Anoxic Depositional Conditions Research Articles

The relationship between total organic carbon and bottom water redox state in North American black shales

It is clear from modern analogue studies that O2-deficient conditions favor preservation of organic matter (OM) in fine-grained sedimentary rocks (black shales). It is also clear that appreciable productivity and OM flux to the sediment are required to establish and maintain these conditions. However, debates regarding redox controls on OM accumulation in black shales have mainly focused on oxic versus anoxic conditions, and the implications of different anoxic redox states remain unexplored. Here, we present detailed multi-proxy sedimentary geochemical studies of major Paleozoic and Mesozoic North American black shale units to elucidate their depositional redox conditions. This is the first broad-scale study to use a consistent geochemical methodology and to incorporate data from Fe-speciation – presently the only redox proxy able to clearly distinguish anoxic depositional conditions as ferruginous (H2S-limited) or euxinic (H2S-replete, Fe-limited). These data are coupled with total organic carbon (TOC), programmed pyrolysis, and redox-sensitive trace element proxies, with almost all measurements analyzed using the same geochemical methodology. Consistent with expectations based on previous geochemical and paleontological/ichnological studies, these analyses demonstrate that the study units were almost exclusively deposited under anoxic bottom waters. These analyses also demonstrate that there is wide variance in the prevalence of euxinic versus ferruginous conditions, with many North American black shale units deposited under predominantly ferruginous or oscillatory conditions. TOC is significantly higher under euxinic bottom waters in analyses of both preserved (present day) TOC and reconstructed initial TOC values, although sediments deposited under both redox states do have economically viable TOC content. While this correlation does not reveal the mechanism behind higher organic enrichment in euxinic environments, which may be different in different basins, it does open new research avenues regarding resource exploration and the biogeochemistry of ancient reducing environments.

Characteristics of oils and their potential source rocks from the Cherokee Platform, Oklahoma

The Woodford Shale is the most prolific source rock in the Anadarko Basin and Cherokee Platform and, over the past decade, it has become renowned for its high unconventional hydrocarbon potential. Recent Woodford Shale studies have emphasized facies heterogeneity based on lithology, sequence stratigraphy, and geochemical analyses. Data presented in this paper focus on the organic geochemical characterization and genetic relationship between the Woodford Shale and oils in the Cherokee Platform area, Pottawatomie County, Oklahoma. The shale samples were screened by total organic carbon (TOC) and Rock-Eval (RE) that indicate a high-quality Type I/II kerogen. Vitrinite reflectance measurements (VR%) of 0.59% for the Woodford Shale indicate the samples are at an early maturity stage in this area. The Woodford Shale generally has been subdivided into three members (Upper, Middle, and Lower), but in this study only the Upper and the top part of the Middle Woodford were characterized. Both intervals alternate between suboxic and anoxic depositional conditions, along with water column stratification and episodic euxinia or photic zone anoxia (PZA). A PZA episode has been described for Upper Woodford for the first time where C40 carotenoids and aryl isoprenoids show maximum abundance.The second part of the investigation involved oils, collected from wells in the area, and a detailed correlation with the Woodford Shale. The oils showed a close correlation with the Woodford, but maturity based on biomarkers may suggest a deeper source rock with equivalent organofacies. Many of the oils examined showed varying levels of biodegradation and possible mixing with a lighter more mature oil. The oils were compared with each other to determine possible relationships and assess the petroleum system on the southern Cherokee Platform.

Expanded marine anoxia at the Cambrian-Ordovician transition: Evidence from lime mudstone I/Ca and δ238U signatures of the GSSP in western Newfoundland, Canada

The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238Ucarb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotopemass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238UOC (−1.22 to −0.25‰) values, estimated from the limestone δ238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the δ238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.

Understanding the provenance and depositional conditions of Triassic sedimentary rocks from the Spiti region, Tethys Himalaya, India

The Spiti region, renowned as the Museum of Indian Geology, is a world-famous sedimentary succession containing well-exposed sequences from Neoproterozoic to Cretaceous age. In this study, Triassic siliciclastic sedimentary rocks of the Lilang Supergroup were chosen to understand weathering history, provenance, paleoclimate, and depositional conditions using a geochemical and isotopic approach. Triassic shales show more or less similar compositions with substantial enrichment in CaO compared to PAAS (Post Archean shales from Australia), which may be attributed to the association with limestones in the region. However, the sandstones display significant depletion in the trace element concentrations signifying the effect of quartz dilution. The relative depletion of mobile elements (Rb, Ba) as against immobile elements (Zr, Nb, Hf) can be noticed in the trace element spider diagram of the shales. The Triassic sedimentary rocks are characterized by enriched LREE and depleted HREE patterns with pronounced negative Eu anomalies. The Chemical Index of Alteration (CIA; 56–86) indicates low to intense chemical weathering in the source area. The unusual decrease in CIA and other weathering indices in the stratigraphically up section is attributed to changes in climate and environmental conditions during the deposition of sediments in the Triassic period. Detangling the signatures is crucial to understanding the mass extinction crisis, particularly the role of anoxia in these events. Triassic black shales represent suboxic to anoxic depositional conditions in the redox-sensitive elemental binary diagrams. The carbon isotope data of the present study is very well supported by the Total Organic Carbon (TOC), which infers that the oceanic biological system tried to recover from the depletion of biological life. The εNd and 87Sr/86Sr systematics record a shift in source terrains from the Early to Late Triassic period. The Early Triassic samples show much older depleted mantle model ages (TDM = 1.94–1.98 Ga) compared to Late Triassic sediments (TDM = 1.76–1.91 Ga). Similar interpretations can be drawn from Th/Sc ratios (from ∼ 6 to ∼ 0.05) and (La/Yb) N ratios (from ∼ 32 to ∼ 5), which record an increase in these ratios from Early Triassic to Late Triassic formations of the Spiti sedimentary rocks. Overall, trace elemental ratios and radiogenic isotopic signatures of the Triassic rocks of the Spiti region point towards Pan African granitic origin with minor impressions from the juvenile mafic-rich sources, such as Panjal Traps, the African craton, and Arabian-Nubian shield.

Maastrichtian Anoxia and Its Influence on Organic Matter and Trace Metal Patterns in the Southern Tethys Realm of Egypt during Greenhouse Variability.

Maastrichtian organic-rich sediments of Egypt were deposited under a warm greenhouse climate along the stable African shelf. This study presents an integrated analysis of the geochemical, mineralogical, and palynological data from Maastrichtian organic-rich sediments in the northwest Red Sea region of Egypt. The aim of the study is to assess the impact of anoxia on the enrichment of organic matter and trace metals and to reconstruct a model for the formation of these sediments. The sediments are hosted within the Duwi and Dakhla formations, covering an interval of ∼1.14-2.39 million years. Our data indicate variable bottom-water oxygen-level conditions for early and late Maastrichtian sediments. The C-S-Fe systematics and redox geochemical proxies (e.g., V/(V + Ni), Ni/Co, and Uauthigenic) suggest dysoxic to anoxic depositional conditions for the late and early Maastrichtian organic-rich sediments, respectively. The early Maastrichtian sediments contain abundant small-sized framboids (average = 4.2-5.5 μm), suggesting anoxic conditions, while the late Maastrichtian sediments have larger framboids (average = 4-7.1 μm), indicating dysoxic conditions. The palynofacies analyses reveal the high abundance of amorphous organic matter and confirm the predominance of anoxic conditions during deposition of these organic-rich sediments. The early Maastrichtian organic-rich sediments have a significant Climate concentration of Mo, V, and U, indicating high biogenic production rates and distinct preservation conditions. Additionally, the data imply that oxygen deficiency conditions and low sedimentation rates were the main factors controlling the preservation of organic matter in the studied sediments. Overall, our study provides insights into the environmental conditions and processes that led to the formation of the Maastrichtian organic-rich sediments in Egypt.

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.

Organic molecules based palaeoenvironmental inferences and U mineralization in Palaeoproterozoic Bijawar basins of Central India

Palaeoproterozoic, siliclastic, intracratonic rocks of the Bijawar and Sonrai basins contain anomalous U concentration and abundant Organic Matter (OM), although, earlier studies documented rare data on commonality between high U content and closely associated OM. Moreover, OM contains several biomolecules of complex carbonaceous compounds with macro-molecular structures, but insignificant information is available on the origin and associated palaeoenvironmental conditions. Thus, present study on organo-uranyl-complexes is relevant and carried out to understand palaeoenvironmental reasons accountable for U mineralization. For this purpose, OM (isolated from each bulk sample) was analyzed for Total Organic Carbon (TOC), n-alkanes, n-fatty acids and Polycyclic Aromatic Hydrocarbon compounds (PAHs). Obtained TOC values range between 0.02 and 0.23 wt%. Mostly, calculated [(C/S)/12] and [(C/N)/12] values are low; indicate algal source and anoxic depositional conditions, but, Rohini carbonate, Bandai sandstone and Chloritic shale high values signify sub-oxic conditions. Also, low (<1.25) U/Th and high authigenic U data of mineralized Chloritic shale imply anoxic conditions and post-depositional enrichment of U ions (mostly as organo-uranyl-complexes), respectively.GC-MS data show high concentration of n-alkane, fatty acid methyl ester (FAME) and PAH molecules in the OM associated with the rocks signifying high U values. The predominance of Short Chain (SC) n-alkanes together with the SC n-fatty acids revealed derivation of OM from marine algae and bacteria. The Rohini carbonate and Chloritic shale show low isoprenoids, Pristane (Pr)/Phytane (Ph), CPI, OEP, Pr/n-C-17, Ph/n-C18, but high TC-/(Pr/Ph), TC/(Pr + Ph), MP, MPI, MPR and VR values, suggest biodegraded and thermally matured OM which has undergone hydrothermal alteration under anoxic to sub-anoxic conditions. Considerably, high degree of OM oxidization occurred synchronously with the reduction of U6+ to U4+ by sulfate-reducing bacteria. The consistent loss of diaromatic PAHs is also noticed from older to younger rock-units. Further, scanty appearance of PAHs in the lower Gorakalan shale and their disappearance in the upper Chloritic and black shale units, indicate progressive maturation of sediments. The Low Molecular Weight (LMW)-PAHs predominate over the High Molecular Weight (HMW)-PAHs. Comparatively, high phenanthrene abundance is also noticed in the Rohini carbonate, Chloritic shale and -black shale (with high U content). The high proportion of LMW-PAHs and absence of HMW-PAHs (>6 rings) is pointing towards the presence of hydrothermally derived bitumen. Moreover, partial combustion of OM (pyrogenic) contributed to the formation of PAHs. The Chloritic shale and Rohini carbonate are coeval with the Bijawars of the Sonrai basin, representing almost similar organo-molecular records. Organo-molecular abundance and their structural attributes when plotted across the stratigraphic successions, revealed alternate humid conditions facilitated U4+oxidation and formation of U6+ rich solutions, although arid/semiarid climatic conditions assisted precipitation, sorption and absorption of U4+. Almost, similar clay mineral attributes for both the basins lend support to cyclic humid to arid/semiarid palaeoenvironmental conditions. Moreover, majority of the OM derived from the algal mats has played critical role in U mineralization by trapping and adsorption of tiny uraninite grains as well as U ions onto the OM.

Geochemical characteristics of the Silurian-Devonian Kroh black shales, Peninsular Malaysia: An implication for hydrocarbon exploration

In this study, the hydrocarbon potential of the Silurian-Devonian Kroh black shales in Peninsular Malaysia was investigated. To this aim, seventy-five samples from the Kroh black shales were analyzed to identify their geochemical and mineralogical composition using X-ray diffraction (XRD) and X-ray fluorescence (XRF), respectively. XRD results showed that the samples consist of two main components: kaolinite and quartz. From the XRF analysis, the most dominant oxides in the Kroh black shale samples were SiO2 (75.1 wt%) and Al2O3 (13.5 wt%). The ratio of Al2O3/TiO2 suggests that the black shales are originated from an intermediate to felsic igneous rock. The ratio of V/(V + Ni) varies from 0.8 to 1, and V/Ni ranges from 4.3 to 7.1. These values suggest suboxic to relatively anoxic depositional conditions and purely marine sources of organic matter (OM). However, a cross-plot of Ni vs. V indicated that depositional conditions of the Kroh black shales are terrestrial oxic to marine-terrestrial oxic-dysoxic. Also, Rock-Eval 6 pyrolysis was applied to assess the hydrocarbon potential of the Kroh black shales and acquire more information on the thermal maturity of organic matter (OM). Data from the Rock-Eval pyrolysis revealed that the total organic carbon (TOC) content of the seventy-five black shale samples from seven outcrops (i.e., KR-1 to KR-7) varies from 0.33 wt% to 8.56 wt%. The TOC values indicate that the Kroh black shale samples have preserved a fair to a good quantity of the OM richness. Moreover, Rock-Eval analysis affirmed that the samples predominantly fall within a weak version of Type III kerogen, as gas-prone window, and Type IV kerogen, as inert solid bitumen. Furthermore, the relationship between hydrogen index (HI) and production index (PI) with vitrinite reflectance (Ro) confirms an immature to overmature of the OM in the Kroh black shales. Those findings bring to light the Kroh black shale samples are at a minimal level of HC generation; hence, commercial considerations for the Kroh Formation might not be worthwhile. It also adds new insight into the peninsular depositional conditions, contributing to the region's paleogeography understanding.

Oil characteristics and source rock potential of Abu Rudeis-Abu Zenima area, Central province, Gulf of Suez, Egypt

The Gulf of Suez region is one of the most important petroleum provinces in Egypt and the Abu Rudeis-Abu Zenima area (ARAZ) located in the central province of the Suez Gulf on the eastern side. The scope of the current work is to evaluate the oils in the ARAZ area to identify oils characteristics and source rock potential using the data of whole oil gas chromatography and gas chromatography-mass spectrometry (GC-MS), in addition to stable carbon isotope analysis and vanadium and nickel contents. The bulk oils composition in ARAZ area characterized by moderate saturates and aromatics contents, high sulfur and NSO &asphaltene contents and heavy and normal API gravity. The composition of oils differs primarily due to their degree of thermal maturity. The oils generated from algal marine material with little terrestrial materials deposited under anoxic depositional conditions suggested by a low Pristane/Phytane (Pr/Ph) ratio. Moreover, the oils generated from carbonate source rocks of Thebes and Duwi rock units indicated by high C31-C35 Hopanes (45-50%) and C35/C34 extended Hopane ratio (0.99 – 1.12). The oils reached the peak-oil generation stage (0.85% Ro) indicated by the sterane isomerization ratio (20S % C29) that is less than 55 %. The oils of ARAZ area are matched the oils of the Gulf of Suez.

Organic petrography of Ordovician - Silurian rocks and their implications to hydrocarbon generation in the Akkas-1 well, western Iraq: New evidence for the Ordovician Gloeocapsomorpha and glaciation events

The Ordovician-Silurian hot shales are proved to be source for hydrocarbon in Akkas Field. New set of core samples of these important strata from Khabour and Akkas formations were examined from Akkas-1 well by organic petrography. The core samples of the Ordovician were found to be enriched in the liptinitic macerals that are mostly telalginite and lamalginite with lower abundances of vitrinite-like, inertinite and non-fluorescing network-forming matrix bituminite macerals. The studied Ordovician samples showed organic matter composition with large liquid hydrocarbon generation potentiality. The telalginite grains belong to the mat-forming algae that are found mostly in clusters and colonies. The early colonies are assigned to the Ordovician cosmopolitan Gloeocapsomorpha cyanobacterium. Several morphological variations in the telalginite particles are noticed through time that verifies their evolution. This is first record of this important oil prone organic matter in this region. In addition, unique micro-laminae are identified and attributed to the record of seasonal changes or varves. These varves indicate the summer/winter cyclic changes accompanied the Late Ordovician glaciation at western part of Arabian Peninsula and its influence on sedimentation in nearby basins. This cyclic pattern is also recorded for the first time as glaciation evidence in the Akkas-1 well. The seasonal changes of the summer/winter cycle are accompanied with some distinctions in the quantity and quality of the telalginite particles. The summer micro-laminae have large abundance of both organic matter and liptinite maceral. The Silurian hot shale has abundant liptinite maceral that are mostly bituminite and lamalginite. In addition, they are rich in redox sensitive trace elements that reveal anoxic depositional conditions. Ultralaminar structures are documented for the time in the bituminite of Silurian shale, which suggest a micro-algal origin. The source rock in the Silurian shale has a large potentiality to generate liquid hydrocarbon.

Depositional history of low-mature coals from the Puyang Basin, Yunnan Province, China

Late Miocene samples from the Xiaolongtan Formation in Puyang Basin were sampled systematically from bottom to top of the main coal seam. Organic petrological as well as geochemical methods have been applied, aiming at the reconstruction of the depositional environment. The studied samples contain abundant detrovitrinite and liptinite macerals with high ash but low to moderate sulfur contents. This data indicates that the samples were deposited in topogenic mires without marine influence, but with fluctuations of water table and/or water influx which are mirrored by ash and sulfur content. The low values of tissue preservation and vegetation index, but high values of gelification index and groundwater index (GWIAC) in the studied coal imply a peat accumulation in inundated marshes under slightly oxic to anoxic depositional conditions with input of herbaceous plants mainly. An extended data set on vitrinite reflectance, volatile matter, Rock-Eval Tmax and HI suggests a coalification ranging from subbituminous C to B coal.The high concentration of long chain n-alkanes in the studied samples is typical for kerogen derived predominately from higher terrestrial plants. The applied biomarker ratios together with the detected hopenes further confirm the low thermal maturity level, as well as high water table and terrestrial organic matter input during peat accumulation. The variable concentrations of terpenoids and sesquiterpenoids suggest a dominant angiosperm input, but with a relatively high proportion of gymnosperms during some periods of peat deposition.

Element characteristics of oil shales and factors controlling the paleo-sedimentary conditions of organic matter, Çeltek Formation oil shales, Sorgun-Yozgat, Turkey

Inceleme alani Yozgat ili Sorgun ilcesinde bulunmakta ve yaklasik 1000 km2’lik bir alani kapsamaktadir. Bolgede Paleozoyik, Kampaniyen-Maastrihtiyen, Eosen, Miyosen ve Kuvaterner yasli birimler bulunmaktadir. Bu birimler icinde Alt Eosen yasli Celtek formasyonu bolgede ozellikle komur icerigi bakimindan dikkat cekmis olup ayni birim icinde petrollu seyl olarak tanimlanan kayaclar yer almaktadir. Dunyadaki petrol ve gaz verimliligi ve anoksik depolanma surecleri dikkate alindiginda organik madde birikimi acisindan Eosen donemi onemlidir. Calisma alani icerisinden derlenen 2 farkli sondaj ve bir adet yuzeyde yapilan olculu stratigrafik kesite (OSK) ait 29 adet ornegin toplam organik karbon (TOC) ve ana-iz element analizleri baz alinarak petrollu seyllerin paleo-sedimanter ortam ve karakteristikleri ile TOC miktarlari arasindaki iliskileri calisilmistir. Bu ornekler, %1.97 -16.17 (ortalama% 6.30) arasinda TOC icerigine sahiptir. Celtek formasyonu petrollu seyllerine (CFPS) ait jeokimyasal verilerde V/Cr, V/(V+Ni), U/Th, δU ve Otijenik Uranyum (AU) degerleri hesaplanarak, petrollu seyllerin degisken paleo-ortam kosullarina sahip oldugu tespit edilmistir. Paleo-tuzluluk icin Sr/Ba oranlari hesaplanmis ve orneklerin genellikle tatli su ortaminda cokeldigi tespit edilmistir. CFPS’nin paleo-iklim kosullari, kimyasal alterasyon indeks (CIA) degerleri ve Sr/Cu oranlarina gore, kuru, sicak ve zaman zaman nemli olarak belirlenmistir. Fe/Ti ve (Fe+Mn)/Ti oranlarina gore ise petrollu seyllerin sedimantasyonu esnasinda ortamda hidrotermal bir etkinligin oldugu soylenebilir. Ayrica Zr/Rb oranlari bakimindan petrollu seyllerin cokeldigi donemdeki paleo-hidrodinamiklerin oldukca zayif oldugu yorumu yapilabilir. Sonuc olarak havzadaki bu farkli jeokimyasal sartlar, farkli paleo-ortam kosullarina neden olmaktadir.

An integrated approach to understanding the depositional environment and organic matter enrichment factor in Carboniferous source rocks, Junggar Basin, NW China

Multiple indicators (trace fossils, trace metals, and δ13Corg) are used to assess the depositional environments and controls on organic carbon accumulation in the Lower Carboniferous Nanmingshui Formation, Junggar Basin, NW China. The majority of trace fossils belong to the Nereites ichnofacies, representing low energy, oxygen‐poor, and relatively quiet bottom water environments. However, the shallow water trace fossil Planolites also appears in the strata, possibly reflecting variations in the bottom water oxygen contents of the turbidite succession. Redox proxies Th/U, V/Sc, and V/(V + Ni) reveal variable redox conditions; most of the samples are in the range from suboxic to anoxic depositional conditions. Ni and Cu show a positive correlation with TOC, but U and V have no correlation with TOC, also suggesting the redox conditions were mainly suboxic. The P/Al and Cu/Al ratios of all samples are higher than those of Post‐Archean Australian Shales (PAAS), and about two‐thirds of the samples' Ba/Al and Ni/Al ratios are close to or higher than PAAS, suggesting the Early Carboniferous Junggar Basin had relatively high palaeo‐productivity levels. δ13Corg values average − 23.75‰, reflecting a mixture of terrestrial organic and marine organic matter. The indexes of Ba/Al, Cu/Al, Ni/Al, and δ13Corg show good positive correlations with TOC, indicating the palaeo‐productivity played an important role for the organic matter accumulation. On the other hand, the TOC correlates inversely with redox proxies (Th/U, V/Sc, and V/(V + Ni)), suggesting the preservation conditions (anoxia) are not the control factor of organic matters accumulation. Using these multiple approaches, our results indicate that productivity was the primary factor controlling organic matter accumulation.

Geochemical characteristics of the Paleogene-Neogene coals and black shales from Malaysia: Implications for their origin and hydrocarbon potential

Paleogene-Neogene coals are widespread in Malaysia, cover a wide age spectrum and are considered the source rock of hydrocarbons in Malaysia. However, they have not been studied systematically up to now. Moreover, the black shales associated with these coals were not taken into consideration in previous studies. Therefore, this study presents systematic inorganic and Rock-Eval analyses of the Paleogene-Neogene coals and their associated black shales to examine their origin, depositional environment and hydrocarbon potential.With the exception of coals from the Tanjong Formation, Sabah, the Paleogene-Neogene coals are characterized by very low ash yields and low concentrations of trace and rare earth elements including hazardous trace elements. The black shales are composed of quartz, illite, kaolinite and traces of pyrite in some samples. Al2O3/TiO2 ratios in the coals (6–62) and black shales (16–34) suggest a mixture of felsic and intermediate igneous rocks as sources for their detrital fractions, which is supported by the Zr-Ti binary plot. V/(V+Ni) ratios average between 0.8 and 0.7, V/Ni between 13.5 and 3, and Ni/Co between 1.7 and 3.8 for coal and black shale samples; respectively, indicating suboxic to anoxic depositional conditions for both groups of samples. Rock-Eval analysis indicates that coal and black shale samples contain mixed Type II–III kerogens, which suggest similar organic input from terrestrial high plants. The samples also contain immature to mature organic matter and can produce gas and oil. These characteristics along with the high TOC contents (very good to excellent) indicate that the Paleogene-Neogene coals and black shales are potential source rocks for oil and gas fields in Malaysia. The coals and black shales from different ages and localities were shown to have similar source area composition, climate conditions during their deposition, terrestrial organic input as well as suboxic to anoxic depositional conditions. However, coals and black shales from the early-middle Miocene Tanjong Formation exhibit higher Al2O3/SiO2 ratios than the rest of coals and black shales suggesting a prevalence of wetter climatic conditions during formation. In addition, coals and black shales from the early-middle Miocene Tanjong Formation and upper Pliocene Liang Formation show higher values for redox proxies such as V/(V+Ni), V/Ni, Ni/Co and Ce* compared with other formations, indicating the dominance of reducing conditions during the deposition of these formations.

Evidences of hydrothermal fluids recorded in microfacies of the Ediacaran cap dolostone: Geochemical implications in South China

The Ediacaran cap dolostones overlying the Marinoan Nantuo diamictite of the Doushantuo Formation in South China contain abundant enigmatic structures. Two generations of carbonate cement (named C1 and C2, respectively) can be observed in these structures at Wuhe of the Guizhou Province. C1 is characterized by dolomite cements that line the wall of the structures, and C2 by equant calcite spars that distribute in the center. The REE + Y patterns and the range of δ13C and δ18O values of C1 are similar to those of the micritic matrix, indicating that C1 precipitated from the same fluid as the matrix. The REE + Y patterns of C1 are characterized by minor MREE-enrichment, positive Ce and Eu anomalies, large variations in Y/Ho ratios. These patterns reflect normal marine origin with the release of Fe-Mn oxides in pore water during early diagenesis. Positive Ce anomalies and Th/U ratios of matrix suggest the anoxic depositional conditions. C2 is characterized by MREE-enrichment, positive Eu anomalies, high Y/Ho ratios and unobvious Ce anomalies, which is different from C1 and matrix. The Eu anomalies indicate that the precipitation fluids of C1 and C2 may have involved in different contribution of high-temperature hydrothermal fluids. The precipitating temperatures of C2 would be from 105 °C to 120 °C based on the δ18O variations of C1 and C2. The positive correlation between δ18O and δ13C values of matrix, C1 and C2 may attribute to the alteration by hydrothermal fluids. However, the negative correlation between δ13C and δ18O values of C2 may result from the oxidation of organic matter and methane seeps. The geochemical features of different microfacies of the cap carbonates reflect enigmatic fluid chemistry after the global Marinoan glaciation.

Rare earth element characteristics of Paleoproterozoic cap carbonates pertaining to the Sausar Group, Central India: Implications for ocean paleoredox conditions

The Paleoproterozoic era is marked by significant changes in the level of oxygen in the atmosphere and chemistry of the ocean, and the presence of world-wide glaciation. Geochemistry of chemogenic sediments provide clues to these changes. The Sausar Group (2478–2250Ma) of Central India is one of the few places in the world where Paleoproterozoic glacial unit overlained by a cap carbonate horizon has been identified. Major, trace and rare earth element (REE) contents of Paleoproterozoic cap carbonates of the Sausar Group have been analysed for their implications for ocean redox conditions during interglacial periods of the Huronian glaciation. The La/La∗, Gd/Gd∗, Lusn/Lasn and Y/Ho ratios of the cap carbonate reflect their pristine marine nature. Conspicuous Ce anomaly (Ce/Ce∗ up to 1.75), accompanied by the enrichment of Fe, Mn, Zn and U, provides insight into the redox state of the ocean at the time of deposition of the cap carbonate succession. Based on these observations, we infer anoxic depositional conditions in the basin during the interglacial period of the Paleoproterozoic glaciation before the Great Oxidation Event (GOE), and the level of oxygen in the atmosphere fluctuating between 10−5 and 10−2 of the present atmospheric level (PAL).

Mudstones and embedded concretions show differences in lithology-related, but not source-related biomarker distributions

The mudstones of the Western Australian Gogo Formation harbour numerous carbonate concretions which often contain preserved fossils of Lagerstätte-like quality. These are especially notable in places where the mudstone has eroded, giving way to nodule fields, which afford valuable paleobiological records. It is, however, a challenge to determine their paleoenvironmental context. Here, we analysed concretions from a core drilled in the Canning Basin Gogo Formation. At two different depths, concretions were compared to the surrounding mudstone found at the same depth. Electron microscopy and X-ray spectroscopy showed that the concretions were carbonate-rich and contained detrital fragments. Biomarker data showed that mudstone and concretions had very similar distributions and presented marine biosignatures including indicators of anoxic depositional conditions, a stratified water column, and photic zone euxinia. The concretions contained higher amounts of C27 steranes, indicating that more labile organic matter such as animal remains could have triggered concretion formation. Statistical analyses of the results showed that concretions and shales differed (p<0.1) in indices diagnostic for lithologies (and often related to maturity), particularly in the sample recovered from the younger section of the core: the diasterane/sterane indices at each of the depths were 0.34 in the shale vs 0.12 in the concretion in the younger set of samples, and 0.21vs 0.19 in the older set. The homohopane isomerisation indices of the mudstone and concretion were 0.66vs 0.58 and 0.62vs 0.60 (47 and 54m depth, respectively). Furthermore, shales contained higher relative amounts of moretanes. Concretions also differed from the mudstone in their methylhopane distributions, with shales showing higher amounts of 3-methylhopanes. Conversely, when biomarker composition was compared at the two depths, they only differed significantly (p<0.1) in the 2-methylhopane index,%C31 homohopanes, and hopane/sterane ratios (p<0.1). Our results show that the biomarker composition of the mudstone at the time of deposition is largely preserved in the concretions with only a minor overprint from diagenesis, bacterial communities involved in concretion formation, or the organic matter nucleus. It is therefore possible to use biomarker analysis on carbonate concretions to determine their provenance if found outside their immediate geological context.

Depositional conditions and revised age of the Permo-Triassic microbialites at Gaohua section, Cili County (Hunan Province, South China)

In many tropical shallow water regions the end-Permian mass extinction event occurs at the top fossiliferous packstone beds and is immediately followed by the development of microbialite facies. Both the age and redox conditions of the microbialite have been debated and both factors are addressed here in a study of the Gaohua section (Cili County, Hunan Province, China): specifically the size distribution of pyrite framboids and high-resolution conodont biostratigraphy. The framboids populations show a broad size range with examples up to 30microns in diameter, and indicating dysoxic but not anoxic depositional conditions. More intense dysoxia is recorded in interbedded laminated micrites but not beds of giant ooids. Both the Hindeodus parvus zone and Isarcicella isarcica zones were established with the microbialite beds being confined to the H. parvus zone. Therefore, the formation of microbialite postdates the end Permian main mass extinction and records oxygen-poor conditions even in a shallow-water setting such as Gaohua section at Cili.

Petrographic characteristics and palaeoenvironment of the Permian coal resources of the Barapukuria and Dighipara Basins, Bangladesh

Abstract Twenty-seven coal samples from the Barapukuria and Dighipara Coal Basins of Bangladesh were analysed for their maceral content, petrographic characteristics and vitrinite reflectance. The most predominant maceral was the inertinite group (mean 40%), followed by vitrinite (mean 31%) and liptinite (mean 22%), with considerable amounts of mineral matter (mean 7%). Semifusinite, fusinite and inertodetrinite were the most common macerals of the inertinite group. Collotelinite, collodetrinite and vitrodetrinite were the most frequently found macerals of the vitrinite group, while sporinite and cutinite were the most common in the liptinite group. Clay minerals occurred in higher concentrations than other minerals. The measured vitrinite reflectance values (%Ro) ranged from 0.71 to 0.80, indicating a high volatile bituminous B ranking. Facies modelling using maceral composition and maceral indices suggested an environment of forest swamps with alternating oxic–anoxic depositional conditions. Microlithotype-dependent depositional modelling indicated evolution in limno-telmatic zones under fluvio-lacustrine control, accompanied by the development of upper to lower deltaic plain conditions. A terrestrial origin with dry forest to piedmont plain conditions was suggested by the Gelification Index (GI) and Tissue Preservation Index (TPI). The lateral variation of the measured TPI values indicated an increase in the rate of basin subsidence. A cross-plot of the Ground Water Index (GWI) vs. the Vegetation Index (VI) suggested mires under ombotrophic to mesotrophic hydrogeological conditions containing herbaceous plants.

Inorganic geochemical evidence for the depositional facies associations of the Upper Jurassic Chia Gara Formation in NE Iraq

Four wells (K-109, Hr-1, Tk-3, and Bj-1) in NE Iraq (including parts of the Kurdistan Region) were selected to study the Tithonian–Berriasian Chia Gara Formation from the inorganic geochemical point of view. The intervals studied in each well occurred at present-day burial depths of Hr-1 3,075–3,310 m; K-109 2,780–3,090; Tk-1 2,770–2,890; and Bj-1 2,150–2,310 m. A total of 16 samples from the four studied wells were investigated geochemically using X-ray fluorescence in order to measure their major element oxides and their trace element contents. Among the major oxides, CaO has the highest weight percentages in all samples as expected in this limestone-dominated formation. SiO2 and Al2O3 show higher concentrations only in well Bj-1 than the other sections, due to its shallower depth of deposition and its marginal location within the depositional basin. The general trace element distribution along all studied well successions showed good similarity. However, the ratios of V/Ni and V/Cr were of higher values in the lower part of the formation, which is considered as a good indicator to the deposition of this part of the formation within anoxic depositional environment. An exception was in well Bj-1, where the V/Ni ratio was lower in the this lower part of the formation than the upper part that may be due to the different lithology in this succession which is lacking distinct shale or calcareous shale beds. However, the V/Cr ratio can still indicate the prevailing of reducing condition because V/Cr ratios in all samples are >2. Also, Th/U ratio in the lower part of the studied successions was lower than the upper part, which is also coinciding with the domination of the reducing geochemical conditions in the depositional environment. It may be concluded from this study that the R-mode cluster analysis of the main oxides indicated to the main mineral constituent of the rock which is calcite. It is also confirmed that the SiO2 and Al2O3 contents in the Chia Gara limestones are especially high in well Bj-1, particularly in the lower and upper parts of the succession. On the other hand, R-mode cluster analysis of the trace elements showed lower amounts of Zr and V in relation to silica and confirmed the random distribution of Sr and Ba in the studied samples. Q-mode cluster analysis indicated that the upper part of the studied formation is pure limestone in well H-1 and such purity of limestone beds also occurs in the lower part of the studied formation in well Tk-3. The elemental distribution within the sections as inferred from the studied samples indicates that the anoxic depositional conditions were prevailed during the Chia Gara Formation, especially during the accumulation of the lower part.