Marine Black Shales Research Articles

Provenance composition and evolution of marine black shales in the Yangtze platform from Ediacaran to Silurian

Provenance is important in understanding the evolution of geological structure and the coupling of the basin-mountain system. Although many geochemical data document the three sets of marine black shales (Ediacaran to Silurian) in the Yangtze Platform, the evolution characteristics of their sediment sources remain unclear. We gather a huge database from 307 published papers totaling 12,342 analyzed samples to establish provenance. Pearson correlation coefficient and multivariate statistical analyses were performed to establish the element and mineral links in the black shale. Nine immobile elemental ratios (Ti/Th, Th/Sc, Cr/Th, Ti/Zr, Sc/Ta, Ti/Nb, Sc/La, Zr/Sc, and La/Ti) were employed to indicate provenance. The results show that the black shales of the Longmaxi and Niutitang formations are relatively enriched in redox sensitive and productivity related elements such as V, Cr, Mo, U, Co, Ni, Cu, Zn, and Ba. The use of provenance analysis proxies, including V, Cr, Mo, Ce, Fe, P, Ba, Cu, Ni, and Si, should be approached with caution due to the potential interference of marine authigenic components. The three black shales have provenance compositions that gradually extend from intermediate to felsic from the Doushantuo to the Niutitang to the Longmaxi shale. This difference in provenance is a response to the Pan-African and Kwangsian orogenic structures which are likely to impact the paleomarine environment by variable biotrophic element inputs.

Mechanism of organo-nickel co-enrichment in marine black shale

Mechanism of organo-nickel co-enrichment in marine black shale

Paleoenvironmental evolution and organic matter accumulation of the lower Cambrian Maidiping marine black shales in the intracratonic basin, western margin of Sichuan Basin, South China

ABSTRACT The Late Ediacaran to Early Cambrian marked a pivotal phase in paleoenvironmental and tectonic shifts. The Sichuan Basin, on the Yangtze platform’s western edge, developed the Mianyang-Changning intracratonic sag due to Xingkai taphrogenesis. The Maidiping Formation’s black shales, the initial fine-grained clastic deposits from the Early Cambrian, span the Sichuan Basin’s western margin. The influence of the intracratonic sag on organic matter accumulation and paleo-marine environment evolution remains unclear. Comprehensive petrological and geochemical analyses on the Qingping section revealed three distinct members within the Maidiping Formation: black shales and silicified dolomite with shales in a restricted basin (lower member), deep-water black shales (middle member), and phosphoritic turbidite sandstone with siliceous phosphorites (upper member). The lower member’s organic accumulation arose from restricted ocean circulation, leading to stratified waters and reduced oxygen at depth. The middle member’s organic build-up was influenced by enhanced intracratonic sag connectivity with the open ocean, sea-level rise, and increased productivity. The submarine hydrothermal activity might have induced anoxic conditions. The upper member saw a decline in organic matter due to phosphoritic debris and oxygenation. The Maidiping Formation augments the Lower Cambrian source rock, vital for oil and gas exploration in the Sichuan Basin’s western margin.

Experimental study on the influence of pore structure on spontaneous imbibition in marine black shale

Experimental study on the influence of pore structure on spontaneous imbibition in marine black shale

Re-Os geochronology of the Middle to Upper Jurassic marine black shales in the Agardhfjellet Formation, Central Spitsbergen, Svalbard: A cornerstone for global faunal correlation and Os isotopic change

Ammonite provincialism makes it difficult to link the Boreal and Tethyan realms during the Late Jurassic to Early Cretaceous time. Absolute ages through radiometric dating offer new age information that is independent and complementary to relative ages based on biostratigraphy. In this study, we report seven new Re-Os ages for black shales from the Agardhfjellet Formation, Svalbard. We also report pyrolysis results, carbon and sulfur stable isotopic data, and initial 187Os/188Os ratios to evaluate the paleoenvironment during organic-rich shale deposition. Re-Os ages are derived from three Boreal ammonite zones: (1) 159.2 and 160.1 Ma from the shale intervals containing Cadoceras sp.; (2) 149.6, 149.9, 150.8, and 151.9 Ma from the Rasenia cymodoce Zone in the Early Kimmeridgian, which is the equivalent of the Sutneria platynota and Ataxioceras hypselocyclum zones in the Tethyan realm; (3) 146.8 Ma from an upper Volgian interval above the occurrence of Laugeites sp.. Within this 14.5 Myr interval, the initial Os profile shows a gradually increasing trend from 0.335 (one of the lowest shale values throughout the Phanerozoic) to 0.529, consistent with previous initial Os studies and Sr isotopic ratio chemostratigraphic studies. The trend of increasing initial Os ratios indicates an increase in continental runoff relative to unradiogenic Os input from mantle sources, such as seafloor hydrothermal activity or ultramafic source rocks. We provide new Re-Os geochronological anchors in the Late Jurassic with new Re-Os ages and propose correlations of specific Late Jurassic ammonite zones between the Tethyan and Boreal realms. In turn, the gradual increase in seawater Os isotopic ratios may signal climate change in the Late Jurassic, indicating the increase of radiogenic Os or decrease of unradiogenic Os, induced by high continental runoff or less hydrothermal/volcanic activity.

Influence of hydrothermal and upwelling events on organic matter accumulation in the gas-bearing lower Cambrian shales of the middle Yangtze Block, South China

With onoing studies on the diversification of animals during the early Cambrian in the Yangtze Block, resource effects associated with environmental changes have started to be elucidated, in which regional sedimentary events might have played an important role. The gas-rich Yichang area in the southern Huangling anticline is a potential area for shale gas development and provides a more suitable target for unravelling the debates on organic matter (OM) accumulation in the Low Cambrian marine black shales. This study integrated elemental geochemical and mineralogy time-series analyses based on core samples drilled in Yichang area on the shallow-water platform to constrain the evolution of the depositional environments and consider the forcing function behind OM enrichment in the Shuijinguo Formation. Distinctive TOC and Cu/Al peaks within the black shale deposited in the Shuijingtuo Member I suggest intensification of bioproductivity level. The chemical index of alteration (CIA), regarded as an indicator of paleoclimatic variation, increased in the upsection, and together with increasing Sr/Cu and quartz/feldspar ratio values, suggest a trend toward warming conditions and a moderate degree of chemical weathering. The geochemical proxies for the degree of water mass restriction (Mo/TOC and Co × Mn) and paleoredox (MoEF, UEF, U/Th and Corg/P) indicate that the shallow-water shelf experienced weakly-to moderately-restricted water conditions under an anoxic/euxinic environment in the Shuijingtuo Member I and transited to a moderately-restricted water mass with suboxic conditions during the late Shuijingtuo and Shipai periods. A positive Eu anomaly within the Shuijingtuo Member I provided robust evidence of hydrothermal activity. We propose here for an opening of deposition conditions on the shallow-water platform which results in enhanced upwelling and hydrothermal activity (Si, Ba, et.), thereby ultimately disrupting the original nutrient cycling for the metabolism and prompting the OM accumulation in the epicontinental sea during the early Cambrian.

Volcanic activity drives lacustrine carbon sequestration after Oceanic Anoxic Event 1a

The Mesozoic was punctuated by relatively brief periods of anomalous warmth associated with pulses of volcanism, intensified weathering, expansive marine anoxia, and increased organic carbon burial. These are known as oceanic anoxic events (OAEs). Several OAEs have also been linked to potentially synchronous episodes of increased carbon burial in lacustrine settings, attributed to increased weathering-derived nutrient fluxes. Here we present the Early Aptian oil shale sequences of western Liaoning Province, north China, combining sediment petrology, zircon U-Pb geochronology, carbon isotope chemostratigraphy, as well inorganic and organic geochemical palaeoenvironmental proxies to assess possible causal links to OAE1a. New zircon 206Pb/238U ages (121.06 ± 0.63 Ma, 119.72 ± 0.40 Ma, and 119.9 ± 0.65 Ma) for three tuffs and δ13Corg chemostratigraphic correlation show that the western Liaoning Province oil shale interval formed shortly after widespread marine black shale deposition characteristic of OAE1a had ceased. Elevated concentrations of alginite (dinoflagellate-derived organic matter) in oil shale samples which contain micron-thick tuff layers suggest that arc volcanism in north China was the major factor driving increased lacustrine carbon burial, via volcanic ash fertilisation. This is supported by high values of Babio, C27/C29 ααα-20R steranes, C28/C29 ααα-20R steranes, 4-methylsterane index, dinosterane index, and tetramethylsterane index in these layers, which imply that the input of volcanic ash contributed to a flourishing of brackish/saline-water adapted dinoflagellates, leading to the formation of organic-rich tuff laminae. This is a novel mechanism that is distinct to the processes known to have contributed to enhanced marine organic matter burial on a global scale during OAE1a. This research not only contributes to the study of lacustrine source rocks in north China, but also provides insights into the influence of intermediate-acidic volcanic eruptions on organic carbon burial more generally.

Modes of occurrence of lithium in black shale in the Nandan area, Guangxi, SW China: Implications for clay-type resources

Lithium is used in rechargeable batteries due to its high electrochemical potential, and the rapid development of electric vehicles is exerting increasing pressure on reserves. Currently, pegmatites and brine deposits are the two main Li sources, although volcanic clay-type and carbonate-hosted clay-type that formed mainly in continental or coastal basins are receiving increasing attention due to their large size and potential as future Li sources. However, mechanisms of clay Li enrichment are still debated. Marine black shale at the top of the upper Carboniferous-lower Permian Maping Formation in the Nandan area, northern Guangxi, SW China, is enriched in Li to levels of ∼1000 ppm. Knowledge of its genetic mechanism would contribute to understanding the geochemical behaviour and enrichment mechanisms of Li in surface environments. The different geological features of the shale, relative to the above clay types, suggest that the Nandan shale is a new type of Li-rich sedimentary rock. Its whole-rock Li contents are positively correlated with K2O and illite contents, indicating that illite (K0.7Al2[(Si,Al)4O10](OH)2) is the carrier of Li in the shale, as supported by high shale Al, Si, K, and Li contents in the in situ images. Mineralogical studies have revealed that the Li-rich illite is of detrital origin and was formed during continental weathering. The widespread occurrence of framboidal pyrite and high contents of TOC in the shale indicate that Li-rich shales formed under anoxic depositional conditions. Our findings may provide reference material for the study of the genesis of clay-type Li resources.

Paleoclimate and the origin of two 1000 km Lower Mississippian facies tracts in southeastern Laurentia (USA): Cool-humid Famennian and Kinderhookian – warm-arid Osagean

Two lithologically distinctive Mississippian facies tracts (MFTs) are described from Lower and Middle Mississippian strata (Kinderhookian and Osagean North American Stages) in southeast Laurentia (SEL). The MFTs extend from the central Appalachian Basin westward across the Cincinnati Arch, through the Eastern (Illinois) Basin, onto the Burlington shelf in central Missouri and into the Western Interior (Forest City) Basin, USA. The MFTs developed under radically different climatic regimes. Kinderhookian climate cycles ranged from 3rd order humid to 4th and 5th order humid-subhumid alternation. These climate fluctuations controlled Kinderhookian sediment flux, evidenced by fluvial-deltaic sandstones, coal beds, and pro-deltaic terrestrial-organic-matter-enriched marine black shales. A dramatic climate shift coincided with the Kinderhook-Osage boundary, from 3rd order cool-humid to 3rd order warm-arid conditions, causing cessation of deltaic sedimentation and the onset of eolian sedimentation at that boundary. This abrupt climate reorganization is reflected in 3rd, 4th, and 5th order continental and eolo-marine loessites that replaced fluvial-deltaic facies. Eolianites, evaporites, and calcareous protosols indicate Osagean aridity. Consequently, we reject the deltaic depositional paradigm for Osagean siliciclastic facies. Osagean eolian sediment consisted mainly of quartz silt with significant pristine (unaltered) feldspar silt (∼ ≤ 10%). Pristine feldspars are consistent with an arid sedimentary source that lacked significant chemical weathering under an arid paleoclimate. Dissolution of the chemically reactive disordered lattice of eolian-abraded quartz in subarkosic loess served as the predominant source of silica for massive amounts of biotic and abiotic chert in Osagean eolo-marine sediments. The ≤20 μm fraction of quartz dust is particularly susceptible to dissolution and re-precipitation as microcrystalline quartz (chert). We conclude that tectonic, eustatic, and climatic (allocyclic) processes all exerted some control on deposition; tectonics and eustasy controlled accommodation space, whereas paleoclimate changes (cycles), driven predominantly by orbital forcing, were the principal control on sediment supply and lithostratigraphy.

A Carboniferous apex for the late Paleozoic icehouse

Abstract Icehouse climate systems occur across an abbreviated portion of Earth history, constituting c. 25% of the Phanerozoic record. The Late Paleozoic Ice Age (LPIA) was the most extreme and longest lasting glaciation of the Phanerozoic and is characterized by periods of acute continental-scale glaciation, separated by periods of ice minima or ice-free conditions on the order of <10 6 years. The late Paleozoic glaciogenic record of the Paraná and Kalahari basins of southern Gondwana form one of the largest, best-preserved and well-calibrated records of this glaciation. In the Carboniferous, the eastern and southern margins of the Paraná Basin and the Kalahari Basin were characterized by subglacial conditions, with evidence for continental and upland glaciers. In the latest Carboniferous, these basins transitioned from subglacial reservoirs to ice-free or ‘ice distal‘ conditions evidenced by the widespread deposition of marine deposits juxtaposed on subglacial bedforms. High-precision U–Pb zircon chemical abrasion thermal ionization mass spectrometry geochronological constraints from volcanic ash deposits in the deglacial marine black shales of the Kalahari Basin and from fluvial and coal successions, which overlie marine deposits in the Paraná Basin, indicate subglacial evidence in these regions is constrained to the Carboniferous. The loss of ice in these regions is congruent with a late Carboniferous peak in p CO 2 and widespread marine anoxia in the late Carboniferous. The permeant retreat of glaciers in basinal settings, despite an early Permian p CO 2 nadir, highlights the influence of short-term perturbations on the longer-term CO 2 record and suggests an ice threshold had been crossed in the latest Carboniferous. A definitive driver for greenhouse gases in the LPIA, such as abundant and sustained volcanic activity or an increased biological pump driven by ocean fertilization, is unresolved for this period. Lastly, the proposed Carboniferous apex for the high-latitude LPIA record is incongruent with observations from the low-latitude tropics where an early Permian peak is proposed.

Organic matter pore characteristics of over-mature marine black shale: a comparison of organic fractions with different densities

Organic matter pore characteristics of over-mature marine black shale: a comparison of organic fractions with different densities

Geochemistry of marine black shale of the Cambrian Qiongzhusi Formation, Yangtze Plate, SW China: implications for provenance and paleoweathering

Geochemistry of marine black shale of the Cambrian Qiongzhusi Formation, Yangtze Plate, SW China: implications for provenance and paleoweathering

Organic-matter-rich Pennsylvanian Black Shales as a Source of Critical Minerals

This study examines Pennsylvanian shales in Indiana as a potential source of selected critical elements: rare earth elements (REE), vanadium (V), zinc (Zn), arsenic (As), and cadmium (Cd). Widespread, organic-matter-rich marine Desmoinesian black shales are the main target, but other discontinuous shale horizons were also sampled in 11 locations. In total, 39 shale samples were analyzed in this study. The shales are early mature and represent the early oil generation window. The average concentrations of REE in the marine shales studied vary from 164.9 ppm in the Excello Shale to 338.7 ppm in the Carrier Mills Shale. REE distribution patterns show that most shales have REE values roughly representative of the upper continental crust (UCC). Only one sample of the Carrier Mills Shale was enriched compared to the upper continental crust (REE concentration/UCC>5). The shales are characterized by high uranium (U) content, with averages for marine shales from 70.8 ppm in the Excello Shale to 133.8 ppm in the Veale Shale. In contrast, non-marine shales grouped under “other shales” category had the lowest U concentration of 3.3 ppm. Molybdenum (Mo) concentrations in the marine shales are very high at places exceeding 1000 ppm. Among the shales studied, the Mecca Quarry Shale is the most enriched in Mo (average 1103.8 ppm), followed by the Veale Shale (876.7 ppm). The “other shales” category has the lowest Mo content (2.5 ppm) due to deposition under oxygenated conditions. The marine shales are highly enriched in Zn, As, and Cd. Specifically, the Mecca Quarry Shale is the most enriched in Zn, with some samples having content as high as 5480 ppm. Furthermore, the Excello Shale and the Veale Shale have Zn concentrations more than ten times higher than the UCC (~70 ppm). The As content is the highest in the Veal Shale, averaging 61.7 ppm, and some samples reach more than 200 ppm — highly enriched compared to 1.8 ppm in UCC. The Excello Shale, Mecca Quarry Shale, and Veale Shale are all strongly enriched in Cd (76.1, 74.2, and 49.2 ppm on average, respectively), and the Carrier Mills Shale has the lowest average value of 8.5 ppm. These data demonstrate that Pennsylvanian marine black shales are a potential resource of V, Zn, As, and Cd, whereas in non-marine shales, the concentrations of these elements are much lower. In turn, the REE potential needs further investigation; more data are needed to better understand REE concentrations and their associations.

Unconventional CO2 Storage: CO2 Mineral Trapping Predicted in Characterized Shales, Sandstones, and Coal Seam Interburden

Summary Carbon dioxide (CO2) capture from industrial sources including coal combustion, gas processing, cement or steel production, blue hydrogen, or direct air capture, and subsequent geological storage is part of the transition to reduce greenhouse gas emissions. Unconventional and conventional reservoirs provide opportunities for beneficial use such as enhanced recovery, supercritical CO2 (ScCO2) fracturing, and storage of gases such as CO2 and ancillary gases, or potentially hydrogen. The purpose of this study is to use Australian unconventional rock packages to understand the controls on CO2 reactivity and mineral trapping (the most secure form of storage) and compare the potential for CO2 storage. Characterization of core from the Surat, Eromanga, and Cooper basins, Australia, is used to populate CO2 and production water-rock reactivity models. Sensitivity to production water composition and temperature was also tested. Coal seam gas (CSG) reservoir interburden ranged from clay-rich mudstones to interlaminated sandstone and mudstone, and calcite cemented sandstones. The coal seam interburden samples contained high plagioclase and chlorite content. They were predicted to alter to carbonates calcite, ankerite, siderite, and dawsonite mineral trapping CO2. After 30 years, net mineral trapping varied from −0.1 to +0.3 kg CO2/m3, and pH was 4.6–4.9. Net mineral trapping after 1,000 years varied from 5.7 to 16.3 kg CO2/m3 and was 17.1 kg CO2/m3 with higher salinity water. The mineral content had the main control with different lithologies decreasing mineral trapping by 41 or 35% compared with a base case. Overlying plagioclase-rich sandstone trapped 17.1 kg CO2/m3 as calcite, ankerite, dawsonite, and siderite after 1,000 years with the pH increasing to 6. For the quartz-rich oil reservoir sandstone, however, only 0.3 kg CO2/m3 was trapped after 1,000 years. Gas shale and marine black oil shales contained high mica, chlorite, and feldspar content that could be converted to carbonate minerals, mineral trapping CO2. A marine black oil shale mineral trapped 8.3 and 13.9 kg CO2/m3 after 30 and 1,000 years, respectively, as siderite and ankerite. Unconventional reservoirs have a strong potential for mineral trapping during CO2 storage.

Multiple controls on the organic matter accumulation in early Cambrian marine black shales, middle Yangtze Block, South China

This study aims to investigate paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Shuijingtuo Formation in the Three Gorges area and clarify the dominating factors on organic matter (OM) accumulation. In the studied SL well in western Hubei Province, the Shuijingtuo Formation was divided into three intervals based on geological and geochemical characteristics. Total organic carbon (TOC) contents were variable, averaging 0.69% in Unit 1, 5.40% in Unit 2, and 1.12% in Unit 3. Enrichment factors (EF) of Cu and Ni indicated higher paleoproductivity in Unit 2. Unit 1 and the lower part of Unit 2 were affected by hydrothermal activity, whereas hydrothermal events receded in Unit 3. Hydrothermal activities could provide nutrients and may also facilitate the reducing water condition. Redox proxies (Corg/P, EF(Mo)-EF(U)) suggested that Unit 1, Unit 2, and Unit 3 were deposited in oxic-suboxic, euxinic, and suboxic environments, respectively. Upwelling/restriction proxies (EF(Co)*EF(Mn), Co (ppm)*Mn (%), Mo/TOC, and Cd/Mo) implied that Unit 1, Unit 2, and Unit 3 were deposited in upwelling, semi-restricted, strong restricted conditions, respectively. In summary, an anoxic environment, semi-restricted water conditions, and hydrothermal events are beneficial for OM enrichment during the early Cambrian.

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.

New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales

Stable thallium (Tl) isotope data from organic-rich siliciclastic sedimentary rocks have the potential to track ocean redox state on a broad scale. Here, we report new Tl isotope data from the Mesoproterozoic Velkerri Formation (Roper Group) and the Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, Northern Territory, Australia, and interpret these in the context of rhenium-osmium (Re-Os) geochronometry on the same sample suite. Previous work has shown that marine black shales from the Velkerri Formation provide evidence for closed-system Re-Os systematics, yielding a precise isochron with an age of 1361 ± 21 Ma that agrees well with independent age constraints for the unit. The isotopic composition of authigenic Tl in euxinic black shales from the upper Velkerri Formation (ε205Tl = −2.4 ± 0.8, 2SD) indicates that the Tl isotope composition of local seawater at 1.36 Ga was within a plausible range for Tl inputs to the ocean. Isotope mass-balance modeling of the Tl isotope system within a Monte Carlo framework suggests that the Tl isotopic composition of seawater at 1.36 Ga was homogenous on a global scale and that the burial of Mn-oxides exerted minimal isotopic leverage on the Tl isotope composition of seawater at 1.36 Ga. Taken together with existing Mo, Cr, and U isotope data from the same samples, these observations are consistent with a low-O2 ocean-atmosphere system during this period of the Mesoproterozoic.Previous work has shown that the Re-Os systematics of black shales from the older (1.73 Ga) Wollogorang Formation are scattered and yield an erroneously young isochron age of 1359 ± 150 Ma, which has been attributed to post-depositional hydrothermal alteration at ∼1640 Ma. We observe no systematic relationship between stable Tl isotope compositions and the extent of alteration as gauged by open-system Re-Os behavior (−4.7 ± 1.4 for the upper Wollogorang Formation and −4.8 ± 0.4 for the lower Wollogorang Formation), in marked contrast to previous observations for the molybdenum (Mo) and uranium (U) isotope systems. The invariant signature of the Tl isotope data suggests the Tl isotope system was largely unperturbed during hydrothermal alteration. However, it remains difficult to definitively rule out the possibility that authigenic Tl isotope signatures have been overprinted by later localized hydrothermal fluid alteration in the Wollogorang Formation shales. These observations highlight the potential insights afforded by evaluating open-system behavior via radiogenic isotope systems together with other stable isotope tracers in efforts to reconstruct the redox landscape of Earth’s oceans over time.

Phreatic uranium mineralisation hosted by Neogene sediments from the Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan: unique exploration targets in a deformed geological setting

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation, part of the molasse sediments (Siwalik Group) deposited in the Himalayan foreland basin of the eastern Sulaiman Range, Dera Ghazi Khan, Pakistan. The Litra Formation is the only rock unit hosting economic-grade uranium occurrences in the stratigraphic record of Pakistan. Surface radioactivity is mainly associated with cemented layers of the host sandstone, known as hard-bands, whereas weakly cemented sandstone is non-radioactive. The outcropping sandstone is dominantly whitish grey owing to bleaching, which is probably related to hydrocarbon migration from the underlying marine black shales. The main sandstone-type uranium mineralisation is phreatic owing to its parallelism with the water-table, whereas the concordantly oriented tabular (roll-type) ore is subordinate. The host sandstone layers have high dips ranging from 70 to 85°E indicating a deformed geological setting that makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to hydrocarbons, and also by Fe–Ti-oxide phases, phyllosilicates (chlorite and biotite), graphitic schist and black shale clasts in the host sandstone. Uranium ore minerals are mainly coffinite, pitchblende and minor brannerite. Uranium in the ore phases is accompanied by the high contents of Ca, Fe, Ti, Si and Al. Radiometric disequilibrium in the phreatic ore is strongly positive, whereas that in the tabular (roll-front) is moderately positive, which in turn suggests the former is younger than the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium within the same uranium deposit, suggests that the phreatic uranium ore has possibly resulted from remobilisation of the tectonically uplifted tabular (roll-type) ore and its reprecipitation at the horizontally oriented redox interface following the water-table in the host sandstone aquifers. KEY POINTS The Taunsa uranium resource is dominantly phreatic that has resulted from oxidation and remobilisation of the earlier tabular (roll-type) mineralisation. The phreatic uranium mineralisation is very young, which is indicated by low radioactivity and high positive disequilibrium in comparison with that within the tabular (roll-type) ore. Uranium phases are mainly pitchblende, coffinite and brannerite, associated with organic matter of hydrocarbon origin, with phyllosilicates and Fe–Ti-oxide phases. Outcrop of the host sandstone in the Taunsa area is pervasively bleached and is marked by flat topography.

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".

Geochemical characteristics of marine shale in the Wufeng Formation–Longmaxi Formation in the northern Sichuan Basin, South China and its implications for depositional controls on organic matter

Marine black shales of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Sichuan Basin are considered as excellent source rocks in South China. According to the geochemical characteristics of source rocks, the formation and preservation conditions of organic matter in source rocks are showed. In order to study the aggregation mechanism of organic matter, geochemical proxies were proposed, including redox proxies (V, V/Al, U, U/Al, Mo, and Mo/Al), paleoproductivity proxies (P, P/Ti, Ba, Ba/Al, and SiXS), paleoclimate proxies (CIA), and terrigenous flux proxies (Al, Zr, and Zr/Al). At present, the average content of TOC in Wufeng Formation is 3.63 wt%, which is in the range of 2.59–4.22 wt%. The average content of TOC in Longmaxi Formation is 1.02 wt%, and ranges from 0.45 to 2.07 wt%. The redox proxies show that the Wufeng shale is mainly accumulated under anoxic conditions. Longmaxi shale is mainly accumulated under suboxic-oxic conditions. The P/Ti and excess silicon (SiXS) values indicated that the whole sedimentary sequence had high paleoproductivity, but it was reduced to some extent in the Longmaxi Formation. The reduced Corg/Ptot ratios implies that the decrease of productivity may be caused by the increase of P (phosphorus) burial. The restricted degree of watermass shows that Wufeng shale deposits under strongly restricted watermass, while Longmaxi shale deposits under weakly restricted watermass. During the sedimentary period of the Wufeng Formation, the strongly restricted of the ocean circulation of the northern Sichuan Basin and the effective stratification of water column is beneficial to the preservation of organic matter. During the Longmaxi shale sedimentary period, tectonic movement and global sea level changes controlled the production and preservation of organic matter, affecting terrestrial clastic material flux, redox conditions and nutrient cycling.