Ooid Shoals Research Articles

Upper Mississippian (Chester) Ooid Shoals: Austin Upper Mississippian Field New Mexico and Mocane-Laverne Field Oklahoma and their Relation to North American Chester Paleogeography

Upper Mississippian (Chester) Ooid Shoals: Austin Upper Mississippian Field New Mexico and Mocane-Laverne Field Oklahoma and their Relation to North American Chester Paleogeography

ABSTRACT: Upper Mississippian (Chester) Ooid Shoals: Austin Upper Mississippian Field New Mexico and Mocane-Laverne Field Oklahoma and their Relation to North American Chester Paleogeography

ABSTRACT: Upper Mississippian (Chester) Ooid Shoals: Austin Upper Mississippian Field New Mexico and Mocane-Laverne Field Oklahoma and their Relation to North American Chester Paleogeography

Upper Triassic reef facies in the Asher-Atlit-1 borehole, Northern Israel: Microfacies, cement stratigraphy and paleogeographic implications

The late Triassic succession of the Asher-Atlit 1 borehole is over 1000 m thick, and is composed of reefal and associated facies interbedded with volcanics of Norian age. Only borehole cuttings are available. Microfacies classification and cement stratigraphy determined by optical and CL microscopy, allowed discrimination of six episodes of reef establishment, progradation, shallowing, and termination. Organic buildups are constructed of reef-building biota (sponges, possible corals, encrusting organisms) typical for the late Triassic of the Tethys. Reef-associated facies include fore-slope, central reef, ooid shoal, lagoonal, and supratidal environments. Cement zoning patterns trace diagenetic signatures which range from early neomorphic skeletal replacements and original marine cements, via characteristic burial sequences; depositional and diagenetic sequences are terminated by marginal marine intra- or supratidal conditions, and subaerial exposure with pedogenic overprints. Volcanic episodes tend to be associated with termination of carbonate sedimentation episodes, while volcanic quiescence and subsidence permit vertical progradation of reefal and associated facies. The depositional and progradational environment, rapid rate of sedimentation, periodicity, association with volcanics, and regional considerations, suggest a depositional setting on the rifted shelf-margin of the nascent Neo-Tethys, with a possible eustatic overprint.

3-D Geometries and Porosity Trends of Subsurface Ooid Shoal Reservoirs in the Mississippian Ste. Genevieve Formation of the Illinois Basin, USA

3-D Geometries and Porosity Trends of Subsurface Ooid Shoal Reservoirs in the Mississippian Ste. Genevieve Formation of the Illinois Basin, USA

Cyclic sedimentation across a Middle Ordovician carbonate ramp (Duwibong Formation), Korea

The Middle Ordovician Duwibong Formation (about 100 m thick), Korea, comprises various lithotypes deposited across a carbonate ramp. Their stacking patterns constitute several kinds of meter-scale, shallowing-upward carbonate cycles. Lithofacies associations are grouped into four depositional facies: deep- to mid-ramp, shoal-complex, lagoonal, and tidal-flat facies. These facies are composed of distinctive depositional cycles: deep subtidal, shallow subtidal, restricted marine, and peritidal cycles, respectively. The subtidal cycles are capped by subtidal lithofacies and indicate incomplete shallowing to the peritidal zone. The restricted marine and peritidal cycles are capped by tidal flat lithofacies and show evidence of subaerial exposure. These cycles were formed by higher frequency sea-level fluctuations with durations of 120 ky (fifth order), which were superimposed on the longer term sea-level events, and by sediment redistribution by storm-induced currents and waves. The stratigraphic succession of the Duwibong Formation represents a general regressive trend. The vertical facies change records the transition from a deep- to mid-ramp to shoal, to lagoon, into a peritidal zone. The depositional system of the Duwibong Formation was influenced by frequent storms, especially on the deep ramp to mid-ramp seaward of ooid shoals. The storm deposits comprise about 20% of the Duwibong sequence.

Stalked crinoids from a Jurassic tidal deposit in western North America

This is the first systematic and paleoecological study of a crinoidal limestone (encrinite) from the Jurassic System of North America. The encrinite is part of a shallow-water tidal facies of the Middle Jurassic Carmel Formation located at Mount Carmel Junction (southwestern Utah, U.S.A.) and may represent one of the youngest shallow-water encrinites in the geological record. In the past, the crinoid at this locality was referred to as Pentacrinus asteriscus, a name used to describe almost all of the crinoid columnals found throughout the Jurassic of the U.S. western interior. However, systematic work indicates that the crinoid is Isocrinus nicoleti and is the first non-endemic crinoid to be reported from North American Jurassic strata. Although articulated pinnules and arms have been found, I. nicoleti occurs predominantly as well-preserved, partially articulated columnals. The crinoids occur within a tidal complex consisting of ooid shoal, tidal channel, and lagoonal facies. The unique environmental and ecological conditions which existed in the southernmost end of the Jurassic North America seaway may have allowed for the development of this crinoid colony and subsequent deposition of the encrinite.

Origin and Paleoecology of Free-Rolling Oyster Accumulations (Ostreoliths) in the Middle Jurassic of Southwestern Utah, USA

Ostreoliths are abundant in the limestones, siltstones, and shales of the upper Carmel Formation (Middle Jurassic) in southwestern Utah. These are circumrotatory accumulations of the oyster Liostrea strigilecula, along with Plicatula bivalves, the bereniciform bryozoan Eurystrotos, the boring Gastrochaenolites, the nestling bivalve Modiolus, and the encrusting inarticulate brachiopod Discinisca. The ostreoliths are mostly spherical to discoidal and range in diameter from less than two centimeters to more than half a meter. They are found in two laterally-extensive horizons, one directly on a hardground and the other passing through ooid shoal and lagoonal facies. The ostreoliths formed in two ways. One group started as Liostrea-encrusted hardgrounds that were eroded and fragmented on the seafloor. These oyster accumulations collected additional oyster recruits on their sides and undersides until final burial. The second group of ostreoliths developed on soft substrates in oolitic shoals where Liostrea encrusts mollusk valves. The oysters grew on the upward-facing convex surfaces; the downward-facing concave sides were encrusted by bryozoans and Plicatula. After the ostreolith rotated, Liostrea again colonized the upper surfaces but died on the new undersides. The dead Liostrea valves on the undersides were encrusted by Plicatula and the bryozoans. Lithophagid bivalves bored the dead valves as well, and Modiolus nestled in vacant borings and between oyster valves. This accumulation process continued until the ostreoliths were buried in tempestites. These ostreoliths represent a mobile community in which faunal composition and diversity was controlled by the rate of overturning and the consistency of the substrate. Morphological features of Liostrea, such as a deep left valve, rapid growth and calcitic composition, were essential for ostreolith development. One laterally-extensive ostreolith horizon was formed when storm currents distributed them across facies boundaries.

Carbonate production and offshore transport on a Late Jurassic carbonate ramp (Kimmeridgian, Iberian basin, NE Spain): evidence from outcrops and computer modelling

Abstract The complex interplay between shallow-water carbonate production, pelagic sedimentation, and sediment erosion and redeposition on a Kimmeridgian carbonate ramp is analysed from field data and computer modelling. Field data come from reconstructed cross-sections near Zaragoza and near Teruel, NE Spain. Inner ramp areas are dominated by coral patch reefs, ooid shoals and bioclastic packstones and grainstones, whereas middle ramp areas are typified by tempestites and pinnacle reefs. Outer ramp areas are dominated by carbonate muds and marls. The large-scale stratigraphic features of the ramp are simulated using the forward modelling computer program carbonate 6. A good match is found between real and simulated stratigraphic thicknesses along the two sections, large-scale internal stratigraphic geometries, amounts of aggradation and progradation, and the location of sequence boundaries, flooding surfaces and systems tracts in the two modelled cross-sections. To investigate the origin of the offshore carbonate mud, two synthetic stratigraphies, which both closely replicate the overall ramp geometries, were generated by computer simulation. These two simulations are based on two different hypotheses that had not been resolved from field investigation: (1) that most of the outer ramp mud was derived from erosion and redeposition from inshore areas; (2) that most of this mud originated from pelagic sedimentation. Detailed comparison between the simulated occurrence of sedimentary facies (characterized in the program by depth and process of deposition) and the real facies occurrence indicates that the closest match is given by hypothesis (1) where most of the offshore mud is produced by resedimentation from inshore areas. Offshore transport and resedimentation is thought to have been favoured by the windward orientation of the ramp, and offshore transport was highest during periods of sea-level highstands.

A late-devonian (Famennian)Renalcis-Epiphyton reef at Zhaijiang, Guilin, South China

Microbial reefs, together with stromatolitic mounds and ooid shoals, constitute massive limestones in Famennian platform marginal strata in Guilin, in sharp contrast to the well-known coral-stromatoporoid reefs in the Givetian and Frasnian. Microbes played a significant and important role as stabilizers in the Famennian carbonate deposits of Guilin. A reef at Zhaijiang was constructed byEpiphyton andRenalcis, and is representative of such carbonate buildups. The reef is situated 10 km west of Guilin and corresponds to a microbe-dominated platform margin carbonate complex. Organisms in the Zhaijiang microbial reef are low diversity and dominated by ostracods and two genera of microbes,Epiphyton andRenalcis. Other microbial genera such asSphaerocodium andWetheredella occur in most of reef facies in Guilin, but their role as reef builder is doubtful because they occur only in minor amounts. The same four genera occur in volumetrically significant amounts in the upper Devonian carbonate complexes of Alberta. Canada and Western Australia. However.Epiphyton is more abundant in the Guilin reefs.

Jurassic aeolian oolite on a palaeohigh in the Sundance Sea, Bighorn Basin, Wyoming

ABSTRACTAeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio‐eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non‐glacial. Numerous lenticular bodies of cross‐stratified oolite lie near the contact between the lower and upper members of the mudstone‐dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north‐central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross‐strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat‐bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand‐free deflation surface. When the Sundance Sea transgressed the dunes, a thin (<1 m thick), wave‐rippled, oolite veneer formed on the upper surface of the aeolianite. Previous workers, primarily on the basis of sedimentary structures in the veneer, interpreted the oolite lenses as tidal sand bodies.The dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the Sheridan Arch. This structural high was delineated by previous workers who demonstrated thinning of pre‐upper‐Sundance Formation strata and localized development of ooid shoals. Ooids that formed in shoals on the windward (southern) side of the palaeohigh were exposed and deflated during lowstand. Thin, scour‐filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites. Farther north (down‐wind), oolitic dunes prograded over thinly laminated lagoonal silts. When relative sea level began to rise, the uncemented dunes were buried under fine‐grained marine sediment as the lee side of a low‐relief island was inundated.

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.

Digital shaded relief image of a carbonate platform (northern Great Bahama Bank): Scenery seen and unseen

A mosaic image of the northern Great Bahama Bank was created from separate gray-scale Landsat images using photo-editing and image analysis software that is commercially available for desktop computers. Measurements of pixel gray levels (relative scale from 0 to 255 referred to as digital number, DN) on the mosaic image were compared to bank-top bathymetry (determined from a network of single-channel, high-resolution seismic profiles), bottom type (coarse sand, sandy mud, barren rock, or reef determined from seismic profiles and diver observations), and vegetative cover (presence and/or absence and relative density of the marine angiosperm Thalassia testudinum determined from diver observations). Results of these analyses indicate that bank-top bathymetry is a primary control on observed pixel DN, bottom type is a secondary control on pixel DN, and vegetative cover is a tertiary influence on pixel DN. Consequently, processing of the gray-scale Landsat mosaic with a directional gradient edge-detection filter generated a physiographic shaded relief image resembling bank-top bathymetric patterns related to submerged physiographic features across the platform. The visibility of submerged karst landforms, Pleistocene eolianite ridges, islands, and possible paleo-drainage patterns created during sea-level lowstands is significantly enhanced on processed images relative to the original mosaic. Bank-margin ooid shoals, platform interior sand bodies, reef edifices, and bidirectional sand waves are features resulting from Holocene carbonate deposition that are also more clearly visible on the new physiographic images. Combined with observational data (single-channel, high-resolution seismic profiles, bottom observations by SCUBA divers, sediment and rock cores) across the northern Great Bahama Bank, these physiographic images facilitate comprehension of areal relations among antecedent platform topography, physical processes, and ensuing depositional patterns during sea-level rise.

Update on the northern Australian Cambrian spongesRankenella, JawonyaandWagima

New material from the Ordian to early Templetonian (early Middle Cambrian) of the Georgina Basin and Daly Basin in northern Australia allows further observations on the anthaspidellid sponge Rankenella mors and the chambered heteractinide sponge Jawonya gurumal respectively. Explanate specimens of R. mors are found to bear closely spaced, rimmed oscules, and the known range of the species is extended from the Ranken Limestone near Soudan to include the Arthur Creek Formation near Ammaroo. Jawonya gurumal from the Tindall Limestone near Claravale is better preserved than type and topotype material, and demonstrates that the genus is two-walled, and not one-walled as originally described. Furthermore, exopore architecture is much more complex than previously envisaged. The co-occurring related genus Wagima is also considered to be two-walled. Rankenella in the Ranken Limestone flourished in a low-energy, shallow subtidal marine environment subject to episodic higher-energy events that generated ooid shoals and flat-pebble conglomerates. In the Arthur Creek Formation the genus occurs in anaerobic calcimudstone deposited in a low-energy shelf area of limited circulation. Jawonya and Wagima are elements of a spongemollusc association in open shelf onkoid limestones of the Tindall Limestone. Rankenella is among the oldest known intact demosponges. Spiculation in Jawonya and Wagima suggests that the history of the heteractinide astraeospongiid-wewokellid lineage was one of Ordovician reduction or loss of original polyactines in favour of octactines, followed by polyactine reinstatement in the Carboniferous.

End‐Triassic bivalve extinction: Lombardian Alps, Italy

A major biotic crisis affecting virtually all major marine invertebrate clades occurred at the close of the Triassic. Species‐level data on bivalves from the Lombardian Alps of Italy record the extinction and suggest a possible causal mechanism. A significant decline in species richness is observed during the lower Rhaetian, where 51% of bivalve species, equally distributed among infaunal and epifaunal filter‐feeders, went extinct. The taxonomic loss at the middle Rhaetian was more severe, where 71% of the bivalve species were eliminated, including all infaunal and 50% of the epifaunal species. The data indicate that the extinction selectively eliminated infaunal bivalves. An initial loss of bivalve species richness during the middle and upper Rhaetian correlates with changes in sedimentary facies related to a fall in relative sea level. This sea level fall is marked by the onset of peritidal micrites and shifting ooid shoals which may have rendered substrates unsuitable for both epifaunal and infaunal bi...

Baselap Patterns and the Recognition of Lowstand Exposure and Drowning--A Mississippian-Ramp Example and its Seismic Signature

ABSTRACT Seismic sequence stratigraphy draws heavily upon analysis of stratal discordances to decipher the depositional history of a basin. Onlap and downlap, which constitute two different classes of baselap, are the most important types of discordances used to identify sequence boundaries. In the absence of a regional grid of seismic lines from which the total stratal geometry can be documented, however, baselapping relationships are often misleading and misinterpreted. Class 1 baselap, or onlap, is frequently identified along seismic sequence boundaries, but it may also he the most ambiguous lapout pattern to interpret. Onlap includes four subclasses: Subclass 1a--platform proximal onlap--onlap in the direction of the source of sediment supply; Subclass 1b--basinal proximal onlap in the dir ction of the sediment source; Subclass 1c--distal horizontal onlap--onlap of horizontal bottomset in a direction away from the source of sediment supply; and Subclass 1d--distal inclined onlap--inclined bottomset onlapping an inclined surface. Class 2 baselap, or downlap, includes three subclasses: horizontal downlap, or Subclass 2a, describes initially inclined toeset strata that downlap an initially horizontal, bottomset surface; Subclass 2b, or climbing downlap, in which toeset strata downlap a toeset and foreset surface inclined upward in the direction of lapout; and Subclass 2c, or descending downlap, describes toeset strata downlapping a foreset-toeset surface inclined downward in the direction of sediment input. The distinction between these types of baselap is important in aiding the reconstruction of relative sea-level and depositional history, as for example, in resolving whether a carbonate platform bounded by baselapping strata was subjected to a sea-level fall or a sea-level rise. A Mississippian carbonate ramp exposed along the late Paleozoic southern margin of North America demonstrates a dual history of lowstand exposure and transgressive drowning. A sequence boundary (SB345) developed at the end of Osagean time by subaerial exposure at the top of a basinward-prograding Early Mississippian carbonate ramp. The resulting hiatus lasted as much as 10 m.y. and left a chert gravel residuum across the shelf. In addition, deltaic and shoreface systems prograded from the northeast and occupied a lowstand accommodation zone along the basinward margin of the ramp near the present Mississippi embayment. These lowstand deposits form a relatively thick downlapping wedge of siliciclastic strata above the Early Mississippian ramp and SB345. A late Meramecian to Chesterian s a-level rise followed, forcing the siliciclastic depositional systems to retrograde landward. Concomitantly, carbonate sedimentation resumed across the flooded shelf and formed ooid and skeletal grain shoals. Carbonate sedimentation was short-lived, however, because large quantities of mud, derived from the Ouachita orogen to the south, encroached the shelf during the Chesterian sea-level rise, forming an onlapping succession of black shale. The rapid increase in accommodation space and the influx of suspended sediment and nutrients combined to submerge, suffocate, and poison the carbonate factory. Subaerial exposure and drowning led to formation of two baselap surfaces: (1) a lowstand unconformity, and (2) a drowning surface. They lie so close to each other on the shelf, however, that s ismic discrimination is impossible. At lower frequencies (35-50 Hz) synthetic seismic sections show reflections baselapping only one surface, the lowstand unconformity when in fact there are two surfaces of very different origin. At higher frequencies (75-100 Hz), baselap is visible at the drowning surface. The paradox is that although the Kinderhookian-lower Chesterian carbonate ramp was terminated by drowning, the visible seismic baselap was due to lowstand exposure.

The ecology of Lower Cambrian buildups from Zuune Arts, Mongolia: implications for early metazoan reef evolution

ABSTRACTReefal buildups in western Mongolia of mid‐Early Cambrian (late Atdabanian) age flourished during a period when shelf seas were globally widespread. The succession at Zuune Arts records the transition from shallow marine siliciclastic sediments (Bayan Gol Formation) to shallow marine, but still clastic‐influenced, carbonates (Salaany Gol Formation). The Salaany Gol Formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow, gently inclined shelf in a rapidly subsiding epicontinental sea. Cycles commenced with the growth of subtidal metazoan‐calcimicrobe aggregative communities on an open shelf. The resultant buildups were commonly engulfed by extensive, massive microbial stromatolites when they grew in agitated intertidal conditions. Latterly, they were smothered by ooid shoals in response to rapid sea level rise.Four types of reefal buildup are distinguished: (1) green‐coloured calcimicrobe (Tarthinia, Epiphyton, Gordonophyton and Renalcis) boundstones; (2) red‐or green‐coloured Cambrocyathellus‐Tarthinia‐Epiphyton bafflestones; (3) red‐coloured Okulitchicyathus bindstones; and (4) red‐coloured radiocyath‐archaeocyath‐cribricyath bioherms. Each is interpreted as having grown at increasing depths and possibly sedimentation rates. The buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones, and are best developed towards the top of the formation, when sea level was high. Thickets of large, solitary archaeocyaths are also inferred in the deeper interbiohermal areas.These buildups had synoptic relief and constructed porous structures with growth‐framework cavities housing diverse coelobiontic communities. Extensive synsedimentary cements are present, including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids. These early reefs thus have geological fabrics similar to later Phanerozoic representatives. It is proposed however, that this ecosystem was largely composed of generalist and opportunistic filter and suspension feeders which were dependent upon a far higher input of nutrients than modern day reefal developments. Bacteria were probably the main primary producers, from both planktic and benthic cyanobacterial communities. The diversity of each buildup assemblage appears to be controlled by primary cavity size, the richest fauna belonging to the highly tiered radiocyath‐dominated community inferred to have lived in the deepest waters. The communities at Zuune Arts can be compared with other buildups from the early Cambrian, and with Lower Ordovician receptaculid‐calcimicrobe‐solitary sponge bioherms known from the USA and Siberia.

Sequence Stratigraphy of Middle and Upper Jurassic Strata of Southwestern Alabama

Middle and Upper Jurassic systems tracts of southwestern Alabama differ from those of the western Gulf rim, showing: (1) profound influence of antecedent topography; (2) low early subsidence rates; and (3) greater clastic influx from adjacent uplands. Werner Anhydrite and Louann Salt represent the earliest marine incursion onto the Gulf rim following initial rifting; they onlap upper Paleozoic basement and garben-filling Eagle Mills red beds. Because basin-wide evaporative drawdowns overprint even higher order eustatic sea level changes, transgressive systems tracts (TST) and highstand systems tracts (HST) are indistinguishable. Anhydrite and shale caps accumulated via interstratal halite dissolution. Oxfordian Norphlet siliciclastics form a continental lowstand systems tract as illustrated by abrupt contact with underlying marine evaporites without intervening progradational marginal marine facies. Marine-reworked uppermost Norphlet sandstone marks the base of a subsequent TST, which includes overstepping lower Smackover lithofacies (laminated mudstone, algal-laminated mudstone, and pellet wackestone). The upper Smackover HST is characterized by formation of rimmed shelves upon which algal mounds and aggrading ooid grainstone parasequences accumulated. Shallow lagoonal carbonate and evaporite saltern deposition occurred behind ooid shoals; fine-grained siliciclastics accumulated in updip areas. Equivalents of Smackover A, Smackover B, Bossier, and Gilmer sequences are largely masked by influx of Haynesvillemore » and Cotton Valley continental clastics. Lack of biostratigraphic data, a consequence of restricted fauna, precludes useful age assignments for these sequences in Alabama. Middle and Upper Jurassic systems tracts of southwestern Alabama are regionally atypical and cannot serve as a model for Gulf-wide sequences.« less

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.

Coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification (Draken Formation, Upper Riphean, Svalbard)

The Draken Formation (120–250 m) of northeast Spitsbergen (Svalbard) forms part of a thick Upper Proterozoic carbonate platform succession. It consists predominantly of intraformational dolomitic conglomerates, with excellent textural preservation. Six main lithofacies were recognized in the field: quartz sandstones, stromatolitic mats, conglomerates with silicified intraclasts, dolostone conglomerates with desiccated mudrocks, oolitic/pisolitic grainstones and fenestral dolostones. A series of five main gradational biofacies were recognized from silicified (and rare calcified) microfossils. Biofacies 1 represents low-energy subtidal benthos (erect filaments) and plankton (acritarchs and vase-shaped microfossils) whereas biofacies 2 to 5 are microbial mat assemblages (with filamentous mat-builders, and associated dwellers and washed-in plankton) ranging from basal intertidal to high intertidal/supratidal. Colour values (a measure of the lightness of the colour shade) of sawn rock samples were quantified using a Munsell chart, and exhibit a pronounced variation (means of major groups varying from 4.0 to 5.95) across the spectrum of subtidal to supratidal sediments as inferred from other criteria. The lightening in progressively more exposed sediments is related to lowering of organic carbon contents, probably mainly by oxidation. Six types of early cement have been recognized. Calcite microspar (type 1) is common as a subtidal cement in many Proterozoic formations, whereas types 2 (subtidal isopachous fringes), 3 (subtidal hardground dolomicrite) and 4 (intertidal meniscus dolomicrite) are very similar to Phanerozoic examples except for their dolomitic mineralogy. Types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena. They characterize the fenestral lithofacies and compare with modern supratidal cements. Consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching, dolomitization, silicification were all significant syndepositional processes altering the original metastable carbonates. The data set provides evidence for a spectrum of peritidal environments including ooid shoals, protected subtidal, tidal sandflats and protected carbonate mudflats. Different sections show a preponderance of particular facies. The coastal lithofacies continuum was completely dolomitized, unlike offshore to ooid shoal facies of adjacent formations. Dolomitization thus bears a relationship to depositional bathymetry. Although hydrodynamics clearly have a role, the potential importance of whiting precipitation in raising Mg/Ca in marginal marine environments is also stressed.