Carbonate Ramp Research Articles

The Diagenesis Processes of the Hartha Formation in Majnoon Oil Field, Southern Iraq

The Hartha Formation is the shallowest of the major Majnoon oil field reservoirs. It is a 120 m thick carbonate platform deposit with an age of the Upper Campanian - Maastrichtian. It overlies the regional top of the Sa'di Formation unconformity and is in turn overlain by the Shiranish Formation. This research concentrated on diagenetic processes and petrophysical properties in the Hartha Formation in the Majnoon Oilfield, located in southern Iraq. The Hartha Formation was deposited on a carbonate ramp setting. It is subdivided into four main zones (A1, A2, B1, and B2), where A2 is the majority reservoir. Sixity of thin sections were made from several wells (MJ-29, MJ-47, MJ-88, MJ-91, MJ-92, and MJ-93). This formation was influenced by a number of digenetic processes, such as neomorphism, cementation, dolomitization, dissolution, and compaction. Dolomitization, dissolution, and cementation are the three main diagenetic processes in the current study. Detailed petrographic analysis of the dolomitization indicates that the dolomite formed is of two types (pervasive and selective) in the formation. The formation is affected by tectonic subsidence which is main controlling factor that achieved the paleogeography of the study area. This is produced different thicknesses of the formation along the structure, and the facies that formed the formation varied as a result of the variation of the relative sea level.This study aims to determine the extent of the impact of diagenetic processes on the porosity of the formation and thus impacts on the quality of the formation.

Integrated Litho-, Chemo- and Sequence Stratigraphy of the Ediacaran Gametrail Formation Across a Shelf-Slope Transect in the Wernecke Mountains, Yukon, Canada

The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE.

The Eocene carbonate platforms of the Ghomaride Domain (Internal Rif Zone, N Morocco): a segment of the westernmost Tethys

The Eocene platform deposits in the Moroccan Ghomarides have been studied. These marine carbonate platforms were located in the westernmost Tethys approximately 30°N and 0°–10°W during the Cuisian to Bartonian. The study includes observations from fossiliferous assemblages (such as larger benthic foraminifera and colonial corals), their palaeoenvironment as well as rock texture and fabric. Eight microfacies were identified that represent different ramp environments in a ‘distally-steepened carbonate ramp’ type of platform. The studied deposits are organised into a transgressive succession composed of three sedimentary cycles: lower Cuisian, middle Cuisian and middle Lutetian to Bartonian. In the lower cycle, photic inner to mid ramp environments in mesotrophic conditions were prevalent. In the second cycle, photic inner ramp (sea-grass) to mid ramp environments in mesotrophic to oligotrophic conditions were observed. The upper cycle, which is more extensive and variable, represents mesophotic mid ramp to aphotic slope environments and changes gradually from oligotrophic to eutrophic conditions. During the Eocene, larger benthic foraminifera were dominant overtaking the zooxanthellate corals in the Tethys regions. Nevertheless, our study and the performed comparison with other Tethyan sectors have revealed that in some areas both coexisted in similar proportions. In some western Tethys regions close to the Atlantic Ocean, coinciding with areas influenced by upwelling currents, larger benthic foraminifera and coral build-ups were replaced by oyster reefs. The Ghomaride Domain represents an intermediate case between fossil assemblages of the northern Tethyan margin and eastern sector of the southern margin of the Tethys, with a dominance of larger benthic foraminifera but with a certain presence of corals as well. A good correlation exists between Eocene warm intervals and carbonate platform deposits in these domains. Contrarily, during cooling ones shallowing and gaps in the sedimentation are registered. Two anomalies have been detected in the Ghomaride Domain during Ypresian and Bartonian times indicating particular climatic conditions or local tectonic interferences.

Geochemistry of the Upper Cretaceous - Miocene strata in the Tekman-Karayazı Basin (Turkey): Tectonic processes, sedimentation patterns, and the source rock quality along the northern branch of the Neo-Tethys Basins and northeastern Arabian plate

Tekman-Karayazı Basin (TKB) is one of the Eastern Anatolia basins which is located in the south of the Izmir-Ankara-Erzincan Suture Zone and is characterized by sedimentary units ranging in age from Upper Cretaceous to Pliocene. The basin began to form in Upper Cretaceous (Maastrichtian) in a tectonic depression, which over time evolved into a collisional foreland basin. While the Jurassic and Cretaceous Oceanic Anoxic Event (OAE) rocks were preserved in the NE of the Arabian Plate (SW of Iran), they are represented by an ophiolitic mélange which was formed as a result of the closure of the northern branch in the TKB and other basins on the Anatolian microplate. Four tectono-sedimentary phases are recognized over the ophiolitic mélange: Phase I (Maastrichtian-Paleocene (?), after obduction) is comprised of submarine fan deposits. Phase II (Paleocene - Lower Eocene, obduction phase) is characterized by major changes in gradually depositional environments in the TKB (clastics-nummulitic limestone), and continental to ramp carbonate to slope facies in others (e.g., Yakacık-Orhaniye, Haymana-Polatlı). Phase III (Middle – Upper Eocene, ongoing collision phase) shows the development of turbiditic and continental facies show that both TKB and/or the coeval basins have poor source rock potential. Phase IV (Oligo-Miocene, ending collision) represents from eastern to central in eastern Anatolian basins alluvial-fluvial to lacustrine and marine units during Oligocene – Miocene, respectively. The geochemical correlation between the bitumen extracted from sandstone sample and the Ağçakoca shales suggests that this formation can be considered as the corresponding source rock unit; however, according to the Tmax values (364–444 °C, mean: 428 °C), this formation is mostly immature, a scenario supported by the dry wells in the region. The regional volcanic activities triggered in Miocene by continent-continent collision seems to have provided the heat which, in turn, caused the thermal cracking of kerogen in Ağcakoca shales and locally induced petroleum expulsion. Mixing with the underground water on its way to reach a reservoir rock, the generated oil formed the oil seep. Therefore, we propose that a speculative Mescitli-Haneşdüzü (?) petroleum system may exist in this basin. We suggest that the combination of factors such as the asymmetric closure of the Tethys Ocean and the escape tectonics of the Anatolian Plate gave rise to the deposition of Paleocene to Miocene organic matter-rich shales in the NE of the Arabian plate, whereas uplift, antecedent topography, and the global paleo-climatic events disturbed the favorable conditions and caused the studied basins to lack the aforementioned generative source units.

Early Ediacaran Positive C-Isotope Anomaly in Limestones of the Chernaya Rechka Formation of the Igarka Uplift (Northwestern Siberian Platform)

The Precambrian Chernaya Rechka Formation (Igarka Uplift) hosts a high-amplitude positive carbonate carbon isotope anomaly (up to 12.4‰) spanning through 500 m of the section and occurring simultaneously with a rise of δ13Сorg values. The similar trends of carbon isotope variations in the carbonate fraction and organic matter are not caused by local sedimentary environments since the studied anomaly-bearing carbonates accumulated in different zones of the carbonate ramp and shallow shelf. Oxygen isotope composition of these carbonates and some other geochemical criteria indicate minimal (if any) impact of post-sedimentary processes on preservation of the isotope systems. Concentrations of trace elements in the carbonate fraction indicate alternating anoxic and oxic environments that did not affected the carbon isotope composition during accumulation of the Chernaya Rechka Formation. The profound positive δ13С anomaly was putatively caused by a global deficiency of 12С in the paleo-ocean related to accumulation of methane hydrates and burial of non-oxidized organic matter. Together with geochronological and stratigraphic data, minimal 87Sr/86Sr values (0.7074) in the Chernaya Rechka Formation reveal the lower Ediacaran (lower Vendian) age of the unit (635–580 Ma). Among the closest stratigraphic analogues of the Chernaya Rechka Formation are the Dal’nyaya Taiga Group (Patom Basin) and coeval stratigraphic sequences in the southern Siberian Platform. Global nature of the positive δ13С anomaly provide its correlation with other coeval C-isotope events worldwide.

High-Resolution Sequence Stratigraphy of the Mauddud Formation Southern Iraq

Inner ramp carbonate and dolomitic limestone made up the late Albian Mauddud reservoir in Iraq. An age estimate for the Mauddud Formation is late Albian to early Cenomanian, with it overlying the Nahr-Umr Formation and underlying the Ahmadi Formation. The integration of microfacies with wireline logging data to predict the high resolution of the sequence stratigraphy framework. The microfacies description included more than 150 thin sections that covered the study area in southern Iraq. That results in five main microfacies. There are divided into eleven submicrofacies, as a result, for the microfacies, the depositional environment is gradual, from mid-ramp to shallow open marine, shoal-restricted lagoon. The study divided the formation into four orders of sequence stratigraphy: MSL1, MSL2, MSL3, and MSL4. The study includes identifying the maximum flooding surface (K110), which is identified as the original transgression in sea level that covered Arabian Plate. MSL1 represents the lower part of formation and evidence of environmental change to shall the marine environment with argillaceous limestone. The maximum flooding surface (K110) is signifying the deepest depositional environment (mid-ramp). Also, the study marked three marine flooding surfaces by changes as increased gamma ray and density logs values, whilst the highstand system reflects decreasing in logs values, This change was used in conjunction with microfacies data to construct the sedimentological environments that led to the documentation of the Mauddud high-resolution sequence stratigraphy framework.

Spatial complexity of burrow attributes and their impact on porosity and permeability distributions in bioturbated reservoirs

Bioturbated carbonate rocks can form high-quality aquifers and reservoirs. Although the controls on the porosity and permeability of these rocks are fairly understood at the well-bore scale, how such characteristics vary spatially across various scales of observation is still a major unsolved problem. This study integrates results of outcrop observations, imaging with scanning electron microscopy (SEM), computer tomographic (CT) scanning, petrographic analysis, and petrophysical measurements to understand how burrow intensity and burrowing style control the spatial variability of porosity and permeability of one stratum accumulated in a carbonate ramp. The CT scan data revealed two burrowing styles, pipe framework (in the upper part of the stratum, surface constrained textural heterogeneity) and bio-retexturing (in the lower part of the stratum, non-surface-constrained textural heterogeneity). The studied stratum is dominated by the micropores (cross-section>10 μm) of the host rock matrix (HRM), which comprise >50 % of the total porosity within the stratum. Porosity in the stratum does not vary substantially in vertical or lateral trends. In contrast, and perhaps most importantly, permeability does vary vertically and laterally depending on the scale of measurement, burrow percentage, and burrowing style. These results suggested that the storage capacity of the studied stratum is independent of bioturbation, whereas the flow capacity is strongly dependent on bioturbation. There is a substantial vertical change in permeability in the stratum in comparison to only small changes in the lateral direction across the layer of interest. The data also suggested that the subtle lateral change in permeability is controlled by bio-retexturing, which resulted in the homogenization of grain-dominated and mud-dominated sediments by bioturbation. Such a process can be destructive for the permeability compared to the pipe-connected burrowing style. We clearly show that the ichnologically-controlled porosity and permeability trends of the studied stratum are relevant at the field scale. Understanding such trends in the context of paleoenvironmental conditions provides insight into the fluid flow quality of bioturbated carbonates, which constitute a large proportion of water aquifers and hydrocarbon reservoirs globally.

Integration of geoscience data to delineate quality of the Asmari reservoir, Iranian part of the Persian Gulf basin

The Hendijan Field is an asymmetric structure located in the northwestern Persian Gulf and consists of several oil and gas-bearing intervals. Oligocene to Miocene successions in the field host large volumes of hydrocarbons. The Asmari Formation comprises the mixed siliciclastic and carbonate members formed in a shallow marine carbonate ramp depositional environment. The siliciclastic part displays the best reservoir quality within the Asmari Formation. Through petrophysical, sedimentological, and geophysical analysis, this study divides the Asmari Formation into six, third-order depositional sequences. By doing so, it clarifies the relationships between depositional sequences and reservoir characteristics, particularly porosity and water saturation. Detailed petrographical analysis, core description, well log analysis, and seismic interpretation distinguish eight microfacies, two petrofacies, and six third-order sequences within the Asmari Formation. 3D reservoir-property models are developed that incorporate elements of the sequence stratigraphical framework. Within the six identified sequences, high reservoir quality zones are associated with AS-4 and AS-5, which comprise mostly grain-supported lithologies associated with shoal facies. The supratidal, intertidal, and lagoonal facies of AS-2, which are often associated with the occurrence of anhydrite and mud-supported fabrics, reflect lower reservoir quality. Hydraulic flow units (HFU) are delineated to assist in characterizing reservoir heterogeneity and porosity/permeability prediction. Five HFU were identified by applying the flow-zone-indicator method to evaluate reservoir quality. Both the transgressive system tract (TST) of AS-2 and early TST plus late highstand system tract (HST) of AS-6 contain elements of HFU-A. The most productive units of AS-4 and AS-5 belong to HFU-E.

Warm, not cold temperatures contributed to a Late Miocene reef decline in the Coral Sea

Evidence shows that in the modern ocean, coral reefs are disappearing, and these losses are tied to climate change. However, research also shows that coral reefs can adapt rapidly to changing conditions leading some researchers to suggest that some reef systems will survive future climate change through adaptation. It is known that there were changes in the area covered by coral reefs in the past. Therefore, it is important to investigate the long-term response of coral reefs to environmental changes and high sea-surface temperatures (SSTs). However, because of diagenetic issues with SST proxies in neritic, metastable carbonate-rich environments, there is an incomplete and sometimes even incorrect understanding of how changes in SSTs affect carbonate reef systems. A good example is the Queensland Plateau offshore northeast Australia next to the threatened Great Barrier Reef. In the Late Miocene, between 11 and 7 Ma, a partial drowning caused the reef area on the Queensland Plateau to decline by ~ 50% leading to a Late Miocene change in platform geometry from a reef rimmed platform to a carbonate ramp. This reef decline was interpreted to be the result of SSTs at the lower limit of the modern reef growth window (20–18 °C). This article presents a new Late Miocene—ased SST record from the Coral Sea based on the TEX86H molecular paleothermometer, challenging this long held view. Our new record indicates warm tropical SSTs (27–32 °C) at the upper end of the modern reef growth window. We suggest that the observed temperatures potentially exceeded the optimal calcification temperatures of corals. In combination with a low aragonite supersaturation in the ocean, this could have reduced coral growth rates and ultimately lowered the aggradation potential of the reef system. These sub-optimal growth rates could have made the coral reefs more susceptible to other stressors, such as relative sea-level rise and/or changes in currents leading to reef drowning. Given that these changes affected coral reefs that were likely adapted to high temperature/low aragonite saturation conditions suggests that reefs that have adapted to non-ideal conditions may still be susceptible to future climate changes due to the interaction of multiple stressors associated with climate change.

Facies Architecture And Depositional Marine Systems of the Yamama Formation in Selected Wells, Southern Iraq

The Yamama Formation is characterized by a wide geographic extension of southern Iraq. Microfacies analysis of this formation was studied in six wells distributed in six fields: Fayhaa, Sindbad, Siba, Zubair, Ratawi and West Qurna. This research aims to determine paleoenvironments by diagnosing biofacies and lithofacies. Miscellaneous marine fauna of foraminifera and calcareous algae, mainly green algae (dasycladacean.) and skeletal bioclasts from gastropods, pelecypods, bryozoans, sponge spicules, and echinoderms were found. Petrographic studies and well logs interpretations led to the identification of five main Microfacies ( Mudstone, Wackestone, Packestone, Grainestone and Rudstone and twelve submicrofacies (Foraminiferal-Lime mudstone submicrofacies, Argillaceous Lime Mudstone submicrofacies, Planktonic foraminiferal – Lime Wackestone submicrofacies, piculites/ Calcisphers – Lime Wackstone submicrofacies, Benthonic Foraminiferal -Lime wackestone/packestone submicrofacies, Algal (desycladecan) Lime wackestone submicrofacies, Algal-Lime packestone submicrofacies, Bioclastic/ Algal-Lime packestone submicrofacies, Algal/ Foraminiferal- Algal -Lime packstone submicrofacies, Peloidal poorly sorted grainstone with bioclasts submicrofacies, Intraclastic grainstone submicrofacies, Pesudo oolitic-Lithocodium aggregatum grainstone submicrofacies and finally Peloidal-intraclastic, and bioclastic grainstone submicrofacies). Microfacies are deposited in the lagoon, shoal, rudist biostrome and open marine gradient to middle and outer ramp environments. Vertical changes in microfacies with depth were reflected by lateral changes in marine depositional systems and the thickness of the formation. Shoal environments' microfacies are characterized by high thickness, while low thickness characterizes the open marine microfacies. The Yamama Formation was deposited on the low-angle homoclinic carbonate ramp, mainly in the inner and middle ramp, with outer ramp conditions in some parts of the formation.

Palaeogeographic and palaeoclimatic reconstruction of the Permian sediments in the Alborz Basin, Iran: Sedimentological and geochemical approaches

The development of the Permian succession in the Alborz Basin has been dramatically affected by tectonic movement and change in the Cimmerian microcontinent position during the Pangea configuration. The Permian succession in the Alborz Basin consists mainly of siliciclastic-carbonate rocks, and is represented by two rock units: the Doroud Formation (Asselian-Middle Sakmarian) at the base, the Ruteh Formation (Artinskian-Roadian) in the middle, and lateritic horizon (Permian-Triassic boundary) at the top. The Doroud Formation was deposited in fluvial and beach environments. The integrated petrographical and geochemical studies indicate a felsic and intermediate origin for the sandstones of the Doroud Formation that were affected by semi-oxidizing to oxidizing environments under warm-humid climatic conditions. These sandstones, mainly quartz-arenites and sub-litharenites, were deposited in a passive continental margin and mostly sourced from a quartzose sedimentary provenance. The geochemical signatures reflect that the Doroud Formation has Gondwanan characteristics – the siliciclastic nature and deposition in a tectonic quiescence before rifting – so it is presumed that the Neo-Tethys spreading occurred after the deposition of the Doroud. Simultaneous with the deposition of the Ruteh Formation, the sea level was gradually increased, and a carbonate ramp was developed. Meanwhile, the configuration of continents of the Pangea began to change from Irvingian to Wegenerian, and Alborz block, as a part of the Cimmerian microcontinent, was detached from the Gondwana and moved northward through the Palaeo-Tethys Ocean. Furthermore, the Ruteh Formation shows Tethayn characteristics, including a warm climate, the presence of large fusulinid, and the dominance of carbonate sediments. A gradual regression occurred following the deposition of the Ruteh Formation and tectonic uplift in the Permian-Triassic boundary, leading to the formation of a lateritic horizon. Around this time, as the Cimmerian approached Eurasia, the Palaeo-Tethys became smaller, while the Neo-Tethys began to develop considerably.

Oxygen isotopic composition of Paleoproterozoic seawater revealed by clumped isotope analysis of dolomite, Vempalle Formation, Cuddapah, India

The Paleoproterozoic (∼2.0–1.9 Ga) Vempalle Formation, in the south-western part of the Cuddapah Basin, is a well-studied Paleoproterozoic succession. Detailed descriptions of sedimentological features, micro-textures, and isotopic compositions of dolomite in the stromatolites and associated lithologies suggest that the succession was deposited on a carbonate ramp. Early burial diagenesis of the dolomite modified the original isotopic signatures. Nonetheless, here we discern evidence for two distinct depositional events based on clumped isotopic signatures preserved within dolomitic stromatolite and the underlying dolomite mud. Dolomitic muds within cherty precipitate record original thermometry signatures from these rocks. Clumped isotope analyses suggest that the lowest temperature of the Paleoproterozoic seawater was 21.8 ± 2 °C, and that its isotopic composition (δ18OVSMOW) during dolomite precipitation at equilibrium was −9.6 ± 0.1‰, which is lighter than that of present-day ocean water. The overlying stromatolitic dolomite exhibits a temperature range of 70.7–109.8 °C, reflecting late-stage diagenetic processes and hydrothermal activity. Hydrothermal fluids were isotopically enriched (up to 4.9‰) suggesting their mantle origin and interaction with evaporative seawater. We evaluated the degree of water-rock interaction with water of heavier isotopic composition that was responsible for the precipitation of late phase dolomite found in the strata overlying carbonate mud.

How petrophysical properties influence the seismic signature of carbonate fault damage zone: Insights from forward-seismic modelling

Seismic imaging is the most common tool to detect subsurface structures in continental and marine settings. Despite technological advancement, seismic analysis of carbonates is still challenging because they are strongly influenced by petrophysical heterogeneities, being this even more difficult when further heterogeneities are added by the presence of faults. In this work, unmigrated forward-seismic models are developed to understand the seismic response changes related to carbonate-bearing fault systems and their deformation behaviour. We focus on the carbonate ramp of the Majella Massif (central Italy), that is characterised by the presence of porous and faulted carbonate lithologies. This carbonate system represents an analogue of carbonate reservoirs worldwide. Field and laboratory data of fault rocks sampled at increasing distances from the slip planes highlight a damage zone/fault core architecture with a decreasing porosity approaching fault planes. This observation is also confirmed by the thin sections analysis and the calculation of the shear modulus. Seismic images of fault rocks presenting lower porosity than the host rock show weak diffraction hyperbolas while diffraction hyperbolas are more evident in the forward-seismic models of fault rocks with greater porosity than the host rock. Moreover, factors such as the variation in the dip angle of the fault or the thickness of the damage zone can enhance or reduce the hyperbolas. Performing the migration process to stacked sections would not provide evidence of increased porosity in the damage zone due to the suppression of the diffractive component. The presence of weak hyperbolas in unmigrated seismic images is thus interpreted as evidence of a lower porosity damage zone with respect to the host rock. This can be related to deformation mechanisms leading to porosity loss in the damage zone for porous rocks as observed in the study area. However, this could also be related to the confining stress or fracture filling that counteract the fracture-related porosity increase when faults are hosted by tight rocks.

Late Albian ammonites from northwest Algeria: Biostratigraphic, paleobiogeographic and paleoenvironmental inferences

Albian rocks of northwest Algeria are commonly clastic deposits with poor dating markers. This study records two late Albian ammonites from northwest Algeria for the first time. They are Elobiceras (Elobiceras) sp. and Pervinquieria (Pervinquieria) pricei (Spath). These ammonites are collected from the Mcharref Formation interpreted here to be deposited south of Tiaret city under a lagoonal carbonate ramp setting based on facies analysis. The recognition of the well-known early late Albian ammonite Pervinquieria (Pervinquieria) pricei biozone from the study area highly improves the assigned age for the studied sections. The unconformity surface separates the lower and upper members of the Mcharref Formation coincides with the upper boundary of this biozone. The correlation of this boundary with the global sequence boundaries in the frame of ammonite biozones reveals that it equivalent to the SB KAl 6 (∼103.8 my ago) that marks the end of the identified biozone everywhere. Therefore, the recorded biozone shows an excellent correlation between the study area and that of Tethyan, European, American, and Asian biozones. Paleobiogeography of the recognized biozone indicates a cosmopolitan distribution that is interpreted here as a response to the widespread distribution of the Tethyan shallow water that reached to form a connection with the northern and southern Atlantic oceans during the late Albian time.

Shallow-water carbonates of the Coimbra Formation, Lusitanian Basin (Portugal): contributions to the integrated stratigraphic analysis of the Sinemurian sedimentary successions in the western Iberian Margin

An integrated stratigraphic analysis of the Coimbra Formation was performed in the S. Pedro de Moel outcrops of the Lusitanian Basin (Portugal). This unit is dated from the lower–upper (Oxynotum Chronozone) Sinemurian and is subdivided into eight informal subunits. Except for its base, consisting of dolostones and microbialites, much of the succession consists of bioclastic and bioturbated micritic centimetric–decimetric limestones (sometimes rich in benthic macrofauna) alternating with millimetric–centimetric marly layers, all deposited in shallow-water carbonate ramp environments. Organic-rich sediments occur throughout, with total organic carbon reaching up to 12 wt%. At a broader scale, the Coimbra Formation is transgressive and part of a long-lasting 2nd-order transgressive–regressive facies cycle ending around the Sinemurian–Pliensbachian boundary. The vertical variation of δ 13 C determined in bulk carbonate is characterized by relatively normal marine values (0–2.5‰); however, several negative shifts are associated with the organic-rich sediments, with a maximum amplitude of ∼8‰ in the Obtusum Chronozone. These shifts in bulk carbonate δ 13 C are interpreted to be of diagenetic origin and, therefore, of local significance. However, it cannot be discarded that a regional/global signal is imprinted on the observed trends.

Sedimentary facies, lithofacies paleogeography, and an evaluation of the Ordovician sequences in the Sichuan Basin, southwest China

Ordovician strata in the Sichuan Basin occur in formations of varying thicknesses. Deposited in intracratonic and craton margin depressions of the Yangtze Block, these strata are rarely exposed in the basin with poorly described facies types and multiple controversies in sequence stratigraphy, which can lead to inaccurate interpretations of depositional settings and paleogeography. We studied outcrops, samples, and thin sections, and conducted interpretations of the environment of deposition and sequence stratigraphy as well as the lithofacies paleogeographic characteristics of different depositional stages of Ordovician units. Macroscopic facies (08) and their associated microfacies (14) were identified from outcrops and thin sections. The depositional environments of the carbonate rocks were interpreted using the standard microfacies types (SMF) of Flügel (2010) as a reference. The Ordovician units in the Sichuan Basin were divided into three 2nd-order sequences, including two complete cycles (SSQ1 and SSQ2) and one semi-cycle (SSQ3), and further subdivided into ten 3rd-order sequences (SQ1-SQ10). Ordovician units were partitioned into seven formations: the Tongzi, Honghuayuan, Meitan, Shizipu, Baota-Linxiang, and Wufeng formations. Based on the variation characteristics of the carbon-13 isotope (δ13C) curve, two large cycles of sea-level rise and fall were detected in the study area. An extensional rifting event occurred in the basin during the first cycle (Lower Tongzi Formation to Early/Middle Meitan Formation). The basin was in a compressive stage during the second cycle (Late Meitan Formation to Late Wufeng Formation). The Early Ordovician Tongzi and Honghuayuan formations were deposited in shallow water. During the deposition of these units, shoal facies occurred in the platform margin. Afterward, rimmed carbonate platforms occurred in the platform margin with a fauna mainly composed of brachiopods, echinoderms, trilobites, bryozoans, and algae. The Middle Ordovician Meitan Formation is characterized by increased clastic components deposited in continental shelf environments with graptolite and trilobite fossils. The Late Ordovician Baota Formation is marked by the development of slope facies deposited in carbonate ramp environments with cephalopod and trilobite fragments. The Late Ordovician Wufeng Formation was deposited in deep water shelf settings characterized by graptolite fossils. Tectonic activities and relative sea-level change mainly controlled the paleogeographic pattern and the evolution of the Sichuan Basin. The results presented in this study provide valuable insights and fill a significant gap in facies analysis, sequence stratigraphic division, and paleogeography of Ordovician units in the Sichuan Basin.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 1. General characteristics, microfacies and depositional history of the Middle to Late Anisian Bulog sedimentary succession in the Inner Dinarides (SW Serbia)

The opening of the Neo?Tethys started in the Middle Anisian and is recorded in the drowning succession of the shallow?water Ravni/Steinalm Carbonate Ramp and the subsequent deposition of deep?marine limestones, e.g., the red nodular limestones of the Bulog Group and equivalents. In the Inner Dinarides of southwest Serbia the continental break?up of the Neo? Tethys Ocean is characterized by the formation of a horst?and?graben topography. The change from deposition of shallow?water carbonates formed in an epicontinental sea (graben stage of the Neo?Tethys Wilson Cycle) to red nodular deep?marine limestones (Bulog Limestone) in the late Pelsonian (Middle Anisian) is relatively abrupt due to the rapid decrease of carbonate production. The deeper?water Bulog Limestone, deposited in the early stage of the passive continental margin evolution of the western Neo?Tethys can be dated by conodonts and in rare cases also by ammonoids quite exactly, and therefore it is possible to reconstruct the Pelsonian to Illyrian sedimentological evolution precisely: 1) The late Pelsonian is characterized by the drowning of the shallow?water Ravni Carbonate Ramp and a rapid deepening of the depositional realm. Extension led to the formation of neptunian dikes in the shallow?water Ravni Formation, filled with deeper?marine red micrite, and the formation of a horst?and?graben morphology. Whereas some of the horsts uplifted and emerged in the grabens near to the newly formed escarpments thick breccia successions were deposited with a fining?upward trend during the early?middle Illyrian. On top of other horsts, the grabens, or the newly formed gentle slopes red nodular limestones were deposited. In cases layers with enriched ammonoids formed (Fossillagerst?tten). 2) The early?middle Illyrian ongoing subsidence resulted in the deposition of more and more condensed red nodular limestones with hardground formation. 3) Around the middle/late Illyrian boundary a new pulse of tectonic motions resulted in the tilting of blocks, the formation of new escarpments and again mobilization of mass transport deposits. In addition, a second generation of neptunian dikes was formed. They crosscut the late Pelsonian to middle Illyrian Bulog Limestone, the Pelsonian Ravni Formation, and the older generation of neptunian dikes in the shallow?water Ravni Formation. All formerly emerged horsts flooded and red nodular limestones were deposited on the karstified shallow?water Ravni Formation after a gap. This second pulse of tectonic motions is related to the widespread volcanism in the Dinarides as visible in the appearance of mm?sized biotite clasts in the late Illyrian Bulog Limestone. In contrast to the Outer Dinarides, where thick volcanics are intercalated in the Illyrian sedimentary succession, in the Bulog Limestone successions of the Inner Dinarides volcanics are missing. In general the Pelsonian?Illyrian sedimentary succession is characterized by a stepwise deepening of the depositional realm.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 2. Sirogojno (Klisura quarry) - the reference section of the Middle to Late Anisian Bulog sedimentary succession in the Inner Dinarides (SW Serbia)

The Middle to Late Anisian sedimentary succession preserved in the Klisura quarry in Sirogojno (Zlatibor Mt., SW Serbia) preserves the most complete deepening depositional history in the Inner Dinarides in Serbia. In this section, extensional tectonic movements that took place in connection with the Neo-Tethys break-up are reflected in the exceptional well-preserved record of the depositional history. The Pelsonian Ravni Formation at the base of the section is dissected by two generations of neptunian dikes formed in A) the late Pelsonian, and B) the late Illyrian. Above the late Pelsonian drowning unconformity a deepening trend of the depositional environment is mirrored by the litho- and microfacies characteristics of the more than 21 meter thick red nodular limestones of the Bulog Formation. Periods of starvation with deposition of Fossillagerstatten beds and hardgrounds mark characteristic changes in deposition. Two phases of distinct tectonic motions result in unconformities and a stepwise deepening of the depositional realm. The first and major unconformity is the drowning surface on the top of the Ravni Carbonate Ramp which led to the nearly total demise of shallow-water carbonate production, i.e. the change in deposition from shallow- to deepwater limestones. The second phase of tectonic motions is expressed by the formation of an angular unconformity after a phase of starvation in the basin. No Bulog Limestone section in the Dinarides, even not in the type region near Sarajevo, preserves all these characteristic sedimentological features as preserved in the Klisura quarry section in Sirogojno. Therefore this section is proposed to be the reference section for the sedimentological evolution and depositional environment reconstruction of the Bulog Formation in the Inner Dinarides, located in SW Serbia.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 3. Lithostratigraphy and Formations of the Middle to Late Anisian Bulog sedimentary successions (Bulog Group) in the Dinarides (Bosnia and Herzegovina, Serbia, Montenegro)

In the Middle Anisian the opening of the Neo?Tethys started and this is characterized by a significant change in deposition in the whole Western Tethys Realm. In the Dinarides the Middle to Late Anisian tectonostratigraphic and basin evolution is mirrored by their sedimentary successions. In the Middle Anisian, the evolution of the shallow?water Ravni Carbonate Ramp ended relatively abrupt in the late Pelsonian: a rapid decrease of carbonate production is accompanied by formation of a horst?and?graben topography. This change is expressed by the change in deposition from shallow?water carbonates to deepwater sedimentary rocks, i.e. the Ravni Carbonate Ramp experienced a drowning. Contemporaneously neptunian dikes were formed in the underlying Ravni Formation, filled with deep?water limestones. In the grabens, near to the newly formed escarpments mass transport deposits accumulated. During the time span late Pelsonian to middle Illyrian a general deepening trend is mirrored in the sedimentological trend and the microfacies characteristics. Fossilla ? gerst?tten (mainly cephalopods) and hardgrounds occur widespread in the deeper?water red nodular limestones formed widespread above the Ravni Carbonate Ramp. Around the middle/late Illyrian boundary a second intense pulse of tectonic motions, related to the onset of volcanic activity, resulted in a crosscut of the older horst?and?graben topo graphy. A second generation of neptunian dikes was formed, the blocks tilted, and again mass transport deposits accumulated near to escarpments. The late Illyrian is characterized by an overall deepening trend expressed also in a change in the microfacies characteristics to radiolarian?rich wacke? to packstones. In cases even radio larites or silicified limestones are characteristic sedimentary rocks in late Illyrian to Ladinian times beside silicified volcano?sedimentary rocks. This paper will define the late Middle?Late Anisian red nodular limestones and related sedimentary rocks with mass transport deposits overlying the Ravni Carbonate Ramp on base of new sedimen tological, stratigraphic and micro facies data following the international rules and standards. The palaeo geographic position together with characteristic litho? and microfacies features cause the lithostratigraphic definition of the different units. Included in the newly introduced Bulog Group are: 1) the emended and formalized Bulog Formation and 2) the newly introduced and defined Komarani Formation. The type?section of the Bulog Formation east of Sarajevo (Han Vidovic, central Bosnia and Herzegovina) is revisited. The Komarani Formation is introduced with the type?section in Komarani village (Zlatar Mt. in SW Serbia). The reference sections for the Bulog Formation in SW Serbia (Klisura quarry on Zlatibor Mt.), Bosnia and Herzegovina (Pridvorica locality at Romanija Mt.) and in Montenegro (Boljevici near Virpazar), are defined. In the reference sections some characteristic lithological and microfacies features are better preserved as in the type?section and included in the emendation and formalization of the Bulog Formation. The Bulog Formation at the type?section and all reference sections are in the Late Triassic overlain by the shallow?water carbonates of the Wetterstein and Dachstein Carbonate Platforms. Age and facies equivalent red nodular limestones deposited in the outer shelf region are part of the Middle to Late Triassic Hallstatt Limestone succession. In the type?region of the Hallstatt Limestones these late Middle to Late Anisian red nodular limestones are named Schreyeralm Limestones. The name Schreyeralm Limestone was introduced earlier as the name Bulog Limestone. It is discussed if also in the Dinarides the name Schreyeralm Limestone should be used for such Anisian red nodular limestones deposited in the outer shelf region, for priority reasons, or if they should be included into the Bulog Formation to avoid confusion, for historical reasons.

Stratigraphy, isotope geochemistry, seismic stratigraphy and paleogeography of the Lagoa do Jacaré Formation, Bambuí Foreland Basin (Ediacaran-Cambrian), Southeast Brazil

The Bambuí Group is a lithostratigraphic unit that comprises a mixed carbonate-siliciclastic platform deposited in a foreland basin. It is a 1st order sequence that filled the São Francisco Basin during the Ediacaran-Cambrian in southeast Brazil. The Lagoa do Jacaré Formation is the second carbonate unit of the Bambuí Group and is related to a 2nd-order regressive hemicycle composed of dark gray shallow water carbonates with high positive ẟ1³C values (>+10‰) related to the upper part of the Middle Bambuí Excursion (MIBE). We described four outcrops in the center-north of Minas Gerais state (Paraopeba, Curvelo, Montes Claros and Ubaí areas) coupled with ẟ13C and ẟ18O, and SEM-EDS analysis in order to support detailed facies analyses. One seismic line and three well logs in the south part of the Januária High were also used to define the stratigraphic framework of the Lagoa do Jacaré Formation in the Bambuí Foreland Basin system. Eight facies were interpreted from the outcrop data and grouped into five facies associations, that represent peritidal zone (FA1), ooidal sand shoals (FA5), tempestites of mid ramp (FA3), fine-grained carbonate of outer ramp (FA4), and fine-grained siliciclastic from offshore (FA2). The strong presence of terrigenous material (quartz, feldspars, muscovite, biotite, apatite) with some authigenic/diagenetic minerals (pyrite, titanite, rutile) in all facies are the main petrographic aspect of the studied carbonate rocks, related to high sedimentary input and some events during early diagenesis. FA1 with lower mean values of ẟ1³C (+9‰) predominates in the north, meanwhile FA3 is the main association in the south with higher ẟ1³C mean values (+12‰). This difference is probably related to the stratigraphic differences between the basal section (at south) and upper section (at north) of the Lagoa do Jacaré Formation. The Januária High was the main controlling element for the deposition of the carbonate ramp in the north sector of the basin, close to the Ubaí and Montes Claros cities. Probably, this structural high also played an important role as source area in the forebulge sector, uplifted by flexure due to the orogeny of the Brasília and Araçuaí (still in development) Belts (∼540-520 Ma). To the east, a N–S carbonate ramp system was developed in the back-bulge sector according to the main paleocurrents measurements. This ramp is related to a 300 m thickness section of the Lagoa do Jacaré Formation that was firstly identified and correlated to the successions on the surface using well logs, possibly registering at the top the decreasing in the ẟ1³C values, after the main stages of the MIBE probably in close association with Ediacaran-Cambrian boundary, related to the assembling of the Gondwana Supercontinent recorded in this region of southeast Brazil.