Stratigraphic Equivalents Research Articles

Paleoclimate and Paleoenvironment Reconstructions from Middle Eocene Successions at Beni-Suef, Egypt: Foraminiferal Assemblages and Geochemical Approaches

The Eocene deposits of Egypt provide a wide variety of shallow marine facies and fossil assemblages, allowing paleoenvironmental reconstructions in this warmhouse climate interval. Forty-three rock samples have been collected from two middle Eocene sections, exposed at southeast Beni-Suef area in northern Egypt. The studied outcrops are lithologicsally subdivided into two rock units named from base to top as follows: (1) the Qarara Formation (Lutetian) and (2) the El Fashn Formation (Bartonian). Most investigated rock samples showed richness in benthic foraminiferal assemblages and only rare occurrences of index planktonic foraminifera. They yielded 160 foraminifera species which belong to 4 suborders, 19 superfamilies, 34 families, and 59 genera. The stratigraphic distribution of the identified species allowed us to define four local benthic bio-ecozones: (1) Bolivina carinata Lowest Occurrence Zone (Lutetian), (2) Cibicides mabahethi/Cancris auriculus primitivus Concurrent-Range Zone, (3) Nonion scaphum Lowest Occurrence Zone, and (4) Brizalina cookei/Nonionella insecta Concurrent-Range Zone (Bartonian). These biozones are described and discussed in detail and correlated to stratigraphic equivalents in Egypt. The rareness of index planktonic foraminifera through the studied sections does not allow a precise biostratigraphic and chronostratigraphic correlation. The shale samples show low TOC values, which may be related to low productivity, high sediment influx, and/or prevailing oxic conditions. Bulk rock geochemistry, consistent with the benthic foraminifera ecological preferences, indicated that the studied sections were deposited in moderate to high oxygen levels and warm climatic conditions in a typical shelf marine setting. The identified species showed strong similarities with nearby southern Tethys areas, reflecting migration via the trans-Sahara seaway, and minor similarities with those identified from the northwestern Tethys and the North Atlantic province attributed to the change in the environmental and climatic conditions, such as cooler, latitudinal zoned climatic conditions along the northwestern Tethys, which was unsuitable for their biological demands. The warming trend identified from Lutetian to Bartonian intervals corresponds to the onset of the Middle Eocene Climatic Optimum (MECO).

Growth of the Tian Shan Drives Migration of the Conglomerate‐Sandstone Transition in the Southern Junggar Foreland Basin

AbstractThe conglomerate‐sandstone transition (CST) preserved in foreland basins records the stratigraphic equivalent of the gravel to sand transition (GST) which migrates in response to shortening in steady state orogen‐foreland basin systems. Two seismic profiles in the southern Junggar foreland basin provide the first seismic record of migrating CSTs and an opportunity to quantify its migration history in response to the growth of the northern Tian Shan. We identify the horizontal positions of the CSTs based on the distinct seismic signature of conglomerate versus sandstone and estimate their ages according to magnetostratigraphic sections. The linear‐regression analysis reveals that the CSTs migrated northward at 0.4 ± 0.1 mm/yr along 84°E from ∼23 to ∼6 Ma and at 4.0 ± 1.2 mm/yr along 87°E from ∼3.1 to ∼0.7 Ma. The discrepancy relates to variations in crustal shortening along strike of the range. Temporal deviations in the CST positions suggest the second‐order impacts of climatic change.

An investigation of a Devonian/Carboniferous Boundary section on the Bolivian Altiplano

AbstractThe Devonian/Carboniferous Boundary (DCB) interval is associated with mass extinction, isotope excursions and a short glacial episode. This study investigates how boundary extinction and environmental change is expressed in the glacial high-palaeolatitudinal record of the Bolivian Altiplano (western Gondwana). A latest Devonian and early Carboniferous section has been investigated using sedimentology, palynology, total organic carbon and bulk δ13Corganic. The Colpacucho Formation is a Late Devonian shelfal–marine siliciclastic sequence. It is overlain in the study area by a unit of coarse sandstones and sandy diamictites, interpreted as glaciomarine. This distinctive glaciomarine unit is at least 7 km wide and 60–120 m thick with a variably incisive basal contact (<100 m). It is of very latest Famennian age and is a stratigraphic equivalent of proven glacigenic deposits across central South America. The offshore marine Kasa Formation overlies the glacigenic unit above a basal flooding surface. The DCB is 12 m above this flooding surface on the last occurrence of Retispora lepidophyta and significant palynological assemblage changes. This includes the loss of the Umbellasphaeridium saharicum phytoplankton bioprovince, endemic to Gondwana. Marine and terrestrial palynological extinctions are synchronous with a 2 ‰ positive carbon isotope excursion interpreted to be reflective of changes in organic matter delivery and preservation during an interval of environmental stress. These results inform wider debates on global environmental change and mass extinction at the DCB.

Sedimentology and carbonatogenesis of the Upper Campanian limestones in Jebel Berda (south central Tunisia)

The Berda Formation corresponds to an inner-shelf stratigraphic equivalent of the Campanian-Maastrichtian outer-shelf Abiod Formation of Tunisia. The present work aims to carry out a sedimentological study of the lower unit (Upper Campanian) of Berda Formation limestones in order to interpret the limestone depositional environment and the origin of the associated micrite particles. This work is based on the study of a geological section surveyed in the Jebel Berda, typic locality, completed by microscopic observation of thin sections that allowed us identifying different carbonate facies and interpreting their depositional environments. The micrite particles have been examined with the SEM. The mineralogical and geochemical proportions and the magnetic susceptibility of the Berda Formation limestone, have been evaluated using respectively XRD, Atomic Absorption Techniques and KLY-3 Kappabridge susceptometer. The studied succession of Berda Formation includes five carbonate facies that developed within different depositional environments (1) calcisphere-rich bioclastic mudstone-wackestone within a circalittoral environment, (2) echinoderm-rich bioclastic wackestone-packstone within a shallow-marine environment (3) bioclastic rudstone within an open-marine environment, (4) peloid-rich bioclastic grainstone within high energy littoral shallow-water marine environment and (5) bioclastic dolomicrite-dolomicrosparite within a continental to marine transition environment. The studied limestones are composed of two kinds of micrite particles, (1) irregularly shaped particles resulting from fossils and micro-fossils desintegration and (2) regularly shaped polyhedral particles resulting from inorganic chemical precipitation. The carbonate facies are arranged into shallowing-up parasequences. The sedimentary facies variation and carbonate accumulation are controlled by eustatic sea-level fluctuation. In this way the biodetrital micrite occurred mainly during the transgressive intervals whereas the inorganic chemical micrite is related to the regressive intervals.

Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation

Early Cambrian black shales are widely exposed along the southeastern margin of the Yangtze Platform in South China for a length of over 1600 km. The basal part of this sequence host a large spectrum of mineralization, including polymetallic Ni-Mo sulfide, vanadium, phosphorite, barite, as well as combustible shale (stone coal). In particular, the polymetallic Ni-Mo sulfide ore layer is stratigraphically equivalent but geographically separated from the V mineralization. In the Zhangjiajie region in Hunan province, the tenor of the polymetallic Ni-Mo sulfide mineralization in the northeastern segment decreases gradually and grades into vanadium mineralization in the southwestern segment. Based on the stratigraphic correlation, we compared the trace element composition and carbon–sulfur-iron system of the polymetallic Ni-Mo sulfide ore, vanadium ore, and their host black shales.The polymetallic Ni-Mo sulfide and V ores display similar PAAS-normalized REE distribution patterns with negative Ce and positive Y anomalies, similar to present-day seawater. The polymetallic sulfide ores show elevated seawater-like Y/Ho ratios (mean: 52.7 ± 6.8) and the V ores with an average Y/Ho ratio of 40.2 ± 3.8 reflect the influence of terrigenous material. Although both the polymetallic Ni-Mo sulfide ore and the V ore formed under anoxic conditions, the degree of oxygen deficiency was variable, i.e., euxinic condition for the polymetallic sulfide ore but euxinic and suboxic conditions for the vanadium ore. This model is supported by the more elevated redox-sensitive elements (e.g., U and Co) in the polymetallic Ni-Mo sulfide ores compared to the V ores. Excess sulfur of the polymetallic Ni-Mo sulfide ores with S/Fe ratios>1.15 suggests that they formed under iron-limited euxinic conditions. In contrast, the V ores formed under euxinic to suboxic conditions with S/Fe ratios close to or lower than 1.15. The host black shales display positive correlations for TOC and redox-sensitive elements, suggesting that they formed under suboxic conditions. We infer that the polymetallic Ni-Mo sulfide ores formed in a sediment-starved semi-restricted euxinic marine basin. In contrast, the V ores formed under both euxinic and intermittently suboxic to euxinic conditions at the slope of the basin with terrestrial input from enhanced continental weathering. Repeated transgressive–regressive episodes led to the formation of V ores interlayered with suboxic black shale with more subdued oxygen deficiency. The results of this study suggest that exploration for V mineralization should focus on the southeastward lateral stratigraphic equivalents of the polymetallic Ni-Mo sulfide mineralization.

Geochemistry of the Cretaceous Chinkeh oil from Maxhamish field and Garbutt black shale in the Liard Basin, Canada: Implications for a liquid-rich shale hydrocarbon resource

The Cretaceous Garbutt Formation shale and crude oils from the Cretaceous Chinkeh Formation in Liard Basin were subjected to geochemical analysis including biomarker fingerprinting. The Chinkeh oils are sweet light oils with API gravity and geochemical markers typical for the late stage oil window maturity. The oils show good correlation to the Garbutt shale extracts for their sterane compositions, while there are some minor discrepancies amongst their terpane distributions. These differences include enhanced contribution of tricyclic terpanes, high C23 tricyclic terpanes versus C30 hopane and low C24 tetracyclic terpane versus C26 tricyclic terpanes for the Chinkeh oil compared with the Garbutt shale extracts. Although not analyzed during this study, the Triassic Toad/Grayling interval, as stratigraphic equivalents of the Montney and Doig formations in Western Canada Sedimentary Basin may account for the terpane compositional features. Therefore, it is likely that the Chinkeh oils are mainly sourced from the Cretaceous Garbutt shale with minor additional contributions from a Triassic source. The Garbutt Shale is immature in the Horn River Basin, while in the Liard Basin in a north to south transect the unit's thermal maturity ranges from early mature oil window to wet gas/condensate window, and thus may be prospective for unconventional liquid hydrocarbon resource recovery.

Revised biostratigraphy and regional correlations of the Urgonian southern Vercors carbonate platform, southeast France

The Subalpine Urgonian platform provides seismic-scale outcrops of interest to constrain the depositional organisation of Lower Cretaceous carbonate systems. Revising the biostratigraphy using ammonites, rudists, calcareous algae and orbitolinids and establishing regional stratigraphic correlations between the Gresse-en-Vercors cliff and the Plateau de Glandasse provide a new vision regarding the Urgonian platform stratigraphic architecture of the southern Vercors. The rudist-dominated inner-platform series cropping out in the northern Gresse-en-Vercors cliff grades laterally southward into the bioclastic-dominated Glandasse Formation (defined in the Plateau de Glandasse series). Our new correlations contradict with the Grenoble University's conceptual platform model which considered the Glandasse Formation as rudist-poor lowstand deposits formed in the southern Vercors and sealed by the transgressive Urgonian limestones formation. Our interpretation better conforms to the Geneva University's conceptual model, suggesting that the Glandasse and Urgonian limestones formations should be regarded as stratigraphic equivalents. Further investigations are needed to address the stratigraphic evolution and controlling mechanisms of the regional Urgonian system at the regional scale.

Reservoir quality of the Surmeh (Arab-D) reservoir in the context of sequence stratigraphy in Salman Field, Persian Gulf

The Upper Jurassic Surmeh reservoir in the northern Persian Gulf is a stratigraphic equivalent of Arab-D reservoir that is one of the best Jurassic reservoir rocks of the Arabian Peninsula. In this study, in the first step the palaeontology, sedimentary environment, sequence stratigraphy and reservoir quality of the Surmeh reservoir was investigated in a core-bearing well in Salman oil field. The palaeontological studies reveal a moderately diverse assemblage of benthic foraminifera. Dominated foraminifera include the agglutinated forms and miliolids and there are also fragments of alga, stromatoporoids, corals, bivalves, gastropods and echinoderms. These recorded fauna and flora resulted in distinction of three zones that characterize the age of undifferentiated Kimmeridgian-Tithonian. Based on petrographic observations, along with palaeontological materials, eight microfacies were identified that represent four sub-environments of a carbonate inner ramp including supratidal, intertidal, lagoon and shoal. These studies combined with well log responses resulted in identification of a 3rd order sequence and parts of an incomplete one that is composed of an incomplete HST. These 3rd order sequences comprise four 4th order subordinate sequences.In addition by integration of petrographic analyses, sub-environments energy regimes and petrophysical data, five reservoir zones were identified. Also, based on core porosity and permeability 6 Hydraulic Flow Unit (HFU) with poor to excellent reservoir quality in ascending trend were identified. The reservoir zone I, which belongs to the sabkha and littoral facies, represents the worst reservoir quality (includes HFU1 to HFU3), while the reservoir zone III with best reservoir quality (includes HFU4 and HFU5) comprises the back shoal grain-supported facies. The introduced sequences and reservoir zones in the studied well were propagated to three other wells throughout the Salman Field via using log responses. Also they were correlated with those introduced for Arab-D reservoir in the southern parts of the Persian Gulf providing a correlation framework. This pattern can be used in the next exploration wells which are going to be drilled especially in the northern parts of the Persian Gulf.

A Multistage Genetic Model for the Metamorphosed Mesoproterozoic Swartberg Base Metal Deposit, Aggeneys-Gamsberg Ore District, South Africa

Abstract The polymetamorphosed Swartberg Cu-Pb-Zn-Ag deposit in the Namaqua Metamorphic Province of South Africa is a major metal producer in the region, yet its genesis remains poorly understood. The deposit comprises several stratiform to stratabound units, namely the Lower Orebody and Dark Quartzite, the overlying Barite Unit, and the Upper Orebody, all of which are folded by an F2 isoclinal syncline and refolded by an open F3 synform. A discordant Garnet Quartzite unit surrounds the Upper Orebody in the F2 hinge, where it overprints the Lower Orebody and Barite Unit. The Lower Orebody comprises sulfidic, pelitic lenses with fine-grained pyrite, sphalerite, galena, and lesser pyrrhotite, hosted by sulfide-poor but magnetite- and barite-bearing siliceous rock. The overlying Barite Unit is poorly mineralized and grades from massive magnetite-barite close to the F2 hinge to distal laminated baritic schist and quartzite. The Dark Quartzite is the stratigraphic equivalent of the Lower Orebody and Barite Unit but comprises siliceous quartzite and schist, with lenses of conglomerate and minor Fe-Mn-Zn phases. The Upper Orebody displays rapid zonations from massive magnetite-rich iron formation in the F2 hinge, rich in coarse galena, pyrrhotite, and chalcopyrite, to sulfide-poor, magnetite-bearing schist and quartzite. The Garnet Quartzite is dominated by quartz and almandine garnet and mineralized with pyrite and chalcopyrite. Geochemical discriminant plots show that the Lower Orebody has a significant detrital component, whereas the Upper Orebody and Barite Unit are strongly zoned, with the greatest chemogenic component close to the F2 hinge. This corresponds to a deposit-scale metal zonation from the Cu-rich F2 hinge to more Pb- and then Zn-dominated areas. Mineral assemblages and paleoredox proxies suggest generally oxic conditions, with a more reduced signature close to the hinge and in the sulfidic Lower Orebody lenses. The Lower Orebody is interpreted as a mixed chemogenic-pelitic unit, with sulfides deposited on or near the seafloor during stage 1 hydrothermal activity. The sulfidic lenses formed from fine mud and clay deposited in quiet seafloor depressions, in which warm, dense, reducing, Pb-Zn-Ba–rich stage 1 brines accumulated, while the siliceous portions formed from higher-energy clastic sediments on aerated seafloor highs. The Barite Unit forms a baritic cap to the Lower Orebody, while the Dark Quartzite is their shallower-water equivalent. Thereafter, clastic sediment with lesser hydrothermal input was deposited during stage 2a exhalations, forming the poorly mineralized portions of the Upper Orebody. During stage 2b hydrothermal activity, hot Cu-Fe–rich fluids invaded part of the Upper Orebody, creating the highly chemogenic protolith to the well-mineralized, magnetite-rich portion. Associated hydrothermal alteration in a discordant subseafloor feeder zone created the Garnet Quartzite protolith. The F2 hinge thus corresponds closely to the original vent zone. Swartberg therefore resembles a deformed and metamorphosed Selwyn-type sedimentary exhalative deposit, with both proximal- (Upper Orebody, Garnet Quartzite) and distal-style (Lower Orebody) mineralization. The close association of these styles suggests that differences in the mineralizing fluids and depositional environment, rather than proximity to a vent, determine the deposit style.

Chromite chemistry of a massive chromitite seam in the northern limb of the Bushveld Igneous Complex, South Africa: correlation with the UG-2 in the eastern and western limbs and evidence of variable assimilation of footwall rocks

The Bushveld Igneous Complex (BIC) is known for its laterally extensive platinum group element–bearing layers, the most famous being the Merensky Reef and the UG-2 chromitite in the eastern and western limbs of the complex. In the northern limb, the Platreef mineralization and a thick chromitite seam below it (referred to as the “UG-2 equivalent” or UG-2E) have been proposed to be the stratigraphic equivalents of the Merensky Reef and the UG-2, respectively. In this study, we compare a suite of UG-2E samples from the Turfspruit project with a UG-2 reference suite from the western limb using petrography, electron probe microanalysis, laser ablation-inductively coupled plasma-mass spectrometry, and Mössbauer spectroscopy. The results show that (a) in Mg# vs. Cr# diagrams, UG-2E chromites have a distinct compositional field; however, when samples of similar chromite modal abundance (≥ 80%) are used, the UG-2E chromites overlap the field that characterizes UG-2 chromites; (b) the UG-2E is more variable in chromite modal abundance than the UG-2; and (c) variations in Mg# and Fe3+/ΣFe in the UG-2E indicate contamination of the magma by metasedimentary rocks of the Duitschland Formation (Transvaal Supergroup) during emplacement, followed by partial re-equilibration of chromite grains with a trapped melt. Thus, we conclude that for chromite modes higher than 80%, the chromite composition retains enough information to allow correlation and that the UG-2E in the northern limb is very likely the UG-2 chromitite.

The nonglacial diamictite of Toutswemogala Hill (lower Karoo Supergroup, central Botswana): Implications on the extent of the late Paleozoic ice age in the Kalahari–Karoo Basin

ABSTRACTAlong the easternmost edge of the Karoo–Kalahari Basin (KKB) of Botswana, the Toutswemogala Hill succession exposes a 30–50-m-thick suite of siliciclastic deposits interpreted by some as glaciogenic in origin tied to the Permo-Carboniferous Late Paleozoic Ice Age (LPIA). Six facies associations (FA) were recognized in this succession, resting unconformably on a highly uneven Archean gneissic basement, and consisting from base to top of: 1) clast-supported breccia made up of angular cobbles and boulders ubiquitously derived from the underlying basement, 2) well-bedded siltstones sealing or locally interdigitated with the underlying breccia, and bearing abundant remnants of Glossopteris sp. leaves, 3) a chaotic to faintly laminated matrix-supported diamictite bearing angular and subrounded clasts and tree logs attributed to the genus Megaporoxylon, 4) cross-bedded conglomerate bearing well-rounded quartz and clasts, 5) planar-laminated to ripple-laminated, poorly sorted, muddy sandstones showcasing dispersed mud chips that grade upward into 6) poorly sorted, cross-bedded coarse-grained sandstones displaying convolute beds and abundant imprints of unidentifiable tree logs.No evidence of glaciogenic processes have been found in this succession, in the form of either pavement or clasts striations. The breccia and diamictite are interpreted as scree and mass-flow deposits, respectively. Along with the age of the deposits, inferred from the plant debris (upper Carboniferous to lower Permian), the stratigraphic position of this sedimentary succession resting on the Archean basement suggests that it corresponds to the Dukwi Formation, a stratigraphic equivalent of the Dwyka Group in the Main Karoo Basin. This would explain the resemblance of the facies to those recovered at the base of the central Kalahari–Karoo Basin and in the neighboring Tuli, Ellisras, and Tshipise basins. The absence of diagnostic criteria for glacial processes in the studied succession raises the question of the extent, in both time and space, of the LPIA-related ice masses over southern Africa and particularly in southeastern Botswana. It is suggested here that during this glacial epoch, spatially restricted ice masses were confined in bedrock valleys (valley glaciers) in an uplifted setting otherwise characterized by non-glaciogenic processes, further strengthening the scenario of fragmented ice masses over southern Gondwana.

Pulses of enhanced continental weathering associated with multiple Late Devonian climate perturbations: Evidence from osmium-isotope compositions

Anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of Late Devonian environmental and biospheric crises, which included a major mass extinction during the Frasnian–Famennian transition (marked by the Upper and Lower Kellwasser horizons). Here, this model is investigated by presenting the first stratigraphic record of osmium-isotope trends (187Os/188Os) in upper Devonian strata from the Kowala Quarry (Holy Cross Mountains, Poland). Changes in reconstructed 187Os/188Os seawater values to more radiogenic compositions are documented at the base of both the Lower (~0.42 to ~0.83) and Upper (~0.31 to ~0.81) Kellwasser horizons characteristic of the Frasnian–Famennian transition, and additionally within upper Famennian shales that record a more minor environmental perturbation known as the Annulata Event (~0.20 to ~0.53). These shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the Kellwasser crises and during the later Annulata Event. The similarity of 187Os/188Os values in this study from Frasnian–Famennian boundary and lower Famennian strata (between 0.4 and 0.5) to those from North American stratigraphic equivalents suggests that the 187Os/188Os values record global trends. These findings support a causal relationship between increased continental weathering (and thus, nutrient supply to the marine shelf) and the environmental perturbations that occurred during numerous Late Devonian events, including both of the biospherically catastrophic Kellwasser crises as well as other, less severe, oceanic anoxic events.

Перспективы нефтегазоносности стратиграфических аналогов баженовской свиты в западных районах Западно-Сибирского мегабассейна

Перспективы нефтегазоносности стратиграфических аналогов баженовской свиты в западных районах Западно-Сибирского мегабассейна

What can be learned about the deposition and compaction of peat from the Miocene lignite seam exposed in the Chłapowo Cliff on the Polish coast of the Baltic Sea?

This study focuses on discussing the relationship between the accumulation of peat and its compaction in the overbank zone of the Miocene river system. The examined 2nd Lusatian lignite seam is characteristically interbedded with sandy deposits and these are less compressible than peat. Therefore, in favourable conditions such as those found at Chlapowo Cliff (on the Baltic Sea coast in northern Poland), it is possible to identify the effects of an even or uneven lowering of the mire surface, resulting from diversified compaction of the underlying phytogenic beds. The research problem was applied to two examples located close to each other within the lignite seam. The first case shows the accumulation of fresh peat layers of almost equal thickness, while the second one presents a reduction in the thickness of fresh peat beds over the fossilised channel-fill sandy body. Only the latter example allows us to calculate the peat-to-lignite compaction ratio. The results of the compaction ratio obtained, Cr ≈ 7.37, mean that the youngest, fresh peat layers were compacted at least 7 times, transforming them into lignite beds. However, such a high compaction value, estimated for the top layers of the Miocene mire, cannot be directly taken into account when reconstructing the total thickness of the peat seam prior to covering it with mineral overburden. The published peat-to-lignite compaction ratios, determined using other research methods, for the 2nd Lusatian lignite seam or its stratigraphic equivalents range from 1.7 to 6.0 in most cases, with averages of 2.2 to 4.0. The differences between the data in the literature and the results given in this paper are due to the fact that the process of peat/lignite compaction is ongoing and cumulative.

Reservoir characterization of the Burgan Formation in northwestern Persian Gulf

Lower Cretaceous sequences of the Burgan Formation are notable siliciclastic reservoirs in the Middle East. Despite of this importance, a little is known about various geological characteristics of this reservoir in Iranian northwestern part of the Persian Gulf. In this study, an integrated sedimentological and reservoir geological evaluation has been carried out on the Burgan reservoir using a complete dataset of drilled cores, core plugs, thin sections and petrophysical logs along with laboratory data including XRD analysis of all components to define grain's mineralogy and porosity – permeability data for reservoir quality evaluation. Depositional facies and environments are interpreted and correlated in sequence stratigraphic framework of targeted reservoir across the studied fields and then compared with stratigraphic equivalents in the Middle East. Seven facies types are recognized that indicate deposition in a transitional environment from siliciclastic-dominated delta setting into shallow-marine carbonate platform. Diagenesis study indicates a simple history of post-depositional events that includes near surface (marine) and shallow to deep burial realms. Diagenetic processes are grouped into two categories based on their effects on reservoir properties. They include porosity enhancing (i.e. dissolution and fracturing) and porosity reducing (compaction and cementation) diagenetic processes. Reservoir evaluation of depositional facies has been carried out by defining petrographic rock-types (PRTs) and hydraulic flow units (HFUs) and their correlation in sequence stratigraphic framework. Results showed that semi-to unconsolidated sandstone facies accumulated in Burgan-B member provides the best reservoir units in the Burgan Formation. Organic-rich (rootled) and pelagic shale facies form several intra-reservoir barriers between the reservoir units, especially in Burgan-A member. These show close correlation with reservoir descriptions of this formation reported from the neighboring areas in the Persian Gulf (i.e. Kuwait).

The Bazhenov Horizon of West Siberia: structure, correlation, and thickness

The type sections of the Bazhenov Horizon and formations recognized within this horizon have been identified based on a comprehensive analysis of paleontological, lithological, geophysical (well-log and CDP seismic data), and geochemical data on the West Siberian Basin. The Bazhenov Horizon was traced throughout the entire West Siberian sedimentary basin. The criteria for the recognition of the top and base of this horizon within the stratigraphic equivalents of the Bazhenov Formation were suggested. The proposed facies-stratigraphic zonation of the Bazhenov Horizon reflects the spatial location of all formations identified within this horizon. As seen on the newly proposed thickness map, the Bazhenov Horizon reaches a thickness of 15-25 m within the Bazhenov and Tutleim Formations, 30-35 m within the Mulym’ya Formation, 30-45 m within the Danilov Formation, 40-65 m within the Mar’yanovka Formation, up to 100 m within the Golchikha Formation, >350 m within the Yanovstan Formation, up to 35 m within the Bagan Formation, and 35-40 m within the Maksimkin Yar Formation. A marginal filter (according to A.P. Lisitzin) has been identified along the East Siberian land.

Some middle and late Cambrian trilobites and brachiopods from the Adamsfield Trough, Tasmania

Jago, J.B., Bentley, C.J., Laurie, J.R. & Corbett, K.B., 26 June 2018. Some middle and late Cambrian trilobites and brachiopods from the Adamsfield Trough, Tasmania. Alcheringa 43, 1-17. ISSN 0311-5518.Cambrian Series 3 and Furongian trilobites and brachiopods are described from the Adamsfield Trough in southwestern Tasmania. The oldest fossils are very poorly preserved trilobites, assigned to Asaphiscidae gen. et sp. indet. from within the Island Road Formation a short distance above the unconformity with the underlying Proterozoic Wedge River Beds. A trilobite species from within the isolated Boyd River Formation is referred to Lioparia sp. The Island Road Formation and the Boyd River Formation are stratigraphically equivalent to the Trial Ridge Beds which have previously been dated as belonging to the Lejopyge laevigata Zone. The Trial Ridge Beds are overlain unconformably by the Singing Creek Formation. In the Adamsfield, Clear Hill and Stepped Hills areas, stratigraphic equivalents of the Singing Creek Formation collectively contain the trilobites Pseudaphelaspis sp., Pseudaphelaspis? sp., Prochuangia sp., Mindycrusta sp., Nepeidae gen. et sp. indet., and Olenidae gen. et sp. indet. plus the brachiopods described herein as Billingsella sp. aff. costata, Billingsella sp. A, Billingsella sp. B and a possible member of the Billingselloidea. The Singing Creek Formation has been previously correlated with the Stigmatoa diloma Zone. The genus Lotosoides Shergold 1975 is placed in synonymy with Prochuangia Kobayashi 1935.James B. Jago* [jim.jago@unisa.edu.au] University of South Australia, School of Natural and Built Environment, Mawson Lakes, SA 5095, Australia; Christopher J. Bentley [bigfossil@bigpond.com] 30 Albert Street, Clare, SA 5453, Australia; John R. Laurie [john.r.laurie@gmail.com] Department of Biological Sciences, Macquarie University, NSW 2109, Australia; Keith D. Corbett [keith.corbett@bigpond.com] 35 Pillinger Drive, Fern Tree, Tas 7054, Australia.

Palaeomagnetic time and space constraints of the Early Cretaceous Rhenodanubian Flysch zone (Eastern Alps)

The Rhenodanubian Flysch zone (RDF) is a Lower Cretaceous-lower Palaeocene turbidite succession extending for similar to 500 km from the Danube at Vienna to the Rhine Valley (Eastern Alps). It consists of calcareous and siliciclastic turbidite systems deposited in a trench abyssal plain. The age of deposition has been estimated through micropalaeontologic dating. However, palaeomagnetic studies constraining the age and the palaeolatitude of deposition of the RDF are still missing. Here, we present palaeomagnetic data from the Early Cretaceous Tristel and Rehbreingraben Formations of the RDF from two localities in the Bavarian Alps (Rehbrein Creek and Lainbach Valley, southern Germany), and from the stratigraphic equivalent of the Falknis Nappe (Liechtenstein). The quality of the palaeomagnetic signal has been assessed by either fold test (FT) or reversal test (RT). Sediments from the Falknis Nappe are characterized by a pervasive syntectonic magnetic overprint as tested by negative FT, and are thus excluded from the study. The sediments of the Rehbreingraben Formation at Rehbrein Creek, with positive RT, straddle magnetic polarity Chron M0r and the younger M'-1r' reverse event, with an age of similar to 127-123 Ma (late Barremian-early Aptian). At Lainbach Valley, no polarity reversals have been observed, but a positive FT gives confidence on the reliability of the data. The primary palaeomagnetic directions, after correction for inclination shallowing, allow to precisely constrain the depositional palaeolatitude of the Tristel and Rehbreingraben Formations around similar to 28 degrees N. In a palaeogeographic reconstruction of the Alpine Tethys at the Barremian/Aptian boundary, the RDF is located on the western margin of the Brianconnais terrain, which was separated from the European continent by the narrow Valais Ocean.

Proterozoic carbonates of the Vindhyan Basin, India: Chemostratigraphy and diagenesis

The Vindhyan Basin of north-central India contains a thick, areally extensive, largely unmetamorphosed succession of sedimentary rocks deposited during the Proterozoic Eon. Despite a wealth of studies aimed at using Vindhyan Basin strata as an archive for Proterozoic global events, fundamental questions persist about basic aspects of Vindhyan Basin stratigraphy. Specifically, we lack detailed age constraints for units within the Upper Vindhyan succession, discrete correlation of units across the basin, and critical understanding of the extent to which diagenetic processes have compromised primary geochemical signals. Here, we focus on Vindhyan Basin carbonates—their chemostratigraphy and diagenesis. We present a new petrographic and geochemical dataset—including carbon isotopes, oxygen isotopes, and trace element signatures—for a variety of carbonate units exposed within the Vindhyan Basin, both in the Son Valley and Rajasthan sectors. Results of our diagenetic assessment indicate a range of preservation states in strata of the Lower Vindhyan sequence. The Bhagwanpura Limestone is pervasively dolomitized (largely fabric-destructive), and has been subjected to multiple generations of burial diagenesis in the presence of iron-rich, reducing fluids that altered oxygen isotope and trace element trends. By contrast, the Nimbahera Limestone is dominantly composed of micritic to microsparitic calcite that preserves largely marine isotopic and trace element signatures. The Tirohan Limestone is also pervasively dolomitized, but unlike the Bhagwanpura Limestone, dolomitization potentially occurred during early marine diagenesis, preserving seawater-like isotopic and trace element trends, with the exception of Sr depletion. Upper Vindhyan sequence carbonate units (the Lakheri and Bhander limestones) are largely characterized by micritic to microsparitic fabric-retentive microstructure, as well as isotopic and trace element trends that suggest diagenetic stabilization in the presence of marine fluids.Carbon isotope chemostratigraphy builds on previous work, and when interpreted in the context of recent geochronology, provides new information on the age of the Upper Vindhyan sequence. In the Lakheri Limestone, a stratigraphically coherent pattern of δ13C values between +2 and +3‰ with a minor negative excursion down to +1‰ is observed. This isotopic profile is consistent with global patterns of muted isotopic variability during the late Mesoproterozoic to earliest Neoproterozoic eras, thus supporting recent geochronology that suggests a much older age for the Upper Vindhyan sequence than traditionally thought, at least in the Rajasthan sector. Similar age revisions have recently been proposed for other ‘Purana’ basins in India. In Son Valley, however, strongly positive δ13C values (between +4 and +6‰) are recorded in the Bhander Limestone, suggesting that the Lakheri and Bhander limestones are not stratigraphic equivalents and that the Bhander Limestone may be fully Neoproterozoic in age. Ultimately, our data provide new diagenetic and chemostratigraphic information that will inform future studies of Vindhyan Basin strata.

Source rock evaluation of Middle Eocene–Early Miocene mudstones from the NE margin of the Black Sea

Abstract This study comprises the source rock evaluation of 122 Late Middle Eocene–Early Miocene mudstones from the NE margin of the Black Sea. Samples are immature to early mature. The majority of samples have moderate to very good organic richness, poor to moderate source potential and a hydrogen-deficient to gas-prone source rock quality. However, a significant proportion of the samples have good to excellent organic richness and source potential, and an oil- and gas-prone quality derived from amorphous-rich kerogens. These samples would generate significant amounts of oil and associated gas where buried to peak maturity. They come from the lowermost (Rupelian) part of the Maykop Series and the late Bartonian–early Priabonian Kuma Suite or its stratigraphic equivalents. The Rupelian source-rock interval(s) in west Georgia is at least 60 m thick and potentially as much as 200 m thick. It has a source potential index (SPI) of 0.7–2.5 t HC m −2 . The thickness of the Kuma Suite-equivalent source rock interval south of the western Greater Caucasus is unconstrained. Maykop Series source rocks occur in the Black Sea Basin. Prospective Kuma Suite-equivalent samples on both the northern and southern margins of the Black Sea imply that similar sediments may also be present in the basin.