Carbonate Platform Research Articles

The Devonian clastic wedge and underlying strata of southwestern Canadian Arctic Archipelago: stratigraphic revisions

The Devonian clastic wedge (DCW) and underlying carbonate platforms and basinal mudrocks of the study area are re-examined using legacy seismic data and X-ray fluorescence surveys of borehole chip samples. The Ordovician–Devonian basinal succession of Melville Island is consolidated under the name Ibbett Bay Group within the Northwest Territories, whereas equivalent strata in Nunavut are grouped into the Cape Phillips Formation. The Kitson Formation black shale is correlative with the upper Ibbett Bay Group. Six horizons with high TOC and high gamma response are traced in the Ordovician–Devonian, with the fourth (4a) approximating the Silurian/Devonian boundary; the upper two (4b, 5) are Emsian and Eifelian. In the direction of progradation, the base of Kitson Formation rises stratigraphically from gamma horizon 4a to 5. The upper Kitson represents basinal toes of westward prograding DCW clinoforms. The Blackley and Cape de Bray formations of Embry and Klovan (Embry, A.F., and Klovan, J.E. 1976. The middle-upper devonian clastic wedge of the franklinian geosyncline. Bulletin of Canadian Petroleum Geology, 24: 485–639. https://pubs.geoscienceworld.org/cspg/bcpg/article/24/4/485/57135) are not traceable enough to warrant their formation rank. We revert to the original usage of Tozer and Thorsteinsson (Tozer, E.T., and Thorsteinsson, R. 1964. Western Queen Elizabeth Islands, Arctic Archipelago. Geological Survey of Canada, Memoir 332: 242 p. doi:10.4095/100556) where these units are members within the Weatherall Formation. The distinctive seismic character of the Cape de Bray in western and central Melville Island warrants its recognition as a formal member; elsewhere, it is informal as it cannot be consistently traced. The Blackey is treated as a formal member in an outcrop area of ∼2000 km 2 where it was defined; it is not recognized in the subsurface. Onset of the DCW is tentatively linked to flexural subsidence and crustal thickening caused by the Romanzof Orogeny in the hinterland.

Creation and fill of an Eocene incision on a leeward margin of an isolated carbonate platform, northeast Sirte Basin, Libya

Creation and fill of an Eocene incision on a leeward margin of an isolated carbonate platform, northeast Sirte Basin, Libya

The resilience of Tethyan planktonic and benthic calcifying algae to Early Cretaceous perturbations: comparison between the Valanginian Weissert Event and the Early Aptian Oceanic Anoxic Event 1a

Abstract During the Cretaceous, the Berriasian–Aptian interval witnessed a transition from a relatively cool climate with intermittent polar ice to a greenhouse state that persisted throughout the Late Cretaceous. These palaeoclimatic changes were associated with the construction of large igneous provinces (LIPs), which significantly perturbed the ocean–atmosphere system by introducing large amounts of CO 2 , trace metals and micronutrients, thereby impacting the biosphere. Our study focused on the Tethyan Ocean during the Early Cretaceous, examining the resilience of planktonic and shallow-water benthic calcifying algae to environmental changes. We observed their adaptation, recovery dynamics and the influence of palaeoCO 2 levels on their resilience. Calcification patterns of calcareous nannoplankton served as a proxy for ecological and engineering resilience. While calcareous nannoplankton as a whole showed high resistance, individual taxa exhibited varying levels of resilience. Nannoconids, particularly narrow-canal ones, were highly sensitive and had low resistance. In contrast, Watznaueria barnesiae showed the least sensitivity and highest resistance, likely due to its adaptive strategies and long lifespan. Nannoplankton calcification recovery (engineering resilience) from the Weissert Event took c. 3 million years. After the Early Aptian Oceanic Anoxic Event 1a (OAE1a), instead, nannoplankton did not return to pre-perturbation conditions. In shallow-water platforms, Dasycladales, aragonitic benthic calcifiers, exhibited lower resilience compared to nannofossils. They experienced a decline in species diversity across both the Weissert Event and the OAE 1a, which could indicate higher sensitivity to reduced carbonate saturation under high atmospheric p CO 2 conditions. After the Valanginian Weissert Event, Dasycladales were able to recover, albeit they show a much lower engineering resilience compared to nannoconids, as it took nearly 10 million years to revert to pre-disturbance diversity. The OAE 1a represented a more intense perturbation: the decrease of species diversity was much more drastic and permanent, and Dasycladales were unable to recover, losing their dominant role as carbonate platform biocalcifiers for the remainder of the Cretaceous. Our study provides an assessment of the resilience of Tethyan phytoplanktonic and shallow-water benthic calcifying algae to disturbances during the Early Cretaceous, with implications for tipping points associated with palaeoCO 2 levels. The differential responses in terms of timing and magnitude and the recovery dynamics contribute to the understanding of the potential impacts of current and future global changes on the resilience of marine ecosystems and the thresholds that may lead to ecological crises.

Ichnological response to the Middle Eocene Climatic Optimum (MECO) in the Bartonian deposits of Kutch Basin, India

The Middle Eocene Climatic Optimum (MECO) represents a short-lived interval of global warming that disrupted the long-term cooling episode extending from the middle to late Eocene time. Biostratigraphically the MECO corresponds to the upper interval of planktic foraminiferal zones E11–E12 and the shallow benthic zone 17. Both the planktic and benthic foraminiferal zones are well-documented within the Bartonian mixed siliciclastic‑carbonate sequence of the Kutch Basin in western India. This case study aims to investigate the impact of MECO on the endobenthic community using ichnology as a proxy. The studied sequence comprises the underlying Harudi Formation, consisting of the marine green and grey shale and bioclastic limestone beds, and the overlying Fulra Limestone characterized by benthic foraminifera-rich limestones. The Harudi Formation is bereft of trace fossils except in one bioturbated firmground and two bioeroded hardground horizons. The MECO corresponds to the thin carbonate packstone, the middle green shale, and the Nummulites obtusus rudstone occurring at the middle to upper part of the Harudi succession, established earlier by a concurrent negative δ13Corg isotopic excursion. The shelly limestone or bivalve coquina bed deposited immediately prior to the beginning of the MECO consists of a monospecific firmground ichnofabric characterized by Balanoglossites triadicus. Out of two bioeroded beds, the lower one coincides with the earliest MECO interval and is marked by a brown carbonate hardground colonized with two ichnospecies of Gastrochaenolites. The upper bioeroded horizon corresponding to the end of the MECO is characterized by a series of top- and side-bioeroded (with Gastrochaenolites, Entobia, and Oichnus) carbonate nodules containing multigenerational septarian cracks. The upper hardground demarcates the top of the Nummulites obtusus coquina bed and the starkest basin-wide sediment starvation, making it a major condensed horizon that developed following a steady glacio-eustatic rise during the warming event. The MECO prompted the subsequent deposition of shallow-marine platform carbonates of the Fulra Limestone by accelerating a sharp rise in the sea-level and proliferation of diverse Larger Benthic Foraminifera in a warm oligotrophic seawater.

Crustose red algae in deep time environments: Palaeoecological insights from northeastern India and Türkiye (Turkey)

Crustose red calcareous algae are key organisms in benthic ecosystems worldwide with critical functions like reef-building and substrate stabilization. Coralline algae thrived as major carbonate producers, with corals and/or larger benthic foraminifera (LBF) in numerous shallow-marine Tethyan carbonate platforms from the early Palaeogene to the Neogene. The Palaeocene–Eocene limestone successions in the Jaintia and Khasi Hills, Meghalaya, northeastern India, encompass two principal community types — algal-foraminiferal and coralline algal, with no associated colonial corals, while the Oligocene–Miocene carbonates from the Sivas basin and Siirt province in Türkiye are characterized by an algal-foraminifera assemblage with bryozoans, corals, and molluscs as other noteworthy components pertinent to the carbonate platform environment. We found that the diversity and ecology of these algal communities were influenced by systematic structure of the algal assemblage, sediment input, substrate stability, water depth, and hydrodynamic energy. The algal assemblages from different epochs and varying localities diverge based on the sedimentary regime, environmental settings, and the predominant taxa. The climatic transition from the Palaeocene–Eocene greenhouse to the Oligocene–Miocene icehouse corresponded to a shift from crustose algal assemblages dominated by Sporolithales to assemblages dominated by Hapalidiales.

Development and chronology of the Late Jurassic shallow-water carbonate deposits of the Holy Cross Mountains area, central Poland

The Late Jurassic shallow-water carbonates with intervening clayey-marly deeper-water deposits in the Holy Cross Mts. area formed over large bank of the elevated part of the Northern Tethyan Shelf during about 12 myr. They comprise three main successions (I, II and III) deposited partly in different environmental conditions, controlled by tectonic and climatic factors, and still preserved in the north-eastern margin, the north-western margin and the south-western margin of the Holy Cross Mountains. The history of sedimentation is presented according to the concept of the large tectono-stratigraphic units COK, LUK and KVB, which owe their origin to variable rates of tectonic subsidence, as introduced by Kutek (1994) for the area of central Poland. The studied deposits of the COK megasequence corresponding to the Upper Oxfordian and the Lower Kimmeridgian up to the Hypselocyclum Zone consist of coral limestones, various grained (including oolitic) limestones, and micritic limestones formed over the gradually enlarging shallow-water carbonate platform of the Holy Cross Mts. This platform was subsequently subdivided into two elevated areas, separated by a depressed zone in the middle, bounded by the Nowe Miasto–Iłża–Bałtów Fault Zone in the north-east and the Holy Cross Fault System in the south. The younger megasequence LUK with it strongly transgressive character marks the successive stages of the marine transgression which entered the central, lowered part of the area of the Holy Cross Mts. from the west, where it appeared already in the early Hypselocyclum Chron. It successively spread across the Holy Cross Mts. area towards the north-east and south bringing everywhere the deposition of various oyster lumachelles and marls with ammonites at the end of the Hypselocyclum Chron and during the Divisum Chron of the Early Kimmeridgian to the Acanthicum/Mutabilis Chron of the earliest Late Kimmeridgian. The following megasequence KVB is represented by the detrital lumachelles and chalky limestones with nereineids of the Eudoxus Chron of the Late Kimmeridgian marking the development of still younger shallow-water carbonate platform in the uplifted areas in the north-eastern and possibly the south-western margins of the mountains, allegedly subdivided by a deeper area of sedimentation of marly deposits. The youngest Late Jurassic deposits of the Holy Cross Mts., are very fragmentarily preserved, mostly because of Early Cretaceous uplift and erosion. They suggest an initial episode of complete drowning of the carbonate platform which became covered by marly deposits during the Early Tithonian, and the subsequent restoration of shallow-water carbonate sedimentation at the end of the Early Tithonian.

Evolution of Coniacian facies and environments in the Iberian basin: a longshore current controlling siliciclastic sand distribution on a carbonate platform

The Coniacian carbonate sediments of the Iberian basin were deposited on a homoclinal ramp that grades upwards into a distally steepened ramp, with a major shoreline siliciclastic fringe. Twenty-three facies have been identified and grouped into three main depositional environments: outer, mid, and inner ramp. The last include barrier (shoal), lagoon, carbonate tidal-flat and shoreface sub-environments. The more prominent biogenic components show a mixture of sunlight-dependent phototrophic organisms (mainly large benthic foraminifera) and nutrient-dependent heterotrophic organisms (mainly rudists), with a remarkable rare occurrence of corals. Nutrients supplied from the emergent mainland probably promoted the development of heterozoan organisms. The vertical evolution of the ramp shows: a basal transgressive stage with facies retrogradation; a maximum flooding stage, and a regressive stage with aggradation and progradation of a distally steepened ramp. The presence and distribution of siliciclastics are problematic, since sands coming into the basin are likely to be rapidly and widely redistributed along the basin, taking into account the common storm, wave, and tidal processes preserved by the sedimentary facies. The presence of a clockwise NW-flowing longshore current is postulated to account for this distribution, which was likely induced by both dominant external currents around Iberia and wind-driven currents. These clockwise gyres facilitated the invertebrate dispersion into this enclosed basin and the local presence of upwelling. This could have been another source of episodic nutrient-rich waters from the deep ramp, which may have favored heterozoan development even in the more proximal and relatively shallower-water facies.

Signs of crustal extension in Lower Jurassic carbonates from central Slovenia

The Lower Jurassic Podbukovje Formation represents a succession of shallow marine carbonate rocks deposited on the former Southern Tethyan Megaplatform and one of its successors, the Adriatic Carbonate Platform. Several outcrops of the Podbukovje Formation from central Slovenia (southern margin of the Ljubljana Moor) are presented, bearing possible evidence of Early Jurassic extensional tectonics. Peritidal facies of the lowermost, Hettangian – Sinemurian, part of the Podbukovje Formation locally interfingers with bodies of matrix supported pervasively dolomitized polymictic breccia, several metres to tens of metres thick and is locally cut by neptunian dykes some few decimetres to metres wide. The same or slightly younger part of the formation locally contains grabens/half-grabens metres to tens of metres deep and filled with poorly sorted pervasively dolomitized matrix supported polymictic breccia. Small miliolid foraminifera are present within the clasts and in the matrix. Finally, partly dolomitized blocky breccia tens of metres thick locally overlies the Pliensbachian – lowermost Toarcian limestone with lithiotid bivalves. Besides completely and partly dolomitized clasts, the breccia contains a variety of limestone clasts and preserves common radial ooids and some bioclasts within the partially dolomitized matrix. The Hettangian-Sinemurian breccias and dykes are presumably related to the early, diffused rifting stage of the Penninic (Alpine Tethys) Ocean, whereas Toarcian breccias relate to the main, focused rifting stage. Together with evolving biota and changing paleo-oceanographic conditions, the extensional tectonics may have been an important factor behind the facies changes observed within the Podbukovje Formation.

Osinskaya subformation of the Usolsky formation of the Lower Cambrian of the central and southern parts of the Nepa-Botuoba anteclise: structure, formation conditions and biostratigraphical characteristics

Relevance. Geological prospecting carried out recently throughout the Lena-Tunguska oil-and-gas province makes studies of carbonates of the Osinskaya subformation particularly relevant, as they make it possible to establish the patterns of hydrocarbon trap formation. Aim. To present the results of the lithological and facial, biostratigraphic and petrophysical studies of the Osinskaya subformation in the southern and central part of the Nepa-Botuoba anteclise. Objects. Carbonate rocks of the middle Osinskaya subformation of the Usolskiy Formation of the Lower Cambrian Tommotian Stage, studied from well cores. Methods. GIS complex, detailed lithological and sedimentological description of the core, sequential stratigraphic analysis, petrographic analysis of 300 samples and examination of paleontological remains taken from the core; analysis. Results. Based on these studies, three sedimentation zones were identified, traced, and described for the first time: deep-water, marginal, and shallow-water. The deep-water zone is characterized by uncompensated sedimentation, the presence of single reefs, and confinement to depressions and troughs. It was flanked by an edge zone of carbonate platform associated with elevated thicknesses of the subformation and distribution of reefs over the area. It was followed by a shallow-water zone, represented by medium and reduced thicknesses of the Osinskaya subformation, its sediments are composed of carbonate grained and clayey rocks with small single reefs. The localized marginal zone of the extended carbonate platform agrees with the previously identified Chambinsko-Altybsko-Mirninskaya and Verkhnetokhomsko-Katsko-Pilyudinskaya zones of distribution of reef-like organogenic structures. On the basis of the studied wells, the structure of three formations according to the lithologic-facial profile was described. The paper introduces the authors’ variant of conducting the boundaries of the members. It was revealed that in the deep-water zone the prospects of searching for reservoir rocks are associated with single carbonate structures, in the marginal zone with area bioherms, and in the shallow-water zone with detrital deposits and shallow bioherms. The paleontological finds are shown to be confined to the selected sedimentation zones and beds. It was determined that algae diagnosed in the boudstones (autochthonous) were involved in the formation of the second and third strata, the edge zone of the carbonate platform. Archaeocyaths were diagnosed in detrital carbonate rocks of the second pack (allochthonous) in well no. 7 and autochthonous in boudstones of well no. 2X. Cribriciates were diagnosed in the boudstones of the third member (autochthonous). Archaeocyaths, cribriciates, namacalatusses, and calcareous algae studied from the cores of wells West Yaractinian no. 45, 361, Bolshetyrskaya no. 7, 3X, 2X, 4X, 5X, 6X were major edificators of Early Cambrian organogenic structures (biostromes, bioherms, bioherms massifs, reef formations).

Depositional Setting, Diagenetic Processes, and Pressure Solution-Assisted Compaction of Mesozoic Platform Carbonates, Southern Apennines, Italy

Pressure solution processes taking place during diagenesis deeply modify the hydraulic properties of carbonates, affecting their mechanical layering and hence the dimension, distribution, and connectivity of high-angle fractures. The formation of stylolites is controlled by the texture of the host rock and therefore by the depositional environment and the diagenetic processes that involve it. This study reports the results of a multidisciplinary study carried out on a Jurassic–Cretaceous carbonate platform in southern Italy. The goal is to unravel the control exerted by single carbonate textures and specific diagenetic processes on the formation of bed-parallel stylolites. Microfacies analyses of thin sections are aimed at obtaining information regarding the composition and texture of the carbonates. Petrographic observations coupled with CL analyses are key to deciphering their diagenetic history. Results are consistent with carbonates originally deposited in a shallow-water realm in which carbonate mud is occasionally abundant. In this environment, early cementation inhibits their chemical compaction. In grain-supported facies, pressure solution is only localized at the grain contacts. During shallow burial diagenesis, precipitation of blocky calcite predates the formation of bed-parallel stylolites in the grain-supported facies. Contrarily, mud-supported facies favor chemical compaction, which results in stylolites showing a good lateral extension and thick sediment infill. A classification of different types of stylolite morphology is attempted in relation to facies texture. In detail, rougher morphology (sharp-peak) characterizes the stylolites nucleated in grain-supported facies, while smoother morphology (rectangular to wave-like) is observed in stylolites on mud-supported facies. Application of this knowledge can be helpful in constraining the diagenetic history of carbonate rocks cored from depth, and therefore predict the fracture stratigraphy properties of carbonates buried at depth.

Facies and geochemical characteristics of the Middle-Lower Ordovician Yingshan Formation in the Tarim Basin, NW China: Implications for the high-frequency sequence stratigraphy in shallow-water carbonate platform

Shallow-water carbonates from the Yingshan Formation are important target of hydrocarbon exploration in Tarim Basin. Detailed descriptions of outcrop, core, and thin sections, isotopic composition analysis, and trace element analysis were conducted. Seven lithofacies which consists of two facies belt groups were identified and used to interpret deposition environments of Yingshan Formation: (1) peritidal carbonate, with relatively lower δ13C values from −4.2% to −1.9%, mainly represented by intertidal to supratidal facies and restricted subtidal facies and (2) open-marine subtidal carbonate, with higher δ13C values (−1.5% to −0.3%), mainly consists of shoal facies and interbank sea facies. On the basis of the lithology sets, four types of meter-scale cycle model (types A–D) were grouped into peritidal sequences and subtidal sequences. On the basis of vertical lithofacies, cycle stacking patterns, and accommodation variations in Fischer plots, two third-order depositional sequences (SQ1–SQ2) were recognized. The sequence boundary between SQ1 and SQ2 is not only a surface of a positive shift of δ13C values, but also the transitional zone of cycle stacking patterns. The sequences can be further divided into four fourth-order sequence sets: Sq1–Sq4. The lower sequences (Sq1–Sq2) are dominated by peritidal facies and characterized by a progressive decreasing in accommodation space probably indicates a longer-term fall in sea level. The upper sequences (Sq3–Sq4) are mainly dominated by subtidal facies and characterized by the accommodation space with a progressive increasement, likely indicates a rising sea level with a longer term. The facies-controlled reservoirs are mainly present in the Sq3, related to the sequence boundary between SQ1 and SQ2.

Sea-level and monsoonal control on the Maldives carbonate platform (Indian Ocean) over the last 1.3 million years

Abstract. 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.

Architecture and exploration target areas of the Senegal Basin, West Africa

The recent discovery of large oil and gas fields in the deep-water of the Senegal Basin has drawn global attention. Despite this, several exploration wells in this area fail, which can be primarily contributed to a lack of understanding of the basin's structures and hydrocarbon accumulation conditions. This study examines these characteristics utilizing gravity, seismic and drilling data, and finally makes a comparison with the Cote d’Ivoire Basin, a typical transform margin basin in the South Atlantic. The results suggest that the Senegal Basin, influenced by multiple transform faults and a weak Paleozoic basement, experienced three evolutionary stages: rifting, transitional, and drifting. Each stage contributed to the development of distinct depositional sequences - syn-rift sequences, sag sequences, and continental margin sequences, respectively. The Triassic - Early Jurassic rifting stage predominantly formed continental deposits, like fluvial, lacustrine, and deltaic deposits, in the syn-rift sequences. The Middle-Late Jurassic transitional stage, influenced by transform faults, witnessed the formation of marginal ridges or submarine uplift zones. These zones, in conjunction with landward high terrains, formed a restricted environment promoting the development of source rocks in the sag sequences. During the drifting stage, three types of reservoirs, namely platform carbonate rocks, deltas, and slope-floor fans were formed. Notably, large-scale hydrocarbon reservoirs have been found in the deltas and the slope-floor fans both in the Senegal Basin and the Cote d’Ivoire Basin. The Upper Jurassic - Aptian platforms exhibit thick carbonate rocks and organic reefs on their edges, suggesting substantial potential for hydrocarbon exploration in the Senegal Basin.

Platform margin belt structure and sedimentation characteristics of Changxing Formation reefs on both sides of the Kaijiang-Liangping trough, eastern Sichuan Basin, China

Abstract This article discusses the types and characteristics of platform margin belts on both sides of the Kaijiang-Liangping trough in the Sichuan Basin during the Late-Permian Changhsingian age combined with the seismic reflection characteristics. The results show that there are four types of platform margin belts on both sides of the Kaijiang-Liangping trough: (1) steep-slope, single-platform margin belts controlled by faults; (2) steep-slope, single-platform margin belts controlled by paleogeomorphology; (3) steep-slope, multiple-platform margin belts controlled by paleogeomorphology; and (4) gentle-slope, multiple-platform margin belts controlled by paleogeomorphology. Reefs developed in the different types of platform margin belts exhibit different morphological characteristics and stacking patterns. Among all the reefs that developed in steep-slope, platform marginal belts are mainly characterized by aggradational accretion and different degrees of lateral migration in different directions. In contrast, the reefs developed in gentle-slope platform margin belts show distinctive characteristics of multiple bandings and lateral extension. Research suggests that the differences in the platform margin belts’ structure and reefs’ development rules are jointly controlled by a combination of syn-sedimentary fault activities, paleogeomorphology, and sea-level fluctuations. These results can be used to explore and predict reef reservoirs on carbonate platform margins.

Early Triassic (Griesbachian) spheroids in Beibei, Chongqing, Southwestern China: Characteristics, cause and implications for palaeo‐oceanic conditions

Following the latest Permian mass extinction (LPME), abundant unusual sedimentary features and fabrics were widely distributed in Early Triassic carbonate platforms. As a unique type of carbonate grain, spheroids from the Lower Triassic are infrequently reported, yet differ significantly from giant ooids and oncoids. Griesbachian micritic spheroids are well preserved in the Lower Triassic Feixianguan Formation at the Baimiaozi (BMZ) outcrop in Beibei, Chongqing, southwestern China. These spheroids (diameter: 3.9–10.8 mm) were in a marl layer interbedded with massive oolites. They were deposited in the wave troughs of ripple marks comprising micritic clots, sparry calcite, clay minerals, organic matter and pyrite. A microbial spheroid origin is implied based on the coccoidal microbes (coccoid‐like microspherules and bacterial clump‐like microspherules), numerous pyrite framboids (indications suggest the presence of many sulphate‐reducing bacteria and the high iron content in seawater can promote nitrogen‐fixing cyanobacteria proliferation) and microbial‐derived microspherules. The dark‐coloured matrix between the spheroids primarily included microcrystalline calcite, clay minerals, organic matter and metazoan fossils. The spherical and ellipsoidal shapes of the spheroids suggested rapid accretion and lithification. As a unique carbonate depositional mode, Greisbachian spheroids may have been recorded for a palaeo‐ocean with dysoxic and calcium carbonate supersaturation shortly after the LPME.

Architecture and sedimentary evolution of the Ladinian Kobilji curek basin (External Dinarides, central Slovenia)

The study area is located in cental Slovenia, and geologically located at the junction between the Alps and the Dinarides. The Middle Triassic of this region is characterised by intense rifting manifested by differential subsidence and volcanism. This led to a major paleogeographic reorganisation of the region, where three paleogeographic domains formed in the Upper Triassic: The Julian Carbonate Platform in the north, the intermediate Slovenian Basin, both parts of the Southern Alps, and the Dinaric (Adriatic, Friuli) Carbonate Platform in the south, which today is a part of the External Dinarides that host the area of investigation. Prior to the installation of the Dinaric Carbonate Platfrom, i.e. in the Ladinian, the entire area of the preset-day External Dinarides broke up into numerous tectonic blocks that were exposed to either erosion or continental, shallow-marine, and deep-marine sedimentation. In this study, we analyse at small scale a complex transitional area between a local carbonate platform and the Kobilji curek basin (depositional area dominated by deeper marine sediments), located in the Rute Plateau in central Slovenia south of Ljubljana. During enhanced subsidence, the basin was filled with volcanic material (tuffs and volcanogenic clays and subordinate extrusive material), while the adjacent platform aggraded. The slope was positioned above active paleofaults. During relative sea level lowstand, the platform prograded across the basin. The study area is divided into four major tectonic paleoblocks. The NW paleoblock experienced the most enhanced subsidence, and the platform prograded twice in this area and was submerged again by the rejuvenated subsidence and/or sea-level rise. The second and third paleoblocks subsided only during discrete major subsidence events, and the carbonates of the platform and slope were soon reinstated therein. In the fourth paleoblock to the east the platform persisted during the Ladinian. In the Carnian, the entire study area became emerged, and continental clastics were deposited. These were then replaced by a uniform shallow marine/intertidal Hauptdolomit (Dolomia Principale) formation at the onset of the Norian. This study provides the first detailed reconstruction of the sedimentary evolution of small-scale Ladinian basin and platforms system in the northern External Dinarides.

The Barremian–lower Aptian of NE Arabia: The Kharaib and Shu'aiba formations in Wadi Rahabah and Wadi Kebdah, Ras Al Khaimah, U.A.E.

We present a high-resolution record of carbonate depositional facies, biostratigraphy, stable-isotope and elemental geochemistry of the Kharaib and Shu'aiba formations exposed in Ras Al Khaimah, northern UAE. This is the first comprehensive study of these deposits in the northeastern Arabian Peninsula, which have previously been studied extensively in Abu Dhabi, Oman, NE Saudi Arabia, and southwestern Iran, and represents the most detailed biostratigraphic and chemostratigraphic record currently available from a continuous outcrop section of the Arabian Plate. The Barremian–Aptian boundary was approximated based on orbitolinid biostratigraphy, while a subdivision of the Barremain was not possible. There is no biostratigraphic evidence that the Shu'aiba Formation in the section studied ranges into the upper Aptian. Previously well-established depositional sequences of the Barremian–lower Aptian of the Arabian Plate are discussed in the context of carbonate textures, siliciclastic influx, and biotic assemblages. There is a general trend of decreasing siliciclastic components during the Barremian and earliest Aptian, and peaks in siliciclastic influx indicate episodes of maximum accommodation. Palaeogeographic implications of our data for the distribution of intrashelf basins and their connection to Neotethys are discussed. The negative carbon-isotope excursion that globally marks the onset of OAE1a is not evident in our dataset, while the positive excursion at the end of OAE1a is present in the late early Aptian. A comparison of carbon-isotope records across the early Aptian of the Arabian Plate highlights issues of correlations that are predominantly based on chemostratography in diagenetically altered platform carbonates.

Upper Ordovician–Lower Silurian deposits in the Zakola section (Ilych River, Northern Urals): Composition, structure, formation environments

Research subject. Section of boundary deposits, exposed in the right side of the Zakola stream (the right tributary of the Ilych River, Northern Urals) with a total thickness of 98 m, presented with rock samples (more than 100 pieces), thin sections (100 pieces), slabs (10 pieces), and chemical analyses (15 pieces). Aim. Detailed description of the most complete section of the Upper Ordovician–Lower Silurian boundary sediments of the Shantym facies complex with a stratigraphic reference of layers, lithological typification of sediments, and restoration of sedimentation conditions. Methods. Conventional lithological and paleontological methods were used to dissect the section. Sampling was carried out after 1 m; however, samples were additionally taken when detailed observations were possible to identify lithological and facial differences. The thin sections were studied by the classic method of optical microscopy. The study of the fauna complex was carried out both by collecting samples of the author’s rocks and by the materials of A.I. Antoshkina. In addition, stock materials were attracted. Results. In the section, 14 packs of rocks were identified, which are represented by 6 lithological types of secondary dolomites. Their distribution along the section, along with the presence of characteristic faunal remains, made it possible to establish three members in the section, which can be correlated with the Katian, Hirnantian, and Ruddanian stages of the General Stratigraphic Scale. Conclusion. The Katian deposits are represented by the Yaptikshor Formation (Strata 1), which was formed under the conditions of the lower sublittoral of a ramp-type carbonate platform with a transgressive carbonate tract and a diverse fauna. The beginning of the Hirnantian (Strata 2) is characterized by shallow water conditions of the upper sublittoral up to the littoral, which reflects a sharp shallowing at the Katian–Hirnantian boundary and corresponds to the beginning of the global early Hirnantian regression. The rocks exposed higher up the section indicate sedimentation with a certain rise in the sea level; however, the correlation of this part of the section with the upper part of the Verkh Ruchey Formation is complicated by the fragmentary nature of the outcrops. The overlying Ruddanian deposits (Strata 3) were accumulated on the outskirts of a ramp-type carbonate platform under the conditions of the lower sublittoral, which corresponds to the beginning of the Early Silurian global transgression.

High-latitude platform carbonate deposition constitutes a climate conundrum at the terminal Mesoproterozoic

During the Mesoproterozoic Era, 1600 to 1000 million years ago, global climate was warm with very little evidence of glaciation. Substantial greenhouse warming would have been required to sustain this ice-free state given 5-18% lower solar luminosity. Paleomagnetic data reported here place voluminous ca. 1.2 Ga shallow marine carbonate deposits from India at an unexpectedly high latitude of around 70° from the equator. Previous studies noted high latitudes, but their implication was never considered. Here, we evaluate the temporal-latitudinal distribution of neritic carbonate deposits across the Proterozoic and identify similar deposits from North China that together with those from India are seemingly unique to the late Mesoproterozoic. A uniformitarian interpretation implies that this is cold-water carbonate deposition, but facies similarity with low-latitude neritic deposits rather suggests a hotter climate and elevated polar ocean temperatures of 15–20° or higher. This interpretation represents a climate conundrum that would require much greater greenhouse warming than documented for the Mesoproterozoic.

Seismic stratigraphic and sedimentary record of a partial carbonate platform drowning, Queensland Plateau, north-east Australia

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.