Shallow Marine Carbonate Platform Research Articles

Success and demise of exceptionally preserved terebratulide brachiopod accumulations in a Jurassic (early Pliensbachian) tropical lagoonal setting (Southern Alps, Italy): brachiopod response to environmental changes

During the Early Jurassic, the shallow marine carbonate platforms of the western-Tethys margins were characterized by highly diverse benthos including larger foraminifera, sponges, bivalves, gastropods, brachiopods, echinoderms, and dasycladalean calcareous algae. In this paper, we document examples of such assemblages within the lower Pliensbachian part of the Rotzo Formation (upper Orbitopsella Zone) of the Southern Alps, Italy. This carbonate succession was deposited in a complex mosaic of marine and brackish habitats within a tropical lagoon of the Trento Platform area. Large terebratulide brachiopod shells form autochthonous accumulations comprising exceptionally well-preserved monospecific assemblages of Lychnothyris rotzoana. These brachiopod-bearing successions were analysed in terms of biotic components, microfacies analysis, shell biofabric (three-dimensional arrangement of skeletal elements), and taphonomic signatures to understand brachiopod response to changing conditions within a highly variable lagoonal palaeoecosystem. Findings show that terebratulide shell accumulations are dominated by adult specimens and juveniles are rare. The brachiopods thrived during low energy conditions that resulted in the accumulation of highly temporally-condensed shell beds. Stabilized by microbialite encrustations, the shells were not re-oriented during the subsequent rapid burial. The abrupt demise of these communities was possibly related to rapid environmental change, and causal factors are discussed. The medium-term response of brachiopods to the relatively instable ecosystem of the tropical lagoon shows that they were not able to adapt to continuous perturbations, and that continuing stress severely compromised the resilience of benthic taxa.

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

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

Evidence of a Cenomanian shallow-marine carbonate platform in the northwestern margin of the Bortziriak-Cinco Villas massif, eastern Basque-Cantabrian Basin

Basin margin evolution of rift basins is not always well constrained. In the northeastern margin of the Basque-Cantabrian Basin, a revision of a carbonate succession permits deciphering the transition from synto post-rift phases in the poorly studied northwestern margin of the Bortziriak-Cinco Villas massif. The sedimentological analysis shows three main lithofacies, attributed to a shallow-marine, non-rimmed carbonate platform. On the other hand, biostratigraphic analysis based on the presence of Caprina adversa rudist indicates a Cenomanian age for the succession. Thus, this study illustrates the existence of a Cenomanian shallow-marine carbonate platform unconformably overlying the Palaeozoic massif. This platform development was the result of a generalized transgression during the early post-rift stage.

Tropical ocean temperatures and changes in terrigenous flux during the Paleocene-Eocene Thermal Maximum in southern Tibet

The Paleocene-Eocene Thermal Maximum (PETM, ∼56 Ma) was an extreme climate warming event of the early Cenozoic. The development and ecology of carbonate platforms are highly sensitive to changes in SSTs, sea level and terrestrial detrital input. However, no record of SST changes during the PETM has yet been reported from a shallow marine carbonate platform. Here, we reconstruct SSTs for two shallow marine carbonate PETM sections in present-day southern Tibet from the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGT). Our dataset demonstrates that the temperature of the tropical eastern Tethys likely increased 3 to 5 °C during the PETM (depending on the TEX86 calibration used), and it supports a reduction of 2 °C in the meridional gradient between middle and low latitudes. Elemental data and quartz content imply an increase in terrigenous flux prior to the onset of the CIE, possibly linked to initial PETM warming. However, inconsistent changes in terrigenous flux at other PETM sections in the eastern Tethys suggest a variable response modulated by sea-level change and hinterland weathering, erosion and sediment delivery.

The Paleocene Hangu formation: A key to unlocking the mysteries of Paleo-Tethys tectonism

The sedimentary rocks of the Paleocene Hangu Formation in the Yadgaar Section of the Upper Indus Basin in northern Pakistan, have been the subject of an integrated field, petrographical, and sedimentological investigation. The goals of this study are to improve our understanding of the sedimentary environment, facies shifts, and the impact of tectonism on the genesis of the investigated sedimentary facies. A better understanding of the intricate relationships between the aforementioned factors will clarify whether the regional tectonic drive has partial control over or complete command of the sedimentation processes. The results of this study shows that the Hangu Formation consists of four facies: bauxite, sandstone, coal, and limestone. The bauxite deposits formed in a karst environment with severe chemical weathering in a humid to extremely humid climatic setting. Thin coal laminae indicate a peatland environment that formed within a humid tropical climate. Sub-arkose, arenite, and arkose sandstone facies mark deltaic (sub-humid), coastal–near shore (humid to hot-humid), and high-energy fluvial (arid to semi-arid) sedimentary environments, respectively. Finally, the occurrence of marly limestone points towards deposition on a shallow marine carbonate platform within a coastal-brackish environment. The facies shift of the sediments provides evidence for a gradual transition from continental to marine conditions within the study area, together with episodic transgressive and regressive cycles as well as changing climatic and geomorphological conditions. In consequence, all these changes are controlled and shaped by the effects of Paleo-Tethys tectonism during the Indo-Eurasian intra-oceanic subduction. This advancement through the current work helps in understanding tectonic-sedimentary mechanics, i.e., how regional tectono-sedimentological processes influence the formation of sedimentary sequences.

Controls on East Shale Basin reservoir distribution within the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

The Duvernay Formation accumulated as an organic-rich basinal mudrock concurrent with shallow marine platform carbonates of the Leduc and Grosmont formations. Historically classified as a major source rock to conventional hydrocarbon production, the Duvernay evolved into an unconventional shale reservoir across Alberta, Canada much like other source rock intervals worldwide. Distributions of the Duvernay Formation are partitioned into the West and East Shale basins by a narrow, linear Leduc Formation reef complex known as the Rimbey-Meadowbrook trend. Since 2011, development has focused on the West Shale Basin, but thermal maturity trends suggest the potential for expanded shale reservoir development within the southern portion of the East Shale Basin. This study characterizes sedimentologic and stratigraphic controls on Duvernay reservoir potential to identify development “sweet spots” within the East Shale Basin. Duvernay geologic attributes mapped within this study include: oil thermal maturity, thick restricted basin facies association occurrence (at least 5-10m thick), high average TOC values (greater than 2.0 wt.%), and high net carbonate thickness (greater than 40m). The geologic attributes are predictive of production potential within horizontal wells, and the distribution of their co-occurrence suggests the potential for expanded development within the southern portion of the East Shale Basin.

Shallow marine carbonates as recorders of orbitally induced past climate changes – example from the Oxfordian of the Swiss Jura Mountains

Abstract. Today and in the geologic past, climate changes greatly affect and have affected Earth surface processes. While the climatic parameters today can be measured with high precision, they have to be interpreted from the sedimentary record for ancient times. This review is based on the detailed analysis of stratigraphic sections of Oxfordian (Late Jurassic) age, with the aim to reconstruct and discuss the climate changes that controlled the sedimentation on the shallow marine carbonate platform that today is represented in the Swiss Jura Mountains. The sediments formed under subtropical conditions in which carbonate-producing organisms proliferated, and ooids and oncoids were common. The sections are composed of hierarchically stacked elementary, small-scale, and medium-scale depositional sequences wherein facies changes imply deepening–shallowing trends. The major sequence boundaries Ox 6, Ox 7, and Ox 8 can be correlated with those of other European basins and place the studied sections in a broader framework. The chronostratigraphic tie points imply that the medium- and small-scale sequences formed in tune with the orbital eccentricity cycles of 405 and 100 kyr, respectively, and the elementary sequences with the precession cycle of 20 kyr. Orbitally controlled insolation changes at the top of the atmosphere translated into climate changes: low insolation generally resulted in low amplitudes of sea level fluctuations at the 20 kyr frequency and in a cool and humid climate at the palaeolatitude of the Jura platform. Terrigenous material was eroded from the hinterland and distributed over the platform. High insolation led to sea level rise, as well as to warm and semiarid to arid conditions in which coral reefs could grow. However, nutrient input favoured growth of microbialites that encrusted the corals. The reconstruction of high-frequency sea level fluctuations based on facies analysis compares well with the curve of insolation changes calculated for the past 550 kyr. It is therefore assumed that the sea level fluctuations were mainly due to thermal expansion and retraction of ocean surface water. Two models are presented that explain the formation of elementary sequences: one for low and one for high insolation. Despite the important lateral facies variations typical of a shallow marine platform, and despite the uncertainties in the reconstruction of sea level changes, this study demonstrates the potential of carbonate ecosystems to record past climate changes at a time resolution of 20 000 years. Relatively short time windows can thus be opened in the deep geologic past, and processes and products there can be compared with those of the Holocene and the Anthropocene. For example, it appears that today's anthropogenically induced sea level rise is more than 10 times faster than the fastest rise reconstructed for the Oxfordian.

Uppermost Triassic Halstätt-like cephalopod limestone (Lilu Facies) and Foraminifera, Timor-Leste

Ammonoid-rich wackestone beds forming a ca 2 m section between shallow-marine limestones of the Bandeira Group (Norian to possibly Rhaetian) and a wackestone–calcareous mudstone succession belonging to the Lower Jurassic Wailuli Group are recorded from the western part of Timor Leste. They are included in the Halstätt-like Lilu Facies of the Bandeira Group. Age-diagnostic involutinid foraminiferal species, not previously recorded from Timor, include Involutina liassica, Trocholina sp. ex. gr. Trocholina turris and Lamelliconus permodiscoides, which collectively indicate a Rhaetian age, possibly ranging down to the Sevatian. The conodont Norigondolella steinbergensis, recorded low in the section, is consistent with this age determination. The Lilu Facies was deposited at a water depth of a few tens of metres, as suggested by the presence of extensive cyanobacterial borings into shell fragments and the presence of microbial laminations including the presence of stromatolites and bacinellid-like fabrics. Among macrofossils, common ammonoids are present in the section, and marine reptile bones are conspicuous in the lower beds. Other biogenic components of the rock observed in acetate peels and thin sections are abundant echinoderm debris (including pentacrinoid columnal plates), bivalve fragments and micro-gastropods, rare siliceous sponge spicules, solitary corals, and brachiopod debris. In the studied region, a major drowning of a shallow-marine carbonate platform took place probably in the Rhaetian or possibly latest Norian. Similar ‘ammonitico rosso limestone’ interbedded with thick-bedded limestones of a shallow-marine carbonate platform is present in the uppermost Triassic of Seram. More detailed biostratigraphical comparisons and correlations of similar units in Timor and other nearby islands may indicate a general marine drowning of Late Triassic carbonate platforms, at least in the northern part of the East Gondwana Interior Rift system, during the Rhaetian or possibly latest Norian.

Palustrine Sediments between Two Isolated Shallow Carbonate Platforms (Aptian–Albian Transition, Prebetic of Jaén, South Spain)

During the Aptian-Albian transition, an extensional phase of the Central Atlantic which affected the Prebetic carbonate platform (South Iberian Continental Margin, northwestern margin of the Tethys) occurred. A graben morphology was developed in the platform coeval to a relative sea level fall. As a consequence, palustrine facies characterized by rhizoliths and some pond deposits of black lutites were established. Over these palustrine sediments, a second shallow carbonate platform was built during the early Albian. However, this process was not abrupt, as several levels with orbitolines and rudists were deposited intercalated between the continental facies, recording the transition to a new shallow marine carbonate platform developped during the Early Albian. The presence of these continental palustrine sediments between two episodes of shallow carbonate platform is described for the first time in the Prebetic. The demise of an upper Aptian isolated shallow carbonate platform drove to the deposition of these palustrine sediments in an extensional tectonic regime.

Two pulses of increasing terrestrial input to marine environment during the Permian–Triassic transition

The destruction of both terrestrial and marine ecosystems generally shows a process of rapid extinction followed by the wipe out of the Permian-type relics in the Permian–Triassic mass extinction, but the relationship between the terrestrial and marine ecosystem collapses is not clear until now. The terrestrial input from the land to marine system is an important link between them. Previous study has demonstrated a substantial increase of terrestrial influx to the marine water most likely corresponding to the Permian–Triassic mass extinction. Our new geochemical data from the Laolongdong section in the shallow marine carbonate platform of South China, reveal that the elevated terrestrial input nearby the Permian–Triassic Boundary (PTB) is actually marked by a two-pulse scenario. The first pulse represents a ~ 3- to ~ 6-fold increase of the terrestrial flux. The onset of the increase took place in the extinction horizon at the basal part of microbialites at Laolongdong, basically corresponding to Bed 24a of Meishan section and prior to the end-Permian mass extinction zenith; then the increase continued and spanned the extinction zenith. The second pulse is characterized a ~ 21- to ~ 35-fold increase in the terrestrial flux, equivalent to the widespread, abundant presence of mudstones in the earliest Triassic and roughly corresponding to the stage where the Permian-type survivors were thoroughly wiped out. A two-pulse terrestrial input is also clearly recorded at Cili, Shangsi and Meishan of South China, western Australia and Arctic Canada. Two episodes of the enhanced terrestrial input provide strong evidence for the devastation of the terrestrial ecosystem and insights into the reasonable explanation for the relationship between terrestrial and marine ecosystem collapses nearby the PTB. • Geochemical data reveal a two-pulse terrigenous input nearby the PTB. • The first pulse was linked to the end-Permian terrestrial mass extinction, as a prelude of the marine mass extinction. • The second pulse terminated microbialite, linked to the devastating of vegetation and soil erosion in the Early Triassic. • The enhanced terrestrial inputs provide insights into the relationship between terrestrial and marine ecosystem collapses.

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

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

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

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

Paleoclimate and sea level response to orbital forcing in the Middle Triassic of the eastern Tethys

The Middle Triassic is thought to have had a greenhouse paleoclimate with a few short humid phases. However, the timing of these humid events, and the extent to which orbital forcing influenced the evolution of climate, are unclear. Here, a cyclostratigraphic analysis has been carried out based on the integrated study of magnetic susceptibility, elemental chemistry and lithofacies from two shallow-marine carbonate platform sections in Guizhou, South China. Combined, these sections (Pohong and Yongyue) span the late Anisian to early Carnian. Spectral analyses of magnetic susceptibility, Fe/Al and P through the studied sections show significant cycles of ~85-m, ~7-m and ~ 4-m. Based on independent age constraints and sedimentation rate modelling, these cycles are interpreted as orbital cycles that reflect 405-kyr long-eccentricity, ~33-kyr obliquity, and ~ 20-kyr precession forcing, respectively. Tuning of the records to the stable 405-kyr long-eccentricity parameter in each section yields a tuned astronomical time scale spanning ~7.4 ± 0.2 Myr, which covers from ca. 244.4 to 237.0 Ma and includes the entire Ladinian Stage. Magnetic susceptibility and Fe/Al data from the studied sections show intervals with relatively high values in the middle-late Anisian (ca. 244.0 to 242.6 Ma), and the latest Anisian to early Ladinian (ca. 241.8 to 240.9 Ma). These geochemical changes, combined with lithological changes, indicate enhanced terrigenous flux caused by the acceleration of hydrological circulation under a warmer and more humid climate. In addition, P data records two sharp falls that appear broadly coeval with the previously postulated late Ladinian ecological crisis. Our new astrochronology also reveals that ~1.2-Myr obliquity amplitude modulation cycles are expressed in the Middle Triassic. The regional sea level changes inferred from sequence stratigraphy and sedimentary noise modelling can be correlated with Tethyan and global sea level change. This work demonstrates the utility and power of sedimentary noise modelling in shallow marine environments for inferring past sea level dynamics. It also furthers constrains the timing of putative Middle Triassic climatic and ecological events.

A diverse gastropod fauna from the shallow marine carbonate platform of the Yangou section (south China) in the immediate aftermath of the Permian–Triassic mass extinction

The Permian–Triassic mass extinction (PTME), the most devastating extinction event in Phanerozoic history, has witnessed the demise of almost 80% of marine gastropod genera. Despite the vast scale of the extinction event, the recovery of some groups of marine gastropods has been curiously fast, contrasting with the delayed recovery of many other marine clades. Understanding the rapid recovery of gastropods after the PTME represents a major palaeobiological conundrum, since oxygen‐dependent taxa are expected to have been severely impacted by anoxic oceanic conditions of the PTME. The controversial microbialite refuge hypothesis has been proposed as a potential explanation of this pattern, arguing that marine invertebrates were associated with oxygen‐producing microbial mats during the PTME, which provided a suitable microhabitat for survival within hostile anoxic conditions. To better understand the dynamics of gastropod recovery after the PTME, we studied a gastropod fauna collected from the basal Daye Formation (earliest Triassic) at the Yangou section in Leping County, Jiangxi Province, south China, allowing us to capture a snapshot of the marine fauna immediately after the PTME. Altogether, 13 microgastropod species belonging to 12 genera were recorded, representing the most diverse Induan (earliest Triassic) gastropod fauna in the world known to date. The fact that the Yangou assemblage, along with the second most diverse Griesbachian gastropod fauna from the Wadi Wasit section in Oman, is not associated with microbial buildups implies that microbial mats probably did not play a key role as refuges for marine invertebrates during the PTME. On the other hand, the Yangou fauna suggests that non‐anoxic shallow lagoon systems were more likely to have acted as refuges for marine invertebrates during the PTME.

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

The Eocene Bryozoans of Bekrit-Timahdite Formation, Middle-Atlas, Morocco: Taxonomy, Paleoenvironments and Paleoclimates Implications

Located in the North-West of the African continent, the Moroccan Middle Atlas constitutes an intra-continental chain within the Atlas domain. It is known by several global geological events such as the Paleocene-Eocene crisis during the Paleogene, which corresponds to one of the hottest periods on earth with Paleocene-Eocene thermal maximum (PETM). A taxonomic study carried out on the carbonate deposits of the Bekrit-Timahdite Formation from three sections of the Bekrit syncline allowed the identification of 5 species of the Cheilostome and Cyclostome Bryozoans (Cellaria rigida, Cellaria sinuosa, Aimulosia lamellosa, Heteropora and Osthimosia) in bioclastic limestones, nodular limestones, marly and sandy limestones. The interest of Bryozoans as indicators of paleoenvironments and paleoclimates was assessed by examining current communities, in terms of depth evolution as richness and abundance, distribution of taxonomic categories zoarium types, and zooidal morphology. The depositional environment analysis of Bryozoans showed a shallow marine carbonate platform with low sedimentation energy where is registered a relatively warm paleoclimate.

Application of stable isotopes, trace elements and spectral gamma-ray log in resolving high-frequency stratigraphic sequences of a mixed carbonate-siliciclastic reservoirs

Integrated methods have been applied for the high resolution sequence stratigraphic analysis of a mixed petroliferous carbonate-siliciclastic succession of the Asmari Formation (Oligocene-Miocene) in the Zagros Basin. Detailed sedimentological (core description) and petrographical (one thousand one hundred fifty thin sections) analysis led to identification of more than 38 carbonate and siliciclastic facies (basin wide) that have been deposited in four types of depositional environments, including inner ramp, mid ramp, outer ramp and basin, in homoclinal ramp type carbonate platform. An integrated multidisciplinary approach including core descriptions, petrography, biostratigraphy, gamma-ray spectrometry (e.g., uranium (U), thorium (Th) and Potassium (K)), oxygen and carbon isotopes (Chemo-stratigraphy) and trace element analysis have been used for stratigraphic interpretations of this succession. Based on petrography, biostratigraphy, lateral and vertical facies changes, and GR log, 6 third-order cycles distinguished, whose boundaries correspond to distinct δ13C and δ18O negative peaks. Trace elements (Fe and Mn) contents along with oxygen and carbon isotopes variations are compared with distribution of sedimentary facies and depositional sequences to investigate and correlate stratigraphic boundaries, which are determined by sequence stratigraphy (GR log and facies analysis) and stable isotope stratigraphy (Chemo-stratigraphic) in shallow marine carbonate platform of the Asmari. Negative isotope and elemental peaks coinciding with the sequence boundaries while the positive isotope and elemental peaks are correlated with maximum flooding surfaces. Changes in the frequency of uranium (U), thorium (Th), and Potassium (K) has been used to investigate the stratal surface and sequence boundaries as well as oxidation and reduction states. Fourth-order sequences that formed during the formation of the third-order sequence could be distinguished and separated by uranium log.

Evaluating the role of coastal hypoxia on the transient expansion of microencruster intervals during the early Aptian

Worldwide, a growing number of modern coastal marine ecosystems are increasingly exposed to suboxic- or even anoxic conditions. Low seawater oxygen levels trigger significant ecosystem changes and may result in mass mortality of oxygen-sensitive biota. The applicability of observations from recent (anthropogenically influenced) suboxic coastal settings to fossil anoxic shallow-marine environments is, however, as yet poorly explored. The test case documented here are upper Barremian to lower Aptian strata in the Lusitanian Basin (Ericeira section, Portugal). These are characterized by the transient demise of rudist–coral communities and the rapid establishment of microencruster facies in the vacant ecological niches. The hypothesis is tested that the temporal expansion of the microencrusting organism Lithocodium aggregatum took place in response to platform-top seawater oxygen depletion. We critically discuss the outcome of a multi-proxy palaeoseawater redox approach (e.g. Rare Earth Elements (REEs), U isotopes and palaeoecology) and put the robustness of the proxies applied here to the test. This is done by considering issues with these methods in general but also emphasizing the significance of terrigenous contamination and fractionation effects. Data shown here document that evidence for coastal seawater oxygen depletion in the prelude of Oceanic Anoxic Event (OAE) 1a is lacking, and hence, anoxia was not the driving mechanism for the demise of rudist–coral ecosystems in the proto-North Atlantic platform setting studied here. In contrast, well-oxygenated early Aptian platform-top water masses are proposed for this site. Geologically short (decades to millennia) fluctuations in seawater oxygen levels cannot be excluded, however. But even if these took place, they offer no explanation for the Kyr to Myr-scale patterns discussed here. The present paper is relevant as it sheds light on the complexity of mechanisms that drive punctuated Early Cretaceous coral–rudist ecosystem turnover, and assess strengths and weaknesses of redox proxies applied to ancient shallow-marine platform carbonates.

Lower Jurassic benthic foraminiferal assemblages from shallow-marine platform carbonates of Mallorca (Spain): stratigraphic implications

The stratigraphic distribution of larger benthic foraminifera and other microfossils, as analysed in thin-sections, has been investigated in three stratigraphic sections from the island of Mallorca (Son Maina, S’Heretat and Cuevas de Arta). These stratigraphic sections represent deposition in a Lower Jurassic shallow-water carbonate platform succession (Es Barraca Member, Soller Formation) of the Llevant Mountains. The microfossil assemblages contain abundant benthic foraminifera and calcareous algae, including the typical Liassic dasycladalean alga Palaeodasycladus mediterraneus (Pia) and the microproblematica Thaumatoporella parvovesiculifera (Raineri), which provide good age constraint of the succession. Three consecutive biozones have been recognized based on the stratigraphic distribution of foraminifera, including the evolutionary bioseries Mesoendothyra—Lituosepta—Orbitopsella spp., which are documented for the first time in the Balearic Basin: Biozone A (interval Zone) is characterized by the occurrence of small Siphovalvulina spp., Mesoendothyra sp. and rare Lituosepta ancestors. Biozone B (lineage Zone), whose lower boundary is defined by the first occurrence of Lituosepta recoarensis Cati and its top by the first appearance of Orbitopsella aff. primaeva (Henson). The uppermost part of biozone B shows an interval with transitional morphotypes between Lituosepta and Orbitopsella, indicating that the change between biozones B and C1 is gradual (B/C1 transition). This interval is characterized by the presence of very primitive forms of Orbitopsella. Finally, Biozone C1 (lineage Zone) is defined by the first occurrence of Orbitopsella aff. primaeva together with L. recoarensis. The upper boundary of the Biozone C1, which is defined by the first occurrence of O. praecursor s.l., is not recorded in the studied sections of the Es Barraca Member. This biostratigraphic zonation is consistent with a Sinemurian age for the Es Barraca Member with its top most likely not extending into the Pliensbachian. The proposed biostratigraphic scheme is comparable with those established for other western Tethyan margins such as the High Atlas of Morocco, the Southern Italian Alps or the Dinaric Alps of Slovenia and Croatia. The reconstruction of a depositional transect across the studied sections shows the progressive loss of the upper biozones towards the northeast, demonstrating the existence of significant hiatuses in some sections of the Llevant Mountains domain. These hiatuses point to an intra-lower Pliensbachian (Carixian) early stage of platform fragmentation, with areas affected by erosion or non-deposition, and areas with marine marly limestone sedimentation followed by deltaic siliciclastic progradation on the north-western part of Mallorca.

Palaeontological framework from Pirabas Formation (North Brazil) used as potential model for equatorial carbonate platform

Abstract The Pirabas Formation (early to middle Miocene) from the equatorial margin of North Brazil is characterized by a shallow-marine carbonate platform with high fossil diversity and abundant micro- and macrofossil remains. The Pirabas Formation represents a unique carbonate system along the Atlantic margin of South America that developed before the onset of the Amazon delta. We studied the palaeontology and lithofacies of outcrops of the uppermost Pirabas Formation and found that was deposited in a coastal marine environment with marginal lagoons under the influence of a tidal regime and tropical storms. The remains of calcareous algae, molluscs, crustaceans, echinoiderms, bryozoans, solitary corals, fish and marine mammals, together with foraminifera, ostracods and other marine microfossils, shaped a biogenic framework, that together with the post-depositional processes of dissolution of skeletal grains, is responsible for the mean packstone-floatstone porosity of 14.9%. The palaeontological framework and the petrophysical characterization of the carbonate rocks from the uppermost Pirabas Formation outcrop represent a baseline to interpret the entire Pirabas Formation in the subsurface stratigraphic sections (cores) of this important Neogene unit. Considering that carbonate rocks account for ~50% of oil and gas reservoirs around the world, this research provides a model for Neogene tropical carbonate deposits useful for carbonate petroliferous reservoirs in the Brazilian equatorial basins.