.S. Geological Survey Coastal Center, 600 4th Street South, St. Petersburg, Florida 33701 Florida 33701 Search for other works by this author on: GSW Google Scholar Randolph P. Steinen; Randolph P. Steinen 2Department of Geology, University of Connecticut, Storrs, Connecticut 06268 Search for other works by this author on: GSW Google Scholar Robert F. Dill; Robert F. Dill 3GeoMarine Inc., San Diego, California 92106 Search for other works by this author on: GSW Google Scholar Richard Major Richard Major 4Bureau of Economic Geology, University of Texas, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (7): 603–606. https://doi.org/10.1130/0091-7613(1993)021<0603:LMLIHE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Eugene A. Shinn, Randolph P. Steinen, Robert F. Dill, Richard Major; Lime-mud layers in high-energy tidal channels: A record of hurricane deposition. Geology 1993;; 21 (7): 603–606. doi: https://doi.org/10.1130/0091-7613(1993)021<0603:LMLIHE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract During or immediately following the transit of Hurricane Andrew (August 23-24, 1992) across the northern part of the Great Bahama Bank, thin laminated beds of carbonate mud were deposited in high-energy subtidal channels (4 m depth) through the ooid shoals of south Cat Cay and Joulters Cays. During our reconnaissance seven weeks later, we observed lime-mud beds exposed in the troughs of submarine oolite dunes and ripples. The mud layers were underlain and locally covered by ooid sand. The mud beds were lenticular and up to 5 cm thick. Their bases cast the underlying rippled surface. The layers were composed of soft silt- and sand-sized pellets and peloids and in some areas contained freshly preserved Thalassia blades and other organic debris along planes of lamination. The beds had a gelatinous consistency and locally had been penetrated by burrowers and plants. Layers of lime mud had also settled on bioturbated, plant-stabilized flats and in lagoonal settings but were quickly reworked and made unrecognizable by the burrowing of organisms. Thicker, more cohesive (and therefore older) mud beds and angular mud fragments associated with ooids from Joulters Cays have similar characteristics but lack fresh plant fragments. We infer that these older beds were similarly deposited and thus record the passage of previous hurricanes or tropical storms. Storm layers are preserved within channel sediments because migrating ooids prevent attack by the burrowing activity off organisms. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

NEW ENDOLITHIC CYANOBACTERIA FROM THE ARABIAN GULF. I.HYELLA IMMANISSP. NOV.1

ABSTRACTHyella immanis, a new species of endolithic cyanobacterium that penetrates carbonate ooid sand grains in the Arabian/Persian Gulf, is formally described. Natural populations of the new taxon were sampled in moving ooid shoals at four locations along the east coast of Saudi Arabia, together with seven other endolithic taxa. The new species was isolated, and its properties were studied under experimental conditions. Small reproductive cells (baeocytes) exhibited positively phototactic gliding motility following release. In culture they grew into colonies forming isodiametric packages (prevalent on agar) and distinct pseudofilaments (prevalent in liquid culture). Carbonate penetration of the cultured strain in liquid culture proceeded at a rate of up to 10 μ· d−1. Agitation of cultures with magnetic stirrers enhanced the frequency of borings and the initial boring rates, but it had no effect on the continuing boring activity. A fossil counterpart of the new species was identified in Upper Proterozoic (700–800 million years old) silicified oolitic and pisolitic rocks of East Greenland.

Sedimentation on a Lower Jurassic carbonate platform flank: geometry, sediment fabric and related depositional structures (Djebel Bou Dahar, High Atlas, Morocco)

Along the southern margin of the High Atlas, Morocco, carbonate platforms of Lower Jurassic age developed in a transtensional basin, the South Atlas Trough. Complete exposure and preservation of the platform margin, flank and basin of the Djebel Bou Dahar, allowed the subdivision of the slope into an upper, mid-, and lower slope, based on geometry, sediment fabric and depositional structures. At ca. 50 m below the platform margin, coral-algal patch reefs and ooid sand shoals grade into the depositional slope. The straight upper slope, showing discontinuous bedding at angles up to 29°, extends down to ca. 250 m and consists mainly of poorly sorted mixtures of sand and gravel, almost devoid of mud. Avalanching, rockfall and shearing of the coarse gravelly sediment are the dominant processes readjusting the upper slope. The middle and lower part of the slope profile, extending down to ca. 500 m below the margin, is separated from the upper slope by a distinct slope break. Slope angles range from 19° near the slope break down to 4–8° where the slope merges with the basin-plain. The irregularly bedded mid slope is characterized by scouring and deposition of muddy sands and non-cohesive gravelly debris flows. Sedimentation on the, dominantly muddy, concave-upwards lower slope can be separated into evenly bedded shales, muddy sands and, mud-free, gravel-sand mixtures, deposited as debris sheets. The results of this study emphasize the relationship between sediment fabric and the slope geometry of a carbonate platform flank. The overall geometry, internal heterogeneity and depositional structures are suggested to be strongly related to the original character of the sediment supplied to the Djebel Bou Dahar platform flank.

Origin and Accumulation of Lime Mud in Ooid Tidal Channels, Bahamas

ABSTRACT Mud layers have been found within the ooid sands of Joulters Cays and Lee Stocking Island, Bahamas. A 1.3-m vibracore extracted from 4 m water depth in the Lee Stocking tidal channel contains a lower unit of dark brown muddy sand (skeletal rich) overlain by two layers of ooid sands intercalated with two layers of creamy, white mud. The two upper mud layers are aragonite-rich, sometimes pelleted, and contain very few skeletal grains. The contacts between the ooid sands and mud layers are commonly sharp, but some contacts show evidence of burrowing and mud clast formation. A mud layer with similar textural, mineralogic and petrographic characteristics was recovered from a 16-cm core from a tidal channel on Joulters Cays, Bahamas. This mud layer was also enclosed within ooid sands. Mud in the Lee Stocking ooid tidal channel is apparently of two origins. Mud in the lower unit of the core contains a more equal distribution of aragonite and calcite (50% each) and abundant skeletal grains of normal marine origin, indicating that the lower unit is lagoonal. Mud that is interlayered with ooid sands in the same core is dominated by aragonite (80%) and contains little sand-sized material, suggesting that it is not a typical lagoon mud. SEM examination also confirms that this mud is quite different from the lagoonal mud found at the base of the core. C-14 dating of the mud from the lagoonal unit shows that this sediment was deposited in water depths of 2 to 3 m approximately 5000 years ago when sea level was 3 m lower than present. C-14 dating coupled with knowledge of Holocene sea level indicates that both the ooid sands and the mud layers were also deposited as subtidal sediments. The mechanism by which the mud layers are accumulated with ooid sands is problematic. Suspension by storms and transport to tidal channels is one possibility; however, evaluation of the data suggests an alternative explanation. The post-lagoon history of deposition of the tidal channel may include nearly continuous restriction during which mud layers were deposited. The restriction was possibly caused by the formation and maintenance of ooid sand barri rs. Ooid sand deposition and burial of these layers may have accompanied barrier destruction.

Elevated Reefs and Related Pleistocene Stratigraphy of the Southern Exuma Islands, Bahamas

At least seven elevated coral reefs are exposed on the lee side of an inner chain of low islands on the eastern margin of Great Bahama Bank in the Southern Exumas. Starting at Fowl Cay in the north, they extend at least to Pigeon Cay in the south, a distance of about 44 nautical mi (81 km). These reefs formed around preexisting Pleistocene core-islands and hardgrounds during a transgressive higher than present stand of sea level, prior to the Holocene. As sea level dropped, the reefs were karstedand capped by caliche crusts, a unique paleosol breccia containing land snails (Cerithidea. sp.) and a now lithified dune system with well-developed rhizomorphs. Shallow sea caves eroded into steep, clifflike notches are often located on ridges shoreward of the reef, with roof heights of up to 5 m above the top of the elevated reefs. The reefs form a 5- to 10-m wide terrace, approximately 1 m above present sea level. Shoreline exposures often exceed 300 m (1,000 ft). Most exposures have a base thicket of interwoven branches of Acropora cervicornus approximately 70-cm thick. The badly corroded branches are surrounded by a cemented matrix of ooid sands, marine cement, and coralline debris. Themore » upper 30 cm is formed by a much more diverse reef community of broken shells and corals (Montastrea, Diploria Agaricia, Porites, and Acropora). Crustose coralline algae and colonial foraminifera (Homotrema) form a cementing crust around many corals. These reefs and their relationship to paleosols within subaerial dune deposits provide stratigraphic markers that play an important role in interpreting the development of Pleistocene deposits of the Great Bahama Bank.« less

Heterogeneity in Grainstone Reservoirs--Investigation of a Modern Analog, Joulters Cays, Bahamas: ABSTRACT

Skeletal. peloidal, and ooid grainstones form major carbonate reservoirs for hydrocarbons throughout the world. Reservoirs in the San Andres and Grayburg formations of the Permian basin provide excellent examples of this reservoir type. Subsurface and outcrop studies have documented extreme permeability variability within thin grainstone units of shoallng cycles that occur within these foundations. The Joulters Cays ooid shoal, located north of Andros Island, Great Bahamas Bank, is the most thoroughly documented modern carbonate sand complex. But even at Joulters, the core spacing of previous studies was approximately one core per square mile, which does not approach the sc le needed to address most problems of interwell heterogeneity. Using the existing regional core framework as a guide, a detailed coring program with well spacing approximating that in mature oil reservoirs (between 20 and 40 ac/well) was undertaken to examine the sediment variation and early diagenesis within some subenvironments of the active shoal. Cores and box cores taken at Joulters are dominantly clean ooid sands (grainstones) with minor muddy sands (packstone). However, changes in depositional style across the shoal and the effect of these changes on the preserved texture are of particular significance. Variations in the penetration of resin through boxmore » cores suggest greater permeability differences that are related to minor changes in grain size. These differences are most evident within graded units of spillover-lobe cross stratification on the bankward side of the shoal. Parallel-laminated seaward-dipping units show less variation. Cemented zones (hardgrounds), ranging up to at least 1 m{sup 2} in extent, also may contribute to the permeability heterogeneity. These types of subtle depositional and early diagenetic fabrics influence subsequent diagenesis and resultant permeability variations in grainstone reservoirs.« less

Geochemical Imprint of Meteoric Diagenesis in Holocene Ooid Sands, Schooner Cays, Bahamas: Correlation of Calcite Cement Geochemistry with Extant Groundwaters

ABSTRACT The geochemistry of meteoric calcite cements and their parent waters on three small Holocene ooid-sand islands in the Schooner Cays, Bahamas, is explicated in terms of observed hydrologic and hydrochemical processes. Dissolved Mg is derived soley from admixed seawater or aerosols. The Mg contents of the cements (0.4 to 3.0 mole %) are thus a salinity indicator and suggest cementation in waters composed of 16 mmoles/liter per yr). Variations in Mg and Sr contents across cement mosaics reflect long-term temporal fluctuations in the amou t of aragonite-to-calcite alteration and incursions of brackish waters. The pore-water oxygen reservoir is water-buffered and derived from meteoric rainfall and seawater mixing, but is seasonally enriched in 18O by evaporation. The 18O values of the cements (-3.4 to -5.0 PDB) reflect the relative proportions of cements formed during and immediately after the rainy season versus those formed during the dry season. The pore-water carbon is derived from organic respiration, atmospheric carbon, and dissolved aragonite. Organic respiration dominates the 18C of the vadose cements (-7.8 to + 1.1 PDB) and the 13C of phreatic groundwaters (-2.2 to -14.0% PDB). In contrast, 13C-enriched phreatic cements (-1.2 to +3.7% PDB) reflect carbon from dissolved aragonite, and are not in carbon isotopic equilibrium with the present groundwaters. This case example reveals the complex geochemical signal that can result from even a very simple diagenetic history, such as a single phase of meteoric diagenesis acting on a purely aragonitic sediment.

Temporal and Spatial Variability of Ooid Sand Shoals: Comparison of Mississippian of Kentucky and Quaternary of Bahamas: ABSTRACT

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

Stromatolites, Ooid Dunes, Hardgrounds, and Crusted Mud Beds, All Products of Marine Cementation and Microbial Mats in Subtidal Oceanic Mixing Zone on Eastern Margin of Great Bahama Bank: ABSTRACT

The interisland channels along the eastern margin of the Great Bahamas Bank contain lithified structures that owe their origin to recent marine cementation. This cementation appears to be commonly associated with a complex microbial community of plants and microorganisms living within a bank-margin oceanographic mixing zone. In this region, reversing tidal and wind-driven currents flow up to 3 knots (150 cm/sec) three hours out of each six-hour tidal period. Here, marine-cement crusted, carbonate mud beds are found interbedded within migrating ooid sand bars and dunes and are associated with growing, lithified stromatolites up to 2 m in height. These laminated mud beds are found with thicknesses of up to 1 m in subtidal depths of 4 to 8 m (12 to 25 ft). The muds appear to be homogeneous, but closer examination by SEM and under a microscope reveals they are composed of pelletoid aggregates of needle-shaped aragonite crystals with diameters of up to 50 ..mu... The size of these soft pellets is similar to the smaller grains of ooid sands that are abundant in the area. This size similarity could explain why both the mud beds are found in similar high-energy hydraulic regimes as the ooid sands, but doesmore » not suggest how or why the aggregates of pure aragonite needles form. A high production of ooid sand within this bank margin environment permits the formation of natural levees along the margins of tidal channels. The back sides of these levees are being lithified by marine cements to form hardgrounds. Skeletal and ooid sand dunes stabilized by Thallasia in channel bottoms also are becoming lithified. Grapestones form at the distributaries of flood tidal deltas of ooid sand. All of these features have a common attribute: they are continually in contact with the turbulent mixing-zone waters.« less

Architectural Elements and Depositional Model of Bahamian-Type Ooid Marine Sand Belts Postulated from Surface Features: ABSTRACT

Modern marine ooids are currently forming along the platform margins of Great Bahama and Little Bahama Bank as strike-oriented marine sand belts. Other workers showed that these types of sand bodies consist of flood ramps and shields, and cross-cutting channels that terminate in spillover lobes. Sediment transport is generally perpendicular to the sand belt's axis. Although ooid sands are transported locally by daily tidal currents and waves, wholesale movement of an entire sand body probably only occurs during hurricanes. A completely preserved Bahamian-type marine sand belt should consist of an elongate (parallel with paleostrike), convex-upward body several miles long, and up to about 15-20 ft thick. Isochores should delineate the ramp and shield areas as the thickest zones and the channels as cross-cutting thin zones. Thin channel fills should terminate near leeward margins and pass into thicker zones representing spillover lobes. Large-scale, unidirectional cross-beds should be present in both the basal part of the sand body and in the spillover lobes, and abruptly overlie bioturbated wackestones and/or mudstones of the platform interior. Smaller bidirectional sets should prevail in the upper parts of the sand body. If the sand body prograded during deposition, its seaward margin may contain a gradational coarsening-upwardmore » sequence.« less