Carbonate Platform Research Articles

Mercury isotope evidence for Middle Ordovician photic-zone euxinia: Implications for termination of the Great Ordovician biodiversification event

The Darriwilian (Middle Ordovician) is marked by a striking peak of the Great Ordovician Biodiversification Event (GOBE) followed by an abrupt ∼50 % decline in invertebrate species. Understanding the ultimate driving mechanism behind this biotic turnover and its correlation with those proximate climatic-oceanic changes that directly result in elevated biotic mortality have garnered considerable attention. One hypothesis posits that oceanic redox evolution actively influenced biodiversification, and progressive expansion of marine anoxia caused the termination of this biotic event. In this study, we present sedimentological and geochemical profiles for three outcrops of different lithofacies across the Middle-Upper Ordovician transition, ranging from carbonate platform to deep-water slope-basinal facies of the Tarim and South China cratons. Mercury isotopes reveal that recurrent and spatially dynamic photic-zone euxinia (PZE) occurred in the slope-basinal facies while being absent in platform facies. Mass balance model results suggest that ∼23 % of local atmospheric Hg was sequestered in marine sediments due to elevated dissolved H2S in surface waters. This finding suggests that destabilized oceanic redox conditions developed during climatic cooling, particularly shoaling and upwelling of deep-marine euxinic waters into the photic zone, coupled with global carbon-cycle disturbances, resulted in biodiversity decline following the peak of the GOBE.

Sequence stratigraphic architecture and depositional evolution of the early Eocene-early Miocene Al Jabal Al Akhdar carbonate successions, N Cyrenaica Promontory, NE Libya – Interplay of tectonics and eustasy

The interactions of tectonics and eustasy on the stratigraphic architectural evolution of the early Eocene-early Miocene Al Jabal Al Akhdar carbonate sedimentary successions were evaluated. This evaluation was achieved through detailed analyses of microfacies, depositional interpretation, and sequence stratigraphic framework of five measured sections located within the Al Jabal Al Akhdar Uplift, northeast Libya. The study interval was subdivided into distinct microfacies based on lithological changes, grain size and components, sedimentary texture and structures, fossil contents, color, and vertical stacking pattern. Six major depositional settings were delineated: restricted lagoon, open marine lagoon, platform margin and shoals, platform-margin reefs, foreslope, and middle to lower slope. Four third-order unconformity-bounded depositional sequences were identified based on vertical and lateral facies distribution, variations in fossil contents, and diagenetic features. Each sequence is composed of a transgressive-regressive interval, except for the younger sequence, which only contains a transgressive unit. Four evolutionary stages are recognized: initiation, growth, high-relief platform, and subaerial exposure. Subsidence resulting from tectonism and sediment loading, coupled with rising sea-level, created the accommodation space for the sedimentation of the studied carbonates. Repeated eustatic sea-level fluctuations controlled the internal stratigraphic heterogeneity and sequence development. Tectonic uplift caused a major sea-level retreat, resulting in the emersion of the carbonate sequences. The pre-existing Mesozoic and Paleocene sequences that underwent major uplift during the Late Cretaceous-early Eocene formed a shallow-water positive-antecedent topography for the formation of the carbonate sediments. This study establishes a carbonate sequence stratigraphic hierarchy of different cyclic sedimentation orders and improves our knowledge of sedimentary and stratigraphic architectural evolution and depositional mechanisms of carbonate platforms in an inverted basin setting. This knowledge can aid in predicting the distribution of lithofacies and depositional systems of economic importance.

Impact of the Jenkyns Event on shallow-marine carbonates and coeval emerged paleoenvironments: the Plitvice Lakes region, Croatia

Early Jurassic (late Pliensbachian–early Toarcian) Large Igneous Province (LIP) magmatism affected the entire ocean-atmosphere system culminating in a cascade of paleoenvironmental perturbations known as the Jenkyns Event which globally impacted marine, transitional and terrestrial paleoenvironments. Carbonate platforms at low latitudes in the Western Tethys realm drowned or shifted to non-skeletal production during the early Toarcian due to sea level rise, global warming, and ocean acidification. Unlike deep-marine deposits, shallow-marine carbonates present a challenge in defining global geochemical signals due to common diagenetic modifications. An integrated dataset including δ13Ccarb and δ18Ocarb, TOC, biomicrofacies, SEM, XRD, and palynological study of two stratigraphic successions in the Plitvice Lakes region, Bjelopolje (BJ) and Plitvice Spring (PS), in Croatia, provides an overview of the paleoenvironmental evolution and lateral facies changes in a peritidal setting during the Early Jurassic on the Adriatic Carbonate Platform (AdCP). The investigated stratigraphic succession starts with upper Pliensbachian, peloid-ooid-bioclastic grainstones alternating with fenestral and massive mudstones overlain by lower Toarcian, lagoonal, bioturbated, “spotted” limestones with horizons indicative of short-lived subaerial emergence during the early Toarcian. A negative excursion in δ13Ccarb, foraminifer assemblages, and the predominance of Classopollis pollen within the “spotted” limestones marks the stratigraphic position of the Jenkyns Event. Global-scale events (sea level variation, climate change, C-cycle perturbation, anoxia) operated simultaneously with local to regional synsedimentary tectonics and eustatic movements that preserved the AdCP carbonate factory from collapse or drowning but resulted in pronounced facies differentiation on the shallow-marine carbonate platforms.

Records of Burdigalian sea level and paleoclimate in the Maldives carbonate system

Tropical carbonate systems are valuable archives of paleoenvironments, as the carbonate growth is intimately affected by water depth and climatic conditions. Geochemical data from the Burdigalian interval in IODP Site U1468 in the Maldives, northern Indian Ocean, were integrated with sedimentological and paleontological data for a more detailed reconstruction of depositional history. Generally, the Sr/Ca values of slope sediments record highstand progradation in both sequence unit and whole Burdigalian interval, while the absence of higher Sr/Ca ratio close to the sequence boundary during the early Burdigalian could be related to the erosion of deeper-water sediments due to the activity of bottom current. From 20.5 to 19.1 Ma and from 17.9 to 17.2 Ma, nutrient level and productivity were moderately elevated due to the terrigenous input by the intensified South Asian Proto-Monsoon, which also helped cause more reducing conditions in the distal slope. Moreover, increased nutrient level facilitated the growths of calcareous algae and sponges, while it was not favorable for coral development. The elevated nutrient level, higher sea level, and monsoon-induced current contributed to the backstepping of the outer margin during the late Burdigalian. Our study shows an example on how a tropical carbonate platform evolved in response to the interplay of sea-level and paleoclimatic conditions. Findings are expected to be applicable to other tropical carbonate platforms.

Microfacies of the San Juan Formation (lower Darriwilian) in the Rio Francia, Central Precordillera, Argentina

In this contribution, we describe and interpret the microfacies of the upper levels of the San Juan Formation in the Río Francia section of the Central Precordillera, Argentina, for the first time. Several studies in this section identified the Lenodus antivariabilis, Lenodus variabilis, Lenodus crassus, and Lenodus pseudoplanus zones, thereby limiting this stratigraphic interval to the early Darriwilian age. Five microfacies were recognized: M1 burrowed bioclastic wackestone, M2 bioclastic mudstone-wackestone, M3 intrabioclastic grainstone, M4 crinoidal wackestone-packstone, and M5 bioclastic wackestone-packstone. These microfacies indicate a shallow subtidal inner platform environment with variations in hydrodynamic energy ranging from low to moderate conditions below the fair-weather wave base level. The vertical arrangement of these sub-environments suggests a gradual transgressive setting during the lower Darriwilian for the carbonate beds of the San Juan Formation. The Baños de Talacasto, Ancha Creek, and Don Braulio sections are correlated with the section studied in this work, where conodont zones were documented, and microfacies analyses revealed shallow subtidal environments with low-energy conditions and occasional high-energy episodes. Differences in redox conditions were observed between the Don Braulio section (anoxic) and the Río Francia section (well-oxygenated), interpreted as the result of a rapid sea-level rise, though diachronic across the platform. Microfacies analysis, combined with precise conodont biostratigraphy, provides a powerful tool for reconstructing the Middle Ordovician carbonate platform.

Discussion On The Development Of China's Regional Carbon Market Under The Background Of Double Carbon

Accelerating China's carbon trading market is one of the important means to deal with climate change and achieve the dual carbon goal. Due to the huge differences in China's regional economic development and carbon resource endowment, the impact of the establishment of the national unified carbon trading market on regional economic development is unstable and uncertain. For local governments to make full use of this empty window period, this paper combs the recent development of the carbon market, and discusses the development prospect of China's regional carbon market under the background of carbon trading. The results show that: ①in the current development of the national carbon emission trading market, we can not achieve the carbon unification of the market and abandon the local carbon trading market; ②In the process of regional carbon trading, it can promote the low-carbon development of local economy; ③In the process of regional carbon trading, it can increase the share of clean energy and improve the efficiency of regional ecological and environmental protection. Then, some suggestions are put forward for the development of local carbon trading: ①appropriately expand the pilot scale of local carbon trading, go up and down, and realize the double cycle development of carbon market; ②Strengthen the training of professionals and create an innovative and entrepreneurial business environment; ③build a carbon platform with big data technology and improve the Regional Internet plus carbon enterprise database; ④Actively promote the development of PPP model, explore the carbon sink transaction of cooperation between the government and social capital, continuously improve the regional green and low-carbon development, and jointly promote the realization of the "double carbon goal".

Paleo-redox conditions during the demise of a carbonate platform in the Tethyan ocean: Evidence from phosphatized and metals (Mn and Fe) rich hardgrounds

Phosphatized Mn and Fe rich hardgrounds and condensed pelagic deposits in carbonate platform successions are precious archives of abrupt climate and environmental changes (redox conditions and phosphorous availability) in the past shallow-water marine environment. While numerous examples have been documented in the Cretaceous successions of the Northern Tethys, the scarcity of similar descriptions from the southern margins suggests differences in sedimentary processes or preservation conditions.In this work we study three phosphatized Mn and Fe rich hardgrounds and pelagic condensed deposits that mark the repetitive demise of the Panormide carbonate platform developed in the Southern Tethyan margin during the Cretaceous. The integration of SEM-EDS, PXRD, and Micro-Raman spectroscopy data shows that these hardgrounds consist of fine-grained Fe (goethite and hematite) and Mn (birnessite and/or vernadite) oxides dispersed in a calcite and apatite matrix. Micro-Raman spectroscopy shows the presence of oxidized Mn species: Mn3+ and Mn4+. The oxidation of Mn2+ → Mn3+/4+ and/or Fe2+ → Fe3+ occurred at the sediment-seawater interface under oxic conditions (where both Mn and Fe oxidize) or suboxic conditions (where only Fe oxidizes). The paleoenvironmental perturbations that triggered the formation of both hardgrounds and condensed pelagic deposits were likely related to pCO2 cycle, upwelling of P-Mn-Fe-rich water masses, eutrophication and phosphatization related to the Cretaceous climate oscillations during the main Oceanic Anoxic Events. These perturbations were likely enhanced by tectonic activity. Moreover, we show that the formation of the phosphatized metals-rich hardgrounds and the recovery of shallow-water sedimentation occurred after long-term periods (6–12 Ma). Thus, the Panormide serves as a remarkable example of resilience amidst significant climatic changes.

Effective application of geological process modeling for unravelling carbonate build-up complexity: A case study from the EX-Carbonate build-up in central Luconia Province, Malaysia

In the dynamic field of carbonate reservoir Exploration, the need for specialized models is vital, assisting as a key tool to facilitate interdisciplinary collaboration and enhance strategies for predicting reservoir behavior. This study examines the use of geological process (GPM), a standard tool to simulate forward stratigraphic modelling, particularly focusing on the Central Luconia Province in the South China Sea. Central Luconia Province stands out for its extensive carbonate build-ups and is recognized as a prolific hydrocarbon reservoir. Forward Stratigraphic Modelling (FSM) leads to a paradigmatic change in geological modeling. Unlike geostatistical models controlled by geometrical parameters, FSM relies on mathematical representations of the physical rules determining erosion, transport, and sedimentation in carbonate growth. This tool, supported by various researchers, has proven its ability in quantitative sedimentary system analysis across diverse temporal and spatial scales. This study located in EX Field showcases the ability of stratigraphic forward modeling in replicating sedimentary complexities and resulting stratigraphic architecture of the EX isolated carbonate platform. The methodology links parameters from existing literature with multiscale data. Additionally, carbonate production laws, contingent on water depth, play a pivotal role in the meticulous modeling process. This work illustrates the efficacy of geological process modelling as a transformative approach for unraveling the complexities of carbonate systems, particularly in regions with prolific hydrocarbon reservoirs like Central Luconia. The incorporation of literature-derived parameters and multiscale data serves as evidence of the model's ability to offer nuanced understandings of the dynamic evolution of carbonate platforms, thereby advancing our understanding of reservoir prediction and future management strategies.

The early–middle Cambrian siliciclastic tide‐dominated succession in eastern Korea

AbstractThe basal Taebaek Group (Myeonsan and Myobong formations) illustrates second‐order transgression on the eastern Sino‐Korean Block during the early–middle Cambrian (Stage 3?–Wuliuan). The irregular palaeotopography of Precambrian basement led to the development of localised alluvial fans and fan deltas. As transgression continued, tidally influenced estuarine and associated shallow subtidal flats developed. Continued transgression resulted in a relatively deep subtidal environment, with postulated carbonate shoals serving as barriers. Stacking of facies indicates a general deepening‐upward trend, transitioning from terrestrial–nearshore to marine environments dominated by tidal effects. The study area subsequently experienced shallowing, leading to the emergence of tidal inlet and dune environments. Siliciclastic input was attenuated, facilitating the development of carbonate shoal complexes. The overall transgressive succession of the basal Taebaek Group aligns with similar sequences observed in the Sino‐Korean Block and western Laurentia, where the lowermost Cambrian strata which formed in various depositional environments on the uneven basement rocks were followed by extensive tide‐dominated siliciclastic successions. These successions were then overlain by epeiric carbonate platforms that developed synchronously across the area as siliciclastic input diminished with the continued transgression. This comparison demonstrates that the transgressive sequences in the Sino‐Korean Block and western Laurentia can be understood within a framework that explains how extensive Cambrian carbonate platforms initially developed due to a second‐order eustatic sea‐level rise during the Cambrian greenhouse period.

Diagenetic and hydrothermal events revealed by an Ediacaran dolomite breccia from the Araras-Alto Paraguai basin, southern Amazon Craton

The establishment of extensive carbonate platforms characterized the Ediacaran period during CaCO3 oversaturation events and the exceptional creation of accommodation space associated with the aftermath of the Marinoan glaciation (∼635 Ma). Carbonate deposits in the Araras Group record the early Ediacaran period, stage 1, in the Amazon Craton. The Serra do Quilombo Formation is an intermediate unit characterized by heavily fractured and faulted dolomites, as well as the presence of dolomite-cemented breccias (CB) overlying thick limestone packages associated with diagenetic and hydrothermal modifications. These features are commonly attributed to structurally controlled hydrothermal dolomitization (HTD) and are of significant economic importance as hydrocarbon reservoirs and Mississippi Valley-type (MVT) lead-zinc mineralization hosts. This study aims to unravel the origin of cemented breccias and the diagenetic/burial processes within the unit, focusing on dolomitization processes. Dolomite samples were analyzed using petrographic, scanning electron microscopy, microprobe, micro-Raman, cathodoluminescence, and isotopic analyses (δ13C, δ18O, 87Sr/86Sr) to unravel their burial history. CBs are sub-vertical to sub-horizontal bodies with complex geometries, generally cutting bedding at high angles, indicating hydrofracturing processes related to vertical flows of hydrothermal fluids (hydraulic breccia) and present the typical cockade texture of expansion breccias in dilatational faults. The substitutive matrix RD1 is the main constituent of the Serra Quilombo Formation, its low correlation between δ13C and δ18O (R2 = 0.009), the well-preserved fabric, and the similarity with the isotopic values (C and Sr) documented for Ediacaran carbonates, suggest that the syndepositional dolomitization. The first generation of dolomite cement (DC1) and the last phase of dolomitic cementation (saddle dolomite - SD) occur, filling pores, CBs, and fractures. The cockade texture of the breccias evidences a low precipitation rate or a pause in precipitation between DC1 and SD. Concurrently, DC1 has isotopic signals of δ18O = −4.34 ± 1.32‰ (n = 18) and 87Sr/86Sr = 0.708831 (n = 2), while SD has values of δ18O = −9.57 ± 2.51‰ (n = 15) and 87Sr/86Sr = 0.711464 (n = 3). The large isotopic fractionation between DC1 and SD suggests different dolomitizing fluids. This relationship shows an increase in 87Sr in the fluid as the temperature increases; moreover, the enrichment in 87Sr of the fluid is explained by the interaction of this fluid with rocks from the crystalline basement. Thus, the main conduit for the ascent of this radiogenic fluid would be faults with deep roots spatially close to tectonically active zones. Lastly, the presence of tectonic stylolites cutting cemented breccias and rotated zebra-like strata-bound structures suggests that brecciation occurred before the installation of fragile post-Ordovician transtensional structures, preceding the establishment of Paleozoic Basins on the South American Platform.

Carbon–sulfur–calcium isotopic variability of lower Cambrian shale-hosted carbonate concretions: Insights into growth mechanisms and calcium cycling

Marine calcium cycling is closely linked with carbon cycling in the ocean, in which authigenic carbonates precipitated in sediments play a non-negligible role. However, calcium cycling during authigenic carbonate precipitation in organic-rich, shaly sediments in geological history remains underexplored. This study focuses on carbonate concretions (aggregates of authigenic carbonates) in the lower Cambrian Niutitang Formation, South China, to provide insights into calcium cycling during their growth. Sedimentological and mineralogical observations suggest that these concretions were formed through concentric growth by authigenic calcite and pyrite precipitation during the early diagenetic stage. Geochemical analyses reveal internal variations in “M-shaped” δ13Ccarb trends (from −11.9 ‰ to −4.4 ‰) and diverse δ34Spyr trends (from 4.7 ‰ to 14.0 ‰) along core-to-rim transects. These findings suggest formation through microbial sulfate reduction by organic matter in a shallow depth beneath the sediment–water interface. In contrast to the dynamic δ13Ccarb and δ34Spyr variations and multi-stage concentric growth, these carbonate concretions display nearly uniform δ44/40Cacarb values (from 0.80 ‰ to 1.03 ‰, average 0.96 ± 0.06 ‰, 1SD) and consistent internal trends, which are further attributed to strongly seawater-buffered porewater calcium geochemistry and small calcium isotope fractionation due to calcite precipitation at slow rates. This study confirms that early diagenetic carbonate concretions in the lower Cambrian Niutitang Formation are characterized by much heavier δ44/40Ca values compared to coeval shallow platform carbonates. In light of abundant authigenic carbonates observed in the lower Cambrian successions, their roles in calcium isotope mass balance in the early Cambrian ocean warrant further investigation in the future. Therefore, early diagenetic carbonate concretions in black shales could provide valuable insights into porewater and seawater calcium isotope signals, as well as early diagenetic and marine calcium cycling in geological history.

Middle Ordovician paleoenvironmental evolution of the western Laurentian carbonate platform: Evidence for persistent oxygenation of the shallow ocean and implications for biodiversification

The Great Ordovician Biodiversification Event (GOBE) encompasses a series of rapid and sustained diversification pulses unparalleled in Earth history. Despite the uniqueness and magnitude of the GOBE, a singular driving force for this critical interval has not been identified. Multiple hypotheses have been presented, some pairing the GOBE as cause and effect with extrinsic factors such as changing global temperatures, ocean chemistry, or paleogeography, while others have pointed to factors intrinsic to biology itself, such as the proliferation of skeletonization, predator-prey escalation, or the development of complex tiered seafloor communities. Sufficient data are not available at present to ascertain what combination of these factors was the key to generating the GOBE.Here, we present new paired carbon and sulfur isotope data (δ13Ccarb, δ13Corg, and δ34SCAS), along with iodine concentration data from a Darriwilian carbonate sequence exposed at Meiklejohn Peak in southwestern Nevada, USA. Within a detailed lithologic and biostratigraphic framework, these new data provide a robust correlation to chemostratigraphic zonation established in Baltica and eastern Laurentia along with insights into the evolution of Middle Ordovician biogeochemical cycling. Carbon isotope data indicate a slight (2 ‰) increase in the fractionation between carbonate carbon and organic matter (Δ13C) during the onset of the middle Darriwilian carbon isotope excursion (MDICE), and new carbonate-associated sulfate (CAS) sulfur isotope data are consistent with a decrease in global pyrite burial during this time. Both of these observations support previous work arguing for a synchronous increase in the [O2] of the global oceans. New iodine concentration data from Meiklejohn Peak also suggest a progressive and sustained increase in the [O2] of regional waters throughout the Darriwilian during a period of relative sea-level highstand. This expansion of stable and well‑oxygenated ecospace could have laid the foundation for rapid diversification during one of the largest pulses of the GOBE.

The Cretaceous post-rift series from the Portuguese onshore ranges of the West Iberian Margin and their history of research

The thick post-rift series that covers the Triassic to Lower Cretaceous sedimentary succession of the Lusitanian Basin, in west central Portugal, is crucial to understanding the Mesozoic evolution of the West Iberian Margin, motivating a long history of studies, including those by Daniel Sharpe (1806–56). They record platform carbonates interbedded with marginal marine and alluvial facies. The post-rift infill was deposited above a late Aptian breakup unconformity and is organized into three unconformity bounded sequences: ‘UBS-4A’ (upper Aptian–upper Cenomanian), ‘UBS-4B’ (lower Turonian–lower Campanian) and ‘UBS-5B’ (lower Campanian–Maastrichtian). The first comprises alluvial siliciclastics and shallow-marine, carbonate sequences rich in Tethyan faunas with ammonites and rudists. During the late Cenomanian, the carbonate platform entirely covered the onshore sectors of the Lusitanian Basin. Several diapiric axes controlled the differentiation of inner platform areas with corals and rudists. The infill of ‘UBS-4B’ occurred after the tectonic uplift and emersion of the sectors located to the south of first-order Nazaré Fault Zone. The platform carbonate facies were overlain by regressive micaceous and alluvial coarse siliciclastics. The sequence ‘UBS-5’ overlies a lower Campanian unconformity and has a Campanian–Maastrichtian thick record of alluvial to lagoonal, reddish pelites, rich in vertebrate assemblages.

Dolomite formation in the Miocene Kardiva platform, Maldives archipelago: a tale of closed-system and open-system dolomitization by current pumping of seawater

Dolomites from two IODP Expedition 359 sites on the northeastern margin of the Miocene Kardiva platform, Maldives archipelago, were examined to explore the question of open versus closed system dolomitization in a drowned carbonate platform. The uppermost approx. 130 m of platform margin carbonate at site U1465 contains less than or equal to 12% dolomite except for five, meters-thick intervals with up to 65% dolomite. All U1465 dolomites consists of decimicron-sized euhedral cement crystals and mimetically replaced peloids and coralline red algal clasts. The abundance and the petrographic features of these dolomites are similar to periplatform and slope dolomite in many other settings that have been interpreted as the product of hydrologically closed-system diagenesis. In contrast, the recovered platform margin deposits at site U1469 are greater than 99% dolomite. Those dolomites are partially fabric retentive with fine-to-medium crystalline, planar subhedral to euhedral crystal mosaics of replacive dolomite and dolomite cement, all with Sr contents that average 256 ppm. Their characteristics are comparable to Neogene platform carbonates universally interpreted to signify hydrologically open-system dolomitization. Sr-isotope ages indicate Miocene dolomitization at both sites after platform drowning, and 18Odolomite values are compatible with dolomitization by cold (10⁰ to 15⁰ C) seawater when the platform margin was, on average, approx..400 m below sea level (mbsl). A current pumping mechanism for the advection of seawater into the top of the platform at site U1469 is proposed and tested with a computational fluid-flow simulation. Current pumping occurs when strong ocean bottom currents flow over sedimentary bedforms and generate lateral pressure differences along the sediment-water interface. The pressure differentials drive seawater through the underlying sediments. The flow simulation shows that the ocean currents that swept large sediment sand waves over and off the drowned Kardiva platform for many millions of years could have vigorously pumped Miocene seawater to sub-seafloor depths of many tens of meters . Mass balance considerations suggest complete dolomitization of the upper 20 m of the platform within 500 Ky or less. The current pumping mechanism could drive dolomitization and mineralogical stabilization below hiatal surfaces, or very slowly accumulating sediments, in any marine setting characterized by strong bottom currents. The greater acidity of Mioc ene seawater relative to younger seawater possibly made the pumping, by any means, of Miocene seawater of normal salinity an effective dolomitizing agent at site U1469 and in undrowned Miocene platforms globally.

Recognition of a cryptic maximum flooding surface in shallow marine carbonate sequences using geochemical (Y/Ho) proxy data

ABSTRACTSince diagnostic primary depositional sedimentary structures and depth‐dependent grain‐size trends are rarely preserved, building a comprehensive sequence stratigraphic framework for the vast majority of the Phanerozoic carbonate platform sequences is pending. Among the two most important sequence stratigraphic surfaces, while the subaerial unconformity can be reliably identified by either karst development or the appearance of siliciclastic materials, the demarcation of the maximum flooding surface remains difficult in lithologically uniform shallow marine carbonate sequences. The present study attempts to identify the globally documented maximum flooding surface within the body of the negative carbon isotope excursion of the Palaeocene–Eocene Thermal Maximum recorded in the shallow marine carbonate platform sequences. The results show that, along with the carbonate microfacies, the yttrium to holmium ratio (Y/Ho ratio) of the carbonate fraction reliably records the sea‐level changes. A Y/Ho ratio between 70 and 80 demarcates the stratigraphic position of the maximum sea‐level state (the most open marine condition in the studied interval) and maximum flooding surface in the studied sections. Since the Y/Ho ratios remain relatively stable throughout diagenesis, they can be used for maximum flooding surface identification in shallow marine carbonate platform sequences. The possibility exists that the same method can also be applied to the mixed siliciclastic–carbonate systems.

New diagenetic constraints for the interpretation of Neoarchaean stromatolitic dolostone geochemical composition and their palaeoenvironmental significance

Abstract The stromatolitic dolostones of the Ramonnedi Formation in the Lower Transvaal Supergroup of Botswana formed along a shallow carbonate platform during the transition between the Archaean and Proterozoic eons. These dolostones are bound to reveal more details of the mechanisms and timing of Earth oxygenation and particularly the role of microbial communities in the formation of carbonate platforms in the Archaean; but only if we are able to discriminate what signature is pristine and what is due to diagenetic overprinting. Here, the focus is on the isotopic composition of stromatolitic dolostone, pyrite grains and chert laminations found within the Ramonnedi Formation. Oxygen and carbon isotopes were investigated from the Ramonnedi Formation stromatolitic dolostones in order to constrain chemo-physical conditions at the time of deposition and, eventually, evaluate to what extent the system was re-set during diagenetic processes. In situ δ34S and δ18O compositions were determined by secondary ionisation mass spectrometry (SIMS) from pyrite and chert, respectively. These novel results complement previous analyses and reveal that: (i) the pyrite grains are not common in the dolostones and when present have sulphur isotope compositions that point toward abiotic formation; (ii) dolostone δ13CVPDB values between -1.2 and -0.7‰ are close to marine values and might reflect pristine Archaean signature; (iii) the dolostone δ18OVPDB values, between -9 and -11‰, on the other hand, might reflect diagenetic resetting or very warm Archaean seawater, in line with the δ18O values from the chert laminae in the upper Ramonnedi Formation. (iv) The correct interpretation of these results has important implications in the interpretation of the geochemical proxies coming from the lower Transvaal Supergroup carbonate platform and for the interpretation of the onset of the Great Oxidation Event (GOE). Aim of this work is to complement the existing data on the Lower Transvaal Supergroup of Botswana and assess to what extent the geochemistry of these dolostone was overprinted during diagenesis/metasomatism. These results indicate that a multi-analytical approach is required to ensure the correct interpretation of geochemical proxy data related to the onset of the GOE.

Rejuvenating exploration opportunities in Egypt's Mediterranean region using basin-scale 3D seismic reprocessing and subsurface evaluations

The Eastern Mediterranean region has proven to be a key province for natural gas resources over the last two decades. Gas discoveries in the Nile Delta Basin, Levantine Basin, and over the Eratosthenes platform have proven that an efficient working petroleum system extends to the deepwater areas and into the distal parts of the Nile Delta and Levantine basins. While the suprasalt biogenic post-Messinian play is adequately imaged in various areas in the Eastern Mediterranean region, seismic imaging of the deeper subsalt pre-Messinian plays is challenged by legacy seismic acquisition and processing techniques. Moreover, the complexity of the Messinian salt structures and the heterogeneity of the lithology of the Messinian layer across the Mediterranean basins represent a key subsurface challenge. In phase 1 of the study presented in this paper, we reprocessed 12 legacy 3D seismic data sets over 12,000 km2 and constructed a consistent basin-scale earth model from the shelf areas offshore Sinai to the deepwater areas around the Zohr carbonate platform. The data processing and imaging sequences incorporate full-waveform inversion and tomography in the earth model building process followed by a basin-to-lead evaluation to delineate exploration opportunities in phase 1. The evaluations resulted in delineating Cretaceous and Oligo-Miocene untested structures. Furthermore, potential deepwater Lower Miocene sand fairways were outlined, supported by improved regional seismic imaging and consistent seismic attributes over multiple legacy surveys. The authors of this paper expect to update the results of their work after completing the earth model building and the prestack depth migration workflows of an additional 13 legacy surveys over 35,000 km2 in phase 2 of the study.

Biostratigraphy of the early Cambrian small skeletal fossils from the Niuniuzhai section of southern Sichuan in China

The Niuniuzhai section in Leibo County, southern Sichuan, China, is part of the western carbonate platform of the Yangtze Block. The section is crucial for correlating early Cambrian successions in the eastern Yunnan and southern Sichuan areas. A comprehensive biostratigraphic analysis of small skeletal fossils (SSFs) from this section identifies three SSF assemblages: (1) the oldest from an interval of uncertain age affinity in the middle and upper Dahai Member of the Zhujiaqing Formation; (2) the Watsonella crosbyi Assemblage Zone (Zone Ⅲ) in the upper Dahai Member of the Zhujiaqing Formation; and (3) the Sinosachites flabelliformis–Tannuolina zhangwentangi Assemblage Zone (Zone IV) in the middle Shiyantou Formation. These SSF assemblage zones in the Niuniuzhai section correlate well with sections from eastern Yunnan and southern Sichuan, indicating a coherent Terreneuvian succession on the the Yangtze Platform and in other continents.

Finite element analysis of early deformations of carbonate platforms driven by differential compaction of basinal unit

AbstractA two‐dimensional numerical analysis based on the finite element method and linear elasticity is used to demonstrate how the differential compaction of the basinal unit can cause the early deformation of a prograding and/or aggrading carbonate platform. Our model investigates the modification of the carbonate platform stratal architecture and stress field driven by the process of differential compaction. We compared the results of our model with observations from two Triassic carbonate platforms in the Italian Dolomites: Lastoni di Formin and Nuvolau Mts. (Passo Giau, Italy). We show that the model can explain the modification of stratal architecture, as well as fault and fracture patterns observed on these platforms. In particular, we show that (1) the slope and slope‐to‐basin transition regions are expected to experience most of the brittle deformation and, differently from what was suggested by previous numerical studies, the formation of platform‐ward dipping faults and major fractures with dip angles that tend to decrease moving dip‐ward. In addition, (2) the inner platform region can exhibit a slightly tensile regime, which may lead to the formation of syndepositional and/or syndiagenetic fractures. Moreover, (3) in the case of predominantly prograding platforms, the results of the model show a general tilting and thickening of the inner platform strata towards the shelf‐slope break.

Microfacies Analysis and Evolution of Porosity of the Albian-Early Cenomanian Mauddud Formation from Selected Wells in Ratawi Oilfield, Southern Iraq

Mauddud Formation (Albian stage-the Early Cretaceous) is an important oil reservoir in Ratawi field of southern Iraq. Four wells, R T-2, R T-3, R T-6, and R T-7, located 70 km northwest of Basra, were selected to study microfacies properties and petrophysical associations with the probability of oil production. Seventy-seven core samples are collected, and thin sections for petrographic analysis. The self-potential, Gamma-ray, resistivity, and porosity logs are used to determine the top and bottom of the Mauddud Formation. Water saturation of the invaded and uninvaded zones, shale volume, and porosity were calculated. The study area results showed that the quantity of shale is less than 15% for most of the wells, and the dominant porosity is the secondary porosity. In contrast, the primary porosity is low in all study wells, and the formation contains varying proportions of producing hydrocarbons. The results suggest the formation comprises light-colored dolomitized lime and pseudo-oolitic white limestone with green to blue-gray shale. The petrographic analyses of 65 thin sections of the Mauddud Formation reveal that most skeletal grains are shallow marine-derived faunas, while non-skeletal grains include peloids and ooids. The facies analysis showed that the Mauddud Formation was deposited within different sedimentary environments ranging from deep to very shallow environments, which was represented by a large variation in Five microfacies and ten submicrofacies, the microfacies "Mudstone, wackestone, packstone, grainstone, and Rudstone "submicrofacies" Peloidal wackestone to packstone, Bioclastic wackestone to packstone, Miliolids wackestone to packstone,Orbitolina wackestone, Peloidal packstone to grainstone, Orbitolina packstone, Mudstone to wackestone,Bioclastic wackestone, Miliolids and Bioclastic wackestone to packstone, Bioclastic packstone, and Peloidai grainstone". This variance suggested that the formation might be deposited within the carbonate platform's variety of environment, such as "deep sea, restricted, shallow open marine, mid-ramp, rudist biostrome, and shoal." Seven different kinds of pores have been found in the carbonate sideman of the Formation: Interparticle, Intraparticle, moldic, Vuggy and fracture.