Carbonate Platform Development Research Articles

Gravitational Potential Energy per Unit Area as a Constraint on Archean Sea Level

AbstractTo constrain the density structure of Archean lithosphere, I assume that gravitational potential energy (GPE) per unit area at Archean spreading centers equals that for continents whose surfaces lay near sea level. The present‐day balance of GPE limits average density deficits in Archean mantle lithosphere due to depletion of iron during its formation to ∼30–45 kg/m3. For an Archean volume of seawater equal to or greater than that today and heat loss approximately three times that today (e.g., at ∼3.5 Ga), plausible Archean structures, constrained by GPE balance, favor submerged spreading centers and continents. Emergent Archean continents would be allowed by a combination of the following (a) heat loss only twice that today, (b) less than approximately half of the present‐day volume of thick crust, either continental or oceanic plateaus, and (c) deep Archean spreading centers ( 2 km below continents), which would require a thickness of Archean oceanic crust 25 km. If the volume of Archean seawater were 50% greater than that today, emergent spreading centers could have existed only as isolated unusual features, and emergent continents would require both limited extent of thick crust and spreading centers deeper that ∼2 km. The observed widespread development of continental margins and carbonate platforms at ∼2.7–2.5 Ga is consistent with heat loss roughly twice that today, a volume of seawater comparable with that today, and either (a) a volume of continental crust comparable to that at present and oceanic crust 30 km thick or (b) thicker oceanic crust, possibly 40–50 km, but with a volume of continental crust less than approximately half that today. Calculated temperatures at the Archean Moho are ∼700–900 °C. These calculations do not rule out a hot early Archean ocean (∼50–80 °C) and do not favor an early Archean emergence of life on land above sea level.

Lithostratigraphy of the upper Turinian – lower Chatfieldian (Upper Ordovician) foreland succession, and a U–Pb ID–TIMS date for the Millbrig volcanic ash bed in the Ottawa Embayment

Three stages of carbonate-platform development are preserved in the upper Turinian – lower Chatfieldian succession of the Ottawa Group in the Ottawa Embayment and represent deposition along the Late Ordovician Taconic foreland interior of paleo-southern Laurentia. Compared with contemporary stratigraphy in the adjacent northern Appalachian (southern Ontario, New York state) and western Quebec basins, the intermediate Stage 2 succession, which brackets the Turinian–Chatfieldian boundary, preserves embayment-specific stratigraphic patterns. These include: (i) dramatic west-to-east thickening of the upper Turinian Watertown Formation that defines differential subsidence along the present axis of the embayment, (ii) post-Watertown base-level fall defined by appearance of shoreface siliciclastics, (iii) early Chatfieldian marine transgression represented by the proposed L’Orignal Formation that is coeval with but lithologically distinct from the Selby Formation in the northern Appalachian Basin, and (iv) platform segmentation that resulted in a depositional mosaic of shallow banks (Rockland Formation) and equivalent deeper water mico-seaways (lower Hull Formation). The latter event immediately follows accumulation of the Millbrig bentonite, here dated at 453.36 ± 0.38 Ma. Bracketing these local stratigraphic patterns are the bounding stages (1 and 3) represented by the upper Turinian Lowville Formation and middle Chatfieldian Hull Formation, respectively, that contain facies attributes in common with the adjacent basins and characterize inter-regional depositional systems of first warm, then cooler oceanographic conditions. Stage 2 identifies a structurally controlled transition between these end-member stages: a far-field response in the foreland interior, localized along the axis of a late Precambrian fault system, to contemporary change in subsidence rates and tectonomagmatic events along the Laurentian margin.

Architecture and sedimentary evolution of the southwestern Gissar carbonate platform (Uzbekistan) during the Middle‒Late Jurassic

Abstract Sedimentological investigations have been carried out in the southwestern part of the Gissar Range (Uzbekistan) in order to document the stratigraphic evolution of a vast Middle–Upper Jurassic carbonate platform developed on the northern margin of the Amu Darya Basin. Two major sequences are distinguished, based on their contrasting facies pattern, platform configuration and carbon isotope signature: (1) a Callovian Sequence and (2) an Upper Jurassic Sequence, potentially Oxfordian to Kimmeridgian in age. A carbonate ramp with a double proximal-distal polarity developed during the Callovian Sequence. Oo-bioclastic grainstones were produced in two permanently agitated shallow water areas developing to the north and to the south. Patch and pinnacle reefs mainly built up close to the storm-wave base, passing distally to pellet- and foraminiferal-oncoid-rich mid-to outer-ramp deposits. The top of the Callovian Sequence is a regional discontinuity that could have formed by subaerial exposure of the platform. This probable hiatus could record a major climatic change associated to the Middle‒Late Jurassic transition. The Upper Jurassic Sequence consists of metre-thick alternations of pellet/algal-pack- to grainstones, microbial oncoid floatstones, ooid grainstones, and mudstones, which have likely accumulated in a vast lagoon protected by a large-scale barrier reef observed south of the study area. Gypsum and anhydrite content gradually increased towards the top of the Upper Jurassic Sequence, ending up with a thick chicken-wire gypsum accumulation recording the progradation of a large-scale sabkha over the lagoon. This phenomenon is related to a progressive aridification and restriction of the Amu Darya Basin that could have begun during the Middle/Late Oxfordian and reached its climax during the Tithonian with massive halite deposition. Comparable events occurred in Jurassic platforms located along the Tethyan margins, pointing to regional and global controls on sedimentation.

The influence of carbonate platform interactions with subduction zone volcanism on palaeo-atmospheric CO<sub>2</sub> since the Devonian

Abstract. The CO2 liberated along subduction zones through intrusive/extrusive magmatic activity and the resulting active and diffuse outgassing influences global atmospheric CO2. However, when melts derived from subduction zones intersect buried carbonate platforms, decarbonation reactions may cause the contribution to atmospheric CO2 to be far greater than segments of the active margin that lacks buried carbon-rich rocks and carbonate platforms. This study investigates the contribution of carbonate-intersecting subduction zones (CISZs) to palaeo-atmospheric CO2 levels over the past 410 million years by integrating a plate motion and plate boundary evolution model with carbonate platform development through time. Our model of carbonate platform development has the potential to capture a broader range of degassing mechanisms than approaches that only account for continental arcs. Continuous and cross-wavelet analyses as well as wavelet coherence are used to evaluate trends between the evolving lengths of carbonate-intersecting subduction zones, non-carbonate-intersecting subduction zones and global subduction zones, and are examined for periodic, linked behaviour with the proxy CO2 record between 410 Ma and the present. Wavelet analysis reveals significant linked periodic behaviour between 60 and 40 Ma, when CISZ lengths are relatively high and are correlated with peaks in palaeo-atmospheric CO2, characterised by a 32–48 Myr periodicity and a ∼ 8–12 Myr lag of CO2 peaks following CISZ length peaks. The linked behaviour suggests that the relative abundance of CISZs played a role in affecting global climate during the Palaeogene. In the 200–100 Ma period, peaks in CISZ lengths align with peaks in palaeo-atmospheric CO2, but CISZ lengths alone cannot be determined as the cause of a warmer Cretaceous–Jurassic climate. Nevertheless, across the majority of the Phanerozoic, feedback mechanisms between the geosphere, atmosphere and biosphere likely played dominant roles in modulating climate. Our modelled subduction zone lengths and carbonate-intersecting subduction zone lengths approximate magmatic activity through time, and can be used as input into fully coupled models of CO2 flux between deep and shallow carbon reservoirs.

Crescentiella-microbial-cement microframeworks in the Upper Jurassic reefs of the Crimean Peninsula

In the Upper Jurassic reef successions of the Crimean Peninsula (Sudak and Jalta areas), the microencruster Crescentiella morronensis (Crescenti), microbialites, and multiple generations of cements, form microframeworks. They were observed in two stages of the carbonate platform evolution, in the Middle–Upper Oxfordian, and in the Upper Kimmeridgian–Tithonian. Generally, in both stages, the features of the microframeworks are similar and consist of densely packed Crescentiella associated with microbialites and branched colonies of the sclerosponge Neuropora lusitanica Termier. The difference between the occurrences of the two stages is the variable amount of nubecularid foraminifera and enigmatic tube-shaped structures forming the central cavities of Crescentiella. The Crescentiella-microbial-cement microframeworks formed under phreatic conditions in the upper slope and seaward marginal depositional settings where intensive synsedimentary cementation took place. They formed in the initial stages of long cycles of restoration and blooming of the reefs. The late Jurassic examples resemble the Permian algae-microbial-cement reefs as well as the Triassic Tubiphytes and cement crust-dominated reefs. Concurrently, all these examples formed a transitional facies zone between typical slope facies to shallow subtidal platform margin facies characterized by high taxonomic diversity of calcified sponges, corals, and microencrusters forming the principal part of the reefs.

Exploring coral reef responses to millennial-scale climatic forcings: insights from the 1-D numerical tool pyReef-Core v1.0

Abstract. Assemblages of corals characterise specific reef biozones and the environmental conditions that change spatially across a reef and with depth. Drill cores through fossil reefs record the time and depth distribution of assemblages, which captures a partial history of the vertical growth response of reefs to changing palaeoenvironmental conditions. The effects of environmental factors on reef growth are well understood on ecological timescales but are poorly constrained at centennial to geological timescales. pyReef-Core is a stratigraphic forward model designed to solve the problem of unobservable environmental processes controlling vertical reef development by simulating the physical, biological and sedimentological processes that determine vertical assemblage changes in drill cores. It models the stratigraphic development of coral reefs at centennial to millennial timescales under environmental forcing conditions including accommodation (relative sea-level upward growth), oceanic variability (flow speed, nutrients, pH and temperature), sediment input and tectonics. It also simulates competitive coral assemblage interactions using the generalised Lotka–Volterra system of equations (GLVEs) and can be used to infer the influence of environmental conditions on the zonation and vertical accretion and stratigraphic succession of coral assemblages over decadal timescales and greater. The tool can quantitatively test carbonate platform development under the influence of ecological and environmental processes and efficiently interpret vertical growth and karstification patterns observed in drill cores. We provide two realistic case studies illustrating the basic capabilities of the model and use it to reconstruct (1) the Holocene history (from 8500 years to present) of coral community responses to environmental changes and (2) the evolution of an idealised coral reef core since the last interglacial (from 140 000 years to present) under the influence of sea-level change, subsidence and karstification. We find that the model reproduces the details of the formation of existing coral reef stratigraphic sequences both in terms of assemblages succession, accretion rates and depositional thicknesses. It can be applied to estimate the impact of changing environmental conditions on growth rates and patterns under many different settings and initial conditions.

Carbonate Platform Development in an Intraoceanic Arc Setting: Costa Rica’s Largest Limestone Sequence - The Fila de Cal Formation (Middle Eocene to Lower Oligocene)

The limestones of the Fila de Cal Formation build the largest carbonate platform sequence of Costa Rica extending over 180 km from southern Costa Rica into the Panamanian boarder area of David. The detailed limestone microfacies analysis and micropaleontologic dating based on the contained larger foraminiferal assemblages of 22 samples selected from the most representative exposures reveal three main carbonate facies: (1) Eoannularia eocenica-bearing spiculite facies from the Middle Eocene, (2) large lepidocyclinid-nummilitid facies from the Upper Eocene lead by the Lepidocyclina chaperi-Lepidocyclina tobleri panamensis assemblage and (3) dominant Lower Oligocene Lepidocyclina yurnagunensis-Lepidocyclina undosa assemblage. These three biostratigraphically successive facies record circa 14 to 19 m.y. of continuous shallow-marine carbonate sedimentation in the Pacific frontal arc region of southern Costa Rica. During this time large communities of benthic organisms: larger foraminifera, calcareous algae, corals, bryozoans, mollusks and equinoderms rapidly grew in warm tropical waters to build the largest and long-lasting carbonate platform on the Costa Rican arc. The carbonate sedimentation ended abruptly with accumulation of thick volcaniclastic marine platform-to-slope deposits of the Térraba Formation. Tectonically, the Fila de Cal carbonate platform development coincides with rapid convergence rates of the Farallón plate beneath southern Central America leading to strong arc uplift and new widespread neritic environments. The correlation with contemporaneous carbonate arc sequences clearly indicates that primarily their proximity to the inner volcanic arc areas and strong subsidence in response to emerging volcanic edifices are probably the main factors that controlled growing and persistence of carbonate environments within the arc setting.

Controls on Neogene carbonate facies and stratigraphic architecture of an isolated carbonate platform – the Caribbean island of Bonaire

Abstract The Neogene carbonate succession on the island of Bonaire (Netherland Antilles) shows complex geometries associated with a sequence of depositional and erosional events which reflects the history of this isolated platform and the interaction between eustasy and tectonics. Three major episodes of carbonate platform deposition are defined which show contrasting depositional styles: 1) aggradational platform (Lower-Middle Miocene) with sediments showing a fining-upward trend from mixed coral rudstone to medium-grained coralgal grain/packstone, partly dolomitized and tilted by tectonic deformation; 2) progradational platform (Upper Miocene-Pliocene) which is formed of several shallowing-upward prograding units mainly composed of reworked red algal grain/packstone, with significant dolomitization, passing upward to shoreline and aeolian deposits formed of cor-algal grain/packstone and large benthic foraminifera grainstone, and 3) flat-topped platform (Pleistocene) with a reefal framework composed of a rich variety of corals in a bioclastic pack/wackestone matrix. These platform episodes exhibit contrasting stacking patterns and are separated by erosional unconformities. Overprinting this depositional succession is a series of Quaternary near-horizontal shoreline erosional terraces and vertical cliffs which have been cut into the island stratigraphy and complicate the stratal field relationships. However, this terrace morphology clearly does not represent depositional episodes, as has been suggested before. The internal architecture of each of the three carbonate platform reflects of the dynamics of sedimentation with allogenic controls. The latter relate to major oceanographic and tectonic events in the region, including changing ocean circulation as a result of the closure of the Panama isthmus, and Caribbean plate dynamics that affected sea-floor and island topography. The Bonaire succession provides a model for understanding and predicting isolated carbonate platform development, as well as architecture, facies and potential diagenetic changes, in an active tectonic setting.

Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.

Depositional environments and cyclicity of the Early Ordovician carbonate ramp in the western Tarim Basin (NW China)

During the Early Ordovician, the Tarim Basin (NW China) was mainly occupied by an extensive shallow-water carbonate platform, on which a carbonate ramp system was developed in the Bachu-Keping area of the western part of the basin. Three well-exposed typical outcrop sections of the Lower Ordovician Penglaiba Formation were investigated in order to identify the depositional facies and to clarify origins of meter-scale cycles and depositional sequences, thereby the platform evolution. Thirteen lithofacies are identified and further grouped into three depositional facies (associations): peritidal, restricted and open-marine subtidal facies. These lithofacies are vertically stacked into meter-scale, shallowing-upward peritidal and subtidal cycles. The peritidal cycles are mainly distributed in the lower and uppermost parts of the Penglaiba Formation deposited in the inner-middle ramp, and commonly start with shallow subtidal to intertidal facies followed by inter- to supratidal facies. In contrast, the subtidal cycles occur throughout the formation mostly in the middle-outer ramp and are dominated by shallow to relatively deep (i.e., intermediate) subtidal facies. The dominance of asymmetrical and incomplete cycles suggests a dominant control of Earth’s orbital forcing on the cyclic deposition on the platform. On the basis of vertical facies and cycle stacking patterns, and accommodation changes illustrated by the Fischer plots from all studied sections, five third-order depositional sequences are recognized in the Penglaiba Formation. Individual sequences comprise a lower transgressive part and an upper regressive one. In shallow-water depositional environments, the transgressive packages are dominated by thicker-than-average subtidal cycles, indicating an increase in accommodation space, whereas regressive parts are mainly represented by thinner-than-average peritidal and subtidal cycles, denoting a decrease in accommodation space. In contrast, in intermediate to deep subtidal environments, transgressive and regressive packages display an opposite trend in accommodation space changes. Sequence boundaries (except the basal and top boundaries of the Penglaiba Formation) are usually represented by laterally traceable, transitional boundary zones without apparent subaerial exposure features. Good correlation of the long-term changes in accommodation space (or sea-level) inferred from vertical stacking patterns of facies and cycles suggests an overriding eustatic control on the formation of meter-scale cycles and third-order depositional sequences as well as platform evolution superimposed with local and/or regional tectonic influence during the Early Ordovician. This study would help understand the controls on the tempo-spatial facies distribution, stratal cyclicity and carbonate platform evolution in the western Tarim Basin during the Early Ordovician, facilitating prediction for favorable subsurface carbonate reservoirs and future hydrocarbon exploration and production in the Penglaiba Formation.

Syn-sedimentary deformation as a mechanism for the initiation of submarine gullies on a carbonate platform to slope transition, Browse Basin, Australian North West Shelf

Syn-sedimentary deformation and submarine channel initiation are both important processes influencing the stratigraphic development and reservoir properties of carbonate platforms. Based on a high-resolution 3D seismic dataset our study shows that platform-margin parallel syn-sedimentary faulting in the vicinity of the clinoform breakpoint can lead to the indention of channel heads that subsequently grow and expand in headward and downslope direction. The carbonate-dominated Miocene succession of the Browse Basin on the Australian North West Shelf reveals a distinct submarine channel system during a time of overall platform progradation. The progradation occurs over a drowned Eocene to Lower Miocene carbonate ramp, which formed a large scale inflection. Previous studies investigated the architecture of the carbonate platform, containing a large Neogene barrier reef system. We present a high-resolution 3D seismic study of the contemporaneous platform to slope transition. Landward of first significant gullies, a distinct network of margin parallel, small scale arcuate faults is observed. Active faulting during appearance of the first gullies is exhibited by growth strata, which are located directly basinward of the faults. These steep (65°–85°), small-scale faults are characterized by throws of 5–25 ms TWT and show a tendency to merge laterally and vertically with adjacent faults. Syn-sedimentary deformation associated with these faults likely triggered slope failure and the generation of turbidity currents finally leading to the growth of the gullies by headward and downslope erosion.

Oligo-Miocene carbonate platform evolution in the northern margin of the Asmari intra-shelf basin, SW Iran

Abstract This study is based on seven outcrops along a regional transect in the central subdivision of the Zagros Foreland Basin. This NE-SW trending transect extends for 140 km and covers the northern margin of the Asmari intra-shelf basin in dip direction (from proximal to distal). Results of facies analysis led to the recognition of eight facies associations (Planktonic Wkt, Nummulitic Eu F/R, Alg LR Pkt, Coralgal F-R/Bdt, LR LPBF P-G, Epy.R PBF P-G, Dol M, and TRGS) along the depositional dip direction. They are dominated by large and flat rotalids (e.g., Nummulites, Lepidocyclina and Spiroclypeus), coral and coralline red algae (e.g., build-up features), porcellaneous benthic foraminifera (e.g., Archaias, Peneroplis, and Borelis), and fossiliferous lime-mud. Spatial and temporal distribution of these facies associations conform five platform types with different depositional profiles: a) a Nummulites-bearing mixed terrigenous/carbonate distally steepened ramp, lower Rupelian, b) a Nummulitidae-Lepidocyclina bearing carbonate distally steepened ramp, upper Rupelian-lower Chattian, c) a Coral bearing flat-topped open shelf, middle-upper Chattian, d) an Aquitanian homoclinal ramp, and e) a very shallow-water Burdigalian carbonate shelf. Amongst these platform types, the Late Rupelian-Early Chattian carbonate ramp additionally consists of three successive phases and the Middle-Late Chattian flatted top open shelf was developed in two phases. The changes in platform type resulted from changes in the carbonate producing biotas, sediment redistribution processes and significant impact of relative sea level changes.

An Albian demise of the carbonate platform in the Manín Unit (Western Carpathians, Slovakia)

Abstract The production of platform carbonates of the Manín Unit (Manín Straits, Central Western Carpathians) belonging to the Podhorie and Manín formations and formed by remains of rudists and benthic foraminifers (Urgonian-type carbonates), was previously assumed to terminate during the Aptian. First, we show that these deposits were primarily formed on the upper slope (Podhorie Formation) and in a fore-reef environment (Manín Formation). Second, biostratigraphic data indicate that the shallow-water production persisted up to the Albian, just as it did in another succession of the Manín Unit. The Podhorie Fm contains colomiellids (Colomiella recta, C. mexicana) and calcareous dinoflagellates (Calcisphaerula innominata) that indicate the Albian age. It also contains planktonic foraminifers (Ticinella roberti, Ticinella cf. primula, Ticinella cf. madecassiana, Ticinella cf. praeticinensis) of the Albian Ticinella primula Zone. The Podhorie Formation passes upwards into peri-reefal facies of the Manín Fm where we designate the Malý Manín Member on the basis of rudists shell fragments and redeposited orbitolinids. Microfacies associations share similarities with the Urgonian-type microfacies from Mediterranean Tethys and allow us to restrict the growth and the demise of the carbonate platform. δ13C and δ18O isotopes change over a broad range of both formations: δ13C is in the range +1.03 to +4.20 ‰ V-PDB and δ18O is in the range −0.14 to −5.55 ‰ V-PDB. Although a close correlation between δ13C and δ18O indicates diagenetic overprint, a long-term increase of δ13C can indicate a gradual increase in the aragonite production and/or increasing effects of oceanic water masses in the course of the Albian, prior to the final platform drowning. Carbonate platform evolution was connected with submarine slumps and debris flows leading to redeposition and accumulation of carbonate lithoclasts and bioclastic debris on the slope. Our study confirms that the growth of carbonate platforms in the Central Western Carpathians was stopped and the platform collapsed during the Albian, in contrast to the westernmost Tethys. A hardground formed during the Late Albian is overlain by Albian - Cenomanian marls of the Butkov Formation with calcisphaerulid limestones characterized by planktonic foraminifers of the Parathalmanninella appenninica Zone and calcareous dinoflagellates of the Innominata Acme Zone.

Three-dimensional stratigraphic-sedimentological forward modeling of an Aptian carbonate reservoir deposited during the sag stage in the Santos basin, Brazil

Stratigraphic-sedimentological forward modeling is a useful technique to test the role of parameters that may act in the depositional processes of sedimentary rocks. The technique was applied to the Santos basin, Brazil, using the software program DIONISOS to test possible depositional scenarios for an Aptian section of a lacustrine carbonate platform developed during the late sag stage (uppermost pre-salt section) of the basin. Different parameters were tested in a set of numerical simulations with the aim of reproducing the carbonate facies spatial distribution and the overall stratigraphic stacking pattern of the platform, as observed in the available well data. Several models were simulated, and four of them are discussed in this paper. Based on the available descriptive data from 10 wells, Model 3 is considered to produce the best fit to explain the development of the 100-m thick Aptian carbonate platform, occurrences of grainstones and stromatolites as proximal and intermediate facies, respectively, and the overall carbonate stacking pattern. Thus, based on 3D stratigraphic-sedimentological forward modeling, the best set of environmental conditions able to explain the pattern of carbonate deposition observed in the study area are (1) a slow carbonate depositional rate (0.08 mm/y over an interval of 2.4 My), which is indicative of a larger microbial contribution, and (2) lake-level oscillations, which were essentially induced by arid climatic conditions. These results also enable new perspectives for the application of stratigraphic-sedimentological forward modeling as a predictive tool for hydrocarbon exploration in carbonates and for the development of depositional models of other sag carbonate platform settings.

Oligocene and Miocene Carbonate Platform Development In the Browse Basin, Australian Northwest Shelf

Abstract: Extensive 2D and 3D seismic-reflection data calibrated by wells provide new insights into the lateral extent, morphology, internal architecture, and age of carbonate platforms in the subsurface of the Browse Basin, Australian Northwest Shelf (NWS). The oldest carbonate build-ups are interpreted as a giant bryozoan build-up complex of Oligocene age (34.03–27.8 Ma). In the late Burdigalian, tropical reef growth began, and numerous reef-rimmed carbonate platforms progressively coalesced into an extensive barrier reef. From the mid-Langhian to the early Tortonian, the Browse Basin contained an elongate (along-margin) barrier-reef system over 500 km long. This barrier reef possibly extended in the southwest into the Northern Carnarvon Basin. After the early Tortonian, reefs on top or landward of this major barrier-reef system were smaller and less connected. This significant reduction in area covered by reef-rimmed platforms likely resulted from cooling following the Mid-Miocene Climate Optimum and relatively high subsidence rates. The reef-rimmed carbonate platforms that formed between the late Burdigalian and the early Tortonian are very similar throughout the Browse Basin. Platform development after the early Tortonian was regionally diverse. High subsidence rates in the northern Browse Basin are interpreted to have promoted the development of thick, aggrading platforms. Lower subsidence in the south led to thinner, more widespread platforms, recurrently shifting location in response to eustatic change. The final phase of reef decline around 6 Ma coincides with an increase in current-driven drift sedimentation. Current-related winnowing of sediments from the lagoons and slopes of the remaining carbonate platforms likely contributed to their final drowning.

Relative sea-level change in western New Guinea recorded by regional biostratigraphic data

We present new biostratigraphic analyses of approximately 200 outcrop samples and review biostratigraphic data from 136 public domain exploration wells across western New Guinea. Biostratigraphic ages and palaeodepositional environments were interpreted from occurrences of planktonic and larger benthic foraminifera, together with other fossils and environmental indicators where possible. These data were compared with existing geological maps and exploration well data to reconstruct the palaeogeography of western New Guinea from the Carboniferous to present day. In addition, we used the known bathyal preferences of fossils to generate a regional sea-level curve and compared this with global records of sea-level change over the same period. Our analyses of the biostratigraphic data identified two major transgressive-regressive cycles in regional relative sea-level, with the highest sea levels recorded during the Late Cretaceous and Late Miocene and terrestrial deposition prevalent across much of western New Guinea during the Late Paleozoic and Early Mesozoic. An increase in the abundance of carinate planktonic foraminifera indicates a subsequent phase of relative sea-level rise during a regional transgressive event between the Late Jurassic and Late Cretaceous. However, sea-levels dropped once more during a regressive event between the Late Cretaceous and the Paleogene. This resulted in widespread shallow water carbonate platform development in the Middle to Late Eocene. A minor transgressive event occurred during the Oligocene, but this ceased in the Early Miocene, likely due to the collision of the Australian continent with intra-Pacific island arcs. This Miocene collision event resulted in widespread uplift that is marked by a regional unconformity. Carbonate deposition continued in platforms that developed in shallow marine settings until these were drowned during another transgressive event in the Middle Miocene. This transgression reached its peak in the Late Miocene and was followed by a further regression culminating in the present day topographic expression of western New Guinea.

Sea-level changes and carbonate platform evolution of the Xisha Islands (South China Sea) since the Early Miocene

Analyses of the biogeochemical branched and isoprenoid tetraether (BIT) index and elemental geochemistry from Well XK1 provide insight into the development of the Xisha Islands carbonate platform in the South China Sea (SCS) since the Early Miocene. BIT is the ratio of branched glycerol dialkyl glycerol tetraethers (bGDGTs) to isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), which are derived from meteoric and marine environments respectively. BIT serves as a novel proxy for tracking sea-level changes.The BIT curve of Well XK1 is characterized by “low–high–low–high” alternating stages, indicating the superposition of sea-level changes on carbonate platform evolution in the SCS. Following the rapid expansion of the SCS in the early stages of the Early Miocene, biogenic reefs began to form, and BIT oscillated intensely. The SCS started to regress after its initial expansion during the Middle Miocene, at which time global sea level was also falling significantly. The combination of both tectonic activity and eustatic variation has resulted in the deposition of strata with reef-beach facies, which are characterized by high BIT values. Relative sea level reached its lowest point in the late stages of the Middle Miocene (~11.6Ma). Subsequent sea-level rise between the Late Miocene and Pliocene induced a negative shift in BIT, and lagoonal facies formed under optimal warm marine conditions. Periodic exposure caused the reefs of the Xisha Islands to suffer from erosion during the Pleistocene, which is consistent with a second elevation in the BIT index. However, modern trends show increasing regional water depths again.Major and trace element curves (Na, Si, P, B, Ga, Mo, Zn), as well as the associated ratios (Na2O/K2O, Na2O/SiO2, B/Ga, Ti/Sr, Zr/Sr, Al/Sr, Th/U), accurately record sea-level variations. Several significant discontinuities correlate with sea-level shifts, and provide important details about the geomorphology, sedimentation patterns, and diagenesis types on the SCS carbonate platform. The agreement between organic and inorganic geochemical data demonstrates that our reconstructions of SCS sea-level changes and modes of carbonate platform development are robust.Our findings also demonstrate that carbonate evolution in the Xisha Islands was shaped by relative sea-level changes, implying that sea level fluctuations in the SCS were controlled by global eustatic factors in addition to regional tectonic subsidence.

Impact of tectonic and volcanism on the Neogene evolution of isolated carbonate platforms (SW Indian Ocean)

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geophysical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that submarine flat-top seamounts correspond to karstified and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well-constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical environments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typified by the establishment of wide lava flow complexes, and (2) to extensional tectonic deformation associated with high-offset normal faults dividing the flat-top seamounts into distinctive structural blocks. Explosive volcanic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long-term evolution of shallow-water carbonate platforms.

Mut Basin, Turkey: Miocene carbonate depositional styles and mixed systems in an icehouse setting

The Mut Basin in southcentral Turkey contains a rich variety of Cenozoic carbonate deposits that developed on a complex pre-Miocene topography and can be studied in seismic-scale, three-dimensional outcrops. These include open and rimmed carbonate shelves with steep slopes, small isolated platforms, and mixed carbonate–siliciclastic systems showing reciprocal sedimentation. The exceptional preservations of the stratal geometries make it possible to easily link large-scale depositional geometry to depositional facies in a variety of platform types and depositional environments. The Mut Basin provides a better understanding of (1) the control of antecedent topography on carbonate platform development, (2) the influence of icehouse conditions (e.g., high-amplitude, high-frequency sea level fluctuations) on the carbonate system, (3) the link between faunal evolution and the style of carbonate platforms and stratigraphic architecture, (4) lateral and reciprocal carbonate and siliciclastic sedimentation, (5) platform asymmetry and local controls on stratal geometries, and (6) the architecture of steep carbonate margin and associated gravitational collapse deposits. These concepts are directly applicable to time-equivalent Cenozoic carbonate reservoir systems in Southeast Asia, the southern Caribbean, and the eastern Mediterranean.

Neogene carbonate platform development in the NW South China Sea: Litho-, bio- and chemo-stratigraphic evidence

The variations of lithostratigraphy, foraminifera, geochemical elements, and organic biomarkers in well XK1 from the Xisha (Paracel) Islands reveal the history of carbonate platform development since the early Miocene in the South China Sea. Our results indicate that reef growth was more active in the middle Miocene than in any earlier periods, decreased in the late Miocene, revitalized in the Pliocene, and active again in the Pleistocene interglacial periods. Unlike those limestone-rich intervals, the late Miocene succession in well XK1 is characterized by dolomitized limestones with a low deposition rate (30–60m/Ma post-compaction). These late Miocene rocks also contain relative high MgO (~20%), more ∑REE, abundant algae and some small sized foraminifera, but rare larger benthic foraminifera, very low land-sourced brGDGTs (0–0.5ng/20g), low BIT index (0.1–0.5), and low TEX86-derived sea temperature (~20–25°C), indicating limited active reef growth in relative cooler mesophotic environments at >30m water depths. Similar dolomitized limestones of late Miocene age also occur in other carbonate platform areas including the Nansha (Spratly) Islands in the southern South China Sea. Together, these South China Sea records provide regional evidence of a recession in global carbonate platform development starting from the late Miocene. While carbonate platform reduction in other regions may have been triggered by global cooling and changing oceanic chemistry, accelerated basin subsidence coupled with monsoon cooling are considered as the main factors causing the reef decline in the late Miocene South China Sea.