Carbonate Cycles Research Articles

Platform configuration, microfacies and cyclicities of the upper Albian to Turonian of west-central Jordan

We present a comprehensive facies scheme for west-central Jordan platform deposits of upper Albian to Turonian age, discuss Cenomanian and Turonian carbonate cycles, and reconstruct the paleogeographic evolution of the platform. Comparisons with adjacent shelf areas (Israel, Sinai) emphasize local characteristics as well as the regional platform development. Platform deposits are subdivided into fifteen microfacies types that define eight environments of deposition of three facies belts. Main facies differences between Cenomanian and Turonian platforms are: rudist-bearing packstones that characterise the higher-energy shallow subtidal (transition zone) during the Cenomanian, and fossiliferous (commonly with diverse foraminifer assemblages) wackestones and packstones of an open shallow subtidal environment. On Turonian platforms high-energy environments are predominantly characterised by oolithic or bioclastic grainstones and packstones, whereas peritidal facies are indicated by dolomitic wackestones with thin, wavy (cryptmicrobial) lamination. Rhythmic facies changes define peritidal or subtidal shallowing-up carbonate cycles in several Cenomanian and Turonian platform intervals. Cyclicities are also analysed on the base of accommodation plots (Fischer Plots). High-frequency accommodation changes within lower Cenomanian cyclic bedded limestones of the central and southern area exhibit two major ‘cyclic sets’ (set I and II) each containing regionally comparable peaks. Accommodation patterns within cyclic set II coincide with the sequence boundary zone of CeJo1. The lateral and vertical facies distributions on the inner shelf allow the reconstruction of paleogeographic conditions during five time intervals (Interval A to E). An increased subsidence is assumed for the central study area, locally (area of Wadi Al Karak) persisting from middle Cenomanian to middle Turonian times. In contrast, inversion and the development of a paleo-high have been postulated for an adjacent area (Wadi Mujib) during late Cenomanian to early Turonian times, while small-scale sub-basins with an occasionally dysoxic facies developed northwards and further south during this time interval. A connection between these structural elements in Jordan with basins and uplift areas in Egypt and Israel during equivalent time intervals is assumed. This emphasises the mostly concordant development of that Levant Platform segment.

05/00017 Technical progress in the development of zero emission coal technologies: Ziock, H.-J. et al. Proceedings of the International Technical Conference on Coal Utilization & Fuel Systems, 2003, 28, (2), 1199–1210

We present an update on the development of technologies required for the Zero Emission Carbon (ZEC) concept being pursued by ZECA Corporation. The concept has a highly integrated design involving hydrogasification, a calcium oxide driven reforming step that includes simultaneous C02 separation, coal compatible fuel cells for electricity production and heat recovery, and a closed loop gas system in which coal contaminants are removed either as liquids or solids. The process does not involve any combustion and as such has neither smokestack nor air emissions. An independent assessment of the concept by Nexant, a Bcchtel affiliated company, suggests a net efficiency of approximately 70% for conversion of the higher heat value fuel energy into electrical output. This is even after the penalties of carbon dioxide separation and pressurization to 1000 psi are taken into account. For carbon dioxide sequestration a variety of options are being considered, which include enhanced oil recovery in the near-term and mineral carbonation as a long-term approach. We report on our early results in the development of sulfur tolerant anode materials for solid oxide fuel cells; a critical analysis of the calcium oxide - calcium carbonate cycle; trace element removal; and the recent results of hydrogasification tests.

Sea level changes versus hydrothermal diagenesis: origin of Triassic carbonate platform cycles in the Dolomites, Italy

The Triassic platform carbonate cycles of the Dolomites, best developed and studied at the Latemar, have been considered key examples for the documentation of eustatic sea level changes. The cycles appear to reflect a low energy lagoonal environment which is, however, difficult to reconcile with the small platforms lacking marginal energy barriers in a tropical, stormy climate. An alternative interpretation to eustatically controlled cyclicity and repeated subaerial exposure is provided by diagenesis in a hydrothermal field. Elevated heat flow 0–2 Ma before the onset of the mafic igneous activity triggered the circulation of diagenetic fluids with a composition similar to seawater by intermittent opening of fractures through some platforms. This caused tepee formation by precipitation of displacive cement, pendant cement and pisolite formation, and favoured cyanobacterial mat formation and their early dolomitization. The abundance of cycles correlates with that of the somewhat younger volcanic dikes crosscutting some platforms. Diffuse venting or seeping in the upper, unconsolidated sediment layers explains the stratiform occurrence of the diagenetic features and alternation with normal marine deposition in deep, but photic water at the wave base.

Shallowing-Upward Carbonate Cycles in the Belingwe Greenstone Belt, Zimbabwe: A Record of Archean Sea-Level Oscillations

ABSTRACT Shallowing-upward carbonate cycles in the 2650 Ma Cheshire Formation, Belingwe greenstone belt (Zimbabwe), closely resemble their Proterozoic and Phanerozoic counterparts. The cycles form part of a karstified carbonate-ramp sequence that is overlain by, and grades basinward into, siliciclastic turbidites. A single section of 74 cycles (1.5 m average thickness) was studied in detail. Two basic cycle types are recognized, both with an asymmetric facies stacking pattern. One cycle type contains open marine, subtidal shale at the base. Shale is intercalated with storm-generated sandstone and grainstone beds that become more common and thicken upward, indicating progressive shallowing. Wave-rippled ooid-intraclast grainstone beds and bedsets overlie shale or form the base of the second cycle type. Grainstone formed at or above fair-weather wave base as shoreface sand sheets in an agitated, shallow subtidal setting. Microbial laminites constitute the top of both cycle types and are interpreted as peritidal deposits. In the upper part of the studied section, microbial boundstones with aragonite pseudomorphs are intercalated with, or overlie, laminites and formed in a supratidal environment. The vertical facies distribution within a cycle is indicative of rapid submergence followed by gradual shallowing of relative sea level. High-frequency eustatic sea-level changes are favored over an autocyclic mechanism and tectonically induced allocyclicity as the controlling mechanism for the cyclicity. Hierarchies of stratigraphic cyclicity occur on different scales and may be a result of the combined effects of several orders of sea-level oscillations. Cycle recurrence ratios correspond well to the Milankovitch frequencies calculated for the Late Archean, suggesting that orbital climatic forcing may have been in operation in Archean times.

Paleoceanographic records in the sedimentary cores from the middle Okinawa Trough

Two gravity piston cores (Cores 155 and 18) involved in this study were collected from the middle Okinawa Trough. Stratigraphy of the two cores was divided and classified based on the features of planktonic foraminifera oxygen isotope changes together with depositional sequence, millennium-scale climatic event comparison, carbonate cycles and AMS(14)C dating. Some paleoclimatic information contained in sediments of these cores was extracted to discuss the paleoclimatic change rules and the short-time scale events presented in interglacial period. Analysis on the variation of oxygen isotope values in stage two shows that the middle part of the Okinawa Trough may have been affected by fresh water from the Yellow River and the Yangtze River during the Last Glacial Maximum (LGM). The oxygen isotope value oscillating ranges of the cores have verified that the marginal sea has an amplifying effect on climate changes. The delta(13)C of benthic foraminifera Uvigerina was lighter in the glacial period than that in the interglacial period, which indicates that the Paleo-Kuroshio's main stream moved eastward and its influence area decreased. According to the temperature difference during the "YD" period existing in Core 180 and other data, we can reach the conclusion that the climatic changes in the middle Okinawa Trough area were controlled by global climatic changes, but some regional factors had also considerable influence on the climate changes. Some results in this paper support Fairbanks's point that the "YD" event was a brief stagnation of sea level rising during the global warming up procession. Moreover, the falling of sea level in the glacial period weakened the exchange between the bottom water of the Okinawa Trough and the deep water of the northwestern Pacific Ocean and resulted in low oxygen state of bottom water in this area. These procedures are the reasons for carbonate cycle in the Okinawa Trough area being consistent with the "Atlantic type" carbonate cycle.

Paleoecology and Taphonomy of an Edrioasteroid-Dominated Hardground Association from Tentaculitid Limestones in the Early Devonian of New York: A Paleozoic Rocky Peritidal Community

Abstract The taphonomy, sedimentology, and paleoecology of a rare peritidal hardground in the Lower Devonian (Lochkovian) Thacher Limestone Member of the Manlius Formation in east-central New York State provides insights into sediment dynamics and modes of life of ancient hard substrate-inhabiting organisms. The hardground occurs near the top of a shallowing-upward carbonate cycle, slightly below desiccation-cracked micritic ribbon limestones. The hard surface developed on a partially exhumed tentaculitid pavement and overlying pelletal calcisiltite; the hardground has a relief of about 2–3 cm and is overlain by a thin siliciclastic mud layer. Tentative correlation of the hardground across a lateral distance of about 35 km perpendicular to depositional strike suggests a very subdued topographic profile for this region and development of an extensive peritidal hardground pavement. Tentaculites gyracanthus occurs in a dense pavement of strongly bimodally aligned shells with an ENE to WSW orientation, parall...

Mixed Clastic–Carbonate Cycles and Sequences: Quaternary of Egypt and Carboniferous of England

There are various types of mixed clastic–carbonate sequences andparasequences in the geological record and sea-level change is oneof the major controls on their development, as well as the obviousavailability of terrigenous material. Mixed-lithology sequences areespecially well-developed in icehouse times, as in the Permo–Carboniferous and Quaternary. The two major types, lower carbonate – upper clastic and lower clastic – upper carbonate sequences, reflect the rates of sea-level change and the source of clastic sediment – whether itis brought across the carbonate platform from the adjacent landmassor supplied axially to the basin. Two examples of carbonate–clastichigh-frequency sequences are discussed: 1) from the Red Sea coastof Egypt, Quaternary transgressive/highstand shallow-water carbonatesand late highstand/falling-stage/lowstand clastics deposited inincised-valley fills and fan deltas, and 2) from northern England,mid-Carboniferous ‘Yoredale’ cycles of transgressive shelf carbonatespassing up into highstand coarsening-up deltaic and shoreline clastics,capped by palaeosoils and coal. Locally, falling-stage incised-valleyssupplied sediments to the adjoining basin. One significant differencebetween these two examples is the palaeoclimate: arid in the Red Sealeading to the episodic supply of sand and gravel, and humid in theCarboniferous resulting in the more continuous supply of mud andsand, each having a different effect on carbonate deposition.

Sedimentological and palaeohydrological responses to tectonics and climate in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain)

ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ18O and δ13C covariance trends in calcite reinforce closed drainage conditions. δ18O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ18O (i.e. from pluriannual to millennial). The small δ13C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ13C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.

Facies associations, sedimentary cyclicity and compartmentalization of the MC-1 Member (Mississippian), Tilston Field, southwestern Manitoba

ABSTRACT The sedimentary rocks of the Mississippian Mission Canyon-1 (MC-1) Member (Tilston Beds) of the Tilston Field, southwestern Manitoba, Canada, were deposited on a low-relief carbonate platform along the northeastern margin of the Williston Basin. Three oil pools occur in the Tilston Field: Mission Canyon 1 A (MC-1 A), Mission Canyon 1 C (MC-1 C) and Mission Canyon 1 D (MC-1 D). The rocks of the MC-1 Member are broken into five facies associations, which stack to form five shallowing-upward subtidal carbonate cycles (TB1, TB3 to TB6) and one shallowing- to deepening-upward subtidal carbonate cycle (TB2). Each cycle was deposited in water depths that varied from fair weather wave base to slightly below storm wave base. Well log analysis of the uppermost Lodgepole Formation indicates at least four shallowing-upward subtidal carbonate cycles (L1 to L4). Argillaceous-rich wackestone to mudstone intervals occur in the basal part of each cycle. Each cycle of the MC-1 Member and uppermost Lodgepole Formation can be correlated across the study area. The MC-1 Member and Lodgepole cycles show a trend of increasing total porosity and decreasing argillaceous content from south to north across the Tilston Field. Cycles TB4 and TB5 have the lowest argillaceous content and the highest porosity, and are the main oil producing units in the Tilston Field. The lack of argillaceous material and the relatively high porosity suggests that reservoir compartmentalization is highly unlikely across the TB4 to TB5 boundary. In contrast, the basal parts of cycles TB3 to TB1 and L1 to L4 are argillaceous rich and characterized by fair total porosity. It is likely that these argillaceous-rich zones form barries to the vertical movement of fluids, thus compartmentalizing the aquifer. In the absence of vertical fractures, the dominant drive mechanism in the Tilston Field oil pools is likely to be edge water drive. Progressive erosional truncation of the MC-1 Member cycles to the north, combined with the presence of two structural saddles, effectively reduces the areal support of the edge water aquifer from south to north. The MC-1 A Pool is likely to have the strongest aquifer support, whereas the MC-1 D Pool has the weakest aquifer support. Significant differences in horizontal well productivity can be partially attributed to the relative strength of the aquifer in each pool. End_Page 244------------------------

Potential and limitations of Sr/Ca ratios in coccolith carbonate: new perspectives from cultures and monospecific samples from sediments.

The Sr/Ca ratio of coccoliths was recently proposed as a potential indicator of past growth rates of coccolithophorids, marine algae, which play key roles in both the global carbonate and carbon cycles. We synthesize calibrations of this proxy through laboratory culture studies and analysis of monospecific coccolith assemblages from surface sediments. Cultures of coccolithophorids Helicosphaera carteri, Syracosphaera pulchra and Algirospira robusta confirm a 1-2% increase in Sr/Ca per degrees C previously identified in Emiliania huxleyi and Gephyrocapsa oceanica. This effect is not due merely to increases in growth rate with temperature and must be considered in palaeoceanographic studies. In light-limited cultures of E. huxleyi, Calcidiscus leptoporus and G. oceanica at constant temperature, coccolith Sr/Ca ratios vary by 10% across the range of possible growth and calcification rates for a given species. Among different species under similar culture conditions, Sr/Ca ratios vary by 30%. Although the highest ratios are in the cells with highest calcification and organic carbon fixation rates, at lower rates there is much scatter, indicating that different mechanisms control interspecific and intraspecific coccolith Sr/Ca variations. In field studies in the Equatorial Pacific and Somalia coastal region, coccolith Sr/Ca correlates with upwelling intensity and productivity. A more dynamic response is observed in larger coccoliths like C. leptoporus (23-55% variation in Sr/Ca) than in smaller coccoliths of G. oceanica or Florisphaera profunda (6-15% variation in Sr/Ca). This response suggests that, despite temperature effects, coccolith Sr/Ca has potential as an indicator of coccolithophorid productivity. If the variable Sr/Ca response of different species accurately reflects their variable productivity response to upwelling (and not different slopes of Sr/Ca with productivity), coccolith Sr/Ca could provide useful data on past changes in coccolith ecology. The mechanism of coccolith Sr/Ca variations remains poorly understood but is probably more closely tied to biochemical cycles during carbon acquisition than to chemical kinetic effects on Sr incorporation in the calcite coccolith crystals.

ABSTRACT: Productivity-Controlled Peritidal Carbonate Cycles in a Super-Greenhouse Climate in the Aftermath of the End-Permian extinction, the Lower Triassic (Olenekian) Great Bank of Guizhou, South China

ABSTRACT: Productivity-Controlled Peritidal Carbonate Cycles in a Super-Greenhouse Climate in the Aftermath of the End-Permian extinction, the Lower Triassic (Olenekian) Great Bank of Guizhou, South China

Late Archaean foreland basin deposits, Belingwe greenstone belt, Zimbabwe

Abstract The c. 2.65 Ga old sedimentary Cheshire Formation of the Belingwe greenstone belt (BDB), central Zimbabwe, has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt. The Cheshire Formation rests sharply on a mafic volcanic unit (Zeederbergs Formation) and comprises a basal, eastward-sloping carbonate ramp sequence built of shallowing-upward, metre-scale sedimentary cycles. The cycles strongly resemble Proterozoic and Phanerozoic carbonate cycles and might have formed by small-scale eustatic sea level changes. The top of the carbonate ramp is represented by a karst surface. The carbonates are overlain by and grade laterally to the east into deeper water (sub-wave base) siliciclastic facies. Conglomerate, shale and minor sandstone were deposited by high- to low-density turbidity currents and were derived from the erosion of Zeederbergs-like volcanic rocks from the east. Shortly after deposition, the Cheshire Formation and underlying volcanics were affected by a northwest-directed thrusting event. Thrusting gave rise to the deformation of semi-consolidated sediments and resulted in the juxtaposition of a thrust slice of Zeederbergs basalts onto Cheshire sediments. The stratigraphy, asymmetric facies and sediment thickness distribution, palaeogeographic constraints and evidence for an early horizontal tectonic event suggest that the Cheshire Formation formed in a foreland-type sedimentary basin.

Sr/Ca variations in Cretaceous carbonates: relation to productivity and sea level changes

We present high resolution Sr/Ca measurements on coccolith-dominated pelagic carbonates of Berriasian–Valanginian, Albian–Santonian, and Campanian–Maastrichtian age from DSDP sites and Tethyan land sections. Carbonate Sr/Ca varies by up to 80% over both long and short timescales and may reflect either temporal variations in seawater Sr/Ca or variable Sr partitioning in biogenic carbonates. Numerical models of seawater Sr and Ca budgets indicate that while sea level changes can cause comparably large variations in the seawater Sr/Ca ratio, the timing and pattern of predicted Sr/Ca variations agrees poorly with the carbonate record. Reef crises unrelated to sea level changes may also elevate seawater Sr/Ca, and in some cases, the record of reef crises corresponds to increasing carbonate Sr/Ca. Changes in Sr partitioning, likely due to changes in calcareous nannoplankton productivity, also appear to control carbonate Sr/Ca in these records. Cretaceous carbonate Sr/Ca broadly covaries with indicators of productivity derived from microfossil ecology. Recent studies in sediment core tops and cultures have shown that Sr partitioning in coccolith carbonate is strongly influenced by changes in coccolithophorid growth and calcification rate. Where the effects of changing seawater Sr/Ca can be constrained by independent geological data, carbonate Sr/Ca measurements could provide a valuable perspective on changes in the productivity of calcareous nannoplankton and variations in the carbonate and carbon cycle in the Cretaceous.

Initiation of clement surface conditions on the earliest Earth.

In the beginning the surface of the Earth was extremely hot, because the Earth as we know it is the product of a collision between two planets, a collision that also created the Moon. Most of the heat within the very young Earth was lost quickly to space while the surface was still quite hot. As it cooled, the Earth's surface passed monotonically through every temperature regime between silicate vapor to liquid water and perhaps even to ice, eventually reaching an equilibrium with sunlight. Inevitably the surface passed through a time when the temperature was around 100 degrees C at which modern thermophile organisms live. How long this warm epoch lasted depends on how long a thick greenhouse atmosphere can be maintained by heat flow from the Earth's interior, either directly as a supplement to insolation, or indirectly through its influence on the nascent carbonate cycle. In both cases, the duration of the warm epoch would have been controlled by processes within the Earth's interior where buffering by surface conditions played little part. A potentially evolutionarily significant warm period of between 10(5) and 10(7) years seems likely, which nonetheless was brief compared to the vast expanse of geological time.

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga. The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale. Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs. The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp. Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.

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.

Marine cements in mid‐Tertiary cool‐water shelf limestones of New Zealand and southern Australia

Large areas of southern Australia and New Zealand are covered by mid‐Tertiary limestones formed in cool‐water, shelf environments. The generally destructive character of sea‐floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates, yet these limestones include occasional units with marine cements: (1) within rare in situ biomounds; (2) within some stacked, cross‐bedded sand bodies; (3) at the top of metre‐scale, subtidal, carbonate cycles; and (4) most commonly, associated with certain unconformities. The marine cements are dominated by isopachous rinds of fibrous to bladed spar, interstitial homogeneous micrite and interstitial micropeloidal micrite, often precipitated sequentially in that order. Internal sedimentation of microbioclastic micrite may occur at any stage. The paradox of marine‐cemented limestone units in an overall destructive cool‐water diagenetic regime may be explained by the precipitation of cement as intermediate Mg‐calcite from marine waters undersaturated with respect to aragonite. In some of the marine‐cemented limestones, aragonite biomoulds may include marine cement/sediment internally, suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences. We suggest that marine cementation occurred preferentially, but not exclusively, during periods of relatively lowered sea level, probably glacio‐eustatically driven in the mid‐Tertiary. At times of reduced sea level, there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters, and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites, which now experienced increased swell wave and/or tidal energy levels, fostering sediment abrasion and reworking, reduced sedimentation rates and freer exchange of sediment pore‐waters. Energy levels were probably also enhanced by increased upwelling of cold, deep waters onto the Southern Ocean margins of the Australasian carbonate platforms, where water‐mass mixing, warming and loss of CO2 locally maintained critical levels of carbonate saturation for sea‐floor cement precipitation and promoted the phosphate‐glauconite mineralization associated with some of the marine‐cemented limestone units.

Abstract: Carbonate Sedimentation Rates in Mixed Siliciclastic and Carbonate Cycles, Cisco Group (Virgilian and Wolfcampian), Eastern Shelf, Texas

Abstract: Carbonate Sedimentation Rates in Mixed Siliciclastic and Carbonate Cycles, Cisco Group (Virgilian and Wolfcampian), Eastern Shelf, Texas

Low‐ and high‐frequency sea‐level changes control peritidal carbonate cycles, facies and dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

The Rock of Gibraltar comprises two tectonically separated limbs of an isolated klippe of Liassic Gibraltar Limestone Formation. Both limbs have similar, c. 400 m thick sequences of inner carbonate platform facies arranged in high‐frequency, metre‐scale, shallowing‐upward, peritidal cycles with emergent, caliche caps. Four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures, lithologies, facies and biota. When compared with other Liassic cycles from fault‐bound platforms of the western Mediterranean region all are found to be of similar scale, facies and cycle type. Likely common origins are through Milankovitch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Within the Gibraltar Limestone high‐frequency cycles are superimposed on a low‐frequency (third order?) cyclicity that is revealed, through the use of Fischer plots, to control the occurrrence of facies, biota, high‐frequency cycle types and dolomitization. Falling sea‐level and lowstand phases, with reduced accommodation space, are typified by restricted, inner platform facies and cycles and by early reflux dolomitization. Transgressive and highstand phases, with more accommodation space, are characterized by the absence of early dolomites, the incoming of inner platform microfossils (i.e. foraminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.

Abstract: Outcrop based analog for shallow-water, high frequency carbonate cycles from the Chesterian from Southwestern New Mexico

Abstract: Outcrop based analog for shallow-water, high frequency carbonate cycles from the Chesterian from Southwestern New Mexico