Carbonate Platform Environments Research Articles

Late Mesoproterozoic iron and manganese deposition on a deep-water carbonate platform (Penganga Group), Adilabad, India: Constraints from C, O and Sr isotopes

Abstract The Penganga Group in the Pranhita-Godavari Rift Valley of central India includes a rather unique example of Late Mesoproterozoic iron and manganese formations deposited in a deep-water below storm wave base distal carbonate platform environment. The carbonate rocks of the Penganga Group are mostly lime-mudstones that have been classified into a number of sedimentary cycles with intervals mostly distinguished by color, ranging from brown/pink and siliceous gray to steel-gray and black. At least two about one-meter-thick Mn-Fe formations occur in siliceous gray limestone of the distally steepened part of the Penganga carbonate platform. A number of northwest-southeast striking thrust faults repeat the succession in down dip sections bringing up distal profiles of the Penganga Group against proximal counterparts at the present exposure level. This contribution provides the first comprehensive record of stable C and O and radiogenic Sr isotopes for proximal and distal profiles, allowing for direct comparison and thus, providing context for the environmental conditions of Fe and Mn deposition in a Mesoproterozoic platform environment. Carbon and oxygen isotope compositions determined from the proximal and distal profiles of the platform yield trends that are attributed to variations in circulation pattern and cycles of warm and cold climatic stages across the platform. Secular trends of Sr-isotope compositions suggest a variation in continental input between cold (0.7065) and warm climatic stages (0.7149), similar to that observed for modern marine sediments. The sedimentation of siliceous deposits and ferromanganese deposits in the distal platform environment was likely favored by the onset of upwelling circulation during cold climatic stages. The moderately negative δ13CPDB values (-5.34‰ to -6.34‰) of the Mn-carbonates and variation in δ13Corg values 31.7‰ to -21.7‰ obtained for early diagenetic Mn-carbonate ovoids indicate oxidation of organic matter in the ferromanganese deposits by Mn-oxides during early diagenesis. We conclude that upwelling and platform-wide deposition of ferromanganese deposits on the Penganga carbonate platform may mark the transition from the stagnant Mid-Proterozoic ‘sulphidic’ ocean to a ‘respiring’ open ocean during the Late Mesoproterozoic Era.

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

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

Depositional environments and source rock potential of some Upper Palaeozoic (Devonian) coals on Bjørnøya, Western Barents shelf

Upper Palaeozoic strata are potential sources for hydrocarbon plays on the Norwegian Barents Shelf. The island of Bjørnøya (Bear Island) is located on the western margin of the Barents Shelf and represents an exposed part of the Stappen High, which makes it an excellent location to investigate the source rock potential of this succession. Here, we investigate the organic geochemical composition and source rock potential of coals and organic-rich mudstones of Late Devonian to middle Permian ages. The thermal maturity of the analysed samples ranges from mature to overmature with some samples being in the late oil generation window, and thus exhibiting higher maturities than age and facies equivalent strata in central Spitsbergen in the NW corner of the Barents Shelf and on the Finnmark Platform to the south. The high maturities and thermal cracking may thus have negatively affected the applied isoprenoid and biomarker-based source facies parameters, which potentially inhibit correct source facies interpretation. However, our results and comparisons with previous studies indicate that the vitrinite- and inertinite-dominated humic coals occurring in the lower part of the succession formed by the accumulation of peat in fluvially-influenced, humid wetlands at paleo-equatorial latitudes during a pronounced episode of rifting in the Late Devonian (Famennian) to earliest Carboniferous (Mississippian). Our source rock evaluation indicates a good potential for liquid hydrocarbon generation for the coals of the Upper Devonian Røedvika Formation prior to maturation. The oil-prone organic matter of these coals is primarily considered as terrestrial and non-aqueous. Because of renewed rifting in the middle Carboniferous combined with a shift to arid climatic conditions, as well as an environmental change from dominantly terrestrial and marginal marine clastic to marine carbonate platform environments, the accumulation of wetland peats effectively came to an end. Thus, the overlying Carboniferous and Permian strata generally seems to yield poor to no hydrocarbon generation potential.

Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin

In recent years, with the discovery of oil and gas in the Carboniferous bauxite reservoir in the Ordos basin, the formation and distribution of bauxite and bauxite reservoirs have attracted the attention of oil and gas explorers. Based on the slightly equidistant core testing of minerals and the porosity on the formation and calibration on the logging curves, we established a logging identification method for bauxite in the study area and fitted the formula for calculating the diaspore content and porosity using logging data. By applying this formula and a large amount of logging data, thickness and porosity maps of the Taiyuan formation of bauxite in the southeastern part of the basin were produced. Then, according to the thickness of the earliest deposited Taiyuan formation on the unconformity surface, we analyzed the paleogeomorphology of the bauxite deposition. Finally, a sedimentary facies map of the Taiyuan formation was developed based on the content of sandstone, coal seams, and carbonate rocks contained there. The results show that the diaspore content of bauxite is positively correlated with the natural gamma logging (GR) values. According to the diaspore contents and the GR values, the aluminiferous rocks in the Taiyuan formation can be divided into three categories: ① Bauxite for GR values > 450 API and diaspore content >70%; ② Argillaceous bauxite for GR values = 300–450 API and diaspore content = 25%–70%; and ③ Bauxitic mudstone for GR values = 220–300 API and diaspore content = 0%–25%. Bauxite mainly occurred in the middle and deep lagoon environments in the lower part of the Taiyuan formation. The lagoon is distributed along the paleokarst groove in the NW strike, with a width of approximately 30–40 km and a length of approximately 150–200 km, among which the thickness of pure bauxite is 9 m, argillaceous bauxite 26 m and alumina mudstone 6 m. Bauxite with a high porosity mainly existed in pure bauxite. The lagoon bauxite in the lower part of the Taiyuan formation gradually changes upward into a tidal flat swamp and carbonate platform environment.

Depositional environments and transgression-regression cycles in the Mural Limestone, central Sonora, Mexico

Results of analytical studies in the Mural Limestone of the Bisbee Group exposed in northwest Mexico, carried out to determine the paleo-bathymetry of the marine depositional environment, reveal that deposition occurred in a restricted carbonate platform environment, varying from inner-neritic to shelf margin with the development of lagoon, shoal and biohermal conditions, reflecting minor transgression-regression cycles. Furthermore, the fossiliferous content and lithological characteristics support the interpretation that the studied section corresponds to the transition between Cerro La Puerta and Cerro La Espina members of the Mural Limestone.

Carbonate fabric diversity and environmental heterogeneity in the late Mesoproterozoic Era

AbstractTime-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantineet al.(2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.

Paleo-facies distribution and sequence stratigraphic architecture of the Oligo-Miocene Asmari carbonate platform (southeast Dezful Embayment, Zagros Basin, SW Iran)

A facies and sequence stratigraphic analysis was made of the Oligo-Miocene Asmari carbonate platform situated in the east and southeast of the Dezful Embayment (Zagros Basin, SW Iran). This study provides new paleofacies and depositional sequence distribution models for the eastern part of the Dezful Embayment and discusses the impact of the factors controlling sediment deposition being eustatic sea-level changes and tectonic processes. The petrographic study reveals that the variations in facies cover three depositional settings, e.g. open marine, lagoon and tidal flats. In total, six depositional sequences (DS) were identified within the sedimentary sequence of the Asmari Formation. DS1 and DS2 were deposited during the Rupelian (Oligocene) within deeper open marine platform settings. Sediments forming DS3 and DS4 were deposited during the Chattian in a carbonate platform environment within an open lagoon to protected lagoon setting. The Miocene DS5 and 6 sequences, Aquitanian and Burdigalian in age, were deposited in shallower environments compared to other DS's, within protected lagoon to tidal flat settings. A comparison with the sedimentary development in other parts of the Dezful Embayment showed that the deposition of the Asmari strata started earlier (Early Rupelian) in the eastern, south-eastern and north-eastern part of the basin. In the western and central parts of the Dezful Embayment it began in the Late Rupelian or even Rupelian/Chattian. The eastern and south-eastern parts of the Dezful Embayment displayed a platform margin setting during the Rupelian while the north-eastern and central areas showed a basin setting. During the Early Chattian, clastic sediments originating from the Arabian Plate were deposited in the west, southwest, northwest and centre of the basin in response to a sea-level lowstand. The two very thin sandstone units occurring in the Late Aquitanian succession across the west of the Bibi Hakimeh oilfield are the only sandstone layers related to a sea-level lowstand throughout E/SE of the Dezful Embayment. During the Chattian the sediments in the eastern parts of the Dezful Embayment were deposited in a proximal inner ramp setting, and corals and coralline red algae (mainly Lithophyllum sp. and Lithothamnion sp.) dominated the carbonate platform in the east, while imperforate benthic foraminifera (e.g. Peneroplis sp., Archaias sp.) and rare perforate benthic foraminifera (e.g. Neorotalia sp., and Amphistegina sp.) were the most frequent sediment contributors in the southeast of the basin (Bibi Hakimeh area). At the same time, sedimentation occurred on a distal inner to mid-ramp settings through the central areas of the Dezful Embayment. A eustatic fall in sea-level and tectonic processes resulted in the isolation of the basin. During the Aquitanian two exposure horizons formed in the western and central Dezful Embayment basin while the eastern part only displays one. During the Burdigalian, sedimentation in the entire Dezful Embayment occurred within an innermost, very shallow ramp setting. The current study shows that sea-level fluctuations and sometimes combined with regional tectonic processes controlled the variability in platform geometry and paleofacies distribution of the Oligo-Miocene sediments across the basin.

Provenance of Middle Jurassic sequences in the Northern Qiangtang: implications for Mesozoic exhumation of the Central Tibetan Mountain Range

ABSTRACT The exhumation history of the Central Tibetan Mountain Range (CTMR) in the Qiangtang terrane is important for establishing the boundary conditions prior to the India-Asia collision. Previous studies suggest that the CTMR is a significant provenance for Jurassic sediments in the Qiangtang terrane. The Middle Jurassic sediments north of the CTMR provide crucial information to constrain the timing of exhumation in the source area. Here we present detailed stratigraphic sections and detrital zircon U-Pb geochronology in the northern Qiangtang basin. Sedimentary facies analyses indicate that the Early-Middle Jurassic Quemoco Formation is mainly deposited in the deltaic system and carbonate-platform environment. Upsection, the Middle Jurassic Xiali Formation was deposited in a prodelta environment. Combined analyses of palaeocurrent indicators, conglomerate clast composition and U-Pb age spectra suggest that the CTMR is the main provenance for the Quemoco and Xialai formations; while the Triassic southern Qiangtang strata and Jinsha suture are the secondary source areas. The changes in the specific position of detrital zircon U-Pb age peaks around 500 Ma indicate evolving source terranes. Detrital zircon U-Pb geochronological data from the base of the Quemoco Formation and top of the Xiali Formation constrain the conservative maximum depositional ages to be 174.8 ± 1.0 and 161.7 ± 2.3 Ma, respectively. Combined with previous geochronology data, structural geology, and provenance studies across the Qiangtang terrane, the CTMR experienced rapid exhumation during Middle Jurassic and Late Cretaceous and slow exhumation since Eocene. The Jurassic uplift and exhumation of the CTMR were accompanied by the uplift of the southern Qiangtang terrane. The deformation and exhumation history is in favour of the hypothesis that plateau growth in the south-central Qiangtang terrane initiated during the Mesozoic, which is an important initial condition to quantify Cenozoic growth of the Tibetan Plateau.

Ordovician carbonate mud mounds of the Baltoscandian Basin in time and space – A geophysical approach

Carbonate mud mounds developed within the Baltoscandian Basin, an epicratonic basin on the Baltica palaeocontinent, during the Ordovician. In the Upper Ordovician succession of the Baltoscandian Basin, a large number of mud mounds are present at three stratigraphic levels namely, the Kullsbergs mounds (late Sandbian-early Katian), Nabala and Rakvere mounds (middle Katian), and the Boda mounds (late Katian). These formed in a subtropical-tropical carbonate platform environment, covered by a shallow epicontinental sea. The mud mounds at these stratigraphic levels beneath and around Gotland have been characterized using a comprehensive seismic and well dataset acquired during a period of hydrocarbon exploration and exploitation which began more than 30 years ago. Interpretation of the largely unpublished seismic data in this study provides details on the distribution of mound complexes in the basin and constraints on the geometry of the mounds. Detailed structure contour maps of the top and the base of the Ordovician succession beneath Gotland based on the seismic interpretation are presented. The results give a comprehensive characterization of carbonate mud mound generation on Gotland which may help in understanding the distribution patterns of similar mound complexes in other parts of the Ordovician world formed in similar environments.

Revisiting the Age of Jurassic Coral Bioherms in the Pieniny Klippen Belt (Western Carpathians) on the Basis of Benthic Foraminifers

Abstract Coral bioherms of the Vršatec Limestone that formed massive, several tens of meters thick complexes during the Jurassic were important sources of carbonate production, with carbonate sediment exported to deeper parts of the Pieniny Klippen Basin (Western Carpathians). However, the age of these carbonate factories remains controversial. New analyses of benthic foraminiferal assemblages occurring in coral bioherms and peri-biohermal deposits of the Vršatec Limestone at five sites in the western Pieniny Klippen Belt (Vršatec-Castle, Vršatec-Javorníky, Malé Hradište, Malé Hradište-Kalvária, and Drieňová Hora) show that these sediments were deposited during the Bajocian and were lateral equivalents of crinoidal limestones and breccias, in contrast to previous studies suggesting that they were deposited during the Oxfordian. First, all sites are characterized by similar composition of foraminiferal assemblages on the basis of presence–absence data, although foraminiferal assemblages in biosparitic facies at Vršatec are dominated by miliolids whereas biomicritic facies at Malé Hradište are dominated by the spirillinid Paalzowella. The composition of foraminiferal assemblages does not differ between the lower and upper parts of the Vršatec Limestone. Second, foraminifer species that were assumed to appear for the first time in the Oxfordian already occur in the Middle Jurassic sediments of the northern Tethyan shelf. Third, the first and last appearances of foraminifers documented in other Tethyan regions are in accordance with stratigraphic analyses and ammonoid occurrences, demonstrating that bioherm-forming coral communities developed on the Czorzstyn Ridge during the Bajocian. Several species of foraminifers of the Vršatec Limestone appeared for the first time during the middle or late Aalenian (Labalina occulta, Paalzowella feifeli) and during the Bajocian (Hungarillina lokutiense, Radiospirillina umbonata, Ophthalmidium caucasicum, O. terquemi, O. obscurum, Paalzowella turbinella, Cornuspira tubicomprimata, Nubecularia reicheli) or appeared for the last time in the Bajocian (Tethysiella pilleri) or Early Bathonian (Ophthalmidium caucasicum, O. obscurum). The composition and diversity of communities with benthic foraminifers of the Vršatec Limestone is similar to the composition of foraminiferal communities on carbonate platform environments with corals of the French Jura and Burgundy during the Bajocian.

Sedimentary architecture, depositional facies and diagenetic response to intracratonic deformation and climate change inferred from outcrops for a pivotal period (Jurassic/Cretaceous boundary, Paris Basin, France)

The aim of this study is to decipher the respective influences of geodynamic and climate disturbances at the Jurassic/Cretaceous boundary on sedimentary facies and carbonate diagenesis in a stable intracratonic basin using isotopic geochemistry and subsidence quantification. Fourteen lithofacies were deposited in a (1) carbonate platform and (2) a delta plain environment. Climate change from cool and wet to warm and semi-arid conditions during the early Tithonian influenced the syn-sedimentary dolomitization process within the carbonate platform during the mid-Tithonian. Architecture and facies reconstructions well-constrained the Jurassic-Cretaceous Unconformity (JCU), which was an important local structural episode marked by (1) an 80 m uplift in the eastern Paris Basin and by (2) the formation of a NW–SE low wavelength 15 km-wide and 30 km-long flexure. This first tectonic event tended to maintain brine ponds and supratidal marsh environments in the platform during the late Tithonian and early Berriasian, forming Purbeckian facies and associated dolomitic facies. A major depositional change occurred between the early and late Berriasian from shallow carbonate platform environments to fluviatile-deltaic clastic deposits (Wealden facies). This facies change is underlain by a major unconformity corresponding to the Ryazanian unconformity. It is marked (1) by erosion processes, karstification of the carbonate substrate, and the development of ferruginous weathering products (goethite), followed by (2) incision processes in a fluvial-deltaic environment. This unconformity is consecutive to a 40 m uplift in the eastern Paris Basin. The rifting phase in the Bay of Biscay, in the Pyrenean Zone, and in the Artic-North Atlantic together with the opening of the Ligurian Sea had a major influence on the northern part of France by causing uplifts (120 m from the Tithonian) and flexuring. Geodynamics played a major role in carbonate demise in the Paris Basin leading to exposure and karstification of the carbonate platform. Added to the generalized uplift, western Tethyan cool and humid conditions from the late Berriasian caused terrigenous influx into large-scale marine domains which was detrimental and unfavorable to carbonate growth.

1:5,000 geological map of the upper Cretaceous intraplatform-basin succession in the "Gravina di Matera" canyon (Apulia Carbonate Platform, Basilicata, southern Italy)

Along the slopes of the Gravina di Matera canyon, below and in front of the Sassi di Matera, i.e., the old Matera town (Basilicata, southern Italy), an about 200 m thick succession of upper Cretaceous carbonates extensively crops out. These carbonates belong to the Apulia Carbonate Platform and, in the official geological map, are referred to the Calcare di Altamura Fm, a lithostratigraphic unit composed of shallow-marine peritidal limestones. In order to study the whole outcropping succession, a geological survey was performed in the area and an unaspected intraplatform shallow-basin suite of limestones has been recognized within the Calcare di Altamura Fm. On the basis of lithologic characters, the whole succession has been divided into informal lithostratigraphic units a-g. The base of the succession (unit a) is represented by a crudely stratified bioclastic floatstone-rudstone locally interested by in situ brecciation, a phenomenon that often is the precursor of deep changes in a carbonate platform. Unit a sharply passes upward to thinly bedded and finely planar-laminated mudstone interested by slumps (unit b), conformably covered by dolostones (unit c). Dolostone gradually passes to cherty limestones (unit d) that in turn pass to wackestones (unit e) covered by a megabreccia (unit f). All these units (a-f) indicate that this area of the Apulia Platform performed a change from classic platform environments to slope- and to basin-ones, before the restarting of typical facies of shallow-marine carbonate platform environments (unit g). This subdivision in units of the outcropping succession led us to informally define a member, the Matera member (ALT1), within the upper Cretaceous Calcare di Altamura Fm (ALT). Most likely, the Matera member (units a-f) represents a shallow-basin developed within the Apulia Platform during Late Cretaceous, most likely related to extensional tectonics. Other examples of slope-to-basin sedimentary suite were previously observed within the upper Cretaceous succession of the Apulia Platform and the occurrence of another shallow-basin in the interior of the same platform opens a new and still understimated regional tectonic- and paleogeographic-scenario. Moreover, it should be highlighted that the geological surveyed area belongs to a Regional Natural Park since the 1990 (Parco Archeologico Storico Naturale delle Chiese Rupestri del Materano also known as Parco della Murgia Materana) and that Matera, besides being in the Unesco Word Heritage list since the 1993, has been voted European Capital of Culture 2019. Therefore, the obtained 1:5,000 geological map, apart from documenting some lithostratigraphic evidences of the intraplatform basin, represents an essential tool for both (i) future studies regarding the upper Cretaceous tectono-stratigraphic evolution of the Apulia Platform, and (ii) a geological base for many visitors interested in the cultural and geological heritage of Matera and its territory.

Palaeoenvironment reconstruction and planktonic foraminiferal assemblages of Campanian (Cretaceous) carbonate succession, Çayırbağ area (Trabzon, NE Turkey)

The Campanian (Cretaceous) carbonate succession, which belongs to the uppermost part of the Mesozoic sequence in the Eastern Pontides (NE Turkey), is mainly composed of Globotruncana-bearing grey and red pelagic limestones. One of the most typical outcrops of the succession is located in the Cayirbagarea (NE Turkey) and it was studied with a combined sedimentological and palaeontological approach. Three microfacies types were identified and interpreted based on petrographic analysis of the depositional textures and fauna. They are (1) allochthonous bioclastic-calcispheral packstone/wackestone microfacies (MF-1), (2) calcispheral packstone/wackestone microfacies (MF-2), and (3) pelagic foraminiferal mudstone/wackestone microfacies (MF-3). These microfacies types correspond to slope, deep self and basin environments, respectively. Based on the planktonic foraminiferal assemblages, including Globotruncana cf. arca (Cushman), Globotruncana arca (Cushman), Globotruncana sp., Whiteinella sp., Archaeoglobigerina sp., Archaeoglobigerina blowi (Pessagno), Globotruncana lineiana (d’Orbigny), Whiteinella sp., Archaeoglobigerina cretacea (d’Orbigny), the age of the pelagic carbonate succession is Campanian. Microfacies and palaeontological characteristics of the studied section suggest that they were deposited in a slope to basin environment. The progressive deepening of the environment is probably caused by the vertical movement of the syn-sedimentary extensional tectonic regime. Moreover, a carbonate platform environment adjacent to the deep-marine environment is inferred to exist during the earliest Campanian because of the presence of rare allochthonous neritic skeletal components in the lower part of the section.

The start‐up of the Dolomia Principale/Hauptdolomit carbonate platform (Upper Triassic) in the eastern Southern Alps

AbstractWide carbonate platform environments developed on the western passive margin of the Tethys during the Late Triassic, after a major climate change (Carnian Pluvial Episode) that produced a crisis of high‐relief microbial carbonate platforms. The peritidal succession of this epicontinental platform (Dolomia Principale/Hauptdolomit, Dachstein Limestone) is widespread in the Mediterranean region. However, the start‐up stage is not fully understood. The original platform to basin depositional geometries of the system have been studied in the north‐eastern Southern Alps, close to the Italian/Slovenian boundary where they are exceptionally preserved. Sedimentological features have been investigated in detail by measuring several stratigraphic sections cropping out along an ideal depositional profile. The analysis of the facies architecture allowed reconstruction of the palaeoenvironments of the Dolomia Principale platform during its start‐up and early growth stages in the late Carnian. The carbonate platform was characterized by an outer platform area, connected northward to steep slopes facing a relatively deep basin. Southward, the outer platform was connected to inner sheltered environments by a narrow, often emerged shelf crest. Behind this zone, carbonate sedimentation occurred in shallow lagoons and tidal flats, passing inward to a siliciclastic mudflat. The Dolomia Principale platform was initially aggrading and able to keep pace with a concomitant sea‐level rise, and then prograding during the late Carnian. This stratigraphic interval was correlated with the Tuvalian succession of the Dolomites, allowing depiction of the depositional system on a wide scale of hundreds of kilometres. This large‐scale depositional system presents features in common with some Palaeozoic and Mesozoic carbonate build‐ups (for example, the Permian Capitan Reef complex, Anisian Latemar platform), both in terms of architecture and prevailing carbonate producers. A microbial‐dominated carbonate factory is found in the outer platform and upper slope. The recovery of high‐relief microbial carbonate platforms marks the end of the Carnian Pluvial Episode in the Tuvalian of Tethys.

New data on the composition and age of rocks from the Bathymetrists Seamounts (Eastern margin of the equatorial Atlantic)

The petrology, geochemistry, and isotope ratios of volcanics dredged during the 43rd cruise of R/V Academik Ioffe on the Bathymetrists Seamounts in the eastern equatorial Atlantic have been studied. These are alkaline volcanics of basic and ultramafic compositions. Spider diagrams of the trace elements of volcanic rocks demonstrate strong fractionation, indicating formation of their primary melts from an enriched mantle source at garnet depth facies. Considering the isotope ratio values of 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr and the character of their variations, the volcanic mantle source was chemically heterogeneous: for various volcanic rocks it was a mixture of the mantle components HIMU with EM–1 or EM–2. Limestones dredged together with the volcanics yielded microfossils suggesting a Middle Eocene age of their formation in a carbonate platform environment.

Marine ecology conditions at Weda Bay, North Maluku based on statistical analysis on distribution of recent foraminifera

Analysis of foraminifera in geology,usually being used to find the age of rocks/ sediments and depositional environment. In this study, recent foraminifera was used not only to determinethe sedimentary environment,but also to estimate the ecological condition of the water through a statistical approach.Analysis was performed quantitatively in 10 surface seabed sediment samples in Weda Bay North Maluku. The analysis includes dominance (Sympson Index), diversity and evenness (Shannon Index), and the ratio of planktonic -benthic. The results were shown in the plotting diagram of M-R-T (Miliolid-Rotalid-Textularid) to determine the depositional environment. Quantitative analysis was performed using Past software (paleontological version Statistic 1:29).The analysis result showed there was no domination of certain taxon with a moderate degree of evenness and stable communities and considerably a moderate diversity. The results of this analysis indicated that research area had a stable water conditions with the optimum level of carbonate content, oxygen supply, salinity, and temperature. The ratio of planktonic and benthic indicate the relative depth, which was deeper the water increased the percentage of planktonic foraminifera. Based on M-R-T diagram showed the distribution of sediment deposited on exposed carbonate (carbonate platform) environment with normal saline.

Foraminiferal biofacies and depositional environments of the Burdigalian mixed carbonate and siliciclastic Dam Formation, Al-Lidam area, Eastern Province of Saudi Arabia

Four outcrops along a west to east transect in the Al Lidam escarpment of Saudi Arabia were investigated to understand the vertical and lateral distribution of foraminiferal assemblages for paleoenvironmental reconstruction. The samples were processed using the standard acetic acid method, which extracts foraminifera from lithified carbonate rocks without destroying the fossil content. The foraminiferal assemblages from the Dam Formation are dominated by calcareous porcelaneous Miliolina (Quinqueloculina, Peneroplis, Triloculina, Cornuspira, Sigmoilinita, Coscinospira, Spirolina, Pyrgo, Borelis), followed by hyaline genera (Elphidium sp., Ammonia, Cibicides, Discorbinella) and a minor percentage of agglutinated forms, e.g., Textularina.The high percentage of calcareous porcelaneous taxa and the absence of planktonic foraminifera indicate that the Dam Formation was deposited in a restricted carbonate platform environment, very shallow hypersaline shallow marine, gently sloping ramp (inner ramp to mid ramp) which ranges from supratidal to subtidal with local reef patches towards the basin. The formation was deposited in an arid subtropical climatic environment with water temperatures ranging from 20 to 35°C. Based on the observed assemblage composition, the present day Arabian Gulf can be considered as a modern analogue for the Dam Formation during Miocene times.

Geological features and geochemical characteristics of Late Devonian–Early Carboniferous K-bentonites from northwestern Turkey

AbstractNewly discovered K-bentonite beds, interstratified with limestones/dolomitic limestones of the Upper Devonian–Lower Carboniferous Yılanlı Formation, are exposed in the northwestern Black Sea region of Turkey, around Zonguldak and Bartın. K-bentonite samples collected from four different locations: the Gavurpınarı and Yılanlı Burnu quarries from the Bartın area, the Çimşir Çukurları quarry from the Şapça area, and the Güdüllü and Gökgöl highway tunnel section near Zonguldak city were investigated using optical microscopy, X-ray diffraction and inductively coupled plasma mass spectrometry in order to reveal their mineralogical and geochemical characteristics and understand their origin and evolution. The K-bentonites occur at different levels in the Yılanlı Formation as 2–40 cm-thick, greenish to yellowish beds cropping out several hundred metres along strike. Preliminary biostratigraphic data suggest that the protoliths of the Bartın (Gavurpınarı and Yılanlı Burnu) and Güdüllü K-bentonites were deposited at around the boundary between the Frasnian and Famennian, whereas those in the Şapça and Gökgöl sections are slightly younger (Devonian–Carboniferous boundary interval). The lithofacies types of the host carbonate rocks suggest an ‘epeiric’ shallow carbonate platform environment. Illite and mixed-layer illite-smectite were the major clay minerals in the K-bentonites. The K-bentonites from the Bartın area display a high degree of illitization and consist mainly of illite indicating high-grade diagenesis, whereas illite-smectite-rich samples from the Şapca and Gökgöl tunnel locations reflect relatively lower diagenetic conditions. According to their geochemical compositions, two groups of K-bentonites were distinguished, one with alkali basalt (Bartın area and Güdüllü locations) and one with trachyte affinities (Gökgöl tunnel and Şapça locations). Geochemical fingerprinting of K-bentonites by trace and rare earth element (REE) data suggest that tephras with alkali basalt composition have been derived by a source formed in a ‘continental back-arc’ setting, whereas the source of K-bentonites with trachytic precursors is related to ‘continental within-plate rifting’. An evaluation of the global Late Devonian and Devonian–Carboniferous volcanism suggests that the bentonite precursors may be related to late-Variscan magmatism in Laurussia.

The Oxfordian/Kimmeridgian stage boundary in Late Jurassic sedimentary rocks of the Swiss Jura range

This paper presents a review of the criteria for and limitations to the definition of a usable long distance correlation scheme for the Oxfordian/Kimmeridgian stage boundary, based on the example of the Late Jurassic of the Swiss Jura range. Two main issues and their stratigraphic consequences are more specifically addressed, namely the interpretation of calcareous oolites in terms of depth of deposition and the dependence of ammonite distribution on the paleoenvironment. Calcareous oolites are not diagnostic for depth of deposition, as demonstrated by the conflict with other detailed sedimentary observations. When the sedimentary record is correctly interpreted in terms of water depth, the distribution of ammonites proves to be dependent on the paleoenvironment. Therefore ammonites cannot be directly used as a time correlation tool in a laterally heterogeneous regional context such as the Swiss Jura range. As a consequence, additional proxies are needed to circumvent the limitations of ammonite biostratigraphy in shallow carbonate platform environments. These proxies can ultimately be calibrated with high-resolution ammonite biostratigraphy in basin setting, which remains the most accurate method for long distance time correlation (provided it is not based on endemic or low diversity assemblages). The analysis of the biostratigraphic and lithostratigraphic schemes for the Swiss Jura range with respect to the position of the Oxfordian/Kimmeridgian boundary leads to the conclusion that the former suggestion of a suitable type section on the slope of Summerhalde (3 km NNW of the city of Schaffhausen) has to be replaced by another section outside Switzerland. Indeed, the Oxfordian/Kimmeridgian boundary in northern Switzerland lies within the Knollen Bed (uppermost part of the Gredingensis Zone) rather than at the base of the Reuchenette, Burghorn and Schwarzbach Formations (boundary between Galar and Platynota Zones with low long distance correlation potential).

Late Carboniferous to early Permian sedimentary–tectonic evolution of the north of Alxa, Inner Mongolia, China: Evidence from the Amushan Formation

The late Paleozoic evolution of the Wulijishanhen (WSH)-Shangdan (SD) area near to the Chaganchulu Ophiolite belt is reinterpreted. Analysis of the upper Carboniferous to lower Permian sedimentary sequence, biological associations, detrital materials, sandstone geochemistry and volcanic rocks indicates that the SD area was an epicontinental sea and rift during the late Paleozoic rather than a large-scale ocean undergoing spreading and closure. This study reveals that the actual evolution of the study area is from the late Carboniferous to the early Permian. The fusulinids Triticites sp. and Pseudoschwagerina sp. in the limestones demonstrate that the Amushan Formation develops during the late Carboniferous to the early Permian. The limestones at the base of the SD section indicate that it is a stable carbonate platform environment, the volcanic rocks in the middle of the sequence support a rift tectonic background, and the overlying conglomerates and sandstones are characteristic of an epicontinental sea or marine molasse setting. The rift volcanism made the differences in the fossil content of the SD and WSH sections and led to two sections expose different levels within the Amushan Formation and different process of tectonic evolution. Moreover, the geochemical characteristics and detrital materials of the sandstones show that the provenance and formation of the sandstones were related to the setting of active continental margin. The quartz-feldspar-lithic fragments distribution diagram indicates that the material source for the sandstones was a recycled orogenic belt. Thus, the source area of the sandstones may have been an active continental margin before the late Carboniferous–early Permian. The characteristics of the regional tectonic evolution of the area indicate that the region may form a small part of the Gobi–Tianshan rift of southern Mongolia.