First discovery of the Frasnian–Famennian (F–F) boundary with black shale deposition in the Carnic Alps, Italy

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

Natural nuclear fission reactors: time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere

20 My of nitrogen fixation during deposition of mid-Cretaceous black shales on the Demerara Rise, equatorial Atlantic Ocean

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.

Comparative organic geochemistry of shale deposits of northern Appalachian Basin

Carbon isotope stratigraphy of the Devonian of Central and Southern Europe

Abstract. The oceans at the time of the Cenomanian–Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria–Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world by providing a 6 Myr long astronomically tuned timescale across the Cenomanian–Turonian boundary. We procure insights into the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, physical properties, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria–Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405 kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the base of the Livello Bonarelli is 94.17 ± 0.15 Ma (tuning 1); however, a 405 kyr older age cannot be excluded (tuning 2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405 kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4 Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.

Nutrient trap for Late Cretaceous organic-rich black shales in the tropical North Atlantic

Climatic variability and silicate weathering are remarkable features throughout the Late Cretaceous period. Late Campanian black shale is considered the most significant silicate source rock in the southern Tethys. Here, we used mineralogical and geochemical data to evaluate the continental weathering intensity and climatic changes as well as their impact on the deposition of the Late Campanian black shale in the Western Desert of Egypt. The studied black shale has a relatively high concentration of Al, Fe, Mg, Ca, Sr, Ga, Co, Cr, and V when compared to the average Post-Archean Australian Shales (PAAS). The studied samples have elevated values of Ga/Rb, and low values of Rb/Sr, Sr/Cu, and K2O/Al2O3, supporting the deposition of Late Campanian shale under warm/humid conditions. Furthermore, the average chemical index of alteration (CIA, 78.6%), chemical index of weathering (CIW; 83.8%), C-value (1.26), Fe/Mn (408), and Mg/Ca (1.54) reveal the predominance of warm/humid climate. The chemical weathering proxies (CIA, CIW, PIA, LnAl2O3/Na2O) and ACNK diagram imply that the Late Campanian samples were exposed to a moderate grade of chemical alteration. The deposition of black shale occurred under high seawater salinity conditions based on Sr/Ba (Avg = 3.6). Additionally, the weathering indices are well correlated with paleoclimatic proxies, suggesting that weathering intensity is strongly affected by paleoclimate. However, chemical weathering during the Late Campanian has a weak influence on oceanic nutrient fluxes. No substantial impact of the paleoclimate during the deposition of Late Campanian black shale on water salinity was reported.

The Toarcian stage of the Early Jurassic witnessed warm global temperatures, oceanic anoxia (Toarcian Oceanic Anoxic Events) and extensive deposition of organic‐rich black shales. While black shale deposition in the Southwest German Basin on the Northwest Tethyan shelf has been well‐studied, the palaeodepositional conditions in the Northwest German Basin remain poorly constrained. The present study aims to bridge that gap by examining a completely cored well covering Upper Pliensbachian to Upper Toarcian sediments in the Hils Syncline. Inorganic and organic geochemical, as well as biostratigraphic information, carbon isotope and organic petrological results are interpreted to reconstruct depositional conditions and organic matter accumulation in a shallow Jurassic Sea. The study results demonstrate a Late Pliensbachian regression leading to the deposition of claystone in shallow water under humid conditions with a high terrigenous and freshwater influx, and well‐oxygenated bottom water environments. A transgression in the Early Toarcian initiated the deposition of laminated and organic matter‐rich calcareous black shale, the so‐called Posidonia Shale. This climatic shift caused deepening of the water column, resulting in salinity stratification and bottom water anoxia that favoured organic matter preservation. Black shale deposition was probably triggered by low oxygen solubility in warm waters and intensified monsoonal rainfall. The lowermost part of the Posidonia Shale (basal Unit I) represents the time of most intense carbonate deposition and strictly anoxic bottom water. The middle part (Unit II) is characterised by periodical, short‐term oxygenation of the sea floor, while the thick upper part (Unit III) represents a time of re‐establishment of anoxic conditions. In the Late Toarcian and Aalenian, bottom water reoxygenation marked the deposition of claystones with moderate contents of marine organic matter, above a decimetre‐thick breccia. The Hunzen section thus offers detailed insights into late Early to Middle Jurassic palaeodepositional changes in central Europe.

The Mangxiang Formation black shales are the most important hydrocarbon source rocks in the Wuyu Basin. Trace and rare earth element (REEs) of the black shales from the Wuyu Basin were studied in order to understand their depositional environments and palaeoclimate. Thirty one black shale samples from the Wuyu Basin were analysed by inductively coupled plasma–mass spectrometry and X‐ray fluorescence. The black shales are characterized by moderate SiO2 (52.55–58.73%) contents and medium K2O/Na2O (2.21–4.83) ratio values but relatively high MgO + Fe2O3 (5.8–6.91%) and Al2O3 (15.1–17.5%) contents. The Chemical Weathering Index of Alternation (CIA) ranges from 57 to 70, together with medium Th/U (2.66–4.97) ratio values, reflecting a weak to moderate degree of chemical weathering of the source area. The palaeoredox condition of black shale was slightly oxic (or dysoxic) during black shale deposition as evidenced by slightly Ce anomalies (0.92–0.99) and Mn enrichment (EF = 1.6). The moderate palaeosalinity values (9.32‰–17.75‰), together with medium B/Ga (3.90–5.57) ratio values, indicate a brackish water environment. The palaeoclimate index ∑(Fe + Mn + Cr + Ni + V + Co)/∑(Ca + Mg + Sr + Ba + K + Na) ranges from 0.28 to 0.50 and low Sr/Ba (0.15–1.22) ratio values, indicating a semiarid to semimoist climatic condition during the sedimentation of the black shale. The high ω(La)N/ω(Yb)N ratio values (1.02–1.09) indicate a fast sedimentary rate during black shale deposition. In this study, a preservation model of the Mangxiang Formation black shale was established. The model indicates that excellent preservation may be the major controlling factor for the accumulation of organic matter. Copyright © 2015 John Wiley & Sons, Ltd.

Black shale deposits are widespread in the Paleogene successions in the Beni Suef area, Egypt. In this work, the mineralogical composition, total organic carbon (TOC), and total sulfur (TS) contents of black shales were determined. The organic matter (OM), thermal maturation, and kerogen type can be determined using Rock-Eval pyrolysis. The results show that smectite and kaolinite are the dominant clay minerals in the analyzed samples. Al2O3 and SiO2 as major elements and Zr as a trace element are dominant constituents of terrigenous origin of the Dabaa and El Fashn formations. The total organic carbon content reaches 2.27 wt% at the El Fashn Formation in the Ghaida Al-Sharqia quarry but 0.36 wt% in the Oligocene Dabaa Formation in the Maydoum quarry. However, the total sulfur content reaches 0.58 wt% in the Dabaa Formation but 1.52 wt% in the El Fashn Formation. These results confirm that (1) the Dabaa Formation was formed in oxidizing environments and oxidized the high volume of OM and (2) the El Fashn Formation accumulated in reducing environments of mostly suboxic conditions. The Rock-Eval pyrolysis results of the black shale deposits reflected a kerogen type III of terrestrial origin of the El Fashn Formation due to the high OM content preservation in suboxic shallow marine environments. The Tmax and production index parameters of Rock-Eval pyrolysis reflected that the black shale deposits in the El Fashn Formation are immature, but with surface retorting, it can generate natural gas.

The Cenomanian/Turonian oceanic anoxic event in the South Atlantic: New insights from a geochemical study of DSDP Site 530A

Biogeochemical controls on black shale deposition during the Frasnian-Famennian biotic crisis in the Illinois and Appalachian Basins, USA, inferred from stable isotopes of nitrogen and carbon

The Aznalcollar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65 ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30 Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcollar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcollar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (VA1) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the VA1 cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (VA1) to hydroclastic subvolcanic conditions for the VA2. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F1), and development of folds that evolved to thrusts by short flank lamination. These thrusts caused tectonic repetition of massive and stockwork orebodies. In Aznalcollar, some of the stockwork mineralization overthrusts massive sulphides. These structures are cut by large brittle overthrusts and by late wrench faults. The original geometric features of massive sulphide deposits correspond to large blankets with very variable thicknesses (10 to 100 m), systematically associated with stockworks. Footwall rock alteration exhibits a zonation, with an inner chloritic zone and a peripheral sericitic zone. Silicification, sulphidization and carbonatization processes also occur. Hydrothermal alteration is considered a multi-stage process, geochemically characterized by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilized in the inner chloritic zones. Three ore types occur, both in stockworks and massive sulphides, named pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Zoning of sulphide masses roughly sketches a typical VHMS pattern, but many alternating polymetallic and barren pyritic zones are probably related to tectonics. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300 °C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350 °C). Fluid inclusion data suggest that hydrothermal fluids changed continuously in temperature and salinity, both in time and space. Highest Th and salinities correspond to inner stockworks zones and later fluids. Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200 °C), intermediate (200–300 °C) and high temperatures (300–400 °C). 34S values in massive sulphides are lower than in stockwork mineralization suggesting a moderate bacterial activity, favoured by the euxinoid environment prevailing during black shale deposition. The intimate relation between massive sulphides and black shales points to an origin of massive sulphides by precipitation and replacement within black shale sediments. These would have acted both as physical and chemical barriers during sulphide deposition. Hydrothermal activity started during black shale deposition, triggered by a rise in thermal gradient due to the ascent of basic magmas. We suggest a three-stage genetic model: (1) low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales; locally, channelized high-T fluid flow occurs; (2) hydrothermal cyclic activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and (3) a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralization, mainly in the stockwork zone.