Shallow Platform Research Articles

Depositional, diagenetic and mineralogical controls on porosity development in the Ungani Field, Canning Basin

A 75 m-thick section of Early Carboniferous Tournaisian Ungani Dolomite reservoir was continuously cored in Ungani Far West 1, an appraisal well 3 km west of the Ungani field located on the southern flank of the Fitzroy Tough in the Canning Basin. The objective of this coring program was to better understand the pore systems, mineralogy, and diagenetic history of the reservoir to allow calibration and extrapolation of the petrophysical evaluation of the reservoir at the Ungani field. Petrography, stable isotopic, XRF, hyperspectral logging, grain density and CT scan studies were conducted on the core. The core consists of a 12m overlying sealing shale and 63 m of vuggy, fractured and dolomitised reservoir. The reservoir is commonly bioclastic-rich but pervasive dolomitisation hinders recognition of earlier depositional features. The upper carbonate facies is interpreted as shallow to moderate depth marine ramp-type deposits while the lower carbonate facies is suggestive of shallow platform top settings with ‘reefal’ constructing organisms. Bio-mouldic, vuggy, fracture, cavern and inter-crystalline porosity resulting from multistage brecciation, fracturing, dolomitisation and dissolution events are all present and critical to reservoir development. Based on hyperspectral logging and thin section petrography, the reservoir is predominantly dolomite with late phase cements comprising of quartz, calcite, gypsum, anhydrite, chalcedony, dickite and pyrite. Variable grain densities that correspond with porosity have been noted throughout the core but no obvious relationship between the mineralogy and porosity can be established. Shallow to moderate burial and marine or evaporative reflux fluids are likely responsible for the pervasive dolomitisation. Subsequent leaching of calcite is also key to reservoir development.

Influence of habitat connectivity and seasonality on the ichthyofauna structure of a riverine knickzone

ABSTRACT Knickzones are originated from basaltic outcroppings, present runs, riffles and pools and are highly influenced by flood pulses, which maintain their natural dynamic. However, the construction of hydroelectric power plants alters or eliminate the dynamism of this area and can affect the resident fauna that may be dependent on it. The aim of this study was to evaluate the organization of a knickzone’s ichthyofauna considering the influence of seasonality and connectivity of habitats. The study was performed in a knickzone located in the Sapucaí-Mirim River, Southeast Brazil. We sampled four rocky pools connected to the river and three isolated pools, during rainy and dry conditions. The analysis of the two factors (connectivity and seasonality) and of their interaction showed a significant influence only for seasonality on ichthyofauna structure, with higher values of abundance in the rainy season. The species that most contributed to the high dissimilarity between seasons were Knodus moenkhausii (50% of contribution) and Astyanax bockmanni (21%). The former is the most abundant species in the rainy season and the later in the dry season. The alteration between low and high water level occurs frequently in knickzones, as it is a rocky shallow platform in the middle of a river, with floods occurring seasonally or in stochastic short-term periods. This hydrological seasonal dynamic, high limnological variability and complex interactions of different habitats (pools, runs and rapids) explain the particular ichthyofauna structure in such small area. Our results also indicate the potential importance of basaltic knickzones for regional fish diversity conservation, especially due to the imminent threat by intensive hydropower reservoir construction.

Evolutionary significance of the microbial assemblages of large benthic Foraminifera

ABSTRACTLarge benthic Foraminifera (LBF) are major carbonate producers on coral reefs, and are hosts to a diverse symbiotic microbial community. During warm episodes in the geological past, these reef‐building organisms expanded their geographical ranges as subtropical and tropical belts moved into higher latitudes. During these range‐expansion periods, LBF were the most prolific carbonate producers on reefs, dominating shallow carbonate platforms over reef‐building corals. Even though the fossil and modern distributions of groups of species that harbour different types of symbionts are known, the nature, mechanisms, and factors that influence their occurrence remain elusive. Furthermore, the presence of a diverse and persistent bacterial community has only recently gained attention. We examined recent advances in molecular identification of prokaryotic (i.e. bacteria) and eukaryotic (i.e. microalgae) associates, and palaeoecology, and place the partnership with bacteria and algae in the context of climate change. In critically reviewing the available fossil and modern data on symbiosis, we reveal a crucial role of microalgae in the response of LBF to ocean warming, and their capacity to colonise a variety of habitats, across both latitudes and broad depth ranges. Symbiont identity is a key factor enabling LBF to expand their geographic ranges when the sea‐surface temperature increases. Our analyses showed that over the past 66 million years (My), diatom‐bearing species were dominant in reef environments. The modern record shows that these species display a stable, persistent eukaryotic assemblage across their geographic distribution range, and are less dependent on symbiotic photosynthesis for survival. By contrast, dinoflagellate and chlorophytic species, which show a provincial distribution, tend to have a more flexible eukaryotic community throughout their range. This group is more dependent on their symbionts, and flexibility in their symbiosis is likely to be the driving force behind their evolutionary history, as they form a monophyletic group originating from a rhodophyte‐bearing ancestor. The study of bacterial assemblages, while still in its infancy, is a promising field of study. Bacterial communities are likely to be shaped by the local environment, although a core bacterial microbiome is found in species with global distributions. Cryptic speciation is also an important factor that must be taken into consideration. As global warming intensifies, genetic divergence in hosts in addition to the range of flexibility/specificity within host–symbiont associations will be important elements in the continued evolutionary success of LBF species in a wide range of environments. Based on fossil and modern data, we conclude that the microbiome, which includes both algal and bacterial partners, is a key factor influencing the evolution of LBF. As a result, the microbiome assists LBF in colonising a wide range of habitats, and allowed them to become the most important calcifiers on shallow platforms worldwide during periods of ocean warming in the geologic past. Since LBF are crucial ecosystem engineers and prolific carbonate producers, the microbiome is a critical component that will play a central role in the responses of LBF to a changing ocean, and ultimately in shaping the future of coral reefs.

Framboidal pyrite evidence for persistent low oxygen levels in shallow-marine facies of the Nanpanjiang Basin during the Permian-Triassic transition

Previous studies suggested that anoxia was a causal factor in the end-Permian mass extinction (EPME), marked by abrupt enrichment of pyrite framboids in the post-EPME microbialites of the earliest Triassic on shallow platforms, and that this dysoxic–anoxic phase followed a time of well‑oxygenated seafloors. Here, we report persistent dysoxia throughout the latest Permian and euxinia just before the EPME, based on a new redox history reconstruction study using framboidal pyrite size distribution as well as sulfur isotopic compositions of pyrites, from the Taiping section on the Pingguo Platform in the Nanpanjiang Basin in China. Further, we show that the EPME was followed here by rapid oxygenation, not an anoxic incursion. This revised redox history might be an unusual localized phenomenon when compared to other platforms, or it could be solid sedimentary evidence for redox oscillations outside the bioclast-enriched photic zone, which broadens our understanding of terrestrial–marine ecosystem interactions before and during the EPME.

A mechanistic model of coral bleaching due to temperature-mediated light-driven reactive oxygen build-up in zooxanthellae

Mass coral bleaching has emerged in the 21st century as the greatest threat to the health of the world's reefs. A sophisticated process understanding of bleaching at the polyp scale has now been achieved through laboratory and field studies, but this knowledge is yet to be applied in mechanistic models of shelf-scale reef systems. In this study we develop a mechanistic model of the coral-symbiont relationship that considers temperature-mediated build-up of reactive oxygen species due to excess light, leading to zooxanthellae expulsion. The model explicitly represents the coral host biomass, as well as zooxanthellae biomass, intracellular pigment concentration, nutrient status, and the state of reaction centres and the xanthophyll cycle. Photophysiological processes represented include photoadaptation, xanthophyll cycle dynamics, and reaction centre state transitions. The mechanistic model of the coral-symbiont relationship is incorporated into a ∼1 km resolution coupled hydrodynamic – biogeochemical model that encompasses the entire ∼2000 km length of the Great Barrier Reef. A simulation of the 2016 bleaching event shows the model is able to capture the broadscale features of the observed bleaching, but fails to capture bleaching on offshore reefs due to the model's grid being unable to resolve the bathymetry of shallow platforms surrounded by deep water. To further analyse the model behaviour, a ∼200 m resolution nested simulation of Davies Reef (18°49′ S, 147°38′ E) is undertaken. We use this nested model to demonstrate the depth gradient in zooxanthellae response to thermal stress. Finally, we discuss the uncertainties in the bleaching model, which lie primarily in quantifying the link between reactive oxygen build-up and the expulsion process. Through the mechanistic representation of environmental forcing, this model of coral bleaching applied in realistic environmental conditions has the potential to generate more detailed predictions than the presently-available satellite-based coral bleaching metrics, and can be used to evaluate proposed management strategies.

Carbonate production, accumulation and sea-level change in fault-block platform setting: a case study on the Albian sequences in Central Tunisia

The Albian series have been deposited on both sides of a major E–W paleogeographic fault, which was inherited in the early stages of the Tethyan-rifting phase. To the south of this fault, a shallow platform domain (footwall block or horst) occurred and is relayed northward by a slope-deep shelf setting (hanging wall block or graben). The platform to basin transition was analyzed northward of this fault nearby M’rhila Mountains. A detailed sedimentological study, coupled with a tectonic analysis, indicates that the Albian sedimentary deposits consist of four third-order depositional sequences. The three lower ones are only present in the deep-shelf setting and include substantial carbonate accumulations. The latter deposits are generally arranged into alternating mud-rich-bedded limestone and conglomeratic levels. Most of this material was exported from the adjacent shallow platform by mass flow and suspension transport processes. In the studied series, the maximum carbonate accumulation is recorded in the deep shelf during the moderate transgressive increasing and the late highstand decreasing rates of sea-level rise. However, the minimum carbonate sedimentation is thought to be linked to both the minimum lowstand and the maximum transgressive sea-level rise. The uppermost depositional sequence—which occurs on both sides of the major fault, is different from the three lower ones as it includes only scarce thin re-sedimented limestone beds. The youngest sequence is related to an important interplay between a pulse of tectonic subsidence and the major see level rise that happened during the upper Albian (100–98.9 Ma), resulting in a regional collapse of the substratum and the drowning of the carbonate factory situated southward. Therefore, the uppermost sequence is considered as a tectonically originated depositional sequence rather than a “pure” eustatic one. The organic-rich deposits associated to this sequence south and north of the major fault correlate well with the global anoxic events. The present study case represents a good example illustrating the role of local tectonics and global eustatic fluctuations in controlling not only the facies and thickness distribution but also the carbonate production and accumulation.

Crescentiella-microbial-cement microframeworks in the Upper Jurassic reefs of the Crimean Peninsula

In the Upper Jurassic reef successions of the Crimean Peninsula (Sudak and Jalta areas), the microencruster Crescentiella morronensis (Crescenti), microbialites, and multiple generations of cements, form microframeworks. They were observed in two stages of the carbonate platform evolution, in the Middle–Upper Oxfordian, and in the Upper Kimmeridgian–Tithonian. Generally, in both stages, the features of the microframeworks are similar and consist of densely packed Crescentiella associated with microbialites and branched colonies of the sclerosponge Neuropora lusitanica Termier. The difference between the occurrences of the two stages is the variable amount of nubecularid foraminifera and enigmatic tube-shaped structures forming the central cavities of Crescentiella. The Crescentiella-microbial-cement microframeworks formed under phreatic conditions in the upper slope and seaward marginal depositional settings where intensive synsedimentary cementation took place. They formed in the initial stages of long cycles of restoration and blooming of the reefs. The late Jurassic examples resemble the Permian algae-microbial-cement reefs as well as the Triassic Tubiphytes and cement crust-dominated reefs. Concurrently, all these examples formed a transitional facies zone between typical slope facies to shallow subtidal platform margin facies characterized by high taxonomic diversity of calcified sponges, corals, and microencrusters forming the principal part of the reefs.

BIOTIC AND ABIOTIC EVENTS IN A SHALLOW CARBONATE PLATFORM (UPPER CRETACEOUS, SOUTH PYRENEAN MARGIN)

A detailed sedimentological and paleontological study of the Late Santonian-Late Campanian carbonate platform deposits of the Serres Marginals area (South margin of the Pyrenean Basin) has been performed. Three depositional shallow carbonate sequences have been distinguished (CI, C2 and C3). Within the shallow platform sequences moderate energy subtidal to intertidal, high energy sand-shoal, protected lagoon and restricted lagoon to lacustrine fades have been identified.

First report of a Palaeozoic fenestrate bryozoan with an articulated growth habit

The main goal of this paper is the description of the erect articulated growth habit for the first time within the Palaeozoic Order Fenestrata, and the interpretation of its occurrence within the evolutionary history of colonial morphology in fenestrates. The present paper is based on a single specimen composed of fifteen disarticulated internodes. The specimen was extracted from the outcrop as two minute slabs, one of which was embedded in a two-component epoxy resin for preparation of acetate peels and the other one preserved without subsequent preparation for observation of exterior features. A new Devonian fenestrate bryozoan genus, Arnaopora, and its type species, A. sotoi, are described from the Emsian-Eifelian Moniello Formation (Asturias, NW Spain). Zooecial characters as well as the development of articulated colonies support the definition of this new genus, which is the first reported representative with an articulated growth habit in the Order Fenestrata, almost exclusively composed of rigidly erect unilaminar forms. The occurrence of Arnaopora in the early Eifelian coincides with a peak of disparity in fenestrate bryozoan growth forms during the transition between the Early and Middle Devonian. While all previously known morphological variations of growth forms in Palaeozoic fenestrates derive from the basic reticulate habit, articulation evolved from a rigidly branching phylloporinid ancestor. Arnaopora is extremely rare within the abundant and diverse bryozoan fauna of shallow platform facies where it occurs, which is in contrast with the relatively common presence of cellariform colonies in assemblages from modern shallow environments.

Patterns of ecological diversification in thelodonts

AbstractHere we explore the spatial, temporal and phylogenetic patterns of ecological diversification for the entire clade of thelodonts, one of the earliest groups of vertebrates and longest lasting of the Palaeozoic agnathans in the fossil record. Parsimony and maximum‐likelihood methods are used to reconstruct ancestral states of their geographical distributions, habitats and lifestyles. Our results support the concept that thelodonts originated during the Middle?–Late Ordovician probably in marine open waters of Laurasia, with a demersal lifestyle on hard substrates being the ancestral condition for the whole clade. Later, thelodonts underwent a complex ecological diversification and palaeobiogeographical history, comparable in many aspects to those of some major groups of living fishes. Different modes of life evolved repeatedly and a wide range of habitats were colonized by distinct groups, including deep waters and brackish marine and/or freshwater environments. Diadromous strategies presumably appeared on nine different occasions. The palaeobiogeographical history of thelodonts reveals significant differences in the dispersal potential of some major groups. Dispersal of thelodontiforms entailed displacements over long distances and the crossing of deep‐water biogeographical barriers, whereas those of furcacaudiforms were always limited to areas interconnected by shallow platforms. We propose that the evolution of pelagic larval stages in thelodontiforms might explain this biogeographical pattern and could satisfactorily account for the greater evolutionary success of this group.

Geochemical, petrographical, and petrophysical evaluations of a heterogeneous, stratiform dolomite from a Barremian oil field, offshore Abu Dhabi (United Arab Emirates)

A stratiform dolomite is developed in the Barremian carbonates of offshore Abu Dhabi, United Arab Emirates. Although the average thickness is only 1.1 m (3.6 ft), it is an important drilling target characterized by heterogeneous porosity–permeability values in the studied oil field. We discuss the origin and development of the dolomite based on geochemical, petrographical, and petrophysical evaluations using 81 cores recovered from the field. The δ13C values of the dolomites are relatively high (>5‰), and 87Sr/86Sr mostly fall in the range of Barremian seawater. The thin stratiform geometry and the geochemical signatures of the dolomite suggest early dolomitization just below the seafloor that was driven by the diffusion of Mg2+ from the seawater on top of a shallow platform. Lateral changes in the degree of dolomitization seem to be controlled by the permeability of the precursors. Dolomitization progressed further in packstone precursors (higher permeability) than in wackestone precursors (lower permeability), which led to greater permeability improvement in the packstone by replacement of fine matrix with larger dolomite crystals. Subsequently, late burial dolomite cementation occurred during oil migration, which preferentially affected the higher-permeability early dolomite, and areally, it progressed more in the flank of the field because of the delayed oil charge there. Dolomite-to-dolomite recrystallization became dominant after the oil emplacement. The recrystallization has been continuing under current burial temperatures (>100°C [>212°F]), fully resetting δ18O values and modifying the Fe and Mn concentrations of the dolomites, but has not significantly affected their petrography and petrophysics in the oil leg.

Clinoform development and topset evolution in a mud‐rich delta – the Middle Triassic Kobbe Formation, Norwegian Barents Sea

AbstractClinoform surfaces are routinely used to mark transitions from shallow waters to deep basins. This concept represents a valuable tool for screening potential reservoir intervals in frontier basins where limited data are available. Variations in the character of clinoform geometries and shoreline and shelf‐edge trajectories are indicators of a range of different factors, such as palaeobathymetry, changes in relative sea‐level and sediment supply. Applications of conceptual and generalized models might, however, lead to erroneous assumptions about the supply of coarse‐grained material to the delta front and basin when superficial similarities between clinoform geometries are not treated holistically. The present study examines the mudstone‐dominated Middle Triassic Kobbe Formation – a potential hydrocarbon reservoir interval in the Barents Sea, where prodeltaic to deltaic deposits can be examined in cores, well logs and two‐dimensional and three‐dimensional seismic data. Despite pronounced acoustic impedance contrast to the surrounding shale, channel belt networks are not observed close to the platform edge in seismic datasets, even at maximum regressive stages. However, sub‐seismic prodeltaic deposits observed on the shallow platform indicate that prodeltaic deposits were sourced directly from the delta plain. Clinoform surfaces with different geometries and scale are observed basinward of the palaeoplatform edge of underlying progradational sequences, correlative to mudstone‐dominated prodeltaic core sections. Results indicate that platform‐edge deltas developed at discrete sites in the basin due to normal regression, but the positions of these deltas are not directly relatable to variations in clinoform geometries. Transitions from third‐order to fourth‐order clinoform geometries record discrete transgressive–regressive cycles but are not necessarily good indicators of sandstone deposition. Because of prolonged periods with high accommodation, channel avulsions were frequent and only very fine‐grained sandstone was deposited in heterolithic units at the delta front. Sandstones with good reservoir properties are predominantly found along basin margins.

Facies-dependent δ13C variation and diagenetic overprinting at the onset of the Sturtian glaciation in North-East Greenland

Time-significant surfaces are used to construct a stratigraphic test of the intrabasinal reproducibility of a 16‰ Neoproterozoic carbon-isotope (δ13C) anomaly in the NE Greenland Caledonides. The δ13C excursion from +6‰ to −10‰ occurs in the carbonate-dominated Andrée Land Group and lies below glacial diamictites of the Tillite Group commonly correlated as Sturtian in age (∼720 Ma) and has been widely interpreted to record a global isotopic event reflecting a perturbation in the carbon cycle preceding the initiation of the snowball ice age. δ13C stratigraphic patterns were determined in two strike sections with relatively shallow platform deposits at Kap Weber and slope deposits on Ella Ø preserved directly below the first evidence for glaciation. The top and bottom of the δ13C profile was bounded by contiguous chronostratigraphic surfaces including an unconformity (sequence boundary) at the base recognisable in both sections and a subaerial exposure surface with a basinal correlative conformity at the top. The δ13C profile at Kap Weber shows δ13C values of +6‰ that drop to −8‰ in fine-grained slope deposits before returning to +6‰ in platform carbonates at the base of the glacial diamictite. The slope section on Ella Ø shows similar values for platformal carbonates of +6‰ with a similar drop to ∼−10‰ in slope deposits, but this section lacks the return to platform deposits evident at Kap Weber beneath the diamictite as well as a return to positive δ13C values. The absence of the platformal sediments and positive δ13C values on Ella Ø cannot be attributed to erosional truncation because the contact in this section shows a conformable, interbedded transition into the glacial deposits indicating that a continuous record is preserved. This disparity in δ13C values below the base of the Tillite Group suggests that the most negative δ13C values recorded in slope mudstone facies (−10‰) occur near synchronously with values of +6‰ on the platform. Elemental mapping of the mudstone facies indicates that carbonate is largely pore-filling and authigenic, representing a secondary phase that is unlikely to record a seawater value. Elevated δ13C values in shallow inner-ramp carbonate intervals may record locally modified seawater enriched in 13C by photosynthesis and evaporation. The excursion in δ13C values in the upper Andrée Land Group is therefore interpreted to result from a shift in facies from platform carbonates to carbonate cemented and diagenetically overprinted mudstone and does not record the secular change in seawater δ13C used for correlation or interpretation of biogeochemical events preceding the snowball ice age.

Ostracods biostratigraphy of the Oligocene-Miocene carbonate platform in the Northeastern Amazonia coast and its correlation with the Caribbean region

Oligocene-Miocene ostracods biozones are proposed for the ∼65 m-thick platform carbonate succession exposed in the Northeastern Amazonia coast, Brazil. Previous biostratigraphic studies based on planktonic foraminifera, nannofossils and palynomorphs are poorly age constrained and have hindered the correlation with other units of the Atlantic Equatorial margin. Tectonic stability of the Northeastern Amazonia coast during Neogene, associated with reduced riverine inflow and continuous sea level rise, allowed the establishment of lagoon/mangroves, tidal channel and shallow platform settings favoring massive proliferation of benthic ostracods. The recurrence of these depositional systems along of entire Oligocene-Miocene succession contributed mainly for the preservation of typical assemblages of lagoonal ostracods. Among thirty-two ostracods genera identified, the most of them are generally polyhalines associated with a mesohaline genus as Perissocytheridea. Rare ostracods genera typical of offshore zone indicate limited oceanic connection with lagoon. Additionally, the rare presence of Cyprideis indicates a relatively stable salinity degree suggesting change in the lagoon dynamic until estuarine conditions. The ostracods comprise more than 100 species, ranging from Upper Oligocene to Lower Miocene with five index species correspondent to a single zone called the Cytherella stainforthi Zone that was subdivided in four sub-biozones: 1) Jugosocythereis pannosa, 2) Quadracythere brachypygaia, 3) Triebelina crumena and 4) Neocaudites macertus. This new zonation was calibrated with the Blow's and Wade's planktonic foraminifers zones (N.3 to N.7 zones corresponding to O6 to M4b of Wade) and with van den Bold's zonation in the Neogene Caribbean region. Ostracod assemblages described here provide an excellent biostragraphic framework for local, intrabasinal and regional correlation for the Oligocene-Miocene deposits of the Northeastern Amazonia coast and Caribbean regions. The strong similarity of to the Caribbean deposits open a discussion about the possible paleobiogeographic affinities between the biotas of the Equatorial Atlantic during the Neogene.

Time hierarchical analysis of the conodont paleocommunities and environmental change before and during the onset of the lower Silurian Mulde bioevent – A preliminary report

The Silurian period, and the Wenlock epoch in particular, experienced a series of powerful bioevents, among which was the Mulde event (upper Homerian). Here, the results of a study of the mid- to late Wenlock conodont paleocommunities in the shallow platform environments of the Lithuanian part of the Silurian Baltic Basin are presented. The analyses are based on a detailed quantitative description of the abundance and diversity patterns of the conodonts, as well as the geochemical and geophysical environmental proxies in the Ledai-179 core (central Lithuania). A model selection analysis of the conodont rank abundance distributions revealed episodic and progressive change from the complex communities of the early Jaagarahu time (lower Homerian), to the simple and much more even communities of the early Gėluva time (lower part of the upper Homerian). Spectral and cross-spectral analyses of the conodont species' apparent richness, evenness and abundance, as well as the δ13C and natural gamma ray trends, revealed transient episodes of 5th order periodic fluctuations. It is shown that the transitions in the community states and the transient episodes of the oscillations approximately coincided with the onset of the Mulde event (latest Jaagarahu and early Gėluva time).

Late Pleistocene gastropods from the raised reefal limestone of Jeddah, Saudi Arabia: taxonomic and palaeoenvironmental implications

A temporarily exposed 6-m succession of raised reefal limestone terrace south of Sharm Obhur (Jeddah) was investigated with regard to macrofossil gastropods and their palaeoenvironmental implications. The studied succession consists of two distinct layers. The lower layer consists of white limestone (4 m thick) of Late Pleistocene age, while the upper layer consists of shell-rich calcareous clay (2 m thick) and belongs to the last interglacial marine isotope stage 5e (MIS 5e). Its lithology was correlated with a nearby temporarily exposed 3-m succession, indicating that the deposition of the MIS 5e was not continuous, as indicated by the occurrence of a 0.4-m-thick weathered layer between the lower white limestone and upper calcareous clay. The studied 6-m succession yielded a very rich gastropod fauna with 90 species belonging to 4 subclasses, 5 orders, 16 superfamilies, 33 families and 64 genera. The clayey layer showed higher species richness (65 species) than the limestone layer (18 species), indicating a rapid change in the coral reef environment during the deposition of the interval of MIS 5e. It is assumed that, after the weathering interval, the sea level could not rebound to the same level as it was during the deposition of the white limestone, leading to deposition of calcareous clay in a shallow platform or embayment, allowing proliferation of gastropod species. Littorinimorpha and Neogastropoda species represent about 75% of the total recorded gastropod taxa, whereas Vetigastropoda and Caenogastropoda represent only 25% of the total recorded taxa. Superfamilies Cerithioidea and Stromboidea dominated the clay layer, whereas no particular species dominated the limestone layer.

Suraqalatia brasieri n.gen., n.sp. (larger foraminifera) from the Maastrichtian of Sulaimani area in northern Iraq

Suraqalatia brasieri n.gen., n.sp. from the family Dicyclinidae Loeblich and Tappan 1964 occurs on the Maastrichtian carbonate platform of northern Iraq. The new genus is recognizable by its large very compressed conical test, up to 55–70 mm in diameter, to 0.3–1.6 mm in thickness, planspiral cooling having very small proloculus in the initial part and later circular chambers including numerous chamberlets with an agglutinated wall. Suraqalatia brasieri n.gen., n.sp. is associated with textulariids, miliolids and rotaliids as Loftusia elongata Cox, L. morgani Douville, Orbitoides medius d’Archiac, O. megaloformis Papp & Kupper, O. gruenbachensis Papp, O. apiculatus Schlumberger, Omphalocyclus macroporus (Lamarck), Siderolites calcitrapoides Lamarck, Sirtina orbitoidiformis Bronnimann & Wirz. The associated macrofauna comprises large and rich giant rudists (Preradiolites sp.), other bivalves (Gryphaea sp. and Glycymeris sp.), gastropods (Acteonella sp.), echinoderms and corals. The fauna indicates shallow marine carbonate platform conditions within the Maastrichtian green house. It is also worth mentioning that the new genus has only been recorded from the Maastrichtian age.

The new occurrence of Marinoan cap carbonate in Brazil: The expansion of snowball Earth events to the southwesternmost Amazon Craton

Carbonate deposits exposed in the border of the Pimenta Bueno and Colorado grabens, western part of Parecis Basin, southwestern Amazon Craton, Brazil, have been previously considered as Paleozoic record. These deposits lying unconformably on Mesoproterozoic crystalline rocks, the basement of the grabens, and consist predominantly by pinkish dolomite overlying glacial diamictites, with average negative values of δ13C of −3,10‰VPDB. The contact between the dolostone and diamictites is sharp and deformed similarly with others Neoproterozoic cap carbonates occurrences in the Amazon Craton, also related to the Marinoan Glaciation (635 Ma). This new occurrence of Marinoan cap carbonate is composed by two facies associations. Facies Association 1 consists of pinkish peloidal dolostone with even parallel and quasi-planar laminations, wavy and megarriple bedding, macropeloid lenses associated with low-angle truncations, interpreted as fairwhether- and storm-influenced shallow platform deposits. Facies association 2 consists in dolostone rhythmically interbedded with shale underlaid by 5 m-thick laminated siltstones, interpreted as moderately deep platform deposits. This retrogradational succession is overlaid in angular unconformity by Early Paleozoic diamictites and locally by Mesozoic volcanic rocks. This cap carbonate precedes the Paleozoic deposits of Parecis Basin and represents a post-glacial event linked to the Marinoan glaciation, extending to the southwesternmost Amazon Craton the phenomena of the Snowball Earth hypothesis.

Les échinides du Turonien de la Formation de l’Azilé du bassin côtier gabonais dans la région de Libreville Sud

Study of Turonian echinoids from Gabon store section allowed us to distinguish 10 species belonging to 2 genera: Mecaster pseudofourneli (Péron et Gauthier 1878), Mecaster toucasanus (d’Orbigny, 1854), Mecaster africanus (Coquand, 1862), Mecaster semicavatus (Peron et Gauthier 1880), Mecaster verneuili (Desor, 1847), Mecaster cf. orbignyanus (Desor, 1857), Mecaster palpebratus (Loriol, 1888), Mecaster sp.1, Mecaster sp. 2, Epiaster michelini (Agassiz) 1847. All species are found for the first time in Gabonese coastal basin. The composition and the distribution of echinoids fauna and the sedimentary characteristics indicate a proximal shallow platform deposit environment, with warm water and soft substrate. Two types of depositional environment are suggested: (1) a deep part of the proximal platform, represented by silty limestones and (2) a shallower part of the proximal platform, represented by limestones.

Linking early diagenesis and sedimentary facies to sequence stratigraphy on a prograding oolitic wedge: The Bathonian of western France (Aquitaine Basin)

To improve the understanding of the distribution of reservoir properties along carbonate platform margins, the connection between facies, sequence stratigraphy, and early diagenesis of discontinuities along the Bathonian prograding oolitic wedge of the northeastern Aquitaine platform was investigated. Eight facies are distributed along a 50 km-outcropping transect in (1) toe-of-slope, (2) infralittoral prograding oolitic wedge, (3) platform margin (shoal), (4) open marine platform interior, (5) foreshore, and (6) terrestrial settings. The transition from shallow platform to toe-of-slope facies is marked in the field by clinoforms hundred of meters long. Carbonate production was confined to the shallow platform but carbonates were exported basinward toward the breakpoint where they cascaded down a 20–25° slope. Ooid to intraclast grainstones to rudstones pass into alternating marl-limestone deposits at an estimated paleodepth of 40–75 m. Three sea-level falls of about 10 m caused the formation of discontinuities corresponding to sequence boundaries. Along these discontinuities, erosional marine hardgrounds formed in a high-hydrodynamic environment at a water depth of less than 10 m, displaying isopachous fibrous cements and meniscus-type cements. The cements pass landward into meniscus and microstalactitic forms along the same discontinuities, which are characteristic of subaerial exposure. During the deposition of transgressive systems tracts, carbonate accumulation remained located mostly on the shallow platform. Energy level increased and carbonates were exported during the deposition of highstand systems tracts forming the infralittoral prograding oolitic wedge. During the deposition of lowstand systems tracts, carbonate production fell to near zero and intraclast strata, derived from the erosion of hardgrounds on the shallow platform, prograded basinward. Early diagenetic cements are related exclusively to discontinuities that are not found within the prograding wedge because of the continuous high sedimentation rate under lower hydrodynamic conditions. This absence of early cementation within the infralittoral prograding oolitic wedge was conducive to porosity conservation, making such features good targets for carbonate reservoir exploration. This study proposes a novel sequence stratigraphy model for oolitic platform wedges, including facies and early diagenesis features.