Skeletal Grainstones Research Articles

Geobiological Aspects of the Earliest Triassic Microbialites Along the Southern Periphery of the Tropical Yangtze Platform: Initiation and Cessation of a Microbial Regime

Abstract Microbialite masses within the lowest Triassic strata along the southern periphery of the tropical Yangtze Platform were produced in a post-Permian microbial regime. These microbialites (the Hindeodus parvus Zone), which are represented by thrombolites, occur only where terrigenous sediment influx was rare, even in shallow-marine settings. The lower parts of the thrombolites lie upon a distinctly unconformable Permian-Triassic boundary and exhibit a thin-bedded to thick-bedded planar structure. In contrast, the upper parts of the thrombolites contain domed macrostructures that interact in complex ways with skeletal grainstones and packstones. Irregular frameworks of thrombolite bodies differ in degree of lateral and vertical accumulation and in the amalgamation of mesoclots of microbial origin; they exhibit marked variations in texture. A transgressive episode occurred in the earliest Triassic following the mass extinctions, and this included the initiation of microbial regimes that usually forme...

Three-dimensional sequence analysis of a subsurface carbonate ramp, Mississippian Appalachian foreland basin, West Virginia, USA

Well-cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three-dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio-eustasy during the transition into ice-house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid-ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth-order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic-prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth-order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea-level changes controlled regional sequence development. Thrust-load induced differential subsidence of fault-blocks, coupled with in-plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.

Facies development of a Late Ordovician mixed carbonate-siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada

The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [∼5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform development followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumulation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectonically influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographically restricted platform-interior facies.

Mississippian Platform and Basin Successions from the Todrha Valley (northeastern Anti‐Atlas), southern Morocco

AbstractCarboniferous rocks of Mississippian age are exposed in Jebel Asdaf and Jebel Tisdafine, south and north respectively of the fault‐controlled Todrha Valley, between the towns of Tinerhir and Tinejdad in the Anti‐Atlas of southern Morocco. The age, as determined by microfossils, and facies of the dominantly siliciclastic successions of the two areas are different. The Jebel Asdaf Formation (245 m exposed; base and top not seen) is a shallow shelf sequence of late Tournaisian age. The Jebel Tisdafine Formation is a succession of turbidites (at least 1500 m thick), which rests unconformably on Devonian limestones and is of late Viséan age. Limestone beds in each formation have different origins. Skeletal grainstones and packstones in the Jebel Asdaf Formation formed when sand supply to the shelf was episodically reduced. Ooid‐rich skeletal packstones and grainstones in the Jebel Tisdafine Formation, characterized also by the presence of well‐rounded, coarse‐sand grade quartz grains, were derived from a contemporary carbonate platform. Individual beds in the Jebel Tisdafine Formation are essentially tabular and can be followed for long distances. Channelling is very rare. The turbidites, which are quartz‐rich, were transported from west to east across a basin floor with low slopes. Jebel Azguine, northwest of Tinejdad, is separated from Jebel Tisdafine by a major shear zone. It is made up of quartz sandstones that differ from those of both the Jebel Asdaf and Jebel Tisdafine formations in not containing any carbonate or body fossils, in its trace fossil assemblage dominated by vertical burrows, and in the presence of very thick beds. It is tentatively suggested that the Jebel Azguine sandstones are older than Carboniferous, to which they are assigned on some geological maps. The conclusions of this research are markedly different from accounts of the geology of the area published previously. Copyright © 2007 John Wiley & Sons, Ltd.

Facies, geometry and geological significance of Late Ordovician (early Caradocian) coral bioherms: Lourdes Formation, western Newfoundland

AbstractLate Ordovician coral bioherms in the Lourdes Formation of western Newfoundland exhibit a complex mixing of architectural elements, including framework, boundstone and suspension deposits. The bioherms occur within a narrow (16 m) stratigraphic interval, and a prominent unconformity truncates the interval of bioherm growth and tops of many of the bioherms. The buildups developed along a carbonate ramp. They occur isolated and in groups, individuals in groups are aligned in parallel orientation. The sizes of the bioherms range from small (50–100 cm) coral piles to columnar and dome‐shaped masses (1–15 m); however, topographic relief was never more than ≈1 m. Bioherm construction reflects: (i) stacking of the tabulate coral Labyrinthites chidlensis, and less common stromatoporoids; (ii) accumulation of microbial‐stromatoporoid boundstone and suspension deposits within shelter cavities between corals; and (iii) detrital bioherm‐flank skeletal grainstone beds. Trypanites borings are common in the tops of coral heads. The bioherms exhibit three growth‐development stages: (i) seafloor stabilization, wherein rare, abraded coral colonies lie scattered within pelmatozoan/skeletal grainstone lenses; (ii) colonization, wherein corals (L. chidlensis), rare stromatoporoids (Labechia sp.), and other biota (bryozoans) produced a bioherm overlying the basal sediment base; and (iii) diversification, which is marked by a more diverse range of fauna and flora as well as occurrence of shelter‐cavity deposits. The diversification stage usually makes up more than 70% of a bioherm structure, and, in some defines multiple periods of start‐up and shut‐down of bioherm growth. The latter is defined by bored omission surfaces and/or deposition of inter‐bioherm sediment. The Lourdes bioherms have a similar ecological structure, biotic diversity and depositional environment to patch reefs in the equivalent Carters Limestone in Tennessee. The mixture of coral stacking and boundstone as architectural elements identify an Early Palaeozoic transition of reef‐design development along shallow‐water platforms that began to displace the muddy (boundstone, bafflestone) carbonate buildups more typical of the Early and Middle Ordovician time.

Impact of long-term climate change and sea-level fluctuation on Mississippian to Permian mid-oceanic atoll sedimentation (Akiyoshi Limestone Group, Japan)

To understand the impacts of long-term climatic episodes and sea-level changes on mid-oceanic atoll sedimentation, the stratigraphic changes of litho- and bio-facies, and stratigraphic levels of freshwater diagenetic alteration and biostratigraphic discontinuity of the Akiyoshi Limestone Group (upper Viséan to upper Capitanian) were reviewed. This group records approximately 70 million years of atoll sedimentation in a mid-oceanic realm of the Panthalassa Ocean, which developed nearly synchronously with the Late Paleozoic Gondwana glaciation. The entire succession of the Akiyoshi Limestone Group was divided into four facies associations; skeletal-oolitic grainstone, muddy limestone, muddy limestone–skeletal grainstone, and reefal limestone associations. Skeletal-oolitic grainstone and reefal limestone associations (upper Viséan to Bashkirian) are characterized by an abundance of oolite and crinoidal grainstone, and reef facies of warm-adapted metazoan reef-builders (corals, Chaetetes, and bryozoans), respectively. Characteristics of skeletal-oolitic grainstone and reefal limestone associations indicate their deposition in a warm climate with elevated sea-level. Muddy limestone association (Moscovian to Kasimovian) records the demise of the warm-adapted metazoan reef builders and frequent emergence events of the buildup. Many geochemical data support the significant drop in temperature during Moscovian through Kasimovian time. These depositional and biotic events of muddy limestone association indicate that its sedimentation was strongly affected by cool climate and generally low sea-level. Muddy limestone–skeletal grainstone association is characterized by an upward-increasing significance of calcimicrobes, calcareous algae, and calcisponges. The succession records less frequent emergence events than that of muddy limestone association II. These characters reflect a warming climate and sea-level rise in Gzhelian to Capitanian time. The climate episodes and sea-level changes interpreted from the Akiyoshi Limestone Group generally correspond to the global trend of Mississippian to Permian long-term climatic change and sea-level fluctuation related to the major glacial events of Gondwana glaciation. The late Viséan to Bashkirian warm climate and elevated sea-levels, Moscovian to Kasimovian cooling and sea-level lowering, and Gzhelian to Capitanian warming and sea-level rise recorded in the Akiyoshi buildup are comparable to the global climate model and sea-level fluctuation, which contain Early to Middle Mississippian pre-glacial warm climate and highstands, Middle to Late Pennsylvanian cooling and related glacio-eustatic sea-level fluctuation at the culmination of Gondwana glaciation, and Early to Middle Permian global warming and sea-level rise due to the retreat of Gondwana ice sheets. Akiyoshi atoll sedimentation was most significantly affected Pennsylvanian global cooling and related sea-level lowering, but their impact was short-lived. The onset of the cooling and frequent emergence events in the Akiyoshi buildup occurred delayed, approximately 10 millions years later than that of comparable episodes on the Pangean shelves. The post-glacial warming in the Akiyoshi buildup started at least several millions of years earlier than in epicontinental seas of the Pangea. This phenomenon may be due to the palaeoposition of the Akiyoshi buildup in a mid-oceanic realm of the Panthalassa Ocean.

Skeletal-Carbonate Neptunian Dikes of the Capitan Reef: Permian, Guadalupe Mountains, Texas, U.S.A.

Sedimentary dikes in the Permian (Guadalupian) Capitan Reef of West Texas and New Mexico are composed of siliceous sand, skeletal carbonate, or cement. Skeletal-carbonate neptunian dikes appear to be a relatively uncommon type; only four are known by the authors, but they are important in providing information about platform growth, sediment sources, and dispersal on the platform, and the biota that were present on the platform. The dikes occur within massive poorly sorted skeletal grainstones typical of the reefal part of the Capitan Formation (the Capitan-massive) that is correlative with upper Guadalupian Yates B strata on the platform. They formed during later Yates B, Yates C, and Tansill deposition on the platform, following closely a major period of progradation of the platform margin. The dikes are approximately vertical, and parallel to and within about 100 m of the platform margin. Fissure formation was a probable consequence of flexing and fracturing of lithified shallow-water Capitan-massive over compacting unconsolidated slope and basinal sediment. At least three stages of opening and filling are recorded in the largest and most complex dike, with each sequential fissure opening forming on the basinward side of the previous cemented filling. In two of the dikes, centimeter-thick deposits of vertically laminated peloidal grainstone, which formed in situ within microbial films on the fissure wall, preceded filling by skeletal sediment. Skeletal fossils within two of the dikes are well preserved and represent organisms that either were living within the fissures or were living on the adjacent sea floor and were washed into the fissures soon after death. Other skeletal grains in these dikes, and all of those in the other two dikes, are abraded, strongly micritized, and encrusted with microbial micrite, indicative of having lain on the sea floor for a considerable time before being deposited in the fissures. Fossils in the dikes are characteristic of the reefal and adjacent outer-shelf facies tracts, but they are a biased record of the biota of that depositional area in that they consist largely of vagile or only weakly attached members of the dweller and baffler guilds living on the reef surface and of organisms living within the fissures; members of the reefal framework communities are largely absent. The absence in the dikes of fossils diagnostic of more distant facies tracts indicates that there was little across-platform movement of carbonate sediment on the platform. Deposition of terrigenous sediment is correlated with a sea-level low-stand, indicating that land-derived sediment moved across the platform otherwise infrequently.

High-resolution sequence stratigraphy of a mixed carbonate-siliciclastic, cratonic ramp (Upper Ordovician; Kentucky–Ohio, USA): insights into the relative influence of eustasy and tectonics through analysis of facies gradients

Detailed facies analysis and event stratigraphy of an Upper Ordovician (Rocklandian–Edenian) cratonic ramp succession in eastern North America yields insights into eustatically driven sequence architecture and localized tectonic instability. Seven, predominantly subtidal, mixed carbonate-siliciclastic depositional sequences (3rd order) are identified and correlated across the length of a 275-km ramp–to–basin profile. Within the larger depositional sequences (3rd order) at least two smaller orders (4th and 5th) of cyclicity are recognizable. Three systems tracts occur within each sequence (transgressive, TST; highstand, HST; regressive, RST) and are considered in terms of their component parasequences (5th order). Generally, TSTs are composed of skeletal grainstone–rudstone facies, HSTs are dominated by shaly nodular wacke-packstone facies, and RSTs are mostly calcarenite facies. Systems tracts, sequence boundaries and their correlative conformities, maximum flooding surfaces, and forced regression surfaces were traced from shallow shelf to basinal settings. This high-resolution framework also provides insight into the timing of tectonic fluctuations on this cratonic ramp during the Taconic Orogeny and documents the relative influence of tectonism on lateral facies distributions and eustatically derived cyclicity.

Cherty carbonate facies of the Montoya Group, southern New Mexico and western Texas and its regional correlatives: a record of Late Ordovician paleoceanography on southern Laurentia

The Upper Ordovician Montoya Group in southern New Mexico and westernmost Texas records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized. The medial unit of the Montoya Group, the Aleman Formation is unique because it contains abundant chert (10–70% by volume). The chert occurs as: (1) thin continuous beds of sponge spicules within mudstone or calcisiltite; (2) discontinuous, lenses or nodules within skeletal wackestones and packstones; or (3) as a replacement of skeletal grains and burrows. Coeval skeletal grainstones and muddy peritidal facies contain little chert. Phosphate (up to 5 wt.%) occurs within the underlying Upham Formation and the Aleman Formation as replacement of fossils and peloids. The abundance of chert and phosphate in these subtidal facies indicates they formed within a region of strong upwelling. Regional correlation with Upper Ordovician cherty units along the periphery of southern Laurentia and other low latitude continents suggests that upwelling was widespread and long-lived during the Late Ordovician. The upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on Gondwana during this period. Late Ordovician relative sea-level curves around the periphery of Laurentia indicate correlative third-order (1–3 my duration) fluctuations that may provide a means for high-resolution global correlations. However, the mechanism(s) that produced these long-term fluctuations are unclear.

Coral variation in two deep drill cores: significance for the Pleistocene development of the Great Barrier Reef

Variations in lithology and coral assemblages in drill cores from outer- and inner-shelf reefs are used to characterize the Pleistocene development of the Great Barrier Reef. Based on petrographic, isotopic and seismic characteristics, the outer-shelf core from Ribbon Reef 5 is divided into three sections: (1) a main reef section from 0 to 96 m is composed of six reef units, (2) a rhodolith section from 96 to 158 m is interbedded with two thin reef units and (3) a basal section from 158 to 210 m is composed of non-reefal skeletal grainstones and packstones. Two distinct coral assemblages identified in this core represent a shallow, high-energy community and lower-energy community. These two assemblages are repeated throughout the main reef section, with some units recording transitions between assemblages, and others composed of only a single assemblage. These coral assemblage data also correlate with transitions recorded by coralline algae. Using similar criteria, the inner-shelf core from Boulder Reef is divided into two sections: (1) an upper carbonate-dominated section from 0 to 34 m is comprised of four reef units and (2) an underlying mud section from 34 to 86 m is composed of siliciclastics and two thin, coral-bearing units. The four reef units in the upper section are dominated by a single coral assemblage representing a community typical of low energy, turbid environments. Taken together, these data indicate that: (1) reef growth on the inner shelf initiated later than on the outer shelf, (2) true reef ‘turn-on’ in outer shelf areas, as represented by the main reef section in Ribbon Reef 5, was preceded by a transitional period of intermittent reef development and (3) the repeated occurrence of similar coral assemblages in both drill cores indicates that the Great Barrier Reef has been able to re-establish itself, repeatedly producing reefs of similar composition over the last 500 ky, despite major environmental fluctuations in sea level and perhaps temperature.

Missing molluscs: Field testing taphonomic loss in the Mesozoic through early large-scale aragonite dissolution

Research Article| March 01, 2003 Missing molluscs: Field testing taphonomic loss in the Mesozoic through early large-scale aragonite dissolution Paul Wright; Paul Wright 1Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Lesley Cherns; Lesley Cherns 1Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Peter Hodges Peter Hodges 2Department of Geology, National Museum of Wales, Cardiff CF10 3NP, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Paul Wright 1Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK Lesley Cherns 1Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK Peter Hodges 2Department of Geology, National Museum of Wales, Cardiff CF10 3NP, UK Publisher: Geological Society of America Received: 23 Jun 2002 Revision Received: 02 Nov 2002 Accepted: 08 Nov 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2003) 31 (3): 211–214. https://doi.org/10.1130/0091-7613(2003)031<0211:MMFTTL>2.0.CO;2 Article history Received: 23 Jun 2002 Revision Received: 02 Nov 2002 Accepted: 08 Nov 2002 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 Paul Wright, Lesley Cherns, Peter Hodges; Missing molluscs: Field testing taphonomic loss in the Mesozoic through early large-scale aragonite dissolution. Geology 2003;; 31 (3): 211–214. doi: https://doi.org/10.1130/0091-7613(2003)031<0211:MMFTTL>2.0.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 As a field test for Cherns and Wright's previously published hypothesis based on Paleozoic faunas—that the paucity of certain molluscan groups in the fossil record may be largely a taphonomic, not ecologic, effect—a typical Mesozoic molluscan-dominated fauna is compared with its early-silicified (lagerstätten) equivalent from a similar depositional setting. A Lower Jurassic offshore carbonate ramp facies in South Wales comprises alternating thin limestones and organic-rich shales; the former are largely diagenetic and carry marine isotopic signatures. Their typical fauna is mainly well preserved, calcitic and bimineralic (calcite + aragonite), epifaunal and semiinfaunal bivalves. Evidence of aragonitic shells is normally restricted to molds, common in inner ramp settings in high-energy skeletal grainstones, but mostly poorly preserved and restricted to deep-burrowing bivalves in offshore deposits. However, locally, early silicification in offshore facies has preserved much more diverse faunas with abundant, formerly aragonitic, molluscs, including many more infaunal burrowers. It appears that major aragonite dissolution normally distorted both apparent diversity (65% decrease in bivalve diversity) and the trophic structure of the offshore facies, providing aragonite that probably sourced the diagenetic carbonates. We suggest that aragonitic shells were selectively dissolved in the upper sediment column in lower-energy settings, where high organic contents favored microbial decay and acidity; such early dissolution was absent from the higher-energy facies that originally had low organic contents. Taphonomic loss through early skeletal aragonite dissolution was an equally important process in Mesozoic offshore shelf environments, and although still leaving depleted molluscan-dominated faunas, resulted in a massive distortion of diversity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Carbonate-filled channel complexes on carbonate ramps: an example from the Peerless Park Member [Keokuk Limestone, Visean, Lower Carboniferous (Mississippian)], St. Louis, MO, USA

The Peerless Park Member [Keokuk Limestone, Lower Carboniferous (Visean, Mississippian)], exposed in the St. Louis, MO, area includes a carbonate-filled channel complex with lithology and faunal elements distinct from under- and overlying strata. The complex is confined within a channel of 100-m apparent width that is under- and overlain by cherty, glauconitic crinoid–bryozoan–skeletal wackestone, packstone and grainstone with abundant hardgrounds and little evidence for wave or current energy. Laterally, the valley includes two subchannels separated by a subtle high. The deeper subchannel truncates up to 5 m of strata. The maximum thickness of the channel fill and associated deposits is 6 m. The channel fill includes a matrix-supported conglomerate locally at its base, non-cherty quartz silty–foraminifer–peloid–crinoid–bryozoan–skeletal grainstone with spectacular lateral accretion foresets up to 5 m thick, and cherty skeletal packstone. A capping foraminifer–skeletal grainstone is erosionally overlain by a 10-cm thick quartz siltstone that grades up into glauconitic crinoid–bryozoan–skeletal wackestone and packstone. On the channel flanks, correlative strata of the Peerless Park Member are less than 1 m thick, and include a lower non-cherty crinoid–foraminiferal–skeletal grainstone, a middle cherty packstone and an upper skeletal grainstone. The upper grainstone is in turn erosionally overlain by the thin quartz siltstone that abruptly grades into the crinoid–bryozoan–skeletal wackestone and packstone of the upper part of the Keokuk Limestone. The strata of the Peerless Park Member are interpreted to be the result of a relative fall of sea level that places shallow water, high-energy inner ramp grainstone immediately above deeper water, outer ramp packstone and grainstone. There is no evidence for gradual downward or basinward migration of facies belts. Instead, shallow water facies appear to have abruptly jumped downdip. Likewise, no field or petrographic evidence for subaerial exposure has been recognized, so the relative fall in sea level may not have subaerially exposed the area. This occurrence of an isolated, downdip carbonate grainstone filled channel complex not accompanied by progradational, forced regressive deposits appears to be unique in carbonate ramp depositional systems and the channel includes fill with character different from many previously documented ramp carbonate-filled channels.

Discovery of a submerged relic reef and shoreline off Grand Cayman: further support for an early Holocene jump in sea level

Abstract Drilling close to the base of a submerged sea-cliff on the terraced eastern shelf of Grand Cayman has revealed a relic Acropora palmata reef at a depth of 21 m below msl. Ten cores from its crest are principally composed of cobble-sized clasts of A. palmata set in a matrix of cemented skeletal grainstone. The clasts have a distinctive succession of encrusters that indicate rapid burial: a photophilic association of crustose coralline algae, foraminifera and vermetid gastropods superimposed by a cryptic association of sclerosponges, foraminifera and serpulids. In addition to rapid burial, U–Th thermal ionization mass spectrometer (TIMS) dating of coral clasts within 1 m of the relic-reef surface indicates minor temporal mixing with ages between 8.9 and 8.1 ka. Such mixing and rapid burial is consistent with a hurricane deposit and is identical to deposits found on the crests of modern reefs. In relation to its age, the preservation of a −18.5-m intertidal notch cut into the submerged sea-cliff on the western shelf of Grand Cayman implies that the crest of the relic reef has been lowered 1.5–2 m by marine abrasion/bioerosion at a rate of ∼0.25 mm yr−1. Reconstructing this eroded section using average Holocene accretion rates indicates that the reef likely ceased accreting at ∼7.6 ka at a depth of ∼19 m. Comparing these data with other relic reefs in the Caribbean indicates that the relic reef on Grand Cayman died within 160 years of relic reefs on Barbados, St. Croix, St. Thomas and north Florida. This narrow interval of reef demise also coincides with the time when modern reefs were establishing themselves some 4–9 m higher upslope—a fact that can only be resolved by invoking a rapid 6-m jump in sea level ∼7.5 ka ago. Such a jump would also account for two unexplained events around this time: the restricted interval of global delta initiation and the catastrophic flooding of the glacially lowered Black Sea.

The sequence stratigraphy, sedimentology, and economic importance of evaporite–carbonate transitions: a review

Abstract World-class hydrocarbon accumulations occur in many ancient evaporite-related basins. Seals and traps of such accumulations are, in many cases, controlled by the stratigraphic distribution of carbonate–evaporite facies transitions. Evaporites may occur in each of the systems tracts within depositional sequences. Thick evaporite successions are best developed during sea level lowstands due to evaporative drawdown. Type 1 lowstand evaporite systems are characterized by thick wedges that fill basin centers, and onlap basin margins. Very thick successions (i.e. saline giants) represent 2nd-order supersequence set (20–50 m.y.) lowstand systems that cap basin fills, and provide the ultimate top seals for the hydrocarbons contained within such basins. Where slope carbonate buildups occur, lowstand evaporites that onlap and overlap these buildups show a lateral facies mosaic directly related to the paleo-relief of the buildups. This facies mosaic, as exemplified in the Silurian of the Michigan basin, ranges from nodular mosaic anhydrite of supratidal sabkha origin deposited over the crests of the buildups, to downslope subaqueous facies of bedded massive/mosaic anhydrite and allochthonous dolomite–anhydrite breccias. Facies transitions near the updip onlap edges of evaporite wedges can provide lateral seals to hydrocarbons. Porous dolomites at the updip edges of lowstand evaporites will trap hydrocarbons where they onlap nonporous platform slope deposits. The Desert Creek Member of the Paradox Formation illustrates this transition. On the margins of the giant Aneth oil field in southeastern Utah, separate downdip oil pools have accumulated where dolomudstones and dolowackestones with microcrystalline porosity onlap the underlying highstand platform slope. Where lowstand carbonate units exist in arid basins, the updip facies change from carbonates to evaporite-rich facies can also provide traps for hydrocarbons. The change from porous dolomites composed of high-energy, shallow water grainstones and packstones to nonporous evaporitic lagoonal dolomite and sabkha anhydrite occurs in the Upper Permian San Andres/Grayburg sequences of the Permian basin. This facies change provides the trap for secondary oil pools on the basinward flanks of fields that are productive from highstand facies identical to the lowstand dolograinstones. Type 2 lowstand systems, like the Smackover Limestone of the Gulf of Mexico, show a similar relationship. Commonly, these evaporite systems are a facies mosaic of salina and sabkha evaporites admixed with wadi siliciclastics. They overlie and seal highstand carbonate platforms containing reservoir facies of shoalwater nonskeletal and skeletal grainstones. Further basinward these evaporites change facies into similar porous platform facies, and contain separate hydrocarbon traps. Transgressions in arid settings over underfilled platforms (e.g. Zechstein (Permian) of Europe; Ferry Lake Anhydrite (Cretaceous), Gulf of Mexico) can result in deposition of alternating cyclic carbonates and evaporites in broad, shallow subaqueous hypersaline environments. Evaporites include bedded and palmate gypsum layers. Mudstones and wackestones are deposited in mesosaline, shallow subtidal to low intertidal environments during periodic flooding of the platform interior. Highstand systems tracts are characterized by thick successions of m-scale, brining upward parasequences in platform interior settings. The Seven Rivers Formation (Guadalupian) of the Permian basin typifies this transition. An intertonguing of carbonate and sulfates is interpreted to occur in a broad, shallow subaqueous hypersaline shelf lagoon behind the main restricting shelf-edge carbonate complex. Underlying paleodepositional highs appear to control the position of the initial facies transition. Periodic flooding of the shelf interior results in widespread carbonate deposition comprised of mesosaline, skeletal-poor peloid dolowackestones/mudstones. Progressive restriction due to active carbonate deposition and/or an environment of net evaporation causes brining upward and deposition of lagoonal gypsum. Condensed sections of organic-rich black lime mudstones occur in basinal areas seaward of the transgressive and highstand carbonate platforms and have sourced significant quantities of hydrocarbons.

Recent sedimentary facies of isolated carbonate platforms, Belize-Yucatan system, Central America

ABSTRACT Distribution of recent sedimentary facies is significantly different in four closely spaced, isolated carbonate platforms off the coasts of Belize and Yucatan (SE Mexico). These differences are largely controlled by variations in submarine topography, which are likely the result of differential subsidence and karstification from platform to platform. A variety of depositional styles and patterns is found in platform interiors that developed during the same Holocene sea-level rise. Composition and texture of surface sediment samples were used to define four major sediment types and to distinguish nine depositional environments within the platforms. Facies maps are based on analysis of 454 sediment samples and high-resolution satellite imagery. Glovers Reef is characterized by a circular facies distribution. The nearly continuous reef margin is composed of a coral-red coralline algae-Halimeda grainstone. Mixed nonskeletal (peloidal)/skeletal wackestones and packstones are present in shallow lagoon parts (< 5 m water depth) behind the margin. The deep interior, from 5 to 18 m water depth, is characterized by mollusk-foram-Halimeda wackestones with over 860 patch reefs of coral-red algae-Halimeda-mollusk packstones. Lighthouse Reef also has a coral grainstone and coral-red algae-Halimeda grainstone rim, but the facies distribution in the interior is strongly asymmetrical. The 8-m-deep eastern lagoon is floored by mollusk-foram-Halimeda wackestones and packstones; the 3-m-deep western lagoon has the mixed peloidal/skeletal wackestone to grainstone facies. A linear trend of coalescing patch reefs and skeletal grainstone facies separates the two lagoons. Banco Chinchorro is similar to Lighthouse Reef, inasmuch as facies distribution patterns of the interior are strongly asymmetrical. Mollusk-foram-Halimeda wackestones-grainstones are present in eastern lagoon areas, whereas shallow (< 5 m) western parts of the lagoon are composed of mixed peloidal/skeletal grainstones, packstones, and wackestones. Most of Turneffe Islands is protected on the east by Lighthouse Reef. Accordingly, Turneffe has a narrower reef and skeletal grainstone rim than the other two platforms. Restricted interior lagoons are up to 8 m deep, and are enclosed by wide mangrove rims and surface sediments dominated by Halimeda-rich wackestone rich in organic matter. Coral patch reefs are absent. In contrast, the unprotected northernmost part of Turneffe has a wide reef and grainstone rim and the open lagoon area consists of mollusk-foram-Halimeda wackestones-packstones with abundant patch reefs.

Constructional dynamics for a Lower Cretaceous carbonate ramp (Gorbea Massif, north Iberia)

Abstract The lower Albian distally steepened Egalezaburu carbonate ramp is composed of four stacked cycles with homogeneous characteristics throughout the section. Relative sea-level changes accounted for cycle formation and changes in facies distribution. During transgression and highstands, fossiliferous boundstones were deposited on the inner ramp, and argillaceous mudstones formed on the mid- and outer ramp. Shallow marine benthic communities flourished in open, moderate-energy environments, controlled by the depth of the photic zone. During lowstands, shallow-water sedimentation shifted basinward, and skeletal grainstones dominated shallow-water environments on the mid-ramp, controlled by the depth of the wave-base. Meanwhile megabreccias accumulated locally in oversteepened sites, at the mid- to outer ramp transition. The resulting cycles differ from the inner to the mid-ramp: shallowing-upward muddy cycles formed in the inner ramp during transgressions and highstands, as deepening-upward cycles with a basal grainy unit formed on the mid-ramp during lowstand to early highstand. Photic zone and wave-base levels, controlled by relative sea-level fluctuations, were the main controls on facies types and distribution and cycle development for the Egalezaburu ramp.

Effects of Differential Cementation on the Sonic Velocities of Upper Cretaceous Skeletal Grainstones (Southeastern Netherlands)

ABSTRACT P-wave velocities measured in Cretaceous skeletal grainstones from Limburg, southeastern Netherlands, are significantly higher in moderately cemented samples than in poorly cemented samples from similar depositional facies. Petrography suggests that this relationship is due to differences in the extent of development of intergranular cementation as influenced by original grain mineralogy, rather than a significant difference in primary porosity. The time-average velocity equation for a calcite matrix fails to predict the cementation effect on sonic velocity at high porosities. Linear and nonlinear time-average regressions show strong correlation between velocity and porosity. Whether the underlying trend is linear or nonlinear is a fundamental question that cannot be determined with this data set. Comparison between carbonate grainstones and siliciclastic sandstones reveals that: (1) the grainstones have gradients in velocity-porosity space similar to those in siliciclastic arenites but have velocities that are up to 0.3 km/s higher than those in clean arenites and up to 0.8 km/s higher than in argillaceous arenites; and (2) grainstones develop a rigid framework that transports elastic waves at significantly higher porosities than arenites, 40-50% compared to 30-40%. Higher sonic velocity presumably results from the higher matrix velocity of calcite versus quartz, and the higher elastic moduli created by the effective cement bridges connecting grains.

Early ordovician microbial reef mounds of the tribes Hill formation, Mohawk Valley, New York

Tribes Hill domal thrombolites, termed here microbial reef mounds, occupied the basal part of meter-scale shallowing-upward cycles. They are part of a high-energy facies that a sharp, transgressive surface, separates from an underlying low-energy peritidal setting. This erosional surface served as the surface on which one of the reef mounds established itself during initial transgression before further deepening. The others overlie a floor of skeletal grainstone reflecting a high-stand sea-level facies tract. Skeletal grainstone composes the fill between the mounds. One channel and several aggrading hummocks occupy inter-reef mound areas resulting from storm events in a subtidal setting.

Sequential Stratigraphy of Outcropping Strata Equivalent to Arab-D Reservoir, Wadi Nisah, Saudi Arabia

ABSTRACT Two outcrops in Wadi Nisah of Central Saudi Arabia expose sedimentary successions of the Jubaila and Arab formations which are stratigraphic equivalents of the Arab-D reservoir in Ghawar field. These carbonate exposures consist of lithofacies and stacking patterns similar to the succession found in Arab-D reservoir. Nineteen vertical sedimentary cycles were identified by collectively evaluating disconformable relationships, textural characteristics, and lithofacies assemblages. These, in turn, define 3 depositional cycle types: (1) stromatoporoid, (2) skeletal bank, and (3) thinning-upward. The lateral distribution of lithofacies within the stromatoporoid cycle shows that stromatoporoid grainstones form organic buildups flanked by burrowed mud-dominated carbonates. Stromatoporoid banks locally exhibit thickness increases of about 3 meters (10 feet) over a horizontal distance of 500 meters (1, 700 feet). Conversely, the five foot-thick skeletal grainstone capping the skeletal bank cycle forms a continuous sheet exceeding 2 kilometers (1.2 miles) in width. Thinly bedded carbonate sand units within the thinning-upward cycle exhibit pronounced lateral discontinuity. Beds pinch out against topographic highs formed by stromatoporoid buildups over distances of less than a hundred meters (328 feet). Results of the outcrop stratigraphic analysis enable visualization of lateral and vertical stratigraphic relationships and potential fluid-flow pathways. This study demonstrates that outcropping cycles and their lithofacies components have several important implications for ongoing subsurface reservoir characterization and modeling of the Arab-D. Firstly, cycle definition identifies stratigraphic units whose bounding surfaces describe envelopes that constrain the interwell distribution of lithofacies. Secondly, the lithofacies define geometries for the interwell distribution of petrophysical characteristics. Thirdly, the understanding of lateral relationships of these carbonates aids in reservoir simulation modeling.

Sequence Stratigraphic-Based Reservoir Architecture in Late Jurassic Outer-Ramp Carbonates, Hanifa Formation, Saudi Arabia: ABSTRACT

The Hanifa Formation (135-160m tk; Kimmeridgian age), in our study area, comprises one 3rd-order, coarsening-upward, type 2 stratigraphic sequence. Sediments formed in outer ramp, ramp-margin and basinal environments defining a transition between the Rimthan Arch carbonate platform and adjacent Arabian intrashelf basin. Quantification of Hanifa reservoir architecture for simulation involved development of field-scale geologic models based on sequence stratigraphic principles. No seismic and biostratigraphic data were available. Sequence interpretations were based on regional facies and parasequence analysis from 32 cores and 142 gamma-ray/porosity logs. In the study area, the Hanifa Formation has basinward-thinning tabular geometry, and contains (1) a lower member of organic-rich muddy carbonates and (2) an upper reservoir member of thick, medium to coarse-grained skeletal packstones, skeletal peloidal grainstones, skeletal intraclast conglomerates, and stromatoporoid boundstones. The Hanifa reservoir consists of, from oldest to youngest: (1) a highstand systems tract: aggrading and prograding, sigmoidal-shaped parasequences and parasequence sets of grainstrines, conglomerates and boundstones, capped by a subaqueous, type 2 sequence boundary; (2) a shelf margin wedge: prograding to aggrading, sigmoidal to tabular-shaped parasequences and parasequence sets of skeletal packstones, grainstones and local boundstones showing maximum basinward progradation; and (3) a transgressive systems tract: backstepping tabular-shaped parasequences of grainstones cappedmore » by a drowning surface. All facies are interpreted to have formed in subtidal settings of water depths from 5 to 150m. No evidence was found for shoal-water bank, lagoonal or peritidal deposition or for subaerial exposure in any facies.« less