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The Maldives Archipelago (Indian Ocean), composed of two rows of atolls that enclose an inner sea, offers an excellent study site to explore the forcings of carbonate production at platforms. Glacial–interglacial sea-level changes have been claimed to be the main factor controlling the carbonate platform factories; however, climatic factors may also have an impact. In this work we used geochemical compositional records, obtained by X-ray fluorescence (XRF) core-scanning from the International Ocean Discovery Program (IODP) Site U1467 in the Maldives Inner Sea, to analyze the orbitally driven fluctuations on the carbonate production and export from the neritic environment into the Maldives Inner Sea over the last 1.3 million years. High Sr aragonite-rich carbonates (HSAC) from neritic settings were deposited in the Maldives Inner Sea during sea-level highstand intervals, increasing the Sr/Ca values. In contrast, low Sr/Ca values are observed coincident with sea-level lowstand periods, suggesting that large areas of the atolls were exposed or unable to grow, and therefore, there was a demise in the carbonate production and sediment export to the Maldives Inner Sea. However, comparison of the Sr/Ca values and the sea-level reconstructions for different interglacial periods before and after the mid-Brunhes event (MBE, ∼ 430 ka) indicates that sea level is not the only factor controlling the production of HSAC during sea-level highstands. The study of monsoon and primary productivity proxies (Fe -normalized, Fe/K , and Br -normalized records) from the same site suggests that the intensity of the summer monsoon and the Indian Ocean dipole probably modulated the carbonate production at the atolls. Moreover, Marine Isotope Stage 11 stands out as a period with high sea level and extraordinary carbonate production in the Maldives platform. This outstanding carbonate production in the Maldives atolls (and in other low-latitude carbonate platforms) probably contributed to the mid-Brunhes dissolution event through a strong shelf-to-basin fractionation of carbonate deposition. [ABSTRACT FROM AUTHOR]

Carbonate platforms are built mainly by corals living in shallow light-saturated tropical waters. The Saya de Malha Bank (Indian Ocean), one of the world's largest carbonate platforms, lies in the path of the South Equatorial Current. Its reefs do not reach sea level, and all carbonate production is mesophotic to oligophotic. New geological and oceanographic data unravel the evolution and environment of the bank, elucidating the factors determining this exceptional state. There are no nutrient-related limitations for coral growth. A switch from a rimmed atoll to a current-exposed system with only mesophotic coral growth is proposed to have followed the South Equatorial Current development during the late Neogene. Combined current activity and sea-level fluctuations are likely controlling factors of modern platform configuration. [ABSTRACT FROM AUTHOR]

Selective dissolution of metastable minerals and certain pore types, such as moldic and oomoldic pores, are widely accepted to indicate diagenesis by meteoric fluids in carbonate rocks. In this study, oomoldic‐rich re‐sedimented carbonates from the Happy Field in the Midland Basin, which contain no evidence of subaerial environments, indicate that such pores may form in other settings. Ooid dissolution in the reservoir was evaluated through petrography, scanning electron microscopy, stable isotopes (δ18O and δ13C), elemental geochemistry, and X‐ray diffraction of bioclasts, ooids and associated cements. Cements are primarily dogtooth and equant low‐magnesium calcite, poikilotopic celestine is present, and late‐stage low‐magnesium calcite occurs in lesser amounts. Equant low magnesium calcite cements are interpreted to reflect the initial stages of pore filling, synchronous with aragonitic ooid dissolution, thus implying that oomoldic pores and equant cements formed in similar fluids. Although the prominent equant low magnesium calcite cement textures mimic those often associated with meteoric fluids, geochemistry suggests a marine origin. The equant cements have δ18O (Vienna Pee Dee Belemnite) ratios between −2.0‰ and −3.5‰, and δ13C (Vienna Pee Dee Belemnite) ratios between 4‰ and 5‰. Magnesium/calcium ratios show values, ranging from 5 to 142 mmol/mol (n = 223, average 23.2 and SD = 20.1). Strontium/calcium ratios present values from 0.3 to 2.6 mmol/mol (n = 223, average 0.71 mmol/mol, SD = 0.35). Geochemical data coincide with typical low‐latitude Permian marine carbonate values and further signify diagenesis within the marine phreatic realm. To explain this process, the results indicate that the dissolution of aragonite ooids and precipitation of early cements occurred in environments ranging between the seafloor to shallow burial dominated by undersaturated marine fluids. Undersaturation was triggered by organic matter decomposition occurring at the seafloor and in the shallow subsurface immediately below. These findings have broader implications for marine and burial carbonate diagenesis as well as diagenesis of re‐sedimented deposits. [ABSTRACT FROM AUTHOR]

Tropical carbonate platforms are edifices built by shallow-water, carbonate-producing organisms. Prolonged suppression or shutdown of tropical shallow-water carbonate factories may result in partial or complete platform demise. Factors triggering the drowning process can relate to rates of accommodation increase exceeding carbonate accumulation and/or the establishment of ecological conditions not favorable for carbonate producers, with increasing water temperatures and high nutrient contents proposed as drivers. More recently, the intensification of ocean circulation and currents has been identified as a major factor in carbonate platform drowning. We tested the latter with seismic reflection and multibeam data collected with RV Sonne in 2022 on the Queensland Plateau (north-east Australia) and by correlating these data with ODP Leg 133 sedimentological and biostratigraphic results. The carbonate platforms of the Queensland Plateau underwent a partial drowning between 13.6 and 12.7 Ma. This partial drowning is coeval with the onset of current-driven sedimentation. Relict platforms, which form the core of the presently still active platforms, were established in two steps: After the demise of a spatially expanded platform, a system with smaller, low-relief banks and mounds established. At around 3.7 Ma, there was a turnover and higher relief, flat-topped platforms established. We propose that the geologic history of the Queensland Plateau represents another example of a carbonate platform evolution controlled by ocean currents. • Seismic reflection and multibeam data collected with RV Sonne in 2022 on the Queensland Plateau • The carbonate platforms of the Queensland Plateau underwent a partial drowning between 13.6 and 12.7 Ma. • The Queensland Plateau represents another example of a carbonate platform controlled by ocean currents during its evolution [ABSTRACT FROM AUTHOR]

The Black River (Upper Ordovician – Sandbian) and Trenton (Upper Ordovician – Katian) groups are traditionally interpreted as a deepening‐upward succession deposited in a progressively subsiding Appalachian Basin margin that contained warm‐water, marine, photozoan deposits that pass upward into cool‐water, marine, heterozoan carbonates. This succession is customarily interpreted to reflect an incursion of cold, high‐latitude ocean waters into the area. This view is herein confirmed for coeval carbonates in the northern part of the basin, particularly the St. Lawrence Platform. They are now well explained in this study on the basis of recent studies of cool‐water carbonates and calcite–aragonite seas. Overall the succession is one of Sandbian photozoan ramp deposits succeeded by Katian heterozoan ramp carbonates that changed back to photozoan ramp deposits prior to the Hirnantian glaciation. The current interpretation, that deposition took place throughout a calcite sea time, seems at odds with this series of strata. Instead it is herein proposed that deposition took place during an aragonite sea time wherein calcite sea‐like sediments accumulated under cold ocean‐water temperatures. Such an interpretation is supported by recent experimental data that supports the importance of seawater temperature on CaCO3 polymorph precipitation. If correct, this means that some of the evidence for calcite sea deposition through time brought about by global tectonics, should be re‐evaluated to make sure it was not simply cool‐water carbonate production. [ABSTRACT FROM AUTHOR]

The properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three‐dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non‐fabric selective dissolution, and then a second‐generation of (post‐dissolution) cement. Models are 3 mm3 with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny‐Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny‐Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny‐Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios. [ABSTRACT FROM AUTHOR]

During the Miocene, extensive carbonate deposition thrived over wide latitudinal ranges in Southeast Asia despite perturbations of the global climate and thermohaline circulation that affected the Asian continent. Nevertheless, the mechanisms of its emergence, adaptability in siliciclastic-dominated margins and demise, especially in southern South China Sea (SCS), are largely speculative and remains enigmatic along with a scarcity of constraints on paleoclimatic and palaeoceanographic conditions. Here we show, through newly acquired high-resolution geophysical data and accurate stratigraphic records based on strontium isotopic dating, the evolution of these platforms from ~15.5–9.5 Ma is initially tied to tectonics and eustasy, and ultimately, after ~9.5 Ma, to changes in the global climate patterns and consequent palaeoceanographic conditions. Our results demonstrate at least two paleodeltas that provided favourable substratum of elevated sand bars, which conditioning the emergence of the buildups that inadvertently mirrored the underlying strata. We show unprecedented evidences for ocean current fluctuations linked to the intensification of the Asian summer monsoon winds resulting in the formation of drifts and moats, which extirpated the platforms through sediment removal and starvation. This work highlights the imperative role of palaeoceanography in creating favourable niches for reefal development that can be applicable to carbonate platforms elsewhere. [ABSTRACT FROM AUTHOR]

Metre‐scale cycles are a common feature in Precambrian and Phanerozoic shallow water carbonate successions, and astronomically forced changes in sea‐level (Milankovitch cycles) may have been an important driver controlling their deposition. Nevertheless, the degree to which potentially low amplitude astronomically paced sea‐level oscillations may have controlled carbonate accumulation in deep time is unclear. In this study, a stochastic model of carbonate accumulation demonstrates how metre‐scale exposure‐bound sequences can be generated under conditions of random sea‐level change. These sequences have characteristic durations close to Milankovitch cycles, despite the absence of any astronomical control on their formation. Metre‐scale sequences with sub‐Milankovitch (millennial‐scale) durations can also be generated by the model, potentially shedding light on the origin of sub‐Milankovitch sequences such as those recorded on the Middle Triassic Latemar platform of Northern Italy. Sensitivity tests demonstrate how shallow water carbonates may be very sensitive to weak (i.e. low amplitude) astronomically forced sea‐level oscillations. Notably, strong statistical evidence (P < 0·01) for astronomical cycles can be preserved in modelled successions even when astronomical forcing contributes <1% of the sea‐level variance on million year timescales. Taken together, metre‐scale cycles with Milankovitch‐scale durations in ancient carbonate successions may reveal very little about the amplitude, or even the existence, of astronomical forcing as a sea‐level driver. [ABSTRACT FROM AUTHOR]

The middle Miocene onset of modern ocean circulation patterns changed the growth style of isolated tropical carbonate platforms because surface and contour currents began shaping the flanks of these edifices. Since then, ocean currents have redistributed the offbank-transported sediment, reduced sedimentation by particle sorting or winnowing, and even eroded slopes. As a result, the flanks of isolated carbonate platforms around the world after 13-10 Ma have not only been constructed by mass gravity deposits, but equally by contourites with distinct drift and moat geometries. These produce specific stacking patterns of platform flank deposits. This flank architecture, produced by combined current and gravity processes, is typical of tropical carbonate platforms growing in the Neogene icehouse world. Comparison of this architecture with geometries in older platforms also has the potential to extract information about the rigor of ocean circulation in deep time where the deep-sea record is missing. [ABSTRACT FROM AUTHOR]

The identification of sediment drifts typically relies on interpretation of reflection seismic data sets. This study sedimentologically analyzed an example of a carbonate delta drift previously identified in seismics in order to provide a catalogue of characteristic features at core and seismic scale for allowing testing the occurrence of this poorly known type of deposit elsewhere. Cores and downhole logs recovered during International Ocean Discovery Program Expedition 359 to the Maldives, in combination with seismic data, were analyzed with this objective. The diagnostic criteria for the sedimentological recognition of a delta drift are: (i) the development of sigmoidal clinoforms that thin out towards proximal and distal settings; (ii) a proximal part characterized by coarse‐grained facies with abundant shallow‐water components and distal areas dominated by fine‐grained facies with rare to absent shallow‐water components; (iii) winnowing of the finer fraction in proximal facies; (iv) extensive fragmentation of most of the bioclasts with minor intervals of well‐preserved components; (v) bigradational intervals in the proximal part and large channels in proximal to distal settings; and (vi) the lobe to delta shaped outline of the sediment accumulation. The characteristic shallow‐water fossil assemblage of the Mid to Upper Miocene delta drift consists of large benthic foraminifera (Amphistegina, Cycloclypeus, Lepidocyclina, Operculina and Heterostegina), fragmented red algae and bryozoans, equinoid debris, and Halimeda plates. The deeper‐water part of the drift bodies consists of fine‐grained planktonic foraminifera‐rich wackestone. Condensed intervals may occur as result of enhanced bottom‐current activity. In contrast to siliciclastic drift bodies, the carbonate delta drift has an important contribution by in situ shallow‐water carbonate production reminiscent of a shoal. In situ carbonate production is proposed as a major controlling factor which is as important as the pelagic settling or the shaping by density and bottom currents in siliciclastic drifts. In the absence of three‐dimensional data and in two‐dimensional views the carbonate delta drift sediment bodies resemble carbonate ramps, which indicates that there may be the need to re‐evaluate various cases of such systems described from the geological record. [ABSTRACT FROM AUTHOR]

Carbonate environments inhabit the realm of the surface, intermediate and deep currents of the ocean circulation where they produce and continuously deliver material which is potentially deposited into contourite drifts. In the tropical realm, fine‐grained particles produced in shallow water and transported off‐bank by tidal, wind‐driven, and cascading density currents are a major source for transport and deposition by currents. Sediment production is especially high during interglacial times when sea level is high and is greatly reduced during glacial times of sea‐level lowstands. Reduced sedimentation on carbonate contourite drifts leads to early marine cementation and hardened surfaces, which are often reworked when current strength increases. As a result, reworked lithoclasts are a common component in carbonate drifts. In areas of temperate and cool water carbonates, currents are able to flow across carbonate producing areas and incorporate sediment directly to the current. The entrained skeletal carbonate particles have variable bulk density and shapes that lower the prediction of transport rates in energy‐based transport models, as well as prediction of current velocity based on grain size. All types of contourite drifts known in clastic environments are found in carbonate environments, but three additional drift types occur in carbonates because of local sources and current flow diversion in the complicated topography inherent to carbonate systems. The periplatform drift is a carbonate‐specific plastered drift that is nearly exclusively made of periplatform ooze. Its geometry is built by the interaction of along‐slope currents and downslope currents, which deliver sediment from the adjacent shallow‐water carbonate realm to the contour current via a line source. Because the periplatform drift is plastered on the slopes of the platforms it is also subject to mass gravity flow and large slope failures. At platform edges, a special type of patch drift develops. These hemiconal platform‐edge drifts also contain exclusively periplatform ooze but their geometry is controlled by the current around the corner of the platform. At the north‐western end of Little and Great Bahama Bank are platform‐edge drifts that are over 100 km long and up to 600 m thick. A special type of channel‐related drift forms when passages between carbonate buildups or channels within a platform open into deeper water. A current flowing in these channels will entrain material shed from the sediment producing areas. At the channel mouth, the sediment‐charged current deposits its sediment load into the deeper basin. With continuous flow, a submarine delta drift is built that progrades into the deep water. The strongly focused current forming the delta drift, is able to rework coarse skeletal grains and clasts, making this type of carbonate drift the coarsest drift type. [ABSTRACT FROM AUTHOR]

Carbonate drifts have so far not been as intensely investigated as their siliciclastic equivalents, especially from an ichnological perspective. The aim of this work is therefore to provide an overview of the different bioturbation styles in carbonate drifts for ichnologists and sedimentologists working in such deposits. Different types of carbonate drifts from the Maldives were studied to address this objective. The cores recovered during International Ocean Discovery Program Expedition 359 were examined to provide the sedimentological and ichnological data for a detailed analysis of the ichnology of carbonate drifts. The ichnological characteristics of the Maldives drifts are compared to other carbonate drifts in order to discuss similarities and differences, and thus provide an overview of the general characteristics of carbonate drift ichnology. These drifts are located in the Santaren Channel which lines Great Bahama Bank, along the Marion Plateau in Australia, in the Limassol and Larnaca basins in Cyprus and in the Danish Basin in Denmark. The common characteristics of bioturbation in carbonate drifts are: (i) the complete bioturbation of the sediment with bioturbation indexes between four and six; (ii) the occurrence of distinctive trace fossils limited to facies contacts or condensed intervals; (iii) a typical ichnoassemblage consisting of Thalassinoides, Scolicia, Planolites, Zoophycos, Chondrites, Phycosiphon and Palaeophycus; (iv) the contiguous occurrence of ichnogenera from different tiers, with only Zoophycos and Chondrites as deep tiers; and (v) distinct infills of the traces including particulate organic matter, pyrite, silica and celestine. In addition, the main ichnofacies of carbonate drifts is the Zoophycos ichnofacies. Ichnofabrics grade from coarse‐grained and completely bioturbated to ichnofabrics with present to rare trace fossils and preserved sedimentary structures. The type and intensity of the bioturbation is controlled by the amount of organic matter and the oxygenation at the sea floor that is determined by the action of bottom currents and the sea‐level fluctuations affecting the carbonate factory in carbonate platforms bordering the basins where the carbonate drifts form. The study of the bioturbation in core and outcrop provides palaeoenvironmental information about carbonate‐drift deposits that complement the classical sedimentological data. [ABSTRACT FROM AUTHOR]

This study investigates the δ13C values of Middle Miocene–Modern drift deposits and periplatform sediments in the Maldives and compares these data with the global δ13C values derived from bulk oceanic sediments and foraminifera. This comparison reveals that while the δ13C values of the early Miocene periplatform sediments in the Maldives appear to track the global record of δ13C values, including increases associated with the Oligocene–Miocene boundary as well as the variations within the Monterey Event, the correlation with the Monterey Event may be coincidental. It is suggested that variations in δ13C values do not reflect changes in oceanic dissolved inorganic carbon, but instead pulses of sediment arising from platform progradation that contribute carbonates with elevated δ13C values derived from the adjacent shallow‐water atolls. This conclusion is supported both by correlations between the seismic sequence architecture and the δ13C values which document progradation of 13C‐rich platform sediments, and also by the continuation of the interval of 13C‐rich sediments past the end of the Monterey Event at 13 Ma within the drift. [ABSTRACT FROM AUTHOR]

The Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats. [ABSTRACT FROM AUTHOR]

Abstract During the Paleocene and Eocene, the western continental margin of the Indian Plate was an equatorial province characterized by widespread attached and isolated carbonate platforms. Using high-resolution seismic data combined with sedimentological data and log-motifs, this study presents the stratigraphic evolution, the morphological features, and the distributions of these isolated carbonate platforms which are located on NE-SW oriented basement highs and volcanic ridges linked to the Réunion Hotspot trace. Seismic stratigraphy allows defining seven seismic units in the platform succession. Stratal stacking patterns indicative of phases of aggradation and of large-scale backstepping alternate with horizons interpreted to indicate repeated phases of emersion. The onset of platform growth and the further platform development were mainly controlled by the subsidence following the Deccan Volcanism and the opening of Carlsberg Ridge. Platforms are asymmetrical in the section with the eastern aggrading margins interpreted as windward and the western margins as leeward. The leeward margins, which through time stepped back appear as structurally modified by basement induced faults. Fault blocks movement also partially influenced the internal platform geometries and the facies distribution. Change in the rate of subsidence possibly augmented the relative sea-level rise during the Early to Middle Eocene and hence contributed to the demise of the isolated carbonate platforms. Isolated carbonate platforms are stratigraphically age-equivalent to the Laki-Ghazij lithostratigraphic unit of the contemporaneous carbonate shelf. As per our knowledge, this carbonate system is the best-known seismic scale example of greenhouse carbonate system thriving during the Paleocene –Eocene. Highlights • This study provides a seismic based characterization of greenhouse carbonate platforms of Offshore Indus Basin Pakistan. • This study may assist in hydrocarbon exploration activities in the region. • The evolution and geological framework of the isolated carbonates platforms are described in the context of the regional stratigraphy. • We elucidate the primary controlling factors on the platform development. • This study serves as a best analogue for the subsurface carbonate platform developed under greenhouse climate conditions elsewhere. [ABSTRACT FROM AUTHOR]

Abstract: The formation of syn‐depositional fractures in carbonate platforms is considered an important feature in the understanding of platform evolution. This study investigates the mechanisms of fracture formation in rimmed flat‐topped carbonate platforms in the very well‐exposed Cariatiz Miocene Fringing Reef Unit, SE Spain. Fracture data were obtained using a combination of LIDAR and field mapping techniques, which proved useful in understanding general fracture trends. The morphological expression of fracture sets, preferred fracture localization, crosscutting relationships and fracture fill are characteristics that provide constraints on the timing of fracture formation and opening. Three dominant fracture populations were identified, amongst which a margin parallel and a margin perpendicular fracture set. Margin parallel fractures localize around the platform margin and form vertically extensive dikes that crosscut facies boundaries. The sedimentary fill of such fractures suggests syn‐depositional fracture formation under marine conditions. Together, fracture characteristics suggest a gravitational driver for the formation of tensile stress and the development of margin parallel fractures along the platform edge. Margin perpendicular structures form sub‐vertical dikes and fracture corridors. Margin perpendicular fractures localize on the platform slope and show two types of fracture fill, indicating marine and continental origins. Based on variations of fracture morphology along the carbonate platform, fracture localization, petrographic analysis of sedimentary fill and stable isotope analysis on sparite cements, we suggest a gravitational control on the formation of these fractures. Two mechanisms for the formation of subvertical margin perpendicular fractures are proposed: (1) principal stress rotation as a result of downslope loading. (2) Differential compaction over buried gulley systems on antecedent clinoform slopes. We suggest that the formation of sub‐vertical margin perpendicular fractures might be a systematic feature in slopes of flat‐topped carbonate platforms. [ABSTRACT FROM AUTHOR]

Hydroacoustic and sedimentological data in the Santaren Channel covering both the leeward slope of Great Bahama Bank and the windward slope of Cay Sal Bank allow new insights into carbonate platform slope sedimentation. The data document the interplay between depositional and erosive processes on both slopes through time and provide information on the current regime and its influence on the slope sedimentary processes. This study emphasizes the diversity and complexity of the slope morphology and the sediment distribution of the youngest high-frequency sequence, which has developed since the last glacial maximum. The processes triggering slope failures and the formation of channels and gullies differ on both slopes. At the leeward slope of the Great Bahama Bank, extensive slope failures occurred primarily during sea-level lowering following an interglacial. These slope failures created a slope morphology that channelizes the exported platform sediments during the subsequent highstand. At the windward slope of Cay Sal Bank, contour currents and the local tectonic regime are responsible for slope failures. During sea-level lowstands, downwelling induces turbidity currents. The interaction of turbidity and contour currents leads to the formation of a system of furrows and slope-parallel sediment ridges. The discovered heterogeneities in slope sedimentation improve the understanding of carbonate slope sedimentation and provide implications for sequence stratigraphic interpretation of carbonate platform slopes. [ABSTRACT FROM AUTHOR]

Seismic and multibeam data, as well as sediment samples were acquired in the South Malé Atoll in the Maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits. Multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15–20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea-level lowstand. Such depocenters are located in current-protected areas flanking the reef rim of the atoll or in infillings of karst dolinas. Much of the 50 m deep sea floor in the lagoon interior is current swept, and has no or very minor sediment cover. Erosive current moats line drowned patch reefs, whereas other areas are characterized by nondeposition. Karst sink holes, blue holes and karst valleys occur throughout the lagoon, from its rim to its center. Lagoonal sediments are mostly carbonate rubble and coarse-grained carbonate sands with frequent large benthic foraminifers, Halimeda flakes, red algal nodules, mollusks, bioclasts, and intraclasts, some of them glauconitic, as well as very minor ooids. Finer-grained deposits locally are deposited in current-protected areas behind elongated faros, i.e., small atolls which are part of the rim of South Malé Atoll. The South Malé Atoll is a current-flushed atoll, where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim. With an elevated reef rim and tower-like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record. [ABSTRACT FROM AUTHOR]

Wind-driven surface currents and deeper water-mass circulation shape carbonate sediment drifts in shallow- to deep-water depositional settings of the Maldives. Drift sediments are located in the platform interior, on top of drowned banks, and around the atolls. They are composed of an admixture of pelagic and shallow-water components. Drift bodies vary in dimension, architecture, and genesis. Small patch drifts and sheeted drifts are present in the platform interior and on top of drowned atolls; large patch drifts preferably form in the current-shadow of large atolls and banks. Small inter-atoll channels and large gateways are dominated by channel-related drifts. The Inner Sea, a large perched basin surrounded by atolls comprises a complex contourite depositional system of giant elongated mounded and sheeted drifts, mixed drifts, as well as delta drifts. This succession of different drift types is organized in sequences, delimited by unconformities caused by changes in the current regime. These may be triggered by a re-configuration of the inter-atoll gateways or by allo-cyclic processes such as climate variability. Effects based on to changes in the global water-mass circulation could not be proven. With exception of the delta drift, carbonate drift types in the Maldives are analogous to their siliciclastic counterparts. By contrast, however, these carbonate drifts are emplaced at a short distance to the shallow-water carbonate factory, which acts as main sediment source. Variability and abundance of drifts in the Maldives emphasize the significant role of current-controlled sediment transport and redistribution in carbonate platform development. • Maldives dominated by carbonate drifts in shallow- to deep-water depositional settings. • Marine passages and gateways providing pathways for water mass circulation. • Drift sedimentation mainly controlled by gateway topography. [ABSTRACT FROM AUTHOR]

Facies architecture and platform evolution of an early Frasnian reef complex in the northern Canning Basin of north-western Australia were strongly controlled by syn-depositional faulting during a phase of basin extension. The margin-attached Hull platform developed on a fault block of Precambrian basement with accommodation largely generated by movement along the Mount Elma Fault Zone. Recognition of major subaerial exposure and flooding surfaces in the Hull platform (from outcrop and drillcore) has enabled comparison of facies associations within a temporal framework and led to identification of three stages of platform evolution. Stage 1 records initial ramp development on the hangingwall dip slope with predominantly deep subtidal conditions that prevented any cyclic facies arrangements. This stage is characterised by basal siliciclastic deposits and a major deepening-upward facies pattern that is capped by a sequence boundary towards the footwall (north-west) and a major flooding surface towards the hangingwall. Stage 2 reflects the bulk of platform aggradation, significant platform growth towards the hangingwall and the development of reef margins and cyclic facies arrangements. Thickening of this stage towards the hangingwall indicates that accommodation was generated by rotation of the fault block and overlying platform. Stage 3 records a major flooding and backstep of the platform margin. The Hull platform illustrates important elements of margin-attached carbonate platforms in a half-graben setting, including: (i) prominent, but limited, coarse siliciclastic input that does not have a major detrimental effect on carbonate production near the rift margin in arid to semi-arid settings; (ii) wedge-shaped accommodation created by syn-depositional rotation of fault blocks and tilting of the hangingwall dip slope, resulting in shallow-water facies and subaerial exposure up-dip of the rotational axis and deeper water facies down-dip; and (iii) evolution of a ramp to rimmed shelf, coincident with a sequence boundary-flooding surface, that is accelerated by tilting of the hangingwall dip slope during fault-block rotation. [ABSTRACT FROM AUTHOR]

Carbonate deposits in Roman aqueducts of Patara and Aspendos (southern Turkey) were studied to analyse the nature of their regular layering. Optical microscopy and electron-backscattered diffraction results show an alternation of dense, coarsely crystalline, translucent laminae composed of bundles and fans of elongate calcite crystals with their c-axes parallel to the long axis, and porous, fine-grained laminae with crystals at near-random orientation. The δ18O and δ13C data show a strong cyclicity and anti-correlation, whereby high and low δ18O values correspond to dense columnar and porous fine-grained laminae, respectively. Geochemical analyses show similar cyclic changes in carbonate composition. Electron microprobe and laser ablation inductively coupled mass spectrometry analyses show that porous fine-grained laminae are enriched in elements associated with detrital material (Fe, Mg, K, Al and Si), whereas the dense columnar laminae are nearly pure calcite. The gradient in major and trace-element distribution, regular changes in crystal type and in oxygen and carbon-isotope composition from porous fine-grained to dense columnar laminae reflect changes in water chemistry, discharge, temperature and biological activity. Because of the strong bimodal cyclicity of the Mediterranean climate in southern Turkey, the observed laminae can be attributed to calcite deposition during the dry (porous fine-grained) and wet season (dense columnar), respectively. This observation implies that, with proper geochemical and microstructural control, lamination in carbonate deposits in Roman aqueducts can be used for relative dating of aqueduct construction and maintenance and to obtain data on external factors that influenced the aqueducts, such as palaeoclimate and natural hazards. Carbonate deposits in Roman aqueducts show properties of both flowstone speleothems and riverine fresh water tufa. As many aqueducts of nearly identical channel geometry are present in different climate zones and with different source water characteristics, they can be used as natural experiment setups to test and improve existing models of how laminated fresh water carbonates record climate on time scales ranging from seasonal to millennial. [ABSTRACT FROM AUTHOR]

Abstract: Periplatform ooze is an admixture of pelagic carbonate and sediment derived from neritic carbonate platforms. Compositional variations of periplatform ooze allow the reconstruction of past sea-level changes. Periplatform ooze formed during sea-level highstands is finer grained and richer in aragonite through the elevated input of material from the flooded platform compared to periplatform ooze formed during the episodes of lowered sea level. In many cases, however, the sea floor around carbonate platforms is subjected to bottom currents which are expected to affect sediment composition, i.e. through winnowing of the fine fraction. The interaction of sea-level driven highstand shedding and current impact on the formation of periplatform ooze has hitherto not been analyzed. To test if a sea-level driven input signal in periplatform ooze is influenced or even distorted by changing current activity, an integrated study using seismic, hydroacoustic and sedimentological data has been performed on periplatform ooze deposited in the Inner Sea of the Maldives. The Miocene to Pleistocene succession of drift deposits is subdivided into nine units; limits of seismostratigraphic units correspond to changes or turnarounds in grain size trends in cores recovered at ODP Site 716 and NEOMA Site 1143. For the Pleistocene it can be shown how changes in grain size occur in concert with sea-level changes and changes of the monsoonal system, which is thought to be a major driver of bottom currents in the Maldives. A clear highstand shedding pattern only appears in the data at a time of relaxation of monsoonal strength during the last 315ky. Results imply (1) that drift sediments provide a potential target for analyzing past changes in oceanic currents and (2) that the ooze composition bears a mixed signal of input and physical winnowing at the sea floor. [Copyright &y& Elsevier]

A piston core from the Maldives carbonate platform was investigated for carbonate mineralogy, grain-size distributions, calcium carbonate content and organic carbon. The sedimentary record was linked to Late Pleistocene sea-level variations, using an age model based on oxygen isotopes obtained from planktonic foramanifera, nannofossil biostratigraphy and 14C age determinations. The correlation between the sedimentary record and Late Pleistocene sea-level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the Maldives carbonate platform. Platform flooding events were characterized by strongly increased deposition of aragonite and mud within the Inner Sea of the Maldives. Exposure events, in contrast, can be recognized by rapid decreases in the values of both proxy records. The results show that sediments on the Maldives carbonate platform contain a continuous record of Pleistocene sea-level variations. These sediments may, therefore, contribute to a better understanding of regional and even global sea-level changes, and yield new insights into the interplay between ocean currents and carbonate platform morphology. [ABSTRACT FROM AUTHOR]

Post-glacial, neritic cool-water carbonates of the Western Mediterranean Sea were examined by means of hydroacoustic data, sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea-level rise. The analysed areas, located on the Alboran Ridge, in the Bay of Oran, and at the southern shelf of the island of Mallorca, are microtidal and bathed by oligotrophic to weakly mesotrophic waters. Seasonal water temperature varies between 13 °C and 27 °C. Echosounder profiles show that the Bay of Oran and the southern shelf of Mallorca are distally steepened ramps, while the Alboran Ridge forms a steep-flanked rugged plateau around the Alboran Island. In the three areas, an up to 10 m thick post-glacial sediment cover overlies an unconformity. In Oran and Mallorca, stacked lowstand wedges occur in water depths of 120 to 130 m. On the Alboran Ridge and in the Bay of Oran, highstand wedges occur at 35 to 40 m. Up to 5 m long cores of upper Pleistocene to Holocene successions were recovered in water depths between 40 and 81 m. Deposits contain more than 80% carbonate, with mixed carbonate-volcaniclastics in the lower part of some cores in Alboran. The carbonates consist of up to 53% of aragonite and up to 83% of high magnesium calcite. Radiocarbon dating of bivalve shells, coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr . The carbonate factories in the three areas are dominated mostly by red algae, but some intervals in the cores are richer in bivalves. A facies rich in the gastropod Turritella, reflecting elevated surface productivity, is restricted to the Mallorca Shelf. Rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m; they form a 10 to 20 cm thick veneer overlying rhodolith-poor bioclastic sediments which, nonetheless, contain abundant red algal debris. This rhodolith layer has been developing for the past 800 to 1000 years. Similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation. Sedimentary successions in the cores record the post-glacial sea-level rise and the degree of sediment exposure to bottom currents. Deepening-upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth-indicative red algae. There are only weak downcore variations of carbonate mineralogy, which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface. These new data support the approach of using the Recent facies distribution for interpretation of past cool-water, low-energy, microtidal carbonate depositional systems. Hydroacoustic data show that previous Pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea-level lowstands and accumulated downslope as stacked lowstand wedges; this suggests that, under conditions of high-amplitude sea-level fluctuations, the stratigraphic record of similar cool-water carbonates may be biased. [ABSTRACT FROM AUTHOR]

Abstract: Ocean Drilling Program Site 1131, located in the central Great Australian Bight, recovered an expanded series of Pleistocene cool-water carbonates. Three distinct facies are arranged in repeating sedimentary cycles. Omission intervals form the base of the cycles and are overlain by laminated deposits. The upper part of the cycles consists of bioturbated sediments. Facies cyclicity parallels glacial–interglacial sea-level changes. Firmground/hardground surfaces formed during latest stages of sea-level falls. Laminated sediments were deposited during later sea-level fall and/or early phases of the sea-level lowstand, and the bioturbated intervals are related to interglacial sea-level highstands. Bottom current activity is interpreted to play an important control on facies and cycle development. During the late stages of sea-level fall, upwelling currents winnowed the slope sediments, leading to hardground formation. During latest sea-level falls and early sea-level lowstands, a relative increase in the ramp sediment supply to the slope leading to the formation of the laminated facies was also a response to the continued action of the near-bottom upwelling currents that reworked previously deposited sediments along the slope. During the early sea-level rises, laminated, upwelling-influenced facies were progressively covered by the bioturbated facies. The adopted approach to decipher sedimentary cyclicity, involving the integrated use of wireline logs, geochemical data, FMS images and sedimentological analysis of the lithofacies has allowed to recognise subtle facies changes in upper-slope settings of a distally-steepened carbonate ramp and their relationship with the sea level, as well as to unravel the importance of the oceanic-current regime in the deposition style on the ramp that otherwise would remain unsolved. This integrated method could be applied to study more precisely apparently homogeneous fine-grained calcareous sediment successions of distal-ramp outcrops. [Copyright &y& Elsevier]

Uppermost Tortonian to lower Messinian temperate carboantes crop out in the Agua Amarga Basin (SE Spain). They consist of four units. The lower three units can be tentatively assigned to the lowstand systems tract of a fourth-order sequence, constituing in turn the lowstand ('megatrough unit'), transgressive ('breccia unit') and highstand ('bedded unit') stages of a higher-order cycle. All these materials were deposited in a small pull-apart basini related to the sinistral Carboneras strike-slip fault system. The best represented is the bedded unit (up to 25 m thick), which consists of bioclastic, bryozoan/bivalve-dominated calcarenites/calcirudites with abundant fragmetns of echinioids, barnacles, benthic foraminifers, coralline algae, brachiopods and solitary corals. Facies trends within this unit are roughly arranged in an E-W direction, with the coastline to the north of the basin. The depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts. Seaward of the shoals was the 'factory area', where most organisms lived and maximum carbonate production took place. From this area some of the skeletons were washed landswards by waves and/or currents during storms and incorporated into the shoals and beaches, and others moved downslope along the ramp as mass-flows, accumulating to form the 'fan-bedded zone'. The factory-area and fan-bedded sediments intercalate five well-defined, thick beds of calcarenites/fine-grained calcirudites. They show bar morphologies (single or amalgamated), or make up sand-waves with very consistent tabular cross-bedding pointing landwards. These beds formed in a very shallow, wave/current-influenced, coastal environment. The bars and sand waves in the fan-bedded zone developed during lowstands, while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages. Prograding beachs, shoals, factory facies and fan-bedded layers developed during the... [ABSTRACT FROM AUTHOR]

Carbonate platforms form informative archives for paleoclimates and their internal structures can also hold crucial information about the tectonic history and carbonate evolution of the ocean basins. The Zhongsha atoll (Macclesfield Bank) forms the largest atoll system in the South China Sea with a surface area of 23500 km2. However, the internal structure and evolution of this atoll system is completely unknown. 2D multichannel seismic reflection data were acquired in 2017 over the Zhongsha atoll in the South China Sea to unravel the stratigraphy, geomorphology, depositional processes, and seismic facies of one of the world's largest atoll for the first time. This Neogene carbonate platform comprises more than 1 km thick carbonate sequence and overlies a metamorphic basement. The southeastern part of the atoll comprises a fault-controlled graben system, which was formed during the Cenozoic rifting stage of the South China Sea. Most of the faults trend NE-SW and E-W and terminate at or slightly above the top of Middle Miocene strata. Atolls and abundant organic reefs initiated on the positive relief and closely mimicked the underlying topography during the Early Miocene. Shallow-water carbonates continued growing through Middle Miocene to present times. Regional uplift led to subaerial exposure, termination of platform growth and karstification during the Miocene. We also reveal a number of fluid-flow features such as vertical sub-bottom venting features (chimneys and pipes), chaotic reflection zones, which provide the first evidence of active fluid venting in the area of Zhongsha atoll. The Neogene sedimentation history of Zhongsha atoll further provide an important paleoenvironmental context for future scientific drilling to better constrain the evolution of Asia Monsoon. • First documentation of seismic stratigraphy and deposition of Zhongsha atoll (Macclesfield Bank), South China Sea. • Reefs and lagoons developed during the post-rift phase, whose onset is marked by a turning point around the time of the formation of reflector T50. • Three drowning events are identified to tentatively imply three episodes of fast sea floor spreading in the study region. [ABSTRACT FROM AUTHOR]

Calcareous aeolianites are an integral part of many carbonate platforms and ramps. Such limestones are particularly common in heterozoan, Late Cenozoic carbonate systems, and it has been postulated that they could contain a particularly sensitive record of their offshore source. This hypothesis is tested herein by documenting and interpreting part of the most extensive and temporally longest such system in the modern world. The deposits are a combination of extraclasts and biofragments. Extraclasts are detrital quartz, relict allochems, older Pleistocene particles and Oligocene-Miocene limestone clasts. Biofragments are penecontemporaneous coralline algae, echinoderms, small benthic foraminifera, molluscs and bryozoans. The aeolianites differ in composition from distant, open shelf sediments because they contain more mollusc fragments and many fewer bryozoans. This difference is interpreted to be due to (i) most sediment was derived from near-shore seagrass meadows and macroalgal reefs; (ii) all sediments were modified by hydrodynamics in near-shore and beach environments; and (iii) fragments of infaunal, beach-dwelling bivalves were added to the sediment at the strandline. Extraclasts should be expected in older Pleistocene and Cenozoic heterozoan deposits, because the limestones are poorly lithified, largely due to the lack of meteoric cementation, and so easily eroded. Thus, cool-water aeolianites ought to contain more extraclasts than their warm-water, tropical cousins. Seagrasses in temperate environments are more productive than in the tropics and thus potentially might contribute many more particles to the beach and dunes than do tropical systems. Although particle breakage in the surf zone cannot be proven, herein the abundance of whole benthic foraminifera and delicate bryozoans implies that suspension and flotsam shoreward transport was an essential process. The similarity of Pleistocene aeolianites over such a long time period herein suggests that the combination of postulated sedimentological, biogenic and hydrodynamic processes could be universally important. [ABSTRACT FROM AUTHOR]

High-resolution bathymetric and shallow seismic data along the northeast Red Sea margin reveal a previously disregarded mechanism for carbonate platform drowning at a steepflanked rift basin. At the seafloor, salt extrusions highlight the influence of extensional salt tectonics, with a salt flow from the southern flank of the Al Wajh carbonate platform that likely originates from below. Salt-flow direction, morphology, and kilometer-sized slumps and rotated blocks indicate platform-margin disintegration and rafting of platform blocks toward the southwest. The outlines of several smaller detached or semi-detached carbonate platforms to the south of the main platform can be refitted to the larger platform margin by counter-moving the direction of mass wasting. Several platforms, reaching heights above the seafloor of up to 650 m, are partially or fully submerged in the mesophotic zone and appear to be in danger of drowning. We conclude that the southern outer rim of the Al Wajh platform is breaking apart owing to salt withdrawal, which indicates that carbonate platforms on top of salt sequences grow on mobile ground, leading to platform disintegration, basinward rafts, and the demise of broken-off, smaller pieces of platform. Salt displacement also controls the growth geometries of individual platform rafts, with keep-up reef growth (growth rate equal to sea-level rise) and drowning occurring in close spatial proximity. Therefore, the interplay between salt diapirism and platform growth is not limited to platforms growing on the apexes of diapirs and is more complex than previously thought. [ABSTRACT FROM AUTHOR]

Forward stratigraphic modelling is a fast-developing modelling approach, used to test conceptual models, and predict stratigraphic architecture and depositional facies from basin to reservoir scales. Published subsurface applications demonstrate its added value by integrating multidisciplinary data as well as geological concepts into its constraints. When applied to carbonate depositional systems, composed of multiple sediment factories, the co-operating and interdependent production mechanisms remain poorly studied. By applying the technique to a well-studied section of the Maldives carbonate platform, a specific model design—adapted to the geological age and setting, and constrained by available data—sheds light on the interaction of its carbonate producers. The results yield a naturalistic depositional facies distribution and offer insight in the changing relationship between biotic communities during the platform evolution. After calibration, the reference model unequivocally links the formerly proposed genetic model to the seismostratigraphic architecture. Furthermore, the results show how environmental changes (seemingly of secondary impact compared to changes in physical accommodation in the stratigraphic record) can induce substantial fluctuations in carbonate production rates of biotic communities, affect the ecological accommodation, and thus impact the platform architecture. Therefore, it is crucial to treat carbonate production rates during periods of environmental change as variables with associated uncertainties in a forward stratigraphic model setup. • Forward stratigraphic modelling is applied for the first time to a section of the Oligo-Miocene Maldives carbonate platform. • Late Oligocene platform drowning occurred under reduced carbonate production conditions. • Carbonate production rates cannot be assumed continuous over time intervals with environmental changes. • Subdividing the carbonate factory in ecological assemblages provides a biotic perspective on platform sequence stratigraphy. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of recent shallow-water benthic foraminifera in surface sediment samples from cool-water carbonate environments of the Oran Bight, Alboran Platform and Mallorca Shelf in the Western Mediterranean Sea was studied. Multivariate statistical analyses resulted in the identification of species assemblages, representing different environmental settings. In all three regions the assemblages show a distinct bathymetric zonation that is mainly attributed to the distribution of rhodoliths and related substrates, but also to water turbulence and the availability of food at the sea floor. The live assemblages (Rose Bengal stained individuals) are characterised by rather low diversity and low standing stocks, likely reflecting seasonal population dynamics. In the Oran Bight, elevated standing stocks of “high food”-taxa suggest the impact of anthropogenic eutrophication on the near-coastal benthic ecosystems of this area. The diversity of the dead assemblages is higher than in siliclastic shelf ecosystems of the Mediterranean Sea but lower when compared to carbonate environments of the Levantine Sea. This regional difference is mainly attributed to lower sea surface temperatures and the lack of Lessepsian invaders in the western Mediterranean Sea. In all study areas, a distinct faunal change occurs between approximately 80–90m water depth. This change coincides with the lower distribution limit of living rhodoliths at the shelf of Mallorca, providing coarse-grained substrates that are dominated by attached taxa. Below this depth interval, the fauna shows regional differences depending on the grain-size and related accumulation of organic material. Fine-grained substrates with infaunal niches are restricted to low-energy environments on the deeper shelf southwest off Mallorca. [Copyright &y& Elsevier]

High-resolution seismic and borehole data were used to investigate the sedimentological characteristics of carbonate accumulations in the Pearl River Mouth Basin (PRMB) of the South China Sea. Based on seismic morphologies and petrologic analysis, three scale types of oligo-mesophotic ICBs were identified to have developed ephemerally in the early Early Miocene. The first ICB type is represented by small ICBs with lateral extension in the kilometer range, characterized by mounded geometry, complex internal architectures and a high-frequency alternation of carbonates and volcaniclastics. The second ICB type has lateral extensions of tens of kilometers and convex-up reflections, featuring obvious vertical thickening compared to the surrounding siliciclastic deposits. The third ICB type consist of extensive platforms with hundreds of kilometers of lateral extension, onlapped by surrounding siliciclastic deposits. The correlation of seismic and well data allows us to reconstruct the evolutionary history of the three types of ICBs in the PRMB: All the ICBs built by large benthic foraminifera and/or coralline algae biota, occurred on or around submarine topographic highs such as volcanic edifices or inherited from basement rises, developed under stable subsidence rates between 120 m/Myr and 180 m/Myr, and extinguished suddenly in the late Early Miocene with an abrupt decrease in the subsidence rate or a change from subsidence to tectonic uplift. It is proposed that post-rift subsidence of the study area controlled the evolution of the ICBs. Except for ICBs, mixed carbonate-siliciclastic deposits are identified in well succession, characterized by two types of mixing, namely, compositional mixing (core-plug scale) and strata mixing (stratigraphic scale). These mixed systems generated high-frequency alternations of carbonate, siliciclastic and/or mixed deposits sequence stratigraphy. This work reveals in detail the typical features of seismic morphologies, lithofacies, internal architectures and evolutionary mechanism of ICBs and mixed systems in the PRMB, and thus has implications for predicting reservoirs in hydrocarbon exploration and understanding carbonate sedimentology in a siliciclastic-dominated passive marginal basin. • Describe in detail the typical features of seismic morphologies, lithofacies and internal architectures of the carbonate accumulations in the PRMB. • Establish the evolutionary history of carbonate accumulations in a siliciclastic-dominated basin. • Reveal abrupt decrease in basin subsidence or a change to tectonic uplift triggered the drowning of the ICBs in a siliciclastic basin. [ABSTRACT FROM AUTHOR]

• Four myr sequence of sapropels in an atoll 1000 s of m above the ocean floor. • Anoxia related to the development of a shallow sill resulting in decreased ventilation. • TOC controlled by cyclic anoxia related to changes in Antarctic ice volume. • IODP Expedition 359 drills sapropel sequence of Late Oligocene-Early Miocene in age. This paper reports on the newly discovered occurrence of thick sequences (∼100 m) of Late Oligocene and Early Miocene (∼24.9 to ∼20 Ma) interbedded organic-rich sediments (sapropels) and pelagic (organic poor) carbonates at Sites U1466 and U1468 drilled in the Maldives archipelago during the International Ocean Discovery Program (IODP) Expedition 359. This occurrence is unusual in that this sequence is located > 1000 m above the surrounding ocean floor within an inter-atoll basin and not linked to any known global oceanic events. Total organic content reaches as high as 35% in the darker layers, while the interbedded carbonates have concentrations of less than 0.1%. Trace elements characteristic of anoxic waters, such as Mo, V, Cr, U, and Pb, correlate positively with concentrations of organic carbon. Nitrogen isotopic data show no evidence that the intervals of high total organic carbon are related to enhanced productivity driven by upwelling. Instead, high organic carbon is associated with intervals of anoxia. We propose that sea-level fluctuations linked to changes in Antarctic ice volume restricted exchange with the open ocean causing bottom waters of the inter-atoll basin to become anoxic periodically. The architecture of the platform at the end of the Oligocene, combined with the global sea-level highstand, set the stage for orbitally-driven sea-level changes producing cyclic deposition of sapropels. The proposed mechanism may serve as an analogue for other occurrences of organic carbon-rich sediments within carbonate platform settings. [ABSTRACT FROM AUTHOR]

The recent discoveries of deeply buried Cretaceous reservoir bodies in the Atlantic Ocean revealed that relationships between the distribution of spring carbonate deposits and faults are poorly understood. The well-exposed Quaternary deposits at Obruktepe (Denizli Basin, Turkey) provide an opportunity to reconstruct the three-dimensional sedimentary architecture of such a system. Integration of sedimentological, lithofacies and geochemical analyses reveals complexity in the lateral relationships between sedimentary environments, faults and geothermal spring carbonates. Five environmental systems are distinguished based on the lithofacies analysis: (i) vent; (ii) smooth slope; (iii) travertine-terrace; (iv) tufa-barrage; and (v) flood systems. Encrusting, baffling and settling sedimentary processes are reflected in data acquired at several scales, from lithofacies observations to the morphology and arrangement of geobodies, together with microfabrics and stable carbon and oxygen isotope data. Mean values of +4·9‰ δ13C and −8·74‰ δ18O Vienna PeeDee Belemnite reflect geothermal circulation of springwaters. The environmental distribution and lithofacies indicate a lateral continuum between travertine and tufa deposits within this hot spring system. This finding supports two depositional models in which water flow variation is the main control on both Ca CO3 precipitation and the resulting formation of travertine and tufa. The proposed models address the factors responsible for the development of these complex mound-shaped carbonate spring deposits, and how they are related to fluid circulation at depth and in association with faults. [ABSTRACT FROM AUTHOR]

Heterozoan temperate-water carbonates mixed with varying amounts of terrigenous grains and muddy matrix (Azagador limestone) accumulated on and at the toe of an inherited escarpment during the late Tortonian-early Messinian (late Miocene) at the western margin of the Almería-Níjar Basin in south-east Spain. The escarpment was the eastern end of an uplifting antiform created by compressive folding of Triassic rocks of the Betic basement. Channelized coralline-algal/bryozoan rudstone to coarse-grained packstone, together with matrix-supported conglomerate, are the dominant lithofacies in the higher outcrops, comprising the deposits on the slope. These sediments mainly fill small canyon-shaped, half-graben depressions formed by normal faults active before, during and after carbonate sedimentation. Roughly bedded and roughly laminated coralline-algal/bryozoan rudstone to coarse-grained packstone are the main lithofacies forming an apron of four small (kilometre-scale) lobes at the toe of the south-eastern side of the escarpment (Almería area). Channelized and roughly bedded coralline-algal/bryozoan rudstone to coarse-grained packstone, conglomerates, packstone and sandy silt accumulated in a small channel-lobe system at the toe of the north-eastern side of the escarpment (Las Balsas area). Carbonate particles and terrigenous grains were sourced from shallow-water settings and displaced downslope by sediment density flows that preferentially followed the canyon-shaped depressions. Roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope, whereas the rest of the carbonate lithofacies were deposited by high-density turbidite currents. The steep escarpment and related break-in-slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse-grained, low-transport efficiency skeletal-dominated sediment in the apron lobes. Accelerated uplift of the basement caused a relative sea-level fall resulting in the formation of outer-ramp carbonates on the apron lobes, which were in turn overlain by lower Messinian coral reefs. The Almería example is the first known 'base of slope' apron within temperate-water carbonate systems. [ABSTRACT FROM AUTHOR]

Sedimentological, paleontological, and geochemical data from core XK-1 on the Xisha Islands have allowed for a detailed documentation of the early carbonate development around the Oligocene-Miocene transition in the northern South China Sea (SCS). The deposits within the fore reef, reef, lagoon environments occurred before 23 Ma, the exposure horizon occurred around 23 Ma, and the sediments within the open lagoon setting dominated after 23 Ma. This evolution indicates a sedimentological response to water depth change, which is the combined result of eustatic sea-level fluctuations and tectonic subsidence. The tectonic subsidence during the latest Oligocene and Early Miocene (especially this period) was responsible for the obvious deepening trend. What's more, substantial physical erosion caused an increased amount of terrigenous input into the Xisha Islands area, with detrimental effects on carbonate development after the Oligocene-Miocene boundary. Over the entire South China Sea, the early carbonate systems along the northern and southern margins displayed specific evolution patterns. After comparing these patterns, we found a close relationship between the carbonate initiation and the subduction of the proto-SCS and the opening of the modern SCS. Therefore, it has been demonstrated that sea level and local tectonics as well as paleoclimatic conditions were the main controlling factors in shaping the onset of carbonate sedimentation in the SCS. • Plentiful corals existed during Oligocene in both northern and southern parts of SCS. • Strong physical erosion favored dominance of algae and foraminifera over corals. • Early growth of carbonates is linked to proto-SCS subduction and new-SCS opening. [ABSTRACT FROM AUTHOR]

The Xuande Atoll, with a discontinuous reef rim caused by platform drowning, is a typical ellipse isolated platform on the northern slope of the South China Sea. Based on the data of drilling holes, the analysis of surface sediments and high resolution acoustic data, six sedimentary facies are identified in the Xuande Atoll, including reef outer slope, reef front, reef flat, aeolianite sand island, back-reef slope and lagoon. Facies map shows that the elevated reef-rimmed margins with islands are more noticeable on the windward of the platform. With an elevated and discontinuous reef-rimmed margin as well as coarse-grained sediment in the interior platform, Xuande can serve as a case of a leaky bucket atoll which is characterized by initially drowning scenarios. The facies architecture of the Xuande Atoll is evidently influenced by complex and interacting parameters including antecedent topography, sea level, wind-driven currents, water depth and tectonic subsidence. Different controlling factors exist in the drowning of the windward and leeward sides of the platform margin. The study of modern depositional settings in the Xuande Atoll will provide insight on predictions for ancient carbonate provinces. • This is the first systematically survey on bathymetry, single channel seismic profile and grab sampler in the lagoon of the Xuande Atoll. • A newly established depositional model in lagoonal settings provides insight on predictions for ancient carbonate provinces. • Facies distribution of Xuande atoll is mainly influenced by theantecedent topographyand wind-driven currents. [ABSTRACT FROM AUTHOR]