Black Shale Deposition Research Articles

Tracing bottom-water redox conditions during deposition of Lower and Upper Jurassic organic-rich sedimentary rocks in the Lusitanian Basin (Portugal): Insights from inorganic geochemistry

The Lower Jurassic Água de Madeiros (AM Fm) and Vale das Fontes (VF Fm) Formations, and the Upper Jurassic Cabaços Formation (Cab Fm) are considered the most important units with petroleum source-rock potential in the Lusitanian Basin, and the likely sources for its widespread hydrocarbon occurrences. Nonetheless, the bottom-water redox conditions during their deposition remain debatable. To resolve this controversy, we investigated whole-rock inorganic geochemical data of the most significant organic-rich levels of the AM, VF and Cab Fms cropping out at basin scale. Significant variations in TOC, TS and redox-sensitive elements contents and their ratios are evident within and between the organic-rich levels of the studied units. A positive covariation between primary productivity proxies, TOC enrichment and paleo-redox proxies for the Lower Jurassic units clearly suggest an interdependence between enhanced primary productivity, organic matter accumulation and bottom-water redox conditions. For the AM Fm, the paleo-redox proxies indicate that the black shales deposition took place beneath bottom-waters that ranged from suboxic to strongly euxinic conditions, with a clear predominance of the latter. A TOC threshold of ~6–7 wt% clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions, and is interpreted as the minimum amount of organic matter (OM) accumulation within sediments required to sustain H2S production at rates that outpaced its consumption through precipitation of sulphides and/or OM sulphurization, allowing the diffusion of H2S to the bottom-waters. For the Vale das Fontes Fm, the paleo-redox proxies suggest a prevalence of suboxic to anoxic bottom-waters, despite all black shales displaying TOC contents greater than the above threshold. The less reducing conditions relative to the AM Fm euxinic black shales are attributed to: i) a larger proportion of continental OM; ii) lower reactivity of the OM reaching the sediment-water interface (decreasing oxygen consumption); and iii) higher availability of reactive iron (increasing the buffer effect to H2S produced through OM remineralization by sulphate-reducing bacteria). Both units display Mo-EF vs U-EF covariation patterns that deviate from the field of sediments deposited under unrestricted open-marine conditions, towards higher Mo enrichment relative to that of U, indicating an accelerated transport of aqueous Mo to the sediments, consistent with the operation of a metal-oxyhydroxide particulate shuttle linked to Mn and Fe redox cycling within the water column in a weakly-restricted basin. The Mo-COT regression-line slope indicates a moderate degree of bottom-water mass restriction, with [Mo]aq concentration of ~60–70% of the present-day seawater value and bottom-water renewal times of ~10–20 years, akin to the present-day Cariaco Basin. The redox conditions of the Cab Fm organic-rich levels are dependent on the location of the depositional environment in the basin, with deposition taking place beneath oxygenated bottom-waters at Cabo Mondego, and under suboxic conditions at Vale de Ventos and Pedrógão, the latter denoting a larger degree of oxygen depletion than the former. In this context, the overall potential for OM preservation should increase from Cabo Mondego to Vale de Ventos and Pedrógão.

Early to Middle Devonian ironstone and phosphorite in the northwestern Gondwana Parnaíba Basin, Brazil: A record of an epeiric margin paleoceanographic changes

Pragian to Givetian successions in the Canindé group, Parnaíba Basin (Brazil), present phosphorites, ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering, ocean oxygen levels and black shale deposition events. These facies are associated with the evolution of the northern Gondwana epeiric sea margin of the Rheic Ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record. We conducted an integrated study based on sedimentology, sequence stratigraphy, petrography, scanning electron microscopy and X-ray diffraction to understand the paleoenvironmental, stratigraphic and diagenetic controls. Lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences. Late Eifelian and Givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones. Pragian-Emsian to Givetian ironstones were represented by authigenic coated phosphatic iron silicate (odinite, chamosite/clinochlore, francolite) and siderite precipitation. These are related to normal regressions under lowstand and highstand systems tract. Oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained Fe, Si and P supply on a storm-influenced to storm-dominated shelf with an additional source of iron also associated with river discharges. Phosphorite was formed in a stratified ocean, near storm wave base, under anoxic (sulfide reducing bacteria microbial communities) and suboxic conditions (productivity fueled by recycled P). Shallower anoxic conditions (near the fair weather wave base) existed, possible related to a shift in the locus of upwelling, and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level, ventilation, terrigenous or marine contribution and progressive microbial degradation. At the beginning of late Eifelian transgression, an event of increased delivery of continental flux and eutrophication can also be proposed. The link between organic-rich intervals and third order black shale events (Daleje, Chotěc and Kačak) suggests that extinction events can be tracked by analyzing sections in South America. Global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of C sink conditions probably contributed to the ocean oxygenation.

Size variations and abundance patterns of calcareous nannofossils in mid Barremian black shales of the Boreal Realm (Lower Saxony Basin)

Abstract We present new quantitative calcareous nannofossil data from a Barremian (Early Cretaceous) black shale interval of the Boreal Realm. Thirty samples from two sections in northern Germany have been investigated. The current study aims to understand the genesis of the Barremian black shales, so far only described from the Boreal Realm, in a supraregional to global context by assessing the diversity patterns, relative abundances and biometry of calcareous nannofossils from a distinctive late early Barremian black shale horizon. For morphometric size measurements, three common taxa, Biscutum constans, Rhagodiscus asper and Watznaueria barnesiae have been investigated. The results show a decrease in size and abundance of the high fertility indicator B. constans for the main black shale horizon (Hauptblatterton), while abundances of the warm water species R. asper and Watznaueria spp. increase. Deep dwelling nannoconids only flourished during black shale deposition. This indicates warm, oligotrophic surface waters and an expanded photic zone during the formation of the Hauptblatterton black shales. Similar observations have been made for the Oceanic Anoxic Event 1d (OAE 1d, Albian) in the Vocontian Basin (Southern France) and for the OAE 2 (Cenomanian) in the Western Interior Seaway and southern England. For W. barnesiae our data confirm a high ecological tolerance and we assume a response in size if a certain limit is exceeded towards more extreme conditions. Nutrients therefore likely play a larger role than water temperature. From our results, it seems likely that nutrient availability controls both, size and abundance patterns of B. constans.

Neritic ecosystem response to Oceanic Anoxic Event 2 in the Cretaceous Western Interior Seaway, USA

Cretaceous oceanic anoxic events (OAEs) were periods of geologically short (<1 million years) global change characterized by elevated temperatures, changes in ocean biogeochemistry, ecological turnover, and the global-scale deposition of black shales. After decades of OAE research, the intensity and spatiotemporal heterogeneity of ocean anoxia and its direct effects on marine ecology remain areas of active study. We present high-resolution organic geochemical and foraminiferal records from the western margin of the Western Interior Seaway (WIS) during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2, ~94 Ma) that indicate reorganization of a neritic ecosystem in response to sea-level rise, and dynamic changes in redox conditions that were likely driven by enhanced marine productivity. A significant transgression prior to OAE2 decreased terrestrial organic matter input and led to enhanced productivity, anoxic bottom waters and sediments, and episodic photic zone euxinia. During the middle of OAE2, further enhanced productivity led to the “Heterohelix shift” in planktic foraminifera, the “Gavelinella acme” in benthic foraminifera, and deoxygenation in sediments, bottom waters, and the upper water column. The combined use of high-temporal resolution algal, bacterial, and terrestrial biomarkers, in addition to foraminiferal records, demonstrates the sensitivity of marine autotrophic and heterotrophic communities to sea-level rise and variable water column oxygenation in marginal marine settings during hyperthermal events. Furthermore, our results reveal the temporal and spatial heterogeneity of anoxia during OAE2 in the WIS. This study highlights widespread shallow marine feedbacks during global change that may affect neritic ecosystems under future warming scenarios.

A Middle Devonian basin-scale precious metal enrichment event across northern Yukon (Canada)

AbstractHyper-enriched black shale (HEBS) Ni-Mo-Zn-Pt-Pd-Au-Re mineralization is geographically widespread across the Richardson trough in northern Yukon (Canada), where it discontinuously outcrops at the regional contact between the Road River Group and overlying Canol Formation. Stratigraphic relationships indicate that the contact is Middle Devonian, but there are no precise age constraints for the HEBS. We apply Re-Os geochronology to HEBS mineralization from two localities that are 130 km apart, the Nick prospect and the Peel River showing, to date directly the age of sulfide mineralization. The Nick prospect yields an isochron age of 390.7 ± 5.1 (2σ) Ma, whereas the Peel River showing yields an isochron age of 387.5 ± 4.4 (2σ) Ma. Within error, these ages are identical and overlap with the biostratigraphically constrained age of the sedimentary host rocks, indicating that mineralization and sedimentation were coeval. Significantly, the ages of the HEBS overlap those of Middle Devonian Kačák, pumilio, and Taghanic global-scale biotic events which are characterized by eustatic sea-level rise and black shale deposition. Linkage of the Yukon HEBS to one (or more) of these bio-events indicates that sea-level rise may have been requisite to formation of basin-scale HEBS mineralization in northwestern Canada during latest Eifelian and Givetian time.

Orbital forcing of Triassic megamonsoon activity documented in lacustrine sediments from Ordos Basin, China

The “hothouse” Triassic climate provides a key window to study megamonsoon systems. However, high-quality paleoclimate proxy records are rarely reported from the Tethys realm, where the seasonal meridional migration of the intertropical convergence zone (ITCZ) should be the most remarkable. Here, continuous and well-dated proxy records are used to study monsoon activity in the northeastern Tethys from the Middle Triassic lacustrine sediments (Ch7 black shale, Yanchang Formation) in the Ordos Basin (China). The proxies, including natural gamma-ray intensity, magnetic susceptibility, and element concentrations and ratios (U, K, Zr/Al, Zr/Rb and Sr/Rb), document a strong precession signal in the sediments. We propose that the periodic deposition of the Ch7 black shale occurred in response to intensifications in a monsoon-driven climate enhanced by the early Ladinian warming event. Climate variability in the Ordos Basin (~25°N) reflects the harmonics of ~100 kyr, ~20 kyr and ~10 kyr cycles and is highly coherent with solar insolation at 9° latitude, suggesting that this monsoon system might have been modulated by tropical solar insolation. The dominance of tropical climate processes in this mid-latitude monsoon region can be attributed to the dramatic dynamics of the ITCZ and intense monsoon circulation in the Tethys realm.

Enhanced Arctic-Tethys connectivity ended the Toarcian Oceanic Anoxic Event in NW Europe

AbstractThe Toarcian Oceanic Anoxic Event (T-OAE, c. 182 Ma) represents a major perturbation of the carbon cycle marked by widespread black shale deposition. Consequently, the onset of the T-OAE has been linked to the combined effects of global warming, high productivity, basin restriction and salinity stratification. However, the processes that led to termination of the event remain elusive. Here, we present palynological data from Arctic Siberia (Russia), the Viking Corridor (offshore Norway) and the Yorkshire Coast (UK), all spanning the upper Pliensbachian – upper Toarcian stages. Rather than a ‘dinoflagellate cyst black-out’, as recorded in T-OAE strata of NW Europe, both the Arctic and Viking Corridor records show high abundance and dinoflagellate diversity throughout the T-OAE interval as calibrated by C-isotope records. Significantly, in the Arctic Sea and Viking Corridor, numerous species of the Parvocysta and Phallocysta suites make their first appearance in the lower Toarcian Falciferum Zone much earlier than in Europe, where these key dinoflagellate species appeared suddenly during the Bifrons Zone. Our results indicate migrations of Arctic dinoflagellate species, driven by relative sea-level rise in the Viking Corridor and the establishment of a S-directed circulation from the Arctic Sea into the Tethys Ocean. The results support oceanographic models, but are at odds with some interpretations based on geochemical proxies. The migration of Arctic dinoflagellate species coincides with the end of the T-OAE and marks the arrival of oxygenated, low-salinity Arctic waters, triggering a regime change from persistent euxinia to more dynamic oxygen conditions.

Organic geochemical signals of freshwater dynamics controlling salinity stratification in organic-rich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil)

The Lower Permian Irati Formation in the northeastern and central eastern Paraná Basin (southern Brazil) was investigated in terms of bulk and molecular organic geochemistry in order to enlighten the complex depositional paleoenvironment during deposition of Irati black shales. The geochemical data reflect temporal and spatial variations of freshwater incursions promoting salinity stratification and high primary productivity in surface waters with the establishment of photic zone euxinia. Overall, highly concentrated C17 long-chain alkylnaphthalenes point to algal blooms as a result of freshwater inflows into the Irati Sea, mostly in the central eastern basin segment, and resemble concentration variations of pristane, phytane and nC17. The freshwater inflows were also retraced by increasing chroman ratios toward the top of the investigated units along with the general decrease of further palaeosalinity indicators, e.g., gammacerane, tetracyclic (C24), β-carotane, and the herein suggested γ-carotane and lexane indexes. The existence of a water column stratification promoting organic matter preservation is supported by the detection of tetrahydrophenanthrene at high concentrations, in addition to the known prominent occurrence of gammacerane. The detection of C10 to C31 aryl isoprenoids in addition to C16 and C18 pseudohomologue aryl isoprenoids points to the presence of photic zone euxinia. In general, samples from the central eastern basin were deposited in a deeper marine setting with lower salinity than samples from the northeastern basin, which displays signals of a shallower marine environment with reducing bottom water conditions, both deposited under freshwater influxes.

Empty cephalopod conchs as substrates for gastropod eggs from the Hangenberg Black Shale (Late Devonian) of the Maïder Basin (Morocco)

In the latest Famennian, black shale deposition occurred in many regions, some suggested a marine transgression as the explanation while others saw a link with higher organic input from the land. In either case, the Hangenberg Black Shale was most likely deposited under low oxygen conditions, which enabled exceptional fossil preservation in some regions such as the southern Maïder Basin of Morocco. Here, we provide the first account of small circular structures, in some cases with thin carbon films, which remained on ammonoid and bactritoid conchs. Due to similarities in their morphology and size with other occurrences documented from Europe, we propose that these structures may represent gastropod eggs. We also discuss whether egg-deposition occurred in the water column or on the sediment surface.

Rhenium‑osmium geochronology of the Toarcian Posidonia Shale, SW Germany

Black shale samples from sedimentary layers below and between the Unterer Stein, Oberer Stein, Inoceramus Bank, and Nagelkalk horizons from the Dormettingen quarry, SW Germany were analysed for their Re and Os isotope composition and content. The ~12-m-thick sedimentary sequence of the Posidonia Shale Formation, composed of multiple layers of black shale and intercalated limestone beds, was deposited during the Early Jurassic (Toarcian), a time during which black shale sedimentation was ubiquitous in Western Europe during a widespread oceanic anoxic event. Both the marl and shale layers beneath the black shales and adjacent to the Oberer Stein limestone layer show signs of bioturbation. The excellent biostratigraphic constraints and the high Re and Os contents (24–290 ppb and 290–1050 ppt, respectively) of the Dormettingen shales thus make these samples ideal for testing the influence of bioturbation on the Re-Os isotope system, as well as refining age constraints for the Toarcian OAE and potential triggers for this event. The calculated isochron age for the Dormettingen shales is 183.0 ± 2.0 Ma, with a low initial 187Os/188Osi of 0.377 ± 0.065, indicating a Pliensbachian to Toarcian age. This age is in agreement with published U-Pb and Re-Os ages for other Lower Jurassic sites, astronomical constraints, and biostratigraphic correlations, suggesting limited disturbance of the isotope system by minor degrees of biological activity. The low 187Os/188Osi is consistent with significant influx of mantle-derived material into the Toarcian ocean, likely from weathering of Karoo-Ferrar Large Igneous Province basalts.

Climatic and oceanic controlled deposition of Late Ordovician-Early Silurian black shales on the North Yangtze platform, South China

Abstract Black shales deposited across the Late Ordovician and Early Silurian transition are considered to be the most important source rock and shale gas reservoirs in the Yangtze region of South China. However, the origin of these sediments remains contentious. In this study, we investigated the systematic changes in total organic carbon (TOC), organic carbon isotopes (δ13Corg), major elements and trace elements on the Ordovician-Silurian boundary strata from Qiaoting, North Yangtze Sea, to reconstruct the climatic conditions, redox changes, primary productivity, and other factors there, promoting the understanding for the formation mechanism of the organic-rich black shales. Element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock, which is similar to granodiorite in composition. Paleoclimatic proxies (CIA, CIW, δ13Corg) suggest that the global climate system experienced significant changes from warm-humid to cold-dry and then to warm-humid climates during the Ordovician-Silurian transition. Paleoredox indices (S/C, U/Th, Ni/Co and V/(V + Ni) ratios) also demonstrate fluctuating redox variations from anoxic to oxygenated, and then to anoxic states during this interval in the Yangtze Sea, which describe an oxygenated Guanyinqiao strata sandwiched between the stagnant and anoxic Wufeng and Longmaxi black shales. Paleoproductivity parameters (TOC, Babio contents and P/Al ratio) suggest that the organic-rich Wufeng and Longmaxi black shales, were deposited with a high biological productivity, while the organic-lean Guanyinqiao sediments were deposited with a low biological productivity. These data demonstrate large climatic and oceanic fluctuations during the Ordovician-Silurian transition, providing essential controlling factors on the oceanic anoxia, primary productivity, and subsequent organic-rich black shale depositions in the Yangtze region during the Late Ordovician and Early Silurian intervals.

The Schandelah Scientific Drilling Project: A 25-million year record of Early Jurassic palaeo-environmental change from northern Germany

With the aim to understand prolonged and repeated marine anoxia after the Triassic-Jurassic mass- extinction event, a continuously cored, 338 metre thick succession of Rhaetian to Toarcian sediments was retrieved close to the village of Schandelah near Braunschweig (Lower Saxony, northern Germany). Here, preliminary biostratigraphical, lithological, sedimentological, geochemical, and geophysical borehole data are presented and discussed. Based on the presence of ammonites, ostracods, benthic foraminifers, calcareous nannofossils, and palynomorphs, all major Late Triassic and Early Jurassic stage boundaries and many of the standard Lower Jurassic ammonite zones could be defined. The deltaic Rhaetian sand- and siltstone succession (Exter Fm) contains evidence for seismic activity probably related to large-scale geodynamic processes. The Hettangian (Lias Alpha) is represented by a thick heterolithic succession composed of shallow marine sandy and silty beds with intercalated organic-rich shale, representing deposition on a shoreface with frequent storm activity. Progressive deepening during the Sinemurian to Toarcian resulted in repeated deposition of laminated organic-rich facies. Periods of relative sea-level fall likely occurred during the Late Hettangian, the Late Sinemurian, and Early Pliensbachian, where a series of hardgrounds occur indicating erosion on the sea floor followed by sea level rise leading to omission, and increased reworking. One of the most conspicuous features of the Lower Jurassic in the Schandelah-1 core is the presence of abundant authigenic carbonates (glendonites,concretions, beef-calcite) within the Upper Pliensbachian with partly very negative C-isotope values (down to –37‰ V-PDB) suggesting the anaerobic oxidation of methane. A high-resolution organic carbon isotope record based on 485 analyses shows two major negative carbon isotope excursions (CIEs) within the lowermost Hettangian and Lower Toarcian, respectively. Both excursions coincide with the onset of black shale deposition and are well-known features of these time-intervals in other regions. Despite the fact that black shale deposition also characterizes parts of the Sinemurian and Pliensbachian in similar facies, no large negative CIEs are apparent. The Schandelah-1 core thus provides an unique archive of sedimentary, biotic and geochemical records of long-term Triassic-Jurassic palaeo-environmental change in the European Epicontinental Seaway.

Источники благородных металлов (Au, Pt, Pd) на Северном Кавказе

Research subject. To date, two different-age regional sources of noble metals have been identified in the North Caucasus: 1) Caledonian, which includes ultrabasites of the Front Range (Au, Pt, Pd) and 2) Hercynian, which features copper-pyrite (Au) and black-shale deposits (Au, Pt, Pd). Noble metals in these sources form large areal accumulations and ore anomalies, thus presenting a great research interest. Materials and methods . This paper reports the results of a long-term geological fieldwork, which consisted in a detailed geological and mineralogical mapping of the Devonian shale and volcanic rocks, as well as of the Ordovician ultrabasites, in the North Caucasus. During this work, maps, diagrams and sections of ore mineralization sites were complied, and collection of lithogeochemical, ore and groove samples was conducted. Laboratory studies included the examination of the obtained samples using geochemical, petrographic, mineragraphic and physico-mineralogical methods. Results . The main sources of Au, Pt and Pd, as well as the genetic types of their deposits were established. Four different-age geochemical anomalies (fields) with a constant total Au, Pt and Pd content of over than 1 g/t were identified. Conclusion . Large-scale Au, Pt and Pd anomalies in the North Caucasus have been discovered, which form a new North Caucasian province of noble metals.

Upper Devonian conodont zonation, sea-level changes and bio-events in offshore carbonate facies Lali section, South China

The Lali section (Guangxi Province, South China) is composed of quiet-water carbonates containing abundant conodonts, which allowed recognition of 12 Frasnian and 20 Famennian biozones. The Kellwasser and annulata events are characterized by transgressive black carbonates, while the Hangenberg Event is represented by black shale deposits. Other transgressive events recognized at Lali include the Middlesex, Nehden, Enkeberg, and Dasberg, as well as the Strunian transgression. Four regressive events were recognized on the basis of lithological and microfacies changes, i.e., the linguiformis regression, Condroz Event, velifer regression, and the Derwer regression. These sea-level change events were of short duration and globally isochronous based on conodont correlations. Microfacies analyses and the abundance of Palmatolepis indicate that the highest Frasnian sea level lies in FZ 12-13 (Upper rhenana Zone), whereas the highest Famennian sea level was in the utahensis Zone.

Stratigraphic and environmental control on marine benthic community change through the early Toarcian extinction event (Iberian Range, Spain)

In the Early Jurassic (~183 Ma ago) global warming and associated environmental changes coincided with an extinction event in the marine realm (early Toarcian extinction event). Anoxia was previously considered to have been the main cause of extinction, but extinctions also occur at localities that remained oxygenated throughout the event, suggesting that other factors, such as temperature, may have played a major role. To test this hypothesis, we integrated quantitative analyses of benthic macro-invertebrates with high-resolution geochemical proxies on the bulk rock (TOC, δ13C, δ18O) and on belemnites and brachiopod shells (δ13C, δ18O) from two sections from the Iberian Range, Spain, with no black shale deposition. The sections are orientated SE-NW along an onshore-offshore gradient deepening to the north. The dominant benthic groups, bivalves and brachiopods, show a different response to the extinction: brachiopods go through a complete species-level turnover, while many bivalve species range through the event. In the shallower section, changes in richness and evenness correlate with TOC (Total Organic Carbon), suggesting that variations in nutrient input from runoff, and the possible local onset of low-redox conditions (TOC > 4 wt%), controlled faunal diversity. In contrast, at the deeper section, community change correlates with changes in δ18O, indicating that temperature variations might have influenced faunal change. Different stratigraphic patterns of extinction occur between the two localities, with last-occurrences clustering at the maximum flooding surface in the shallower section, and at the transgressive surface in the deeper one. The observed differences between the two localities highlight the important role of local sedimentary and stratigraphic processes in controlling the shape of the geochemical and fossil record, and the need for studying multiple sections along onshore-offshore gradients in order to extrapolate regional and global patterns.

Multi-proxy analysis of organic matter accumulation in the Upper Ordovician–Lower Silurian black shale on the Upper Yangtze Platform, south China

Abstract There is no doubt that organic matter plays an important role in shale gas accumulation and storage. Thus, successful evaluation and production strategies of organic-rich shale deposits require an understanding of the range of factors that contribute to the accumulation and preservation of organic matter in these deposits. This paper reports results of a multi-faceted study of two Ordovician–Silurian transition sections of the Upper Yangtze Platform of South China. Organic and inorganic geochemical data are used to reconstruct paleoenvironmental changes across this critical interval of Earth history and to identify those factors that were most important to organic matter accumulation. Deposition of organic-rich shale of the Katian lower Wufeng Formation was favored by rising sea level and diminished clastic sediment content. Black shale of the upper Wufeng preceded Hirnantian glaciation whereas the overlying lower Longmaxi Formation accumulated in association with a post-glacial rise of sea level as well as the peak of Caledonian tectonism. The carbonaceous deposits appear to reflect the combined effects of elevated primary productivity as suggested by Si/Al and Ti/Al ratios, especially in the upper Wufeng, and consequent depletion of water column oxygen (preservation). Consideration of the relationships of redox-sensitive trace element concentrations and total organic carbon content suggest that bottom water conditions fluctuated between anoxic and euxinic during this period, perhaps related to rapid oscillations of sea level. Moreover, the basin appears to have remained connected with the global ocean in spite of Caledonian uplift of bordering areas. Thus, black shale sedimentation on the Upper Yangtze Platform at the Ordovician-Silurian boundary reflects the interplay of climate change and tectonism and their influences on sea level, clastic sediment content, and paleo-productivity.

Extreme sulfur isotope fractionation in the Late Devonian Dry Creek volcanogenic massive sulfide deposit, central Alaska

The Dry Creek Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, in east-central Alaska, occurs in a Late Devonian sequence of peralkaline rhyolite tuff, minor graphitic argillite, and local peralkaline quartz-porphyry rhyolite intrusions. Principal mineralized facies are semi-massive and massive sulfide in variably silicified and graphitic rhyolite tuff, massive sulfide in graphitic argillite, disseminated sulfides in graphitic and non-graphitic rhyolite tuff, and vein-hosted sulfides in a subvolcanic, peralkaline quartz-porphyry intrusion. In situ analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in δ34S values from −48.0 to 23.1‰. This remarkable 71.1‰ variation is more than twice the largest range known for sulfides in nearly all individual VMS deposits, both modern and ancient, which typically is <25‰. Based on results of geochemical reaction modeling presented here, extreme sulfur isotope systematics in the Dry Creek deposit is attributed to two main processes: low-temperature (<100 °C) kinetically controlled bacterial sulfate reduction (BSR) in anoxic to locally sulfidic bottom waters and pore fluids, and higher temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater sulfate during water-rock reactions. Deposition of organic-rich sediments prior to and during mineralization provided the key environment that promoted microbially mediated fractionations with Δ34Ssulfate-sulfide of up to 70‰. During BSR reaction progress modeling, residual sulfate evolved by Rayleigh distillation in pore water, producing δ34SSO4 values up to 58‰; normal Late Devonian seawater sulfate had δ34SSO4 = 22‰. BSR produced FeS precipitates with δ34S values from −50 to −15‰. Integrated textural and δ34S data also suggest that isotopically light H2S produced in pore fluids by BSR was incorporated into hydrothermal fluids and early disseminated sulfide minerals during the emplacement of peralkaline quartz-porphyry rhyolite intrusions in the shallow subsurface, broadly coeval with VMS mineralization. Modeling of moderate- to high-temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater-rock interaction predicts sulfide minerals with δ34S values from −8 to 19‰.High-precision CA-ID-TIMS U-Pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 206Pb/238U date of 363.02 ± 0.43 Ma (2σ total uncertainty). This date indicates that the formation of graphitic and sulfidic sediments at Dry Creek, and contemporaneous VMS mineralization, occurred at least 3.6 m.y. before, and hence are unrelated to, widespread black shale deposition during the Hangenberg Event at ca. 359 Ma.

The geochemistry of the sedimentary rocks from the Huadi No. 1 well in the Wufeng-Longmaxi formations (Upper Ordovician-Lower Silurian), South China, with implications for paleoweathering, provenance, tectonic setting and paleoclimate

Abstract The Late Ordovician-Early Silurian transition was a critical interval in Earth's history, marked by mass extinction, the Gondwana glaciation and volcanic events. Here, we present mineralogical and geochemical characteristics across the Ordovician-Silurian boundary (Wufeng, Guanyinqiao and Longmaxi formations) from the Huadi No. 1 well, in the Sichuan Basin, in order to explore the paleoweathering, provenance, tectonic setting and paleoclimatic conditions of the source area. Thin sections, XRD (X-ray diffraction) data and major element results reveal that the rocks were abundant in clay minerals, quartz, minor feldspar, pyrite and carbonate minerals. The samples have similar chondrite-normalized REE patterns to those of PAAS, which are characterized by slight LREE enrichments and accompanied by flat HREE trends with weakly negative Eu anomalies. Paleoweathering indices (CIA and PIA) suggest that the sediments experienced weak to moderate chemical weathering in the source area. CIA values in the lower Wufeng formation range from 67.11 to 72.17, indicating a warm and humid climate, the CIA values in the upper Wufeng formation decrease from 70.35 to 66.15, and the CIA values in the Guanyinqiao and lower Longmaxi formation are persistently low (56.64–62.87), suggesting a cold and dry climate, which is consistent with the widespread presence of glaciation in the Hirnantian period. Further upward in the upper Longmaxi formation, the CIA returns to a high value (65.59), which may reflect that the climate started getting warm and glaciation had finished. Fluctuations in CIA values reflect that the climate shifted from warm to cold and then to warm again during the end Ordovician-early Silurian time period. The A-CN-K triangular diagram and SiO2-Al2O3/TiO2, TiO2-Zr, and La/Th-Hf bivariate diagrams indicate that the provenance of the rocks from the three formations was primarily felsic igneous rocks. Discriminant-function diagrams and La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10 triangular diagrams show that the clastic sediments of the Wufeng-Longmaxi formations were derived most likely from an active continental margin. Finally, across the Late Ordovician-Early Silurian, tectonic activity and the end of the Ordovician glaciation dominantly controlled the deposition of black shales and siltstones in the Wufeng-Longmaxi formations of South China.

Response of cephalopod communities on abrupt environmental changes during the early Aptian OAE1a in the Middle Russian Sea

Significant influence of anoxic events on faunal turnovers in marine communities is well-established. However, many studies are focused on the impact of anoxic conditions on benthic organisms, while coeval changes in pelagic cephalopod assemblages remain relatively poorly understood. In the present paper we discuss the trends in cephalopod assemblages coinciding with the onset of the early Aptian black shale deposition in European Russia coeval with OAE1a. In few sections of the Saratov Volga area (central part of the Russian Platform), representing both offshore and more proximal nearshore lithofacies of the epicontinental Middle Russian Sea, we have recognized simultaneous changes in ammonite and belemnite successions. Belemnites, represented by the late members of the family Oxyteuthididae, are common in the interval directly preceding anoxic event, but totally disappear with the onset of the black shale deposition. For the Deshayesites ammonites shell size reduction across the mudstone – black shale boundary (maximum shell diameter of adults reduces from ∼20 cm to 7–8 cm) can be observed. Some other ammonites become numerous (Sinzovia) within the black shale interval or show a first occurrence in it (Koeneniceras and Volgoceratoides). In our opinion diminishing of Deshayesites shell size during the early Aptian OAE could be caused by coupling of palaeoenvironmental factors such as progressive warming and regional input of brackish water. Preliminary results of carbon isotope studies of aragonite deriving from the ammonite nacreous layer are also provided.

Geochemical Characterization of Exposed Black Shale Sediments of Matamuhari Anticline, Bangladesh

Matamuhari anticline is one of the largest anticline situated at Bandarban district of Bangladesh which is 60 km long and 20 km wide in the part of Bangladesh. Like other part of Bangladesh the whole sequence of exposed rocks of Matamuhari anticline is composed with Neogene sedimentary rocks of shale, sandstone, siltstone, silty shale, tabular and spheroidal calcareous concretions. Seven outcrop black shale samples were collected during February 2013 from the different parts of the anticline namely Boro Bari Jhiri, Ochir Jhiri and Tak Chara area were mineralogically and geochemically examined in order to better understand the depositional environment in the studied area. The major and trace elements were analyzed using XRF spectrometry. Two distinct lithofacies assemblages, mud and silty-clay are found in the collected sediments. The shales are predominated by quartz, kaolinite, chlorite, mica (muscovite and biotite), feldspar (both K-feldspar and plagioclase), and in addition to a variable ratio of vermiculite. Lithologial characteristics and geochemical data demonstrated that sediments are enriched with TOC contents ranging from 0.39 to 0.67 wt.% and shows distinctive correlation to concentration of TOC and particle size distribution. These shales are characterized by a low Mg and K as well as high Al and Fe contents. Studies also show that the values of MgO/Al2O3 and K2O/Al2O3 ratio ranges from 0.15 to 0.17 and 0.18 to 0.20 respectively. The relationship of log MgO/Al2O3 and log K2O/Al2O3 values reveals that all of the studied samples fell within the marine environment. Ni/Co ratio of the studied shale samples ranges from 2.84 to 3.88 (average 3.36). These low values of Ni/Co ratio suggest that the sediments were deposited under oxic conditions. However, the above observations suggest that the sediments of Matamuhari anticline area were deposited in a shallow marine environment under oxic conditions.