Search Results (8)

The depositional environment of the Permian Longtan shale (LS) in southwestern Guizhou Province, China, has been analyzed using mineralogical and geochemical approaches. Macroscopic observations of those studied LS samples showed that the LS is rather homogeneous and interbedded with coal strips, suggesting a relatively stable and shallow water environment. A detailed microscopic analysis demonstrated that higher land plants contributed the predominant proportion of organic matter in the LS. Inorganic geochemical analysis revealed a mixed source of materials with relatively larger proportions of basalt and andesite. Semiarid to humid and warm climates corresponding to an overall intensive weathering were deduced in the late Permian periods. The LS was deposited in a brackish-to-marine water environment with an oxic to dysoxic redox condition. Sea level rise/down coupled with changes in climate, water salinity, and redox condition jointly controlled the formation of the Longtan shale. Mineralogical composition indicates that the LS mainly comprises of argillaceous with minor siliceous facies, which will likely bring challenges for hydraulic fracturing. Full article

Many organic-rich marine mudstones, which are key hydrocarbon sources, were deposited on continent margins in mid-water oxygen-minimum zones (OMZs) that expanded and intensified during oceanic anoxic events (OAEs). Other marine hydrocarbon sources include platform and forearc black shales that record trans-continental, long-erm anoxic/dysoxic environments with no modern analog. Their explanation as recording deep-water, Black Sea-type basins or low-oxygen upwelling is not satisfactory for occurrences on shelves that lack significant epeirogenic activity, while modern studies show that upwellings do not cross the shelf break. The alternative is the Bakken model, which concludes that regionally extensive shelves and forearc organic-rich mudstones are shallow-water facies. These Bakken facies reflect hyper-warming conditions with high sea-levels, high water temperatures with increased insolation and low oxygen solubility, turbid water due to algal blooms and mud eroded from orogenic highlands, and possible LIP activity. Early Paleozoic black shales indicate that increased nutrients presumed to accompany the Devonian appearance of forests with deep roots that enhanced weathering simply cannot explain older Cambrian–Ordovician shelf anoxia/dysoxia. Shallow-marine deposition by the Bakken model is mandated by black shales deposited on subaerial unconformities that show high-energy facies (wave cross beds, HCS) and common bioturbation. The Bakken model explains shallow anoxia/dysoxia with high Paleozoic sea levels and tropical distribution of large continents. It is based on the Upper Devonian–lower Mississippian Bakken Formation (western U.S. and adjacent Canada). Rising temperatures, diminished oxygen solubility, and eustatic rise with deglaciation accompany modern climate change and mean that near-future platform seas will feature the reappearance of low-oxygen Bakken facies and environments. Full article

Geochemical proxies are a reliable tool in deciphering the paleoenvironment and diagenetic alteration in carbonate rock units. The Lower Cretaceous Yamama Formation (LCYF) is an important carbonate unit of the Saudi Arabia region which has been studied in detail to evaluate the paleoenvironment and diagenetic alteration through geochemical studies. This study presents new data on petrography, stable isotopes, and trace and rare-earth elements to enhance our understanding on paleoenvironments, redox conditions, and paleosalinity during the deposition of these carbonate units. Field studies show that the formation is composed of thick-to-thin-bedded limestone. Petrographic studies show that the formation is mostly composed of mudstone, wackestone, packstone, and grainstone facies. The stable isotopic values of carbon (δ¹³C V-PDB = +0.58‰ to +2.23‰) and oxygen (δ¹⁸O V-PDB = −6.38‰ to −4.48‰) are directly within the range of marine signatures. CaCO₃’s dominance over SiO₂ and Al₂O₃ indicates minimal detrital contribution during the LCYF precipitation. The REE pattern suggests coeval marine signatures which include (i) a slight LREE depletion compared to HREEs (av. Nd/Yb_N = 0.70), (ii) negative Ce anomalies (av. Ce/Ce* = 0.5), and (iii) a positive La anomaly (av. La/La* = 1.70). Micritic limestone has low Hf (bdl to 0.4 µg/g), Sc (bdl to 2.5 µg/g), and Th (bdl to 0.8 µg/g) content, which suggests negligible detrital influence. The Ce content of different facies (Ce = 1u.80 to 12.85 µg/g) suggests that their deposition took place under oxic to dysoxic conditions. However, there is moderate variation during the deposition of MF-I, with higher Ce values as compared to MF-II, MF-III, and MF-IV, which suggests that the deposition of MF-I mostly took place in anoxic to dysoxic conditions. Full article

Paleoenvironmental conditions significantly influence the distribution patterns and organic matter enrichment of shale. This study investigated the vertical variations of major elements, trace elements, and total organic carbon (TOC) in the Lower Carboniferous marine shale from the Yaziluo Rift Trough, South China, to understand the paleoenvironmental conditions, including redox conditions, terrigenous detrital input, paleoproductivity, and paleo-seawater depth. The Lower Carboniferous formation consists of three sedimentary facies: basin facies, lower slope facies, and upper slope facies. From the basin to the lower slope and then to the upper slope facies, TOC, quartz, and pyrite contents gradually decrease, whereas the carbonate mineral content shows an increasing trend. A continuous decline in paleo-seawater depth transformed a deep-water anoxic environment with high paleoproductivity and low detrital input in the basin facies into a semi-deep-water environment with dysoxic-oxic conditions and moderate detrital influx in the lower slope facies, evolving further into a suboxic environment with high detrital flux in the upper slope facies. The geochemistry results suggest that anoxic conditions and high paleoproductivity were the primary controls on organic matter enrichment in the siliceous shale of the basin facies. In contrast, redox conditions significantly influenced organic matter accumulation in the mixed shale of the lower slope facies, attributed to relatively low paleoproductivity in a more restricted marine setting. Additionally, the adsorption of carbon components by clay minerals facilitated the preservation of organic matter in the calcareous shale of the upper slope facies. Full article

The late Paleocene–early Eocene warm greenhouse conditions, characterized by elevated pCO₂ levels in the atmosphere and a dramatic increase in sea surface temperature, prompted abundant authigenic glauconite formation within the shallow marine sediments worldwide by lowering the net sedimentation rate, increasing organic productivity and expanding the oxygen minimum zones to shallow oceans. The early Eocene marginal marine Cambay Shale Formation experienced episodes of marine inundation represented by limestone–green shale alternations. The shales host abundant authigenic light-green, dark-green, and brown pellets. A detailed characterization of the pellets of the Valia and Vastan lignite mines, integrating the sedimentological, petrographical, mineralogical, and mineral geochemical data, suggests two distinct varieties of Fe–silicate formation, viz. glauconite and chamosite. While the glauconitic green pellets are ubiquitous to Valia and Vastan mines, brown chamosite pellets are confined within the basal part of the green shale facies alternating with fossiliferous limestone in the Vastan mine. The glauconites of the Valia mine manifest a ‘nascent’ to ‘slightly evolved’ maturation stage of glauconitization, whereas the glauconites of the Vastan mine represent the ‘evolved’ type. The limestone–green shale alternation in the Valia mine is overlain by a ~4 m-thick spherulitic mudstone facies comprising monomineralic sideritic spherulites, reflecting a pure FeCO₃ composition. The glauconites in the Cambay Shale Formation transformed from kaolinite-rich clay pellets under dys-oxic depositional conditions. The increasing anoxicity within the microenvironment, possibly amplified by the rapid oxidation of continent-derived organic matter, facilitated chamosite formation instead of glauconite. The increased freshwater influx into the marginal marine depositional environment resulted in immature, K-poor glauconites of the Valia mine. The formation of siderite spherulites overlying the limestone–green shale alternation relates to the beginning of the regressive phase of sedimentation. Full article

Organic-rich rocks of the Marcellus subgroup in the study area consist of a diverse suite of mudstone lithofacies that were deposited in distinct facies belts. Lithofacies in the succession range in composition from argillaceous to siliceous, calcareous, and carbonaceous mudstone. Heterogeneities in the succession occurs in the form of varying mineralogical composition, slightly bioturbated to highly bioturbated chaotic matrix, organic-rich and organic-lean laminae, scattered fossil shells in the matrix, and fossils acting as lamination planes. Lithofacies were deposited in three facies belts from the proximal to the distal zone of the depositional system. Bedded siliceous mudstone (BSM) facies occur in the proximal facies belt and consists of a high quartz content in addition to clay minerals and pyrite. In the medial part of the facies belt lies the laminated argillaceous mudstone (LAM), bedded calcareous mudstone (BCaM), and bedded carbonaceous mudstone (BCM). The size of detrital mineral grains in the lithofacies of the medial facies belt is larger than bedded argillaceous mudstone (BAM) of the distal facies belt, characterized by clay-rich matrix with occasional fossil shells and horizontally aligned fossils. Two types of horizontal traces and one type of fecal string characterize the proximal mud-stone facies, whereas only single horizontal trace fossil is found in the mudstones of the medial and distal facies belt. Parallel alignment of fossil shells and fossil lags in lithofacies indicate that bed-load transport was active periodically from the proximal source of the depositional system. Bioturbation has heavily affected all of the lithofacies and presence of mottled burrows as well as Devonian fauna indicate that oxic to dysoxic conditions prevailed during deposition. The deposition of this organic-rich mudstone succession through dynamic processes in an overall oxic to dysoxic environment is different from conventional anoxic depositional models interpreted for most of the organic rich black shales worldwide. Total organic content (TOC) varies from top to bottom in the succession and is highest in BCM facies. The brittleness index, calculated on the basis of mineralogy, allowed classification of the lithofacies into three distinct zones, i.e., a brittle zone, a less brittle zone, and a ductile zone with a general proximal to distal decrease in the brittle behavior due to a decrease in the size of the sediments. Full article

Turbidite-associated black shale of the Semanggol Formation is extensively distributed in the northwestern part of the Western Belt, Peninsular Malaysia. The black shale occurs as a dark grey to black and thick to medium-bedded deposit. It represents the distal part of submarine fan system (outer-fan) overlying interbedded sandstone to shale facies of the mid-fan and conglomeratic pebbly sandstone facies of the inner-fan. Field observations and its widespread occurrence have resulted in the black shale being considered as a potential analog for a source rock in offshore Peninsular Malaysia. The present study includes detailed mineralogical (XRD, SEM, and EDX analysis), inorganic geochemical (major oxides, trace elements TEs, and rare earth elements REEs), and Rock-Eval pyrolysis analyses of the black shale samples, collected from the Gunung Semanggol, Bukit Merah, and Nami areas in northwestern Peninsular Malaysia. The primary focus of this study is to investigate the provenance, paleoredox conditions, paleoclimate, sedimentary rate, paleoproductivity, and upwelling system that would be helpful to understanding the role of these parameters in the enrichment of organic matter (OM) in the black shale. The Rock-Eval analysis shows that the black shale of the Semanggol Formation comprises type-III kerogens, which suggests organic input from a terrestrial source. The black shale also contains mature to postmature organic matter. Based on the mineralogical analysis, the mineral composition of the black shale comprises illite and kaolinite, with abundant traces of quartz and feldspar as well as few traces of titanium and zircon. Inorganic geochemical data designate black shale deposition in a passive margin setting that has experienced moderate to strong weathering, semi-arid to hot arid climate, and moderate sedimentation rate. Ratios of Ni/Co, U/Th, and V/(V+Ni) along with slightly negative to positive Ce* anomalies and U_EF-Mo_EF cross-plot unanimously indicate anoxic/dysoxic water conditions that are suitable for organic matter preservation. Geochemical proxies related to modern upwelling settings (i.e., Cd/Mo, Co vs. Mn) show that the deep marine black shale was strongly influenced by persistent upwelling, a first-order controlling factor for organic matter enrichment in the distal part (outer fan of the submarine fan system) of the Semanggol Basin. However, productivity-controlled upwelling and a high sedimentary rate, as well as high-productivity in oxygen-depleted settings without strong anoxic conditions, has played an essential role in the accumulation of organic matter. Full article

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca_{98.35–99.34}Mg_0.66–1.65(CO₃). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/Yb_N and ave. 0.73 of Pr/Yb_N), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81; ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02; ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65; ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74; ave. 1.68) and La/Yb ratios (0.68–1.35; ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00; ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history. Full article