Carbonate Platform Setting Research Articles

THE GENUS ORBITOLINA D’ORBIGNY, 1850 (LARGER BENTHIC FORAMINIFERA) AND ITS CONSTITUENT SPECIES: NOTES ON IDENTITY AND STRATIGRAPHIC RANGES

Following a literature review of the species of Orbitolina and its allies, the identity criteria and stratigraphic ranges of the six potential constituent species of the commonly encountered mid-Cretaceous foraminiferal genus Orbitolina d’Orbigny, 1850 (sensu stricto) are reviewed based on published material. It is noted that correct taxonomic assignment of Orbitolina species requires observation and measurement of both the embryonic apparatus in centred axial thin-sections and of the chamber passage shape in tangential sections. As a consequence, many illustrations purportedly of Orbitolina sensu stricto in the published literature cannot confidently be identified to species level, and such specimens (including new material) should be described or re-described as Orbitolina spp. The genus Orbitolina has a range of late Albian – middle Cenomanian. Species of Orbitolina, when identifiable, have shorter ranges. Orbitolina concava (Lamarck 1816) is restricted to the early – middle Cenomanian, and its inception thus forms a potential proxy for the base of the Cenomanian in carbonate platform settings, but care needs to be taken in its identification and separation from other species of Orbitolina that have at least partial late Albian ranges.

Morphology and evolution of submarine canyons around the Zhongsha Platform, South China Sea: Implications for sedimentary processes in a modern isolated carbonate setting

Recent high-resolution multibeam bathymetry and seismic data from the platform-top to the abyssal plain of the Zhongsha Platform allow for a detailed investigation of the morphologies, spatial distribution, and trigger mechanisms of submarine canyons, submarine landslides, and associated sedimentary features along modern isolated carbonate slopes. The newly observed Zhongsha Canyon System provides a natural laboratory for reconstructing the source-to-sink sedimentary processes in a pure carbonate setting. This study reveals that there are thirty-four submarine canyons at water depths between 300 and 4100 m on the northern and western slopes of the Zhongsha Platform. Two morphologically different submarine canyon types are identified: (1) dendritic canyons, which exhibit abundant tributaries with scallop-shaped failures at the canyon heads, and (2) linear canyons, which feature rare tributaries with elongated failures at the canyon heads. The dendritic canyons are more complex in morphology than the linear canyons as a result of the interaction among numerous tributaries. Canyon initiation and evolution pass through three phases: (1) initial stage: off-platform sediment transport and platform margin failures contribute to erosive gravity flows; (2) developmental stage: initiation and incision of submarine canyons along platform margin failures; and (3) mature stage: numerous mature canyons along the platform margin. Off-platform sediment transport, density cascading, gravity flows, monsoon currents, and deep circulation play an essential role in shaping the slope morphologies. In addition, submarine landslides are extensively observed along the entire slope of the Zhongsha Platform at water depths of 600 to 4200 m, including canyon-wall failures, slope landslides, canyon-front landslides, and slope-toe failures based on their location and genesis. These processes can steepen the platform slopes by upward retrogressive and downward progressive erosion. On a larger scale, the persistent submarine canyons and occurrence of landslides around the Zhongsha Platform contribute to the uniqueness of this landscape among modern carbonate slopes. The morphologies and evolutionary processes of Zhongsha Canyon System present significant differences from the global carbonate submarine canyons in terms of their dimensions and trigger mechanisms. The findings of this work provide novel insights into the morphological features and sedimentary processes of submarine canyons in modern isolated carbonate platform settings.

Tectonic, eustatic and climate controls on facies architecture during the transition to the Neoproterozoic icehouse in the Adelaide Superbasin, Australia

The Tonian to Cryogenian (ca. 1000–635 Ma) marks a crucial turning point in Earth’s history, where tectonic reorganisation and fluctuating oceanic and atmospheric geochemistry plunged the globe into icehouse conditions. This was followed by a postglacial warming period that delivered large volumes of nutrients to the oceans and stimulated eukaryotic evolution. The Adelaide Superbasin in South Australia hosts a thick repository of Neoproterozoic and lower Cambrian sedimentary successions that preserve the depositional conditions during this unique time. In this study, detailed sedimentological data was collected from over 8,350 m of measured section at seven locations across the northern Flinders Ranges. Tonian deposits reveal a carbonate platform setting, where deposition was controlled by basin geometry and proximity to uplifted source areas. In the early Cryogenian, sedimentary successions were affected by the Sturtian glaciation, characterised by two glacial advance-retreat phases that coincide with climatically driven regression. The end of the Sturtian glaciation was marked by basin subsidence and widespread transgression into a more distal subaqueous environment. Despite the lithostratigraphic and sequence stratigraphic similarity between Tonian–Cryogenian successions globally, their correlation remains contentious. The influence of local tectonic regimes during the Tonian created a potential oceanic restriction between developing basins, which challenges the chemostratigraphic correlation between these deposits. Further, limited geochronological ages and opposing interpretations of glacial cyclicity puts into question the timing and extent of the Sturtian glaciation. Conversely, the post glacial transgression appears to be the most globally consistent as it results from climatically controlled sea level rise that was driven by melting ice sheets.

Revisiting the upper Visean mud mounds from Derbyshire (UK): the role of brachiopods in their growth

Several brachiopod-rich mud mounds occur in the upper Visean (Brigantian) of the Derbyshire Carbonate Platform succession in UK. The re-evaluation of the lithofacies architecture of a Derbyshire mud mound complex, developed in an intraplatform middle-ramp environment, led to the recognition of three lithofacies associations: (a) a 10 m thick basal unit of automicrite boundstone with siliceous sponge spicules and brachiopod–bryozoan packstone to wackestone beds; (b) a 10 m thick, 250 m wide, lens-shaped, convex-up massive core of clotted peloidal micrite and fenestellid bryozoan boundstone with sponge spicules; (c) inclined brachiopod–bryozoan–crinoid packstone flank beds. In the mud mound complex core, most of the carbonate mud with clotted peloidal and structureless micrite fabric is the result of biologically induced and influenced in-situ precipitation processes (automicrite). Brachiopods are not, as previously thought, limited to storm-scoured “pockets” in the mud mound complex core but are abundant and diverse in all lithofacies and lived on the irregular mud mound complex surface concentrating in depressions sustained by automicrite boundstone and the growth of bryozoans and sponges. The upper Visean Derbyshire mud mounds are, thus, representatives of a newly defined fenestellid bryozoan–brachiopod–siliceous sponge mud mound category, occurring in various middle–upper Visean Western European sites, a sub-type of the fenestellid bryozoan–crinoid–brachiopod Type 3 buildups of Bridges et al. (1995). These mud mounds, and other types of brachiopod-rich buildups, developed in carbonate platform settings between fair-weather and storm wave base, in dysphotic environments with dispersed food resources during the Visean. Brachiopod mud mound colonisation was favoured by moderate water depth, availability of food resources, and diverse substrates.

Stratigraphy of the Thanetian rotaliids limestone, northern Lorestan, Iran - Lorestan, SW Iran

In this research, biostratigraphy of the Taleh Zang Formation, located in northern Lorestan, SW Iran, is discussed. The benthic foraminiferal contents of the upper Paleocene successions were examined in a carbonate platform setting in the context of European standard zonation (Shallow Benthic Zones). The co-occurrence of Glomalveolina levis (Hottinger) and Miscellanea miscella (d’Archiac & Haime) characterizes Zone SBZ 4, indicating a Thanetian age. Based on biofacies analysis and faunal associations, with emphasis on benthic foraminifera and coralline red algae, tidal flat and marginal marine environment exposed to salinity fluctuations (short-term salinity fluctuations or fully marine conditions), dominated by small benthic Foraminifera (Kathina) with slightly mesotrophic conditions, are determined for the deposition of the Poshte Jangal section. The highly translucent, shallowest part of the inner shelf dominated by representatives of Miscellanea and Opertorbitolites. The biotic assemblages representing warm tropical waters with oligotrophic conditions were observed in Palganeh and Pasan sections. Keywords: Taleh Zang Formation, Paleocene, Foraminifera, Biostratigraphy, Thanetian.

Middle Permian Mixed Siliciclastic‐Carbonate System on the Northwestern Margin of the Indian Plate, Pakistan: Implications for Paleoclimate and Carbonate Platform Evolution

AbstractA mixed siliciclastic‐carbonate system that responds to changes in Permian climate and subsequent carbonate platform evolution is investigated using microscopic details of the Middle Permian Amb Formation (Fm.), in Saiyiduwali section, Khisor Range, northern Pakistan. Thin sections were made from rocks throughout the stratigraphic section of the Amb Fm. and analyzed with an emphasis on carbonate and clastic microfacies, and the latter interpreted within the existing chronostratigraphic framework. Outcrop observations reveal that the units comprise coarse‐grained, channelized, ripple‐marked, and burrowed sandstone and sandy, fossiliferous limestone with minor marls and shale intercalations, suggesting deposition in a subaqueous tide‐dominated delta to beach barrier. Based on the determined seven microfacies coupled with outcrop observation, the Amb Fm. was deposited in a tide‐influenced subaqueous delta to middle shelf environment under fluctuating sea level. The deposition of compositionally mature sandstone in the lower part of the formation suggests reworking of detritus from the rift shoulders and an adjacent source area with an ambient warm and humid climate. The stratal mixing of carbonates and compositionally mature siliciclastic units in the middle part suggest deposition under tectonic and climate‐induced terrigenous and carbonate fluxes to the basin. Thus the deposition shows a perfect transition from clastic‐dominated deltaic to pure carbonate platform settings as a result of warm climate and tectonics. This Middle Permian warming is confirmed by sea‐level rise and the presence of a temperature‐sensitive fusulinid fauna in association with photozoan‐based ooids. Deposition of the Amb Fm. and establishment of a carbonate platform are envisaged to be associated with major rifting of northern Gondwana, which subsequently resulted in the development of a rift basin at the passive margin of the NW Indian Plate then in northern Pakistan.

Integrated biochemostratigraphy of the Permian-Triassic boundary beds in a shallow carbonate platform setting (Yangou, South China)

Investigation of the end-Permian mass extinction (EPME) has been greatly facilitated by high-resolution conodont biostratigraphic studies. To date, however, a detailed biostratigraphic framework has been lacking for shallow-marine successions of the Tethys region. In the present study, we analyzed carbonate microfacies, conodont biostratigraphy, and carbonate carbon isotopes of the Yangou Quarry section, a shallow-marine EPME succession that accumulated on an isolated carbonate bank on the northern margin of the Yangtze Platform. Microfacies analysis yielded four facies and 8 microfacies that record a seawater deepening immediately following the EPME. Ooids were abundantly produced during the EPME and earliest Triassic mass extinction (ETME) but were replaced by microbial mats, algae, bivalves, and brachiopods during an earliest Triassic transgression. A total of three conodont zones are recognized from the uppermost Changhsingian to the lowermost Griesbachian, in ascending order, the Hindeodus praeparvus Zone, H. parvus Zone, and Isarcicella staeschei Zone. The EPME and PTB (Permian-Triassic boundary) horizons are marked by the first occurrences of H. praeparvus at 6.18 m and H. parvus at 6.57 m, respectively, above the base of the section. The conodont biostratigraphic framework of the Yangou Quarry section was refined through intercalibration with its δ13Ccarb profile. This section records two discrete negative carbon isotope excursions (CIEs) that are correlative with the EPME and ETME horizons, and that can be correlated with similar CIEs at Meishan D and other PTB sections globally. Our study provides the first integrated, high-resolution biochemostratigraphic framework for a shallow-marine PTB succession in the eastern Tethys region, which will facilitate paleoenvironmental studies of similar facies elsewhere as well as correlations with deeper-water facies.

First report of Acanthochaetetes (Porifera: Demospongiae) from the Cretaceous Khalsi Formation, Ladakh Himalaya, India

AbstractThe Cretaceous chaetetid sponge Acanthochaetetes huauclillensis Sánchez-Beristain and García-Barrera is reported for the first time from the Aptian–Cenomanian Khalsi Formation, Ladakh Himalaya, India. Its low- to high-domical growth form could suggest an adaptation to either an environment with constant sedimentation rates, or to an irregular substrate. However, these growth forms also may indicate an absence of important environmental/sedimentological changes during the lifespan of the sponges. In addition, the growth form of this species suggests a calm, non-turbulent, reef-like microenvironment. Along with the other faunal assemblages, such as the rudists, corals, and the gastropod Nerinea, A. huauclillensis indicates a tropical to subtropical shallow marine carbonate platform setting. This new finding extends its stratigraphic range from the upper Hauterivian to the Aptian–Cenomanian interval in the eastern Tethyan realm.

Petro-mineralogical and geochemical characteristics of Shahaba Limestone from Gogi-Kanchankayi sector, Bhima Basin, Karnataka with reference to Uranium mineralisation

The Shahabad Limestone Formation of Bhima Basin from Gogi-Kanchankayi area occurs in heterogeneous forms like massive/blocky limestone, argillaceous/ siliceous limestone and laminated/ flaggy limestone. These limestones are primarily composed of micrite, which often alters into sparry calcite on diagenesis with associated impurities of quartz, feldspar, barite, chlorite, glauconite, sulphides and carbonaceous matter. Geochemically, these limestones comprises of variable CaO with low MgO and P2O5 content. Trace elements concentration shows elevated Ba, Rb and depleted Sr. The current study classified these limestones as non-dolomitic and non-phosphatic types deposited in shallow marine carbonate platform setting with low energy conditions. Post-sedimentation, basin tectonics has resulted in reactivation of the basin margin fault causing intense fracturing of limestone. Subsequent hydrothermal movement along those fractures has resulted in re-mobilisation and re-precipitation of sulphides and carbonaceous matter, and along with alteration has facilitated the precipitation of the uranium bearing minerals.

Stratigraphic framework, redox history, and organic matter accumulation of an Early Cambrian intraplatfrom basin on the Yangtze Platform, South China

Details of the Lower Cambrian succession of the newly-discovered intraplatfrom basin of the Yangtze Platform (South China), source rocks of the giant Ediacaran-Cambrian gasfields of the Sichuan Basin, including its stratigraphic framework, redox and organic matter accumulation histories, remain unresolved. This paper describes results of a petrological and geochemical investigation of Lower Cambrian strata near the center of the Sichuan Basin with the goal of elucidating those processes that controlled the sedimentary history of the succession. We recognize two intervals of organic-rich source rocks deposited in the intraplatfrom basin, including the Maidiping Formation and overlying Qiongzhusi Formation. The Maidiping accumulated in a carbonate platform setting during Fortunian-Early Stage 2 time (~541–526.5 Ma) and comprises a variety of rock types, including carbonate, chert, phosphorite, shale, and Mn-ore layers. Total organic carbon (TOC) value ranges from 0.47 to 5.06 wt.%; the highest quality source rocks are found in its lower member. The Qiongzhusi Formation was deposited as part of a clastic shelf during Middle Stage 2-Middle Stage 3 (~526.5–515 Ma) time. This unit is dominated by shale and argillaceous siltstone and displays TOC values varying from 0.30 to 4.28 wt.%. The most organic-rich deposits are found in its lower and upper members. Redox and hydrographic proxies suggest that the intraplatfrom basin was dominated by a strongly restricted environment and anoxic bottom water conditions that experienced intermittent episodes of euxinic or dysoxic-oxic conditions during the Fortunian-Middle Stage 3 time. However, bottom water conditions appear to have been oxic during Late Stage 2 time. Accumulation of Lower Cambrian source rocks across the Yangtze Platform appears to have been closely linked to depositional setting. Our findings suggest that organic matter accumulation in the intraplatfrom basin was fundamentally redox-controlled whereas productivity exerted the greatest control on deposition of organic matter near the slope-deep basin of the Yangtze Platform. In summary, Early Cambrian high-quality source rocks preferentially accumulated in topographic lows, including the intraplatfrom basin and slope-deep basin, an association useful to targeting conventional and unconventional hydrocarbons in the Sichuan Basin.

A late Permian–Triassic trench‐slope basin in the Central Qiangtang metamorphic belt, Northern Tibet: Stratigraphy, sedimentology, syndepositional deformation and tectonic implications

AbstractRecords of sedimentation and deformation in trench‐slope basins contain valuable tectonic information about the associated oceanic subduction zone. Here, we present a multidisciplinary study on newly discovered late Permian–Triassic sedimentary successions in the >500‐km‐long Central Qiangtang metamorphic belt (CQMB) to better understand the type of basin and the concomitant tectonism. The Mayer Kangri succession contains lithofacies associations of submarine fan siliciclastic rocks, slope‐environment limestone, deep marine chert and minor olistostromes from the forearc basin. The conodont assemblages and sandstone and andesite interlayers yield continuous stratigraphic ages from the Lopingian to middle Norian. The clastic sediments had two provenances, including the epicontinental arc in the North Qiangtang block (NQB) and synchronous volcanism in the accretionary wedge. Moreover, a large suite of the Anisian–early Carnian radiolarian cherts (>30 m thick) was discovered in the Lanling area. Regionally, the CQMB shows evident spatiotemporal variations in late Permian–Triassic sedimentation, with a general depositional trend of southward deepening and getting younger. The three identified subzones include a bathyal setting, a carbonate platform setting and a deep marine setting from north to south. These observations indicate that the late Permian–Triassic sedimentary successions in the CQMB were deposited in a trench‐slope basin environment during the northward subduction of the Longmu Co–Shuanghu Tethys Ocean beneath the NQB. Generally, the CQMB and the concomitant trench‐slope basin is among the well‐preserved ancient analogs characterized by extensional tectonism. The syndepositional horst‐graben‐like structure, forearc basin‐derived olistostromes, abyssal radiolarian cherts and synchronous volcanism provide new implications for the geological evolution of the trench‐slope basin.

Distribution and controlling factors of the oolitic shoal deposits in the sequence stratigraphic framework: a case study of the Lower Triassic Feixianguan Formation, eastern Sichuan Basin, China

As one of the main reservoirs types in the Sichuan Basin, the granular shoal reservoirs of the Feixianguan Formation have great potential for oil and gas exploration. Therefore, it is very important to study the sedimentary evolution characteristics and the distribution of shoal deposits of the Feixianguan Formation. In this paper, the sequence stratigraphy, sedimentary characteristics and distribution characteristics of the granular shoal deposits of the southwestern section of the Kaijiang–Liangping Trough in the Sichuan Basin were investigated, through core observations, thin section analysis, and field profiles measurement and sampling, combined with well-logging data and interpretations of two and three-dimensional seismic data of the local area. According to the identification marks, three sequence boundaries (SB1, SB2, and SB3) and two third-order sequences (SQ1 and SQ2) were identified, and a transgressive systems tract (TST) and highstand systems tract (HST) were distinguished in each third-order sequence. The analysis of the Feixianguan Formation sedimentary facies indicated that this formation developed in a carbonate platform setting. In the depositional period of SQ1, the study area inherited the sedimentary pattern of the Changxing Formation and developed carbonate platform depositions. During the SQ1-HST stage, the platform margin shoals and the intra-platform shoals in the study area were relatively well developed. During the SQ2-TST period, oolitic shoals were developed in the region, but their thicknesses were smaller than those of SQ1-HST. In addition, the distribution pattern of sedimentary facies was similar to that of SQ1-HST. The platform marginal zone was relatively narrow at that time, whereas the open platform was more widely distributed, and the distribution areas of platform margin shoals and intra-platform oolitic shoals were relatively limited. During the depositional period of SQ2-TST, the sea level dropped to the lowest level of the studied interval, and limited and evaporation platform facies developed in the study area. Based on previous studies, the influence of different factors on the deposition of the shoals in the Feixianguan Formation were evaluated. The results showed that under the influence of sea level, basement faults, and hydrodynamic conditions, the platform margin area in the study area was more favorable to the development of granular shoals, and the growth model of the granular shoals was “accretion before migration”.

A multi‐proxy based high‐resolution stratigraphical analysis of the possible Palaeocene–Eocene boundary interval, Salt Range, Pakistan

The Palaeocene–Eocene boundary (PEB) interval has been recognized in the Patala Formation, Nammal Gorge Section, Pakistan using lithostratigraphy, stable carbon isotope stratigraphy and integrated biostratigraphy. Four distinct lithological units are identified in the Patala Formation in stratigraphic order from bottom to top; unit‐1 to unit‐4. The top of unit‐3 hosts three closely spaced unconformities demarcated as U1, U2, and U3. The PEB interval is represented by a negative shift of 1.61‰ in δ13CV‐PDB representing the carbon isotopic excursion (CIE) in unit‐2. Based on the encountered foraminifera, dinocysts, and calcareous nannoplanktons, the following biozones are identified across the PEB; two larger benthonic foraminiferal (LBF) zones (SBZ4 and SBZ5), the zonal boundary slightly postdating the CIE, two smaller benthonic foraminiferal zones (BB1 and BB2), three planktonic foraminiferal zones (P4/P5 + E1), four dinocysts zones (Pak‐DV, Pak‐DVI, Pak‐DVII, Pak‐DVIII), and two calcareous nannoplankton subzones (NP9a and NP9b). The following protist, dinocyst and calcareous nannoplanktons encountered in the studied interval advocates for the placement of the possible PEB at the level between samples NAM‐SH‐07 and 08; 1—presence of the smaller benthonic foraminifera Angulogavelinella avnimelechi diagnostic of the base of the CIE and the benthonic extinction event (BEE) Tethys wide, 2—presence of the cosmopolitan and the PEB diagnostic dinocyst species ‘Axiodinium augustum’, 3—presence of the PEB diagnostic Rhomboaster‐Discoaster calcareous nannoplankton assemblage. The LBF species Alveolina vredenburgi [a marker species of the Larger Foraminiferal Turnover (LFT) associated with the Palaeocene Eocene Thermal Maximum (PETM) and PEB in western Tethys] occurs slightly above the onset of the CIE. Previously reported compositional change in the LBF assemblage from the Salt Range [i.e. the replacement of the Palaeocene orbitoidiform LBF (Setia and Orbitosiphon) by the Eocene LBF (Orthophragminids, Alveolina, Nummulites and Assilina)] also postdates the PEB interval here. The late appearance of Alveolina vredenburgi and the compositional change in the LBF after the onset of the CIE are attributed to the facies variation across the PEB reflecting fluctuating depositional environment due to the active tectonic across this interval. This fluctuation is evident from the smaller benthonic and planktonic foraminiferal dominating assemblages in unit‐2 representing bathyal settings followed by LBF dominating unit‐3 representing shallow carbonate platform settings. The change in LBF assemblage closely resembles the well‐known LFT associated with the PETM as well as the carbonate platform stage III and the PEB reported from other Tethyan sections.

Pycnodont fishes (Actinopterygii, Pycnodontiformes) from the Upper Cretaceous (lower Turonian) Akrabou Formation of Asfla, Morocco

The Upper Cretaceous (upper Cenomanian–middle Turonian) Akrabou Formation of Asfla, southeast Morocco is renowned for exceptionally preserved, often three-dimensional bony and cartilaginous fish fossils. Teleosts, rare holosteans and chondrichthyans are well known from the so-called ‘Goulmima ichthyological assemblage’, however pycnodonts (Actinopterygii, Pycnodontiformes) have received scant attention and remain undescribed until now. Five nominal species are recognised in the assemblage, including two new forms: Neomesturus asflaensis gen. et sp. nov. and Paranursallia cavini sp. nov. The Italian genus Polazzodus is reported in Morocco for the first time. Specimen taphonomy is examined in the context of substrate consistency and oxygenation of an outer shelf carbonate platform setting. Diversity and ecological disparity of the Goulmima assemblage is revised in regard to trophic partitioning with durophagy seemingly more diverse than previously recognised. Comparisons with other pycnodont-bearing horizons of similar age demonstrate strong faunal affinities between the Asfla pycnodonts and those in the western Tethys and South Atlantic.

Classification of the Paleoproterozoic Gulcheru Formation: Implications on Early Paleogeographic and Tectonic Evolution of the Cuddapah Basin, India

Abstract The Papaghni sub-basin of the Cuddapah basin is an arcuate NW-SE trending intracratonic basin, stretching from Kurnool in the NW to Guvvalacheruvu in the SE, in southern peninsular India. Despite of large number of researches, an understanding of the evolution of the basin is still elusive. The sub-basin preserves records of repeated opening and closing of Paleoproterozoic rifts along the zone of NW-SE trending axis in the East Dharwar craton. Present study was carried out along the southwestern margin of the Papaghni sub-basin, in and around Tadpatri-Gooty-Dhone area, where the successive litho-units are represented by two major unconformity-bound sequences. Each of these sequences are characterized by a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation in an extensional basin. The basin initiated at about 2 Ga and shortly evolved into a shallow sea, with development of wide siliciclastic shelf in an open marine condition, the Gulcheru Formation. Gradually, with time the siliciclastic shelf was replaced by an extensive stromatolitic carbonate platform, the Vempalle Formation. Dedicated fieldwork exemplifies that the initial basin-fill is characterized largely by several cycles of simple, but amalgamated and stacked fanglomerates (alluvial fan to fan-delta) systems developed from several single stream breaches onto the proximal setting of the basin, in a syn-rift setting. These fanglomerates display a fining upward sequence, which progressively passes up to high-energy coastal, tide-storm dominated shelf environments of post-rift setting. This study justifies the subdivision of the Gulcheru Formation into two formal lithostratigraphic members, the Daditota Member and the Karvipalle Member. The Daditota Member directly overlies the granitoid basement and grades upward to Karvipalle Member. Karvipalle Member, comprising predominantly of medium to fine-grained, reddish brown and white quartz arenites of stable shelf deposits, distinctly differs from the conglomerate and very coarse-grained mass-flow dominated deposits of the Daditota Member. The Vempalle Formation overlies the Gulcheru Formation with a gradational contact and consists of stromatolitic dolomite, oolite and shale, developed in a ramp type carbonate platform setting. Gradual transition from siliciclastic to carbonate sequence points to passive margin setting developed during the breakup of the supercontinent Kenorland. Disposition of proximal to distal sequences and northeastern, eastern and southeastern paleocurrent of the sequences advocate a NW-SE shoreline and eastern paleo-slope of the basin.

Hurricane Deposits on Carbonate Platforms: A Case Study of Hurricane Irma Deposits on Little Ambergris Cay, Turks and Caicos Islands

AbstractThe study of modern hurricane deposits is useful in both identifying ancient hurricane deposits in the rock record and predicting patterns of deposition and erosion produced by future storm events. Hurricane deposits on carbonate platforms have been studied less frequently than have been those along continental coasts. Here we present observations of the characteristics of deposition and scour caused by Hurricane Irma on Little Ambergris Cay, a small uninhabited island located near the southeastern edge of the Caicos platform in the Turks and Caicos Islands. Hurricane Irma passed directly over Little Ambergris Cay on 7 September 2017 as a Category 5 hurricane. We described and sampled multiple types of hurricane deposits and determined that the washover fans were the best sedimentological records for hurricane conditions, as they were subject to very little reworking over time. We compared different model predictions of storm tide and wave height with eyewitness reports and distributions of scour. Examining the washover fans allowed for the construction of a conceptual model for hurricane deposits formed in a high‐energy storm event on a carbonate platform. Characteristics of the washover fans were their small size, the lack of sedimentary structures, and very well sorted sediment. The size and distribution of carbonate boulders eroded and transported by the storm are consistent with depth‐averaged flow velocities in the range of 1.5–5.3 m/s. The strength of the storm and the low‐lying topography, distinct features of a carbonate platform setting, contributed to high levels of sediment bypass and high flow velocities, resulting in small, unstructured deposits.

Early Devonian ammonoid faunas in the Zeravshan Mountains (Uzbekistan and Tadjikistan) and the transition from a carbonate platform setting to pelagic sedimentation

Thick Early Devonian carbonatic sedimentary successions, exposed in the Zeravshan Mountains of Uzbekistan, display a transition from a reefal to a pelagic facies. This allows us to document and analyze the history of sedimentation and changes in marine faunas of this region. The late Pragian succession of Bursykhirman Mountain is documented with the transition from platform carbonates to pelagic sediments. Lithology and microfacies through the early Emsian sedimentary sequence of two ammonoid-bearing sections were investigated with a focus on the Dzhaus Beds. In addition to this sedimentological analysis, we discuss the palaeobiogeographically peculiar situation of Uzbekistan (palaeocontinent Kazakhstania). Many species found in the Kitab State Geological Reserve are endemic and at least restricted to the South Tien Shan. We suggest a moderately close relationship to southern Chinese and Vietnamese faunas, even though more palaeontological data from the latter two regions is needed for a test. We also revise the cephalopod fauna from the Kitab Reserve and introduce the following new taxa: Beckeroceras gen. nov., Uzbekisphinctes gen. nov., Ivoites meshchankinae sp. nov., Kitabobactrites salimovae gen. et sp. nov., and Metabactrites rakhmonovi sp. nov. • Key words: Pragian, Emsian, carbonate microfacies, endemism, Ammonoidea, palaeogeography

Cyclic anoxia and organic rich carbonate sediments within a drowned carbonate platform linked to Antarctic ice volume changes: Late Oligocene-early Miocene Maldives

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.

Mg isotope response to dolomitization in hinterland-attached carbonate platforms: Outlook of δ26Mg as a tracer of basin restriction and seawater Mg/Ca ratio

Magnesium isotopes in early diagenetic dolomite have been proposed as a potential tracer for seawater chemistry and global Mg cycles. However, the applicability of Mg isotopes of early diagenetic dolomite in studies of ancient seawater requires a detailed understanding of the behavior of Mg isotopes during dolomitization in a variety of geological settings. Hinterland attached carbonate platform is an important sink of seawater Mg through dolomitization, and basin restriction is a common feature in hinterland attached carbonate platforms, yet its effects on Mg isotope systematics in carbonates have not been well documented.The upper Albian Hevyon Formation in southern Israel was deposited in a typical hinterland attached carbonate platform setting and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions. The abundance of dolomite increases up-section in the Hevyon Formation and correlates with the appearance of microbial deposits and disappearance of metazoans, reflecting water level fluctuations in multiple exposure and recharging events. In conjunction with sedimentary facies analysis, multiple geochemical proxies (Ni/Co, V/Cr, Ce/Ce∗, and δ13C) indicate the development of anoxic conditions. These two lines of evidence, together with textural indicators and 87Sr/86Sr ratios, suggest dolomitization was microbial and penecontemporaneous in a shallow low circulation water body. δ26Mg of dolomite increases from −1.99 ± 0.12‰ in the lower part of the sequence to a value of −1.52 ± 0.02‰ in the middle interval, then decreases back to −2.06 ± 0.18‰ in the upper unit.Overall, Mg isotope compositions co-vary with δ13C, Mg/Ca ratios, as well as crystal size of dolomite, which implies that variations in δ26Mg value reflect syn-depositional changes in water chemistry as a result of dolomitization in episodically restricted basins. δ26Mg tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction. Furthermore, by pairing Mg isotope variability of the dolomite with a Rayleigh distillation model and elemental mass balance in aqueous solution, it is possible to constrain the Mg/Ca ratio of seawater. Based on the records from the Hevyon Formation, Mg/Ca ratio of seawater during the Albian is constrained between 1.5 and 1, which is consistent with results from other records.

A new Permian–Triassic boundary brachiopod fauna from the Xinmin section, southwestern Guizhou, south China and its extinction patterns

Wu, H.T., He, W.H., Shi, G.R., Zhang, K.X., Yang, T.L., Zhang, Y., Xiao, Y.F., Chen, B. & Wu, S.B., XX.XXXX.2018. A new Permian–Triassic boundary brachiopod fauna from the Xinmin section, southwestern Guizhou, south China and its extinction patterns. Alcheringa 00, 000–000. ISSN 0311-5518.A new brachiopod fauna comprising 31 species in 19 genera is described from a Permian–Triassic boundary section in Xinmin, Guizhou Province, Southwestern China. The brachiopods were collected from the Changhsingian (latest Permian) Changxing (=Changhsing) and Dalong (=Talung) formations and the lower Griesbachian (earliest Triassic) Daye Formation, which were deposited, respectively, in a shallow-water carbonate platform, upper offshore and carbonate platform settings. Among the brachiopods described and illustrated, a new species Juxathyris subcircularis is proposed. In addition, some species Araxathyris previously reported in south China have been discussed in detail and revised, with new morphological information. In particular, internal structures are provided for the first time for Orthothetina and Araxathyris species reported from south China. In addition, important clarifications are also provided on the morphology and diagnoses for Haydenoides, Martinia, Crurithyris and Transcaucasathyris, as well as for Paryphella transversa.Huiting Wu School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China and School of Life and Environmental Sciences, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia; Weihong He [whzhang@cug.edu.cn] State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China; G. R. Shi [guang.shi@deakin.edu.au] School of Life and Environmental Sciences, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia; Kexin Zhang State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China; Tinglu Yang Faculty of Geosciences, East China University of Technology, Nanchang 330013, PR China; Yang Zhang School of Earth Sciences and Resource, China University of Geosciences, Beijing 100083, PR China; Yifan Xiao and Bing Chen School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China; Shunbao Wu, School of Earth Sciences, China University of Geosciences, Wuhan 430074, PR China.