Shallow-water Carbonate Platform Research Articles

U-Pb geochronology reveals that hydrothermal dolomitization was coeval to the deposition of the Burgess Shale lagerstätte

Fault-controlled, hydrothermal dolomitization often occurs at margins between shallow-water carbonate platforms and deep-water sedimentary basins. In western Canada, for example, the platform margin between the Cathedral Formation and the Burgess Shale Formation has been dolomitized at temperatures up to ~200 °C, with local magnesite, talc, and clinochlore mineralization. At the same time, the Burgess Shale Formation includes exceptional fossils that provide key evidence of the radiation of the animal phyla during the Cambrian Period (541 to 485.4 Ma). This lagerstätte and Mg-rich minerals within the adjacent and underlying strata, however, have been critically understudied. Here we show, using carbonate U-Pb geochronology, that western Canada was tectonically active and subject to hydrothermal dolomitization during the Middle Cambrian (Miaolingian Epoch) to Middle Ordovician (488.1 ± 18.8 Ma). These results extend the latest stages of rifting along the western margin of Laurentia into the Paleozoic, while also suggesting that the dolomitization of the Cathedral Formation occurred at the same time as the deposition of the Burgess Shale lagerstätte.

Early Mississippian global δ13C excursion is not a diagenetic artifact

Shallow-water platform carbonate δ13C may provide a record of changes in ocean chemistry through time, but early marine diagenesis and local processes can decouple these records from the global carbon cycle. Recent studies of calcium isotopes (δ44/40Ca) in shallow-water carbonates indicate that δ44/40Ca can be altered during early marine diagenesis, implying that δ13C may also potentially be altered. Here, we tested the hypothesis that the platform carbonate δ13C record of the Kinderhookian-Osagean boundary excursion (KOBE), ∼353 m.y. ago, reflects a period of global diagenesis using paired isotopic (δ44/40Ca and clumped isotopes) and trace-element geochemistry from three sections in the United States. There is little evidence for covariation between δ44/40Ca and δ13C during the KOBE. Clumped isotopes from our shallowest section support primarily sediment-buffered diagenesis at relatively low temperatures. We conclude that the δ13C record of the KOBE as recorded in shallow-water carbonate is consistent with a shift in the dissolved inorganic carbon reservoir and that, more generally, ancient shallow-water carbonates can retain records of primary seawater chemistry.

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.

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.

Salt-rich versus salt-poor structural scenarios in the central Northern Calcareous Alps: implications for the Hallstatt facies and early Alpine tectonic evolution (Eastern Alps, Austria)

One of the most remarkable features of the central Northern Calcareous Alps (Eastern Alps, Austria) is the widespread presence of Upper Triassic deep-water carbonates (the Hallstatt facies) and Permo-Triassic evaporites resting on deep-water Middle Jurassic strata and their underlying Upper Triassic shallow-water carbonate platform successions. The Hallstatt facies and accompanying evaporites have been classically interpreted to originate either from a location south of the time-equivalent carbonate platforms, or to have been deposited in deeper water seaways within the broad platform domain. To date, this dispute has been addressed mostly through the analysis of Triassic and Jurassic facies distribution in map view, which, however, is subject to some degree of ambiguity and subjectivity. In this contribution we present, for the first time, sequentially restored regional cross-sections through the central Northern Calcareous Alps to understand the implications of the contrasting paleogeographic models. We present (a) an interpretation based on a highly allochthonous origin of the Triassic deep-water units and (b) an interpretation based on their relative autochthony in which we incorporate the potential influence of salt tectonics in the central NCA. The restored cross-sections provide a framework within which the alternative scenarios and their paleogeographic implications can be better understood. Through this analysis we propose that salt tectonics in the central NCA can provide a valid explanation for apparent inconsistencies in the relative autochthony scenario and thus constitutes a reasonable alternative to the currently accepted allochthony scenario.Graphical abstract

The Turonian–Campanian rudist bivalve succession in the Central Iberian Basin

Four rudist assemblages: lower Turonian, upper Turonian, upper Coniacian, and Santonian–?Campanian, are distinguished in the shallow water carbonate platform successions of the Iberian Basin, nowadays Iberian Range, a Mesozoic intra-continental basin in the western margin of the Mediterranean Tethys. Because of the depositional evolution of the Iberian Basin, the occurrence, abundance, taxonomic diversity, diagenetic processes, and shell preservation, for each assemblage, is linked, both, to the shallow character of these carbonate platform successions, and the successive high and low frequency sea level falls. Twenty identified rudist taxa are described and figured: Hippuritidae, six species of two genera; Radiolitidae, ten species of six genera, one new, Hoyosites tozoi gen. et sp. nov; Requieniidae, one genus. The knowledge of the shell characters of some taxa has been improved and the taxonomic, biostratigraphic, and palaeobiogeographic significance of most of them increased. The precise positioning of the first three rudist assemblages in high-frequency depositional stacking pattern (parasequence sets) and their correlation and calibration with ammonite biozones provide biostratigraphic datums of great importance. This fact notably improves the chronostratigraphic framework of the Cretaceous sedimentary successions of the Iberian Basin, especially towards the coastal margins, and allows the accurate quantification of the hiatuses associated with the parasequence sets boundaries, so enabling their precise hierarchization.

Devonian–Carboniferous transition in various facies of Northeast Laurussia (North Urals)

This study analyzed 14 stratigraphic sections exposing the Devonian–Carboniferous boundary interval in the North Urals, representing different facies from shallow-water carbonate platform to bathyal, in order to assess the possibility of recognizing regional levels corresponding to the base of the conodont zones S. sulcata and Protognathodus kockeli. The base of the S. sulcata Zone is regionally recognized by the first appearances of the conodonts Siphonodella sulcata (Huddle), S. semichatovae Kononova and Lipnjagov and Patrognathus crassus Kononova and Migdisova, roughly correlated with the first appearance of the foraminifer Tournayellina pseudobeata Reitlinger and Kulagina, and with the first appearances of the ostracods Cribroconcha primaris Kochetkova, Chamishaella grekoffi Tchizhova, Glyptolichvinella spiralis (Jones and Kirkby), Armilla uralica Sobolev, Calcarofera ex gr. media Schornikov, Compositocostata cumina Sobolev, Spinoalacia tschigovae Sobolev, Richterina latior Rabien. The base of the S. sulcata Zone occurs in the interval between two positive inorganic carbon isotope excursions. The base of the Protognathodus kockeli Zone is marked by the first appearance of Protognathodus kockeli (Bischoff) in a very narrow facies belt on the slope of the carbonate platform, recognized regionally by the onset of the last phase of the positive inorganic carbon isotope excursion, the beginning of the transgression after the terminal Famennian regression, the beginning of the unilocular foraminifera Bisphaera bloom and probably the appearance of the ostracods Monoceratina sp. 1. The level corresponding to the base of Pr. kockeli Zone is poorly supported biostratigraphically, but is characterized by clear isotope-stratigraphic and cyclostratigraphic data, although the eustatic signal may be obscured in places by local tectonics.

Sedimentary environments in the prelude to the lagerstätten conditions of the Tlayúa Formation (Albian) in central Mexico: A microfacies approach

This contribution presents the first detailed microfacies analysis of the Lower Member of the Albian Tlayúa Formation outcropping in southern Mexico. The basis for this study consists of two measured stratigraphic sections named Tlayúa and Barranca Abuelo. The Tlayúa section is a composite section ∼48 m thick that includes the sedimentary record of the Lower Member and the first 12 m of the Middle Member. This carbonate succession directly precedes the fossil Lagerstätten deposits of the Middle Member exposed at Tlayúa Quarry. A second stratigraphic section of ∼23 m outside of Tlayúa Quarry includes the transitional contact between the Lower and Middle members and is documented herein for the first time. A detailed microfacies analysis combined with stratigraphic and sedimentologic data suggests that the Tlayúa deposits are part of a tropical shallow-water carbonate platform similar to the modern Great Bahama Banks.Microfacies analysis of the studied sections reveals the evolution of a protected carbonate setting affected by recurring storm events of different intensities. The succession records shelf lagoon sedimentation for most of the Lower Member and its gradual change to tidal flat deposition in its upper part. Shelf lagoon deposits consist of a diverse set of subtidal microfacies characterized by a carbonate fabric with low taxonomic group diversity that indicates stressed environmental conditions. Tidal flat deposits consist of shallowing upward cycles, each less than 1 m thick. Commonly, the peritidal cycles are incomplete and consist of mud-rich shallow-subtidal intertidal microfacies, which are clearly observed at the top of the Lower Member. Both microfacies are similar to those observed in the complete cycles described below. The complete cycles consist of mud-rich shallow-subtidal microfacies with low diversity of bioclasts that gradually change to intertidal microfacies characterized by the association of microbialites with desiccation features that suggest intermittent subaerial exposure. Intertidal microfacies record an important change in the composition of the carbonate factory because microbial processes generated most of the sediments in these microfacies. Supratidal deposits characterized by the presence of subaerial exposure structures and calcified pedogenic products, complete the peritidal succession and were identified at the base of the Middle Member. Thus, the presence of soil pisoid-like structures, clotted micrite, possible Microcodium structures, and alveolar textures indicate a sufficiently long subaerial exposure for the formation of pedogenic products, and therefore, a break in the marine sedimentation of the Tlayúa Formation.

Integrated analysis of a mixed carbonate-siliciclastic reservoir in the Iranian Asmari intra-shelf Basin: Insights from well data, seismic profiles, and outcrops

A multidisciplinary approach (facies analysis, biostratigraphy, cyclostratigraphy, and seismic stratigraphy) was employed in this case study to assess the depositional geometry and sequence stratigraphic architecture of a mixed carbonate-siliciclastic reservoir in the Oligo-Miocene Asmari intra-shelf basin (SW Iran). The integration of both the surface and subsurface datasets provides an excellent opportunity for this analysis. Previously, this reservoir was assumed to consist of flat-lying strata with no significant gradient, primarily because of the emphasis on the lithostratigraphy correlation technique and, to some extent, biostratigraphy resolution. However, these aspects have been discussed and improved in this study. Subsurface data, including well data and seismic data, correspond to the southern flank of the Asmari Basin (Kupal Oil Field), which was compared with the exposed sections (Lar and Dashtak anticlines), exceptionally found in its northern counterpart. The Oligo-Miocene rock successions studied here are Chattian-Burdigalian in age and comprise 14 sedimentary facies distributed along different settings of a depositional profile, ranging from deep-water basin (F.1, F.2) and shallow-water carbonate platform (F.3 to 13) to continental settings (F.14). Using cyclostratigraphy, 19 significant timelines were identified along the oil field and calibrated with age dating and seismic data. The integration of well-to-seismic data ties and outcrop observations allowed for the identification of five seismic facies and four seismic packages. Consequently, the combination of these techniques and methods established a sequence stratigraphy architecture comprising four 3rd-order depositional sequences (sequence. I, II, III, IV). The transgressive system tract (TST) and highstand system tract (HST) are the main packages identified within these sequences. Due to the resolution of seismic data and the spatial position of the Oligo-Miocene depositional setting in the Kupal Oil Field, Lowstand system tract (LST) can only be observed in the sequence. I. within this sequence, three 4th-order depositional sequences (cycles. I-a, I-b, I-c) were hierarchically nested into the HST package.

Benthic biota (nummulites) response to a hyperthermal event: Eccentricity-modulated precession control on climate during the middle Eocene warming in the Southern Mediterranean

Earth's future environment depends critically on how the marine ecosystems react to the current greenhouse conditions. Past warming events from the Middle Eocene can shed light on the nature of these reactions. Middle Eocene sedimentary archives from the Tethyan margins record the Middle Eocene Climatic Optimum (MECO) with varying carbon isotope excursions from site to site. The present study integrates stable isotopes, the abundance of benthic fauna, and astrochronology to look for the timing of the MECO peak warming and the maximum abundance of nummulites in the Southern Mediterranean (Tunisia). A mesotrophic environment during the lower Bartonian with high eustatic sea level promotes the formation of a shallow water carbonate platform in central Tunisia from 40.6 Ma to 39.8 Ma. Nummulites occur in low abundance in the Lower Reneiche Limestone unit (LRL, Bey et al., 2015) from 40.6 to 40.2 Ma, then increase, reaching maximum abundance at the Upper Reneiche Limestone unit (URL, 40.1 to 40 Ma). Maximum accumulation of nummulites and negative excursion of δ18O in the URL occurred during the MECO's warming peak (40.07 to 40 Ma), 200 kyr after the middle Eocene third-order maximum flooding surface. The nummulitic beds represent warmer climates and higher sea levels (arid to semi-arid climate), and the short marl intervals reflect enhanced detrital input (humid climate and lower sea level). The eccentricity modulations were extracted using a lowpass filter applied on a wide-band precession filter then we computed the instantaneous amplitude using Hilbert Transform. The Spearman-rank correlation assesses the potential introduction of amplitude modulations during tuning. Power Decomposition Analysis (PDA) indicates a dominant short eccentricity forcing on the regional climate during the MECO with a minor obliquity component. Variations in magnetic susceptibility and CaCO3 content suggest a combined control of climate and sea-level cycles controlled by eccentricity-modulated precession cycles.

Characteristics and Geological Significance of High-Frequency Cycles in Salinized Lake Basins: The Paleogene Kumugeliemu Group in the Xinhe Area, Northern Tarim Basin

Salinized lake basins have distinctive sedimentary response characteristics, similar to marine shallow-water carbonate platforms. High-frequency cycles can also be used to reveal more sedimentological information, such as relative lake-level fluctuations, lithofacies sequence combinations, and paleogeographic evolution. In this article, a comprehensive study on the stratigraphic shelf delineation and high-frequency cycles of the Paleozoic Kumugeliemu Group in Xinhe area, northern Tarim Basin, was performed using drilling cores, logging curves, and seismic analyses. As a result of the study, the following data were obtained: the three sets of marker beds in the Kumugeliemu Group in the study area could be divided into a bottom sandstone component (E1-2 km1), a lower gypsum mudstone component (E1-2 km2), a salt rock component (E1-2 km3), and an upper gypsum mudstone component (E1-2 km4) by petrology vertical overlay combination and isochronous tracking correlation, which constituted two third-order cycles (ESQ1, ESQ2). They were further divided into seven fourth-order cycles (Esq1–Esq7). Due to the droughty and saline lacustrine depositional system background, the internal rock fabric changed frequently and showed a periodic vertical overlay pattern. Stratified gypsum salt, gypsum mud (sand) rock, and gypsum rock were used as the cycle interface. A single cycle was mainly characterized by an upward shallower depositional sequence of rapid lake transgression followed by a slow lake regression, composed of massive sandstone–lamellar mudstone–lime dolomite–gypsum rock, massive sandstone–lamellar mudstone–gypsum rock (gypsum salt), massive sandstone–massive gypsum mud (sand) rock–gypsum rock, and other cycle structure types. The complete sedimentary cycle was superposed by a single cycle and compared by the inter-well thickness difference, indicating that the study area had a paleogeomorphology pattern of “West-Low–East-High”. The thickness of the cycles decreased gradually from bottom to top vertically, and five sedimentary stages were determined, i.e., freshwater, brackish, brackish water, salt lake, and semi-saltwater, reflecting the evolutionary process of increasing salinity, lake basin filling, and gradual salinization and shrinkage.

Orbital forcing of Upper Jurassic (Tithonian) shallow-water carbonates, Tethyan Adriatic Platform, Croatia evaluated using synthetic vs. real data sets

A continuing debate surrounds the relative dominance of random vs. Milankovitch (astronomical) forcing of the stratigraphic record, particularly in the genesis of shallow-water carbonate platform successions. Using time series analysis, we examined a ∼ 7 myr duration, 700 m thick, internally conformable Upper Jurassic (Tithonian) section from the Adriatic Platform, southern Croatia. The Tithonian succession has a lower interval of dominantly subtidal parasequences and an upper interval of oolitic, laminite-capped peritidal parasequences. We combined this investigation with an analysis of synthetic data sets of water depth derived from numerical models of carbonate sedimentation containing differing amounts of random variance with superimposed Milankovitch forcing. Average Spectral Misfit (ASM) was calculated for each synthetic time series of preserved water depth and our Tithonian data to determine the most likely sedimentation rates and significance levels for rejection of the null hypothesis of no astronomical forcing. The ASM-determined sedimentation rates could then be compared to known long-term rates to again reject or accept the null hypothesis of no astronomical forcing. Using this approach, random synthetic data series showed considerable difference from astronomically-forced synthetic data sets. Multitaper method (MTM) spectral analysis of the Tithonian Dunham rank series tuned to the 100 kyr eccentricity cycle shows variable forcing by short (100 kyr) eccentricity, obliquity, and precession. The Tithonian parasequence thickness series was examined using wavelet analysis for bundling of parasequences and showed that long- (405 kyr), short-eccentricity and obliquity, plus obliquity modulation (∼200 kyr) influenced parasequence bundling. The results indicate that the ASM analysis can differentiate between successions dominated by random processes and those strongly influenced by Milankovitch processes.

The Early Cretaceous C-isotope record of shallow-marine carbonates: a transect across southern Alps, Apennines and Dinarides

Shallow-water carbonate platforms host a precious record of carbonate- associated proxies of palaeoceanographic conditions. The Apennine section of San Lorenzello, Valanginian–Barremian in age, exhibits interesting variations of geochemical, palaeontological and sedimentological proxies in the time interval after the Weissert and before the Selli OAEs. The C-isotope correlation, performed across a transect going from the Urgonian platform in the Subalpine Chains to the Apennine Carbonate Platform in the southern Apennines and to the Adriatic Platform in the Dinarides shows that the δ13C profile of the studied section potentially preserves the global marine C-isotope signature, but the above carbonate platforms show lateral thickness variations of time equivalent discrete stratal intervals and a not strictly synchronous distribution of some biomarkers. Biostratigraphy, carbon isotopes and sequence stratigraphy allow a chronostratigraphic calibration of the studied section and the interpretation of its depositional sequences (superbundles) and biotic events in the framework of global sequences and eustatic cycles. A recovery of the neritic biota is observed at the end of the Weissert OAE, coinciding with the Valanginian–Hauterivian sea- level rise and with a return to warmer temperatures after the late Valanginian cooling. The superbundles, considered also as expression of 400-kyr orbital cycles, can contribute to create a floating orbital time scale which quantifies the durations of the Hauterivian and Barremian stages.

A pilot study of upper Yangtze shallow-water carbonates of the Paibian global marine euxinia: Implications for the late Cambrian SPICE event

The SPICE (Steptoean Positive Carbon Isotope Excursion) is a significant event in the Late Cambrian, recording global carbon cycle perturbations. Its driving mechanism is still controversial, but mainly believed to be related to an increase of organic carbon burial caused by ocean anoxia. The shallow-water platform carbonates of the Upper Cambrian Loushanguan Formation of Yankong outcrop (Upper Yangtze) spans the event interval. Their primary geochemical proxy signatures (trace elements, carbon and strontium isotopes), have been utilized to better understand the driving mechanism of the SPICE event. The variations of paleoenvironmental proxies in the upper Loushanguan Formation exhibit a rapid transformation of redox conditions similar to those of deep-water regions, thus reflecting an expansion of anoxic seawater to shallow-water column. The increase of 87Sr/86Sr, Al, and ∑REE during the SPICE suggests an enhancement in nutrient inputs from continental weathering that might have promoted primary productivity, which preferentially consumed light 12CO2 of surface waters, resulting in 13C-enriched DIC. The occurrence of PGMEE (Paibian global marine euxinia event), immediately before the enhancement of continental weathering, suggests that it was the onset-driving mechanism of the SPICE event and the increase in continental weathering might have promoted primary productivity to further intensify carbon burial and play a superimposed effect on that of the SPICE event.

Record of a dense succession of drowning phases in the Alpstein mountains, northeastern Switzerland: Part II—the Lower Cretaceous Schrattenkalk Formation (late Barremian)

The Schrattenkalk Formation represents a complete succession of Lower Cretaceous shallow-water carbonate platform series cropping out in the Alpstein massif of north-eastern Switzerland. The Schrattenkalk Formation is traditionally divided into two sedimentary units, the “Lower” and the “Upper” Schrattenkalk, separated by the more marly Rawil Member. The “Lower” Schrattenkalk is habitually dated to the late Barremian, while the Rawil Member and the “Upper” Schrattenkalk are dated to the early Aptian. New field observations, however, call the lithostratigraphic dichotomy of the Schrattenkalk into question, as the neritic carbonates are disrupted by several key surfaces associated with karstic episodes and/or transgressive sediments, corresponding to ammonite-rich hemipelagic deposits on the distal shelf. A large number of ammonites were collected in the Drusberg Member as well as rare ammonites from the Schrattenkalk Formation. These ammonites as well as the neritic macrofauna from the Schrattenkalk Formation allow a precise dating of the onset of the Schrattenkalk Formation across the Alpstein massif and its successive phases of progradation. Three successive carbonate bodies and a fourth sedimentary intermediate rock body at the top of the Schrattenkalk platform are defined, based on new biostratigraphic data and updated interpretations of the sequence stratigraphy and geochemical data. The data shows a progressive onset of the Schrattenkalk carbonate platform along the studied transect, following a SE progradation over time. The oldest deposits refer to the upper Barremian T. vandenheckii Zone and the youngest carbonates to the uppermost Barremian M. sarasini Subzone. The new dating of the discontinuity surfaces and key-beds highlight three successive flooding events. The first drowning phase, which correlates with the "Sartousiana" event, dates from the middle late Barremian (upper T. vandenheckii—lower G. sartousiana Zone). The second phase, represented by the Rawil Member, is an incipient drowning, which seems to coincide with the latest Barremian Taxy event (usually reported to the I. giraudi and lowermost M. sarasini zones) according to rare ammonite discoveries. The final demise of the Schrattenkalk platform, situated close to the Barremian-Aptian boundary, is related to an exposure and consecutive drowning event.

Simplorabanitina gen. nov. (type species S. simplex sp. nov.) provides further evidences for the high diversity of Nezzazatidae (Foraminifera) in Cenomanian shallow-water carbonate platforms

ABSTRACT A new genus of the Nezzazatidae is here introduced as Simplorabanitina simplex gen. et sp. nov. from the Cenomanian Sarvak Formation of south-western Iran. The main architectural constraints used to discriminate the new taxon from its close ally Rabanitina Smout lies in the complete lack of skeletal elements collectively called the ‘median plate’ or ‘complex longitudinal plate’, and by the occurrence of pseudo-keriotheca characterising the chamber wall. The new occurrence increases the number of species of Nezzazatidae throughout the shallow-water carbonate platforms of the Arabian Plate prior to the extinction related to the Oceanic Anoxic Event 2 of the Cenomanian-Turonian boundary, indicating high diversity. Unlike some other Cenomanian Nezzazatidae that have been commonly found from Mexico to the Middle East, the biogeographic affinity of Simplorabanitina n. gen. seems to be restricted to the Arabian Plate and perhaps might also be found in the coeval successions of NE Africa. 37F231ED-05B8-4BCA-9F9D-2B38AFA55296

Three-dimensional morphometric analysis and statistical distribution of the Early Kimmeridgian Hanifa Formation stromatoporoid/coral buildups, central Saudi Arabia

The high porosity and permeability stromatoporoid/coral facies partly characterize some of the most productive supergiant Late Jurassic carbonate reservoirs in the Middle East. Reservoir models often overlook detailed morphology and distributions of this facies due to the limited resolution of subsurface data. 3D morphological architecture and distribution of the Late Jurassic stromatoporoid/coral buildups are poorly understood as not many studies specifically investigate these aspects. This study performs a full three-dimensional outcrop investigation of the stromatoporoid/coral complex part of the Late Jurassic Hanifa reservoir analog in Wadi Birk, Saudi Arabia. We investigated spatial correlation, clustering tendencies, scaling relationships, and comprehensive statistical description over a 1.2 × 1 km area analog to a small-scale sectoral model of a typical Middle Eastern oilfield. This study utilizes integrated datasets including measured sections, drone-based digital outcrop model (DOM), core, ground penetrating radar (GPR), and shallow seismic to perform a full three-dimensional outcrop investigation. We identify and map three levels of the scaling hierarchy of the buildups: S1, S2, and S3. S1-scale buildups (a few meters in length) combine to form S2 clusters (∼35 m in length), which in turn may grow into larger S3 complexes (∼128 m in length). S1-scale buildups are near-circular and isotropic. However, as the S1 buildups cluster and grow into S2 and S3, their circular shape evolves into pseudo-ellipsoids with a strong preferential orientation towards northwest-southeast. The S2 and S3-scale amalgamated buildups are associated with asymmetric flank accumulation towards the northwest. We propose that the orientation of the S2 and S3-scale buildups and associated flanks are attributed to hydrodynamic currents driven by paleo-trade winds. This study also performs a brief static connectivity assessment to demonstrate the subsurface implications of these facies to field development planning. Results show that a vertical 5-spot pattern with a 1 km spacing can, in the best case, only access 14% of the Gross Rock Volume of the build-up facies due to the extreme lateral heterogeneity. Some 84% of the build-ups would be connected via flank and other facies with significantly different reservoir quality. This result underlines the extreme heterogeneity of the late Jurassic shallow-water platform carbonates and the potential for substantial quantities of bypassed hydrocarbons remaining in subsurface analog reservoirs.

Spatial heterogeneity in carbonate-platform environments and carbon isotope values across the Paleocene–Eocene thermal maximum (Tethys Himalaya, South Tibet)

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is a large negative carbon isotope excursion (CIE) that testifies to a massive perturbation of the global carbon cycle and has been considered to be an ancient, deep-time analogue for present and future climate change. However, the environmental and carbon isotopic response to the PETM in shallow-water carbonate platforms has remained largely elusive. This study presents new sedimentological, biostratigraphic, and carbon isotopic data to evaluate the impact of the PETM on a shallow-water carbonate platform from more proximal southern parts of the northern Indian continental margin. Detailed biostratigraphy and carbon isotope data indicate that the PETM occurs 3 m below the boundary between nodular and thin-bedded limestones, is maintained up to the thin-bedded limestone and calcareous marl interval, and is followed by recovery at the base of thick-bedded limestone interval. Microfacies analysis testifies to a regression from open to restricted shallow-marine environments at the transition from PETM onset to PETM core, and shallowing continued through the PETM core. Restricted lagoonal deposition was renewed during PETM recovery. These environmental changes were associated with two major turnovers of shallow-water biota. We infer that the first sudden biotic change at the PETM onset may relate to intensified continental weathering, whereas the second biotic change at PETM recovery may have been caused by sea-level fall. The smaller magnitude of the CIE observed in proximal, shallower-water than in distal, deeper-water environments is ascribed to increased primary productivity due to increased nutrients' supply associated with intensified continental weathering. • A regression at the PETM onset from open to restricted shallow-marine environments, and restored to restricted lagoon at the recovery • The sudden environmental changes across PETM were associated with major changes of shallow-water biota • The smaller magnitude of the CIE observed in shallower-water than in deeper-water strata is ascribed to increased primary productivity

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

Proliferation of Chondrodonta in upper Cenomanian shallow-water limestones of the Adriatic Carbonate Platform (Croatia) as a proxy of environmental instability

The occurrence of the opportunistic, oyster-like bivalve Chondrodonta is widely documented in late Cenomanian shallow-water carbonates of the Tethyan Realm. Despite its high abundance and widespread geographic distribution, the precise relationship between the time of its proliferation and the environmental perturbations that precede the Cenomanian – Turonian anoxic event, has not been yet investigated. Stratigraphic and geochemical analyses of the upper Cenomanian Chondrodonta accumulations within the inner platform limestones of the Adriatic Carbonate Platform (AdCP) are performed to assess the timing and the environmental controls on the Chondrodonta proliferation.In the study area, Chondrodonta appears as sparse to common individuals within radiolitid rudist beds and reaches a phase of proliferation and predominance in the benthic community. This biotic event has been correlated via C- and Sr-isotopes to a late Cenomanian environmental stress phase, occurred in shallow-water settings and in the deep counterparts.This environmental stress phase on the AdCP was characterized by high nutrient levels, fluctuating seawater oxygenation, and more terrigenous inputs that favored the flourishment of Chondrodonta and its transient predominance over the less resilient rudists. This bioevent seems to be recurrent at the central Tethys scale, allowing to consider the proliferation of Chondrodonta in shallow-water carbonate platforms as a stratigraphic marker for a late Cenomanian environmental stress phase preceding the anoxic event.