Black Shale Deposition Research Articles

Carbonate shutdown, phosphogenesis and the variable style of marine anoxia in the late Famennian (Late Devonian) in western Laurentia

The shutdown of carbonate productivity and its replacement by black shale deposition is often observed in the geological record and yet the factors driving such a change are debated. The latest Famennian of western Laurentia (present day North America) provides a good example of a limestone – black shale transition that is roughly contemporaneous with the global Hangenberg Crisis – a series of environmental and biotic changes associated with the widespread development of black shale deposition. However, in western Laurentia limestone deposition ceased and black shale deposition began prior to this crisis. Examining the type section of the Exshaw Formation at Jura Creek (Alberta) reveals that declining seafloor oxygen levels were likely responsible for the loss of carbonate productivity, followed by phosphogenesis of the youngest carbonates. The top surface of the limestone is a hard ground that was reworked, possibly by internal wave action, producing a lag of phosphatic clasts and pyrite grains including giant framboids (reaching 100 μm diameter) and polyframboids. The subsequent redox history of the lower Exshaw Formation shows substantial variations. Initially, the occurrence of small framboids, and elevated FeHR/FeT and Fepy/FeHR combined with enhanced U, Mo and Re concentrations indicates intense euxinia in the water column. These conditions were replaced by ferruginous anoxic conditions recorded in radiolarian-rich black shales that have moderate concentrations of trace metals, and low pyrite content, as confirmed both by low Fepy/FeHR ratios and scanning electron microscope (SEM) observation. Mo-U covariation trends suggest that Mo enrichment under ferruginous conditions was promoted by drawdown in association with Fe minerals precipitated in the water column. The return of better ventilated conditions around the Devonian/Carboniferous boundary is marked by a decrease in trace metal content and the loss of syngenetic pyrite. The Hangenberg Crisis is thus marked by an improvement in ventilation, from euxinic to ferruginous conditions in our study, and evidence from other basins in western North America suggests an even greater increase in oxygenation at this time.

Rare earth and trace elements enrichment and implications in black shales of Safaga-Qussier sector, Egypt

The electronic revolution in recent years points out the importance of rare earth elements (REEs) and trace elements (TEs) in these strategic industries. This study aimed to assess REEs and TEs in black shales from Duwi Formation of two mines, Um Al-Huwaitat and Hamadat, located in the Safaga-Qussier region. The chemical results indicated oxic marine along with warm and humid climate conditions of this sediment deposition. Um Al-Huwaitat Mine was highly enriched with total REEs (average 255.3–325.3 ppm) than Hamadat (average 157–207.4 ppm). Thus, it revealed that the total REEs decreased from Safaga to Qussier direction. The average content of REEs has nearly the same decreasing order of elements; they are arranged in decreasing order: Ce> Y> La> Nd> Sc> Sm and Ce> Y> La> Nd> Sc> Sm for Hamadat and Um El-Huwaitat Mines, respectively. The REEs and TEs concentrations are normalized to the UCC to investigate their distribution patterns. REEs and TEs concentration in the studied samples were higher than UCC, especially Y, Sc, Cs, Co, Zn, Zr, Mo, Cd, Hf, Ta, W, Bi, Se, Br, I, Th and U. The enrichment factor showed that Um Al-Huwaitat black shale is extremely enriched with Y, Cs, Sr, Mo, Cd, W, Bi, Se, I and U; and very high enriched with Ce, Nd, Sc, Co, and Br. La, Sm, Cr, Mn, Ni, Zn, and Th revealed significant enrichment. Clay minerals, especially montmorillonite, have been the major contributor to the enrichment of shale with REEs and TEs. Such findings point towards shale mineralisation with these elements. The current results shed light on the benefit optimization of ceased phosphate mines in the study area through using the associated black shale deposits as a source of REEs and TEs.

Mineralogy and geochemistry of the Middle Permian Pingdiquan Formation black shales on the eastern margin of the Junggar Basin, north‐west China: Implications for palaeoenvironmental and organic matter accumulation analyses

The Middle Permian black shales from the eastern Junggar Basin of northern Xinjiang, north‐west China are one of the richest and thickest lacustrine hydrocarbon source rocks in China. The palaeoenvironmental settings and controlling factors of organic matter accumulation along the margins of the Middle Permian lacustrine basin remain unclear, although the same works have been undertaken in the centre of the Middle Permian lacustrine basins. Here, we performed a combination of mineralogical and geochemical analyses on the black shales of the Middle Permian Pingdiquan Formation from the eastern margin areas of the Junggar Basin. The results show that the Pingdiquan Formation black shales in the study areas are organic‐rich, with total organic carbon (TOC) contents of 0.74–17.61 wt% (avg. 5.81 wt%). Salinity proxies including Sr/Ba and Rb/K ratios, combined with traces of gypsum and halite in a few samples, suggest brackish to saline water conditions. Palaeoclimate proxies indicate a warm‐humid environment during the deposition of the Pingdiquan Formation black shales. The redox proxies V/(V + Ni), V/Cr and U/Th ratios, and Eu and Ce anomalies together indicate dysoxic‐anoxic conditions of lake water. Strong positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and TOC contents imply that palaeoproductivity may have played an essential role in organic matter accumulation during the deposition of the black shales. In contrast, negative correlations between terrestrial detrital input proxies (Al and Ti) and TOC contents indicate that terrestrial detrital input may have acted as a diluent to organic matter accumulation. Additionally, although the correlation between redox proxies [V/(V + Ni) and U/Th] and TOC contents is not straightforward, dysoxic‐anoxic conditions of lake water are an essential prerequisite for organic matter accumulation in the Pingdiquan Formation black shales.

Multielement Imaging Reveals the Diagenetic Features and Varied Water Redox Conditions of a Lacustrine Dolomite Nodule

The genesis of dolostone has long been puzzling for more than two centuries. Although much work has been done on investigating the process of dolomitization, little emphasis has been put on examining the diagenetic water redox condition with the wealthy geochemical information preserved in primary dolomite, which is believed to archive the aqueous environment as well as biotic and/or abiotic effects during formation. In situ interpretation with high resolution is a prerequisite in refined research of dolomite. Here, we reported the multielement imaging results of a lacustrine dolomite nodule with the host black shale from the Songliao Basin, northeast of China. Micro X-ray fluorescence (μ-XRF) with a spatial resolution down to 10 μm was used for in situ scanning. Two key parameter settings of the μ-XRF, including single-point exposure time and spatial resolution, were optimized to achieve a better result in a reasonable scanning time scale. The final imaging data graphically revealed dynamic variation of elemental distributions, including elements enriched in dolomite (e.g., Ca, Mg, Fe, and Mn), clastic quartz (Si), and clay minerals (e.g., Al and K) and redox-sensitive trace elements (e.g., Cr, Mo, V, and U). The well-preserved laminated structures inside the nodule and the features with a magnesium-rich core wrapped with an iron-concentric outer layer and a manganese-concentric shell together indicated its primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite. The elemental variation indicates a varied bottom water redox condition, which involved from sulfidic to ferruginous and manganous zones. Here, we propose that the intermittent supplies of sulfate and Fe-/Mn-oxidized minerals interrupting the black shale deposition while favoring dolomitization might be brought by the oxidized and salted seawater. And this lacustrine dolomite is expected to be a potential fingerprint mineral in tracking the seawater intrusions to the Songliao Basin which happened 91 million years ago.

Temporal and spatial differences between predicted and measured organic carbon in South Atlantic sediments: Constraints to organic facies modelling

The evaluation of total organic carbon (TOC) content from wells is a routine practice for assessing source rock potential, and empirical relationships to estimate TOC content have been proposed and used in organic facies modelling. However, no critical evaluations address how well the available TOC-predictive equations estimate the TOC content in recent and past sediments of the South Atlantic. In order to investigate the performance of the available TOC-predictive equations in this study, we compiled a large South Atlantic dataset composed of new and published modern TOC, dry bulk density, and sedimentation rate with the associated satellite-derived net primary productivity. We repeat this exercise for another time interval using the same input parameters but estimating net primary productivity using transfer functions. Our results indicate that all equations overestimate the modern TOC distribution, mainly at the continental margins and at the edges of the subtropical gyre. However, we must point out that this is partly due to our primary productivity values used as inputs, which exhibit higher values than the ones used in the calibration datasets of the TOC-predictive equations. In addition, the equations perform better, in terms of correlation, in the eastern compared to the western South Atlantic. We observe that in the west, the equation of Müller and Suess (1979) exhibited the highest correlation between predicted and measured TOC values, while in the east, the “Definition” equation using the carbon flux equation of Antia et al. (2001) and the burial efficiency equation of Betts and Holland (1991) showed the best results. In addition, we indicate that lateral transport of organic carbon at the margins might be influencing the correlation negatively. Overestimations the TOC content were also apparent for the downcore data, although with reduced magnitude, and correlation between predicted and measured TOC values was lower compared to the modern dataset. This can be explained by more sparse data in the glacial dataset and a strong lateral transport during this interval of low sea level. The reasons behind the different performances of equations are explained by the way they incorporate the input parameters into their formulations, which is especially relevant for primary productivity, sedimentation rates, and the inclusion of processes such as the carbon flux. We highlight that carbon flux equations should also be used with caution in very shallow and high productive systems, since they can produce unrealistic results. Our compilation indicates that the use of a single equation is not adequate to resolve the broad spatial TOC distribution in marine environments and that organic facies modelling software should consider regional aspects in order to produce adequate quantitative estimations of TOC content. Additionally, we used our dataset to investigate the optimal environmental conditions linked to organic-rich sediments in the South Atlantic. Our results show that high productivity, shallow depths, and optimal sedimentation conditions are required for the occurrence of organic-rich sediments. This can be viewed as important factors preconditioning the formation of black shales in the South Atlantic. Nevertheless, we highlight that orbitally-driven changes in hydroclimate and its impact in weathering-derived nutrient input probably played a major role on boosting productivity and preservation of marine organic carbon during sedimentation of high-TOC black shales in the South Atlantic.

Hydrocarbon seepage in the mid-Cretaceous greenhouse world: A new perspective from southern Tibet

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained δ13Ccarb value of −30.5‰ is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including 13C depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates – the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system – agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous.

Cyclic variations in paleoenvironment and organic matter accumulation of the Upper Ordovician–Lower Silurian black shale in the Middle Yangtze Region, South China: Implications for tectonic setting, paleoclimate, and sea-level change

During the Ordovician–Silurian transition, the Yangtze Block in South China experienced intense tectonic movements and climate change and was covered by the widely distributed black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation, which recorded key information about these major geological events. By geochemical and petrological analyses of three profiles located in the Middle Yangtze Region, at least four long-term sedimentary cycles can be identified in the Upper Ordovician–Lower Silurian black shale. These cycles are mainly controlled by paleoclimate changes and tectonic activity. The Late Ordovician tectonic subsidence related to the Kwangsian Orogeny promoted a gradual sea-level rise and the deposition of the Wufeng black shale in the Middle Yangtze Region, while a significant sea-level fall related to the Hirnantian intensifying glaciation terminated this process. Subsequently, post-Hirnantian warming climate and glacier ablation triggered a significant sea-level rise, reduced the water restriction, increased the productivity, and intensified the anoxic environment, promoting the deposition of black shale in the second sedimentary cycle. However, after the short period of climate warming, global climate cooling occurred again with a regional tectonic uplift in the study area. This led to a gradual sea-level fall, reduced anoxic environment, and increased water restriction during the Rhuddanian, as well as the deposition of black shale with a decreased amount of organic matter. Moreover, intensified glaciation occurred during the Rhuddanian–Aeronian transition significantly accelerated the falling speed of the relative sea level, resulting in a stratigraphic gap and a small thickness of the Silurian black shale in the Middle Yangtze Region. Afterward, the study area experienced a significant climate warming event during the Early Aeronian, which led to a large-scale glacier ablation and a short period of sea-level rise. This promoted the deposition of the Aeronian black shale in the third sedimentary cycle. Nevertheless, the intensified weathering condition related to a continuously warming climate and tectonic uplift led to sufficient supply of terrigenous deposits, sustained sea-level fall, and finally terminated the development of Ordovician–Silurian black shale in the Yangtze region. Thus, these cyclic variations of the black shale not only recorded the shift of the Ordovician–Silurian tectonic movement (from Ordovician subsidence to Silurian uplift) in the Middle Yangtze area and the Hirnantian glaciation, but also provide strong evidence for the global intensified glaciation during the Rhuddanian–Aeronian transition and the subsequent climate-warming event. In addition, this study also provides new perspectives for the understanding of global sea-level change during the Ordovician–Silurian transition.

Increased biomass and carbon burial 2 billion years ago triggered mountain building

The geological record following the c. 2.3 billion years old Great Oxidation Event includes evidence for anomalously high burial of organic carbon and the emergence of widespread mountain building. Both carbon burial and orogeny occurred globally over the period 2.1 to 1.8 billion years ago. Prolific cyanobacteria were preserved as peak black shale sedimentation and abundant graphite. In numerous orogens, the exceptionally carbonaceous sediments were strongly deformed by thrusting, folding, and shearing. Here an assessment of the timing of Palaeoproterozoic carbon burial and peak deformation/metamorphism in 20 orogens shows that orogeny consistently occurred less than 200 million years after sedimentation, in a time frame comparable to that of orogens through the Phanerozoic. This implies that the high carbon burial played a critical role in reducing frictional strength and lubricating compressive deformation, which allowed crustal thickening to build Palaeoproterozoic mountain belts. Further, this episode left a legacy of weakening and deformation in 2 billion year-old crust which has supported subsequent orogenies up to the building of the Himalayas today. The link between Palaeoproterozoic biomass and long-term deformation of the Earth’s crust demonstrates the integral relationship between biosphere and lithosphere.

Application of molybdenum and thallium isotopes as indicators of paleoredox conditions and genesis of hyper-enriched black shale deposits, Peel River, Yukon, Canada

ABSTRACT Hyper-enriched black shale (HEBS) deposits in northern Yukon, consist of thin (<10 cm), laterally extensive (tens of thousands of km2) stratiform sulfide mineralization layer(s) that are hyper-enriched in Ni, Mo, Zn, platinum group elements, Re, and Au. The genesis of HEBS deposits and the ambient paleoenvironment in which they formed are the subject of vigorous debate. Non-traditional stable isotopes, particularly molybdenum and thallium, are robust paleoredox indicators and we have employed these isotope systems in this study of Yukon HEBS. Systematic sampling and Mo and Tl isotopic analysis of a continuous 200 m stratigraphic section through the Yukon HEBS mineralization and footwall and hanging-wall strata at the Peel River north and south bank showings (spaced five km apart) give δ98Mo –1.24 to –0.53‰ and –8.1 to –5.2 ε-units for the mineralization and –0.70 to 0.60‰ and –6.5 to –2.0 ε-units for the unmineralized strata. These values preclude a hydrothermal origin and strongly suggest that redox processes were responsible for the Yukon HEBS mineralization. The isotopic compositions, together with rare earth element (REE) systematics (REE profile, Y positive anomalies, Ce negative anomalies, and Y/Ho values) and other bulk geochemical redox indicators (Mo, V, Re/Mo, Ni/Co, U/Th, and V/Cr) indicate that the Peel River HEBS mineralization formed because of metal scavenging from seawater in a quiescent, euxinic basinal paleoenvironment.

Age of the lower Cambrian Vanadium deposit, East Guizhou, South China: Evidences from age of tuff and carbon isotope analysis along the Bagong section

Abstract The early Cambrian is a critical interval of dramatic oceanic and biochemical changes in geological history. The black shale deposits, which are rich in Mo, Ni, V, and platinum group elements (PGE), are a reflection of that interval. Among all known Cambrian black shale deposits in South China, the vanadium deposits are poorly constrained by geochronology. The newly discovered tuff layer in the Sansui Bagong vanadium deposit in Guizhou Province can provide excellent constraints on the age of vanadium deposits. In this study, we obtain a new zircon U–Pb isotopic age, which can constrain the age of the vanadium deposit. This tuff occurs in the middle part of the ore bed, and the age of the tuff layer can reflect the mineralization age of the V deposit. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used in this study to obtain the zircon U–Pb age of the tuff and indicates that the event of volcanic activity took place at 520.9 ± 1 Ma. This age is close to the boundary between the Cambrian Terreneuvian and Series 2. This result provides a good constraint on the age of the vanadium deposits in South China and makes the vanadium deposits to be comparable with other Mo–Ni–PGE deposits. The alteration of organic carbon isotope (δ 13Corg) values can reflect changes in the marine environment and is widely used in stratigraphic correlation. The analysis of the δ 13Corg values of the ore bed in the present study reveals a positive excursion at the bottom of the deposit and a negative excursion in the V-enriched layer. The δ 13Corg values in the Bagong section are comparable to those in the Xiaotan and Longbizui sections. The ages of the Sansui vanadium deposit constrained by the U–Pb isotopic age of the tuff and the δ 13Corg values are consistent. All of the data lead us to infer that the vanadium deposit formed at approximately 521 Ma.

The influence of paleoclimate and a marine transgression event on organic matter accumulation in lacustrine black shales from the Late Cretaceous, southern Songliao Basin, Northeast China

Lacustrine black shales are widely developed within the Late Cretaceous Nenjiang Formation (k2n) of the Songliao Basin. These black shales provide an ideal natural laboratory for reconstructing the paleoclimate and paleoenvironment during the Late Cretaceous in Northeast China and have great significance in the exploration for potential oil and gas resources. According to the systematic analysis of organic carbon isotopes, elemental geochemistry and clay mineral composition, the organic matter (OM) source, paleoenvironment, paleoclimate, paleoproductivity and marine transgression event of the Nen 1 member (k2n1) were investigated, and the influence of the paleoclimate and marine transgression event on OM accumulation in lacustrine black shales was revealed. Black shales in the lower unit are characterized by a high OM abundance (max. 4.19 wt%, avg. 3.02 wt%), I-II1 kerogen (with abundant lamalginite) and low maturity. The element ratios (Fe/Mn, Mg/Ca and Sr/Cu), paleoweathering indexes (CIA and PIA), δ13Corg values and clay mineral composition together indicate a warm paleoclimate in the Late Cretaceous. The humid paleoclimate promoted the flourishing of lacustrine microalgae and surrounding terrestrial vascular plants and enhanced the lake level, conducive to favorable sources and preservation conditions for OM accumulation. Based on paleontological and radioisotope dating, the global paleotemperature rise in the Late Santonian stage probably led to the marine transgression event. It can be explained by the high lake level and sea-level conditions under the global humid climate background that resulted in the Songliao paleolake connecting with the paleo-Pacific Ocean. Alkaline and saline seawater incursion into the paleolake increased the salinity of the lacustrine water column and further aggravated the anoxic conditions of the lake, and abundant marine nutrient inputs promoted the primary bioproductivity and contributed to the accumulation of OM. The discovery of the OM accumulation mechanism under the influence of the warm-humid paleoclimate and intermittent marine transgression will add critical geological constraints on further studies of geological events in the deposition of late Cretaceous black shale.

Graphite from Palaeoproterozoic enhanced carbon burial, and its metallogenic legacy

AbstractThe episode of widespread organic carbon deposition marked by peak black shale sedimentation during the Palaeoproterozoic is also reflected in exceptionally abundant graphite deposits of this age. Worldwide anoxic/euxinic sediments were preserved as a deep crustal reservoir of both organic carbon, and sulphur in accompanying pyrite, both commonly >1 wt %. The carbon- and sulphur-rich Palaeoproterozoic crust interacted with mafic magma to cause Ni–Co–Cu–PGE mineralization over the next billion years, and much uranium currently produced is from Mesoproterozoic deposits nucleated upon older Palaeoproterozoic graphite. Palaeoproterozoic carbon deposition has thus left a unique legacy of both graphite deposits and long-term ore deposition.

Geology and gold content of carbonaceous shale in Baikal mineralization site, Southern Ural

The authors analyze gold mineralization in Baikal site inside Kumak ore field, adjoined to Bredin black shale sediments (С1bd). It is found that the gold mineralization within the site adjoins mostly bands of quart–micaceous–tourmaline metasomatically altered carbonaceous shale. Float sampling of all local rock varieties shows commercial content of gold (to 6.5 g/t) and stable high content of silver (to 7.6 g/t). Gold is mostly finely dispersed, and is connected with two basic mineral associations: gold–bismuth–telluride and native gold intergrown with tourmaline. The analysis of compositions of gold particles reveals their high carat type. Chemically, tourmaline belongs to dravite and foitite, and is similar to tourmaline from the deposits of orogenic gold and goldbearing sulfide ores. Tight accretion of fine needle-like tourmaline and gold implies their synchronous formation, and allows assessing Baikal site as the quartz–tourmaline gold ore formation. The study was carried out under State Contract Topic No. 0246-2019-0078, the microprobe analyses were performed under State-Financed Topic No. AAAA-A19-119072390050-9 and were supported by Orenburgneft JSC in the framework of a corporate grant, 2019–2020.The authors are extremely grateful to S. A. Yagudina, T. I. Chernikova and I. V. Smoleva for the analysis execution, to E. O. Kalistratova for the help in heavy concentrate description, as well as to R. S. Kisil and V. S. Panteleev for the assistance in field works.

Dramatic attenuation of continental weathering during the Ediacaran-Cambrian transition: Implications for the climatic-oceanic-biological co-evolution

The Ediacaran–Cambrian (E-C) transition witnessed dramatic tectonic, climatic, oceanic, and biological co-evolutions at the dawn of the ‘Cambrian explosion’. Although glaciations were reported in some mid-high-latitude continents during this interval, large uncertainties remain concerning timing and extent, as well as their effect on the entire Earth's surface system. We here report magnesium (Mg) isotopic compositions, along with chemical index of alteration (CIA), Al/Ti ratios from a time-dated, nonglacial E-C boundary section on the mid-low-latitude Yangtze Block, South China, to constrain chemical weathering intensity and associated climatic, oceanic and biological co-evolution during this critical interval. Our data demonstrate a rapid and significant negative δ26Mg excursion (from +1.28 to −0.34‰) along with similar CIA and Al/Ti decreases across the E-C boundary, reflecting an apparent attenuation in chemical weathering intensity likely in response to severe climate cooling. This finding further affirms the far-field glaciation reported in the mid-high-latitude continents during the E-C transition. Under this condition, the intense climate cooling could have enhanced oceanic circulation and/or upwelling, and simultaneously caused prominent variations of seawater geochemistry (including alkalinity elevation) and nutrient flux as well as ecological stresses on metazoans. All these could reasonably account for extensive deposition of phosphorites and organic-rich black shales, and faunal turnover (extinction of Ediacaran fauna and appearance of skeletonized small shelly fauna) across this interval. These novel datasets thus shed new insights into the climatic-oceanic-biological interactions and time-specific deposition around the E–C transition at the prelude of the Cambrian explosive bioradiation.

Nannoplankton and 13C and 13O stable isotope stratigraphy record of the mid Cretaceous sequences, Zagros Basin (western Iran)

Deposition of black shales and dark argillaceous limestones across the Cenomanian to Turonian interval has been recorded in many parts of the world. In this regard, a stratigraphic section in the Iran's hydrocarbon field in the Kermanshah region in the Zagros Basin has been studied in detail to reconstruct palaeoceanographic conditions under a reliable biostratigraphic framework. Biostratigraphy based on calcareous nannofossils integrated with high resolution carbonate and oxygen stable isotope analyses were carried out on the Cenomanian-early Turonian interval of the upper part of Garau Formation. In this formation, 24 species of calcareous nannofossils were identified in the pelagic marine deposits. Our study allowed the definition of nannoplankton bio zones from the upper part of Eiffellithus turriseiffelii Zone to the lower part of Quadrum gartneri Zone in the mid Cretaceous. This research first recorded the upper part of the Garau Formation at site and it is consistent with the previous bio zones identified in the Tethyan domain in the region. The middle Cretaceous events of calcareous nannofossils proposed here correlate with the isotope global profile for the early Cenomanian-earliest Turonian interval and with those, reported for the southeastern France and south England reference curves, represented by the positive Cenomanian to Turonian δ13C excursion. Thus, the sequences of dark argillaceous limestone rich in organic matter in Kermanshah region can be considered as a continuation of the global trend in the Tethyan domain, where the Cenomanian-Turonian Oceanic Anoxic Event constrained to about 91.5 ± 8.6 millions of years, occurred in an anoxic pelagic sedimentary environment correlates with both, global and local sea-level rises, represented in this study by the characterization of the nannoflora assemblage reflected in a variety of widespread paleoenvironmental changes that coincides with significant shifts in the stable carbon and oxygen isotope signature.

First evidence of the early cretaceous oceanic anoxic events (MBE and OAE1a) in the southern Tethyan margin (NE-Tunisia): biostratigraphy and shale resource system

The Jebel Oust region (north-eastern Tunisia) recorded two levels of marine black shale in the Lower Cretaceous marly series. Geodynamic evolution, biostratigraphic and Rock–Eval analysies allow classifying those black shales as unconventional shale oil resource systems that were deposited during two oceanic anoxic events: the Middel Barremian Event "MBE" and the Early Aptian Event "OAE1a". Paleogeographic evolution highlights two transgressive–regressive cycles: the first one is Valanginian-Early Barremian, and the second is Late Barremian–Early Aptian. Each black shale deposit occurs at the end of the transgression that coincides with the highest sea level. During the Barreman–Aptian interval, sedimentation was controlled by extensional faults in a system of tilted fault blocks which were reactivated several times. Kerogen is of type I, II origin in black shales and of type III origin in marls. Tmax values indicate "oil window" stage. Average transformation ratio is around 67% and 82%, respectively, in the Lower Aptian and Middel Barremian source rock related to the relatively high thermal maturity degree due to the deep burial of the later. Estimated initial hydrocarbon generation potential is moderate to high. Oil saturation index records an "oil crossover" indicating expelled and migrated hydrocarbons from the organic-rich to the organic-poor facies. The petroleum system of the two mature source rocks with a high hydrocarbon generation potential enclose all elements characterizing a "shale oil hybrid system with a combination of juxtaposed organic-rich and organic-lean facies associated with open fractures".

Features of the Formation of the Golets Vysoschaishii Gold-Ore Black-Shale Deposit (Bodaibo Ore District)

Features of the Formation of the Golets Vysoschaishii Gold-Ore Black-Shale Deposit (Bodaibo Ore District)

Driving mechanisms of organic carbon burial in the Early Cretaceous South Atlantic Cape Basin (DSDP Site 361)

Abstract. Extensive black shale deposits formed in the Early Cretaceous South Atlantic, supporting the notion that this emerging ocean basin was a globally important site of organic carbon burial. The magnitude of organic carbon burial in marine basins is known to be controlled by various tectonic, oceanographic, hydrological, and climatic processes acting on different temporal and spatial scales, the nature and relative importance of which are poorly understood for the young South Atlantic. Here we present new bulk and molecular geochemical data from an Aptian–Albian sediment record recovered from the deep Cape Basin at Deep Sea Drilling Project (DSDP) Site 361, which we combine with general circulation model results to identify driving mechanisms of organic carbon burial. A multimillion-year decrease (i.e., Early Aptian–Albian) in organic carbon burial, reflected in a lithological succession of black shale, gray shale, and red beds, was caused by increasing bottom water oxygenation due to abating hydrographic restriction via South Atlantic–Southern Ocean gateways. These results emphasize basin evolution and ocean gateway development as a decisive primary control on enhanced organic carbon preservation in the Cape Basin at geological timescales (> 1 Myr). The Early Aptian black shale sequence comprises alternations of shales with high (> 6 %) and relatively low (∼ 3.5 %) organic carbon content of marine sources, the former being deposited during the global Oceanic Anoxic Event (OAE) 1a, as well as during repetitive intervals before and after OAE 1a. In all cases, these short-term intervals of enhanced organic carbon burial coincided with strong influxes of sediments derived from the proximal African continent, indicating closely coupled climate–land–ocean interactions. Supported by our model results, we show that fluctuations in weathering-derived nutrient input from the southern African continent, linked to changes in orbitally driven humidity and aridity, were the underlying drivers of repetitive episodes of enhanced organic carbon burial in the deep Cape Basin. These results suggest that deep marine environments of emerging ocean basins responded sensitively and directly to short-term fluctuations in riverine nutrient fluxes. We explain this relationship using the lack of wide and mature continental shelf seas that could have acted as a barrier or filter for nutrient transfer from the continent into the deep ocean.

High-frequency redox variation across the Ordovician–Silurian transition, South China

Mass extinction and extensive black shale deposition were associated with widespread ocean anoxia during the Ordovician–Silurian transitional interval, yet uncertainty among trace-metal proxies has precluded an improved understanding of redox dynamics at this time of Earth history and their relationship with animal evolution and accumulation of organic-rich shale. Herein, we describe results of a geochemical investigation of the Qianqian #1 section of South China that preserves the stratigraphic record of the inner shelf of the Upper Yangtze Platform. Specifically, newly acquired total organic carbon (TOC), total sulfur (TS), iron (Fe) speciation, and major element and trace metal abundances are considered with existing data from the studied section. Our results reveal that redox trends suggested by Fe speciation data are generally consistent with those based on Corg/P ratios and redox-sensitive trace element (RSTE) enrichment. However, documented differences among redox histories based on Fe speciation, Corg/P ratios, and RSTE enrichment may reflect the influence of high-frequency redox fluctuations. Comparison of bimetal ratios (U/Th, V/Cr, Ni/Co, and V/(V + Ni)) to Fe speciation, Corg/P ratios, and RSTE enrichment suggest that commonly cited thresholds of bimetal ratios for various redox states are unreliable, though variations in U/Th, V/Cr, and Ni/Co may provide a more robust indication of relative redox changes. A documented strong correlation of changing redox conditions and fossil distributions suggests that largely dynamic and ferruginous, perhaps euxinic, conditions contributed to biotic crisis and recovery. A strong coupling of TOC and redox proxies suggests that protracted anoxia or euxinia is critical to the accumulation and preservation of organic matter. Our study demonstrates that redox evaluations that rely upon a single geochemical proxy or elemental ratio are likely to be uncertain. Rather, multiple geochemical proxies are required to explore relationships among redox states, animal evolution, and deposition and preservation of organic-rich shale.

Novel watermass reconstruction in the Early Mississippian Appalachian Seaway based on integrated proxy records of redox and salinity

Redox state and salinity are fundamental properties of watermasses, and in modern environments, detailed analysis of spatial variability in redox and salinity is possible through direct measurement. Watermass reconstruction is difficult in deep-time systems, however, because the sedimentary record of ancient watermasses is often incomplete or difficult to access on spatial scales large enough to permit basin-scale reconstruction. During the Middle Devonian to Early Mississippian periods, the Appalachian Basin of eastern North America was characterized by a variably restricted watermass, leading to extensive deposition of organic-rich black shale associated with the regional development of bottom-water anoxia. Watermass reconstruction during deposition of these units has previously been attempted, but only on small spatial scales, with most studies focused on trace element proxies applied to individual study sections. Here, we focus instead on a broader geographic reconstruction of watermasses in the Lower Mississippian (Tournaisian) Sunbury Shale and present a new geochemical dataset that spans five drill cores across a basin transect from northeastern Ohio to southern Kentucky (USA). Iron speciation and trace metal abundances reveal a strong paleoredox gradient that can be related to water depth and basin hydrography. In the northeastern-most core of Ohio, iron speciation reveals largely ambiguous redox conditions; however, oxic conditions are potentially indicated by very low total organic carbon and trace metal contents. These oxygenated conditions were likely related to proximity to the Catskill deltaic wedge complex that fed freshwater into the basin and lowered the position of the regional pycnocline. In contrast, in central and southern Ohio, paleoredox indicators reveal the development of ferruginous conditions associated with rapid deepening to the south into the central axis of the Appalachian Basin. Further to the south in Kentucky, our data reveal dominantly euxinic conditions, which developed as environments shoaled towards the basin-bounding Cumberland Sill. We also investigated paleosalinity across the Appalachian Basin using strontium/barium (Sr/Ba) ratios. Our data reveal a subtle paleosalinity gradient from low-brackish (i.e., close to freshwater) conditions close to the Catskill Delta complex to increasingly brackish conditions to the southwest. This trend was disrupted by large paleosalinity variation across a shallow mixing zone in the southwestern-most core. Lastly, strong enrichments of redox-sensitive trace metals in both ferruginous and euxinic environments are parsimoniously attributed to the operation of an iron-manganese particulate shuttle that continuously supplied oxide-sensitive metals to the Appalachian Basin. Ultimately, this study is among the first to present detailed watermass reconstruction in an ancient epeiric sea using both redox and salinity proxies applied over a large lateral transect. We suggest that a similar approach may be useful in other deep-time systems of geobiological, paleoclimatic, and paleoceanographic significance.