Mudstone Deposits Research Articles

Industrial Assessment of Mudstone Deposits of Gercus Formation as a Raw Material for Bricks Industry in Erbil Governorate, Northern Iraq

This paper evaluates the mudstone deposits' physical, chemical, and mineralogical properties and the possibility of use as a raw material to manufacture building bricks. Four sites are selected from the Gercus Formation in Shaqlawa district and the samples are tested. The mudstone samples are analyzed to define the grain size, Atterberg limits, chemical, physical and mechanical characteristics of the clay bricks. The grain size analysis of the mudstone samples showed they contained a high percentage of clay and silt and a low percentage of sand. The chemical analysis showed that silica (SiO2) was the major component in the mudstones. XRD analysis showed the presence of non-clay minerals i.e. calcite, quartz, albite and clay minerals of illite, palygorskite and chlorite. Raw samples of all studied sites were treated to prepare brick samples without addition, and others with 10% standard sand added. The results showed that the bricks prepared from the B to B10 and C to C10 stations have varying water absorption and compressive strength, while the efflorescence has light salts. All these properties are within the requirements of the Iraqi specification for brick manufacturing. The sample sites A, D, A10, and D10 are not compatible with the Iraqi standard in terms of absorption and compression.

The Cibarco anticline of the northernmost San Jacinto fold belt: Possible influence of mud diapirism on the Neogene to the recent evolution of the structure where the first oil and gas wells were drilled in Colombia

The Cibarco anticline, located in the northwestern South American convergent margin, is a long (approximately 40 km) and narrow (2.5 km wide), south-southwest–north-northeast-trending structure that separates two much wider depocenters, the Tubará-Juan de Acosta depocenter (approximately 20 km wide) to the west, and the Sabanalarga syncline (approximately 11 km wide) to the east. Detailed geologic surface mapping and subsurface seismic interpretation suggest that the formation and evolution of the Cibarco anticline have been strongly influenced by mud diapirism, which has been focused along a west-southwest–east-northeast-trending, inverted pre-Oligocene extensional fault. Seismic interpretation and outcrop analyses also confirmed that the source of mobile shales is the Upper Oligocene marlstones of the Lower Ciénaga de Oro Formation. We suggest that the main mechanism that caused the overpressures that triggered and controlled shale mobilization was disequilibrium compaction related to the sedimentary loading of impermeable seals and the shortening and inversion of pre-Oligocene extensional faults. Growth strata indicate that the Upper Oligocene mud started moving in Early Miocene times during the deposition of the thick Porquero mudstones, and it stopped migrating in Latest Miocene times. After an uplift and erosion event, mud diapirism remained inactive in the Early Miocene, and it was then reactivated in Late Pliocene times. Slight mud migration focused on the inverted fault continued throughout the Pleistocene and to the present. Today, the slight mud migration to the surface is observed in volcanoes located in the northern strand of the Cibarco anticline, although the rest of the anticline has become welded. Our new interpretations, carried out more than a century after the first hydrocarbon exploration activities began in Colombia, will hopefully allow us to increase our understanding of shale tectonics, petroleum systems, and plays in this portion of the convergent margin.

Sedimentary Expressions of the Early Jurassic Jenkyns Event in an Inland Lacustrine System in the Yin'gen–Ejinaqi Basin, North China

AbstractThe Jenkyns Event, more widely known as the Toarcian Oceanic Anoxic Event (T‐OAE), is marked by globally distributed negative carbon‐isotope excursions, widespread oxygen depletion, and large‐scale organic carbon burial, which indicate major climate/environmental perturbations in Earth's surface systems during the Early Jurassic. Although extensive research has been conducted in European continental settings, particularly in the western peri‐Tethys regions, the impacts of this event beyond Europe remains largely unexplored. Here, a multiapproach study including investigations into the spore‐pollen assemblages, pyrite framboids, clay minerals, total organic carbon (TOC) levels, and organic carbon isotope (δ13Corg) levels in a lacustrine borehole section (MED1) from the Yin'gen–Ejinaqi Basin, North China, provides evidence of the occurrence of the Jenkyns Event and its extensive sedimentary responses in the eastern Tethys terrestrial systems. Two distinct spore‐pollen assemblages have been identified in MED1 (drilling depth: 982.4 m to 1267.5 m), with the Cycadopites–Protopinus–Osmundacidites assemblage in the lower part (1267.5 m to 1132.9 m) indicating a middle Early Jurassic age and the Classopollis assemblage in the upper part (1132.9 m to 985.7 m) suggesting a Toarcian age. Framboidal pyrite data suggest more anoxic conditions during the deposition of black mudstone and shale intercalations in the lower part of the Classopollis assemblage (1132.9 m to 1066.9 m), which combined with organic carbon enrichment and negative δ13Corg excursions, are considered the paleoenvironmental response to the Jenkyns Event in the study area. Furthermore, the evolution of vegetation groups changed from plant groups characterized by bisaccate and cycad pollen, as well as fern spores, to vegetation groups represented by Cheirolepidiaceae pollen across the Jenkyns Event, as evidenced by spore‐pollen data, together with the clay mineral assemblage change characterized by a notable increase in illite at the expense of kaolinite, suggests that while a subtropical‐temperate climate persisted, a change toward warmer and drier conditions most likely occurred in the early Toarcian in the study area. In contrast to the humidification evidenced in many coastal settings, this aridification trend in the Yin'gen–Ejinaqi Basin aligns with the conditions in many inland areas. It is hypothesized that the underlying cause of these divergent changes may be linked to certain patterns of spatially variable water availability on land, potentially driven by extremified hydrological conditions.

Geochemistry and mineralogy of the shale-hosted vanadium Van Property deposit, Mackenzie Mountains, Northwest Territories

Abstract Shales and mudstones are fine-grained rocks formed in sedimentary basins throughout Earth’s history. These lithologies are increasingly important targets for mineral deposit exploration since they can have economic resources of critical minerals including vanadium (V), an essential component of redox-flux batteries in solar cells. However, many Paleozoic shale-hosted V deposits are metamorphosed and deformed. This commonly obscures primary features including V-bearing host phases and the original composition of organic material. In this study, we present geochemical and mineralogical data from the Paleozoic Van Property deposit, Northwest Territories, Canada, to show that V can be released from organic matter during metamorphism and incorporated in clay phases such as illite. The siliceous argillites at the Van Property hosts up to 0.69% V2O5 and were metamorphosed to (sub-)greenschist facies. Their mineralogy is dominated by quartz with minor graphite, illite, muscovite, pyrite, sphalerite, rutile, and carbonates. Although, some illite (i.e., high-V illite) can have up to 13 wt% V2O3 and rutile can have up to 4.4 wt% V2O3, mass balance calculations are insufficient to explain V enrichment at the Van Property utilizing only illite and rutile. The third V host is inferred to be carbonaceous matter in which V accumulated syn-deposition on the seafloor. Subsequent metamorphism led to the demetallation of V-bearing geoporphyrins and the release of vanadyl ions (VO2+), some of which were then incorporated into high-V illite and rutile. This process of V enrichment highlights the role of organic matter in scavenging V from superficial reservoirs and the importance of metamorphism on subsequent V release and its incorporation into inorganic phases. The geochemistry of siliceous, V-rich argillites at the Van Property is also compared to other V-enriched shale and mudstone deposits, highlighting the diversity of shale-hosted V deposits, and emphasizing the need for further research to close the knowledge gaps related to variations in composition, mineralogy and V enrichment processes.

Lacustrine environment evolution in the Mesozoic North Yellow Sea Basin, eastern China: Insight from the transition of Jurassic grey mudstones to Cretaceous red successions

During the Cretaceous, terrestrial red beds appear on a global scale. Previous reports have suggested that these red beds were intricately linked to hot paleoclimates and unique paleogeographic configurations. This study focuses on petrological, mineralogical, and geochemical analyses conducted on samples that capture the transition from grey to red mudstone successions dating from the Middle Jurassic to Early Cretaceous within the North Yellow Sea Basin, with a focus to investigate the evolution of lacustrine environments and paleoclimates during their deposition. The findings revealed four episodes of hydrothermal activity occurring during the deposition of the Jurassic grey to Cretaceous red mudstone successions. Some mudstone samples have identified positive Eu anomalies, but the hydrothermal signal may not be as strong due to the open lake environment. Hf vs La/Th and ∑REE vs La/Yb graphs indicate that most grey mudstones originated primarily from upper crustal felsic sources, while certain red mudstones contain mantle material, and the provenance of both has changed significantly. Noteworthy indicators like Sr/Ba, B/Ga, U/Th, Ni/Co, V/Cr and V/(V + Ni) values denote that the sedimentary environments of both grey and red mudstones were oxidized, deviating from conventional viewpoints. However, this deviation may stem from hydrothermal contamination impacting trace elements such as Ni, Co, and Sr. Assessment using indicators like Rb/Sr and Sr/Cu reflects a transition from a warm and wet Jurassic climate to a hot Early Cretaceous climate. Variations in lake levels, prompted by climate fluctuations, led to increased oxygenation on the lake bottoms, resulting in the deposition of red mudstones. These findings suggest a close correlation between the red mudstones and the hot climate, driven by the tectonic evolution of Eastern China. The transformation from grey to red mudstones during the Middle Jurassic to Early Cretaceous periods signifies a response to paleoclimatic and paleoenvironmental changes in East Asia.

FIRST EVIDENCE OF PALAEO-TETHYAN UPPER TRIASSIC CALCAREOUS NANNOFOSSILS IN NORTH DOBROGEAN OROGEN (ROMANIA)

This study presents the analysis of Upper Triassic sediments from five locations in North Dobrogea(Romania) and the Black Sea. Microfacies analyses on thin sections from the Frecăţei log reveal a shallowing trend and likely an increase in energy upward into the section. The oldest part of the log is characterised by deposition of mudstones transitioning to wackestones, with an increase of bivalves. Similar wackestones are observed in the Izvoarele and Rândunica logs, i.e. microfacies with abundant bivalves, some foraminifera and echinoderms. The offshore boreholes 816 and 817 Lebăda Vest (core CM 9 and CM 31) seem to have been deposited in a basinal or distal marine shelf environment indicated by the presence of mudstones with rare bioclasts. In contrast, sample CM 10 from borehole 816 LV is a micritised grainstone suggesting a deposition in a shallower, higher energy environment.Scanning electron microscope observations reveal a moderate diagenetic alteration in all studied samples, mainly due to dissolution. Two calcareous nannofossil species: Prinsiosphaera triassica triassica and Eoconusphaera zlambachensis were commonly observed in sample F of the Frecăţei log, allowing for assignment of the sediments to the Rhaetian age. Two coccoliths were also observed in sample F of Frecăţei log and sample CM 9 of the borehole 816 LV indicating the presence of coccolithophorids within the study region. This discovery constitutes the first confirmed record of well-preserved, determinable Upper Triassic calcareous nannofossils in the Palaeo-Tethys Ocean.

Stratigraphic and geochronologic investigation of the Muddy Creek Basin: Implications for the Eocene tectonic evolution of southwest Montana, USA

Sedimentary basins record crustal-scale tectonic processes related to the construction and demise of orogenic belts, making them an invaluable archive for the reconstruction of the evolution of the North American Cordillera. In southwest Montana, USA, the Renova Formation, considered to locally represent the earliest accumulation following Mesozoic−Cenozoic compressional deformation, is widespread but remains poorly dated, and its origin is debated. Herein, we employed detrital zircon U-Pb and (U-Th)/He double dating and sanidine 40Ar/39Ar geochronology in the context of decimeter-scale measured stratigraphic sections in the Renova Formation of the Muddy Creek Basin to determine basin evolution and sediment provenance and place the basin-scale record within a regional context to illuminate the lithospheric processes driving extension and subsidence. The Muddy Creek Basin is an extensional half graben in southwest Montana that is ∼22 km long and ∼7 km wide, with a >800-m-thick sedimentary package. Basin deposition began ca. 49 Ma, as marked by multiple ignimbrites sourced from the Challis volcanic field, which are overlain by a tuffaceous fluvial section. Fluvial strata are capped by a 46.8 Ma Challis ignimbrite constrained by sanidine 40Ar/39Ar dating. An overlying fossiliferous limestone records the first instance of basinal ponding, which was coeval with the cessation of delivery of Challis volcanics−derived sediment into the Green River Basin. We attribute initial ponding to regional drainage reorganization and damning of the paleo−Idaho River due to uplift and doming of the southern Absaroka volcanic province, resulting in its diversion away from the Green River Basin and backfilling of the Lemhi Pass paleovalley. Detrital zircon maximum depositional ages and sanidine 40Ar/39Ar ages show alternating fluvial sandstone and lacustrine mudstone deposition from 46 Ma to 40 Ma in the Muddy Creek Basin. Sediment provenance was dominated by regionally sourced, Challis volcanics−aged and Idaho Batholith−aged grains, while detrital zircon (U-Th)/He (ZHe) data are dominated by Eocene cooling ages. Basin deposition became fully lacustrine by ca. 40 Ma, based on an increasing frequency of organic-rich mudstone with rare interbedded sandstone. Coarse-grained lithofacies became prominent again starting ca. 37 Ma, coeval with a major shift in sediment provenance due to extension and local footwall unroofing. Detrital zircon U-Pb and corresponding ZHe ages from the upper part of the section are predominantly Paleozoic in age, sourced from the Paleozoic sedimentary strata exposed in the eastern footwall of the Muddy Creek detachment fault. Paleocurrents shift from south- to west-directed trends, supporting the shift to local sources, consistent with initiation of the Muddy Creek detachment fault. Detrital zircon maximum depositional ages from the youngest strata in the basin suggest deposition continuing until at least 36 Ma. These data show that extension in the Muddy Creek Basin, which we attribute to continued lithospheric thermal weakening, initiated ∼10 m.y. later than in the Anaconda and Bitterroot metamorphic core complexes. This points to potentially different drivers of extension in western Montana and fits previously proposed models of a regional southward sweep of extension related to Farallon slab removal.

Study on the Differential Distribution Patterns of Fine-Grained Sedimentary Rocks in the Lower Third Member of the Shahejie Formation in Zhanhua Sag, Bohai Bay Basin

Due to the diverse types of sedimentary source supply systems, the research on the distribution patterns of fine-grained sedimentary rocks in rifted lake basins is relatively underdeveloped. The authors of this article selected the lower third member of the Shahejie Formation (Es3L) in Zhanhua Sag as a research object. Using rock core, thin section, and other data, we analyzed rock types and genesis mechanisms, identified the spatiotemporal distribution patterns of facies within the sag, and established a fine-grained depositional model for the rifted lake basin. Five lithofacies were identified, revealing the differential distribution patterns of fine-grained sedimentary rocks in the region. The northern steep slope zone and the deep depression zone are characterized by the deposition of gravity flow-formed terrestrial layered-pebbly mudstone and gravity flow-formed mixed-source pebbly mudstone, respectively. In the central lake basin zone, mixed-source laminated mudstone with vertical rhythmic interlayers has developed. In the southern gentle slope zone, biologically cemented endogenic irregularly bedded algal limestone and chemically precipitated endogenic laminated limestone have developed. The characteristics of strong northern and weak southern terrestrial inputs in the study area control the differential distribution of lithofacies, and based on this, a fine-grained rock depositional model for the rifted lake basin was established.

Controls on organic matter enrichment of the early-middle Miocene lacustrine oil shale, central Tibet (SW China): New insights from clay minerals and geochemistry

The Lunpola oil shale is the largest lacustrine oil shale deposit in Tibet (SW China), which deposited in a Cenozoic lacustrine rift basin and contains abundant organic matter (OM), whereas the crucial factors that controlled the OM enrichment remain elusive. Here, we present new datasets of total organic carbon (TOC) contents, clay minerals and whole-rock geochemistry of the early-middle Miocene Middle Dingqinghu Formation oil shale from the Lunpola basin, central Tibet, in order to elucidate the paleoclimate, paleo-environment condition and the controlling factors on the OM enrichment of the lacustrine oil shale.Oil shales from the Lunpola East section have high TOC contents (avg. 9.60%), while the contiguous mudstones contain low TOC contents (avg. 0.44%). During the oil shale deposition, high smectite and kaolinite contents, low illite contents and low Sr/Cu ratios suggest strong chemical weathering and hence warm and humid climate, which may probably have close relationship with the mid-Miocene climatic optimum (MMCO). However, during the mudstone deposition, the relatively high illite and low smectite contents and higher Sr/Cu ratios suggest weak chemical weathering, indicating relatively arid climatic condition. The paleo-environment indexes (Corg/P, MoEF and UEF) and MoEF-UEF covariation reflect an anoxic bottom water condition during the oil shale deposition, while the mudstone formed in oxic-dysoxic water environment. The weak and moderate positive relationship between bio-productivity proxies (P/Ti and Cu/Ti ratios) and TOC contents reveal that the bio-productivity condition is not the crucial factor for the OM enrichment. Moreover, the water column environment was brackish and stratified during the oil shale deposition, and the sedimentation rate was moderate, which were also conducive to the OM preservation. The positive correlations between the TOC and the paleoclimate and paleoenvironment proxies demonstrate that warm and humid climate during the MMCO and anoxic water conditions play predominant roles in OM enrichment of the Lunpola lacustrine oil shale. Besides, modest sedimentation rate can prevent the dilution of OM, which can also promote the OM enrichment. This study can not only deepen the understanding of OM enrichment mechanism in lacustrine rift basin but also provide important information for predicting the distribution of the oil shale in the Lunpola Basin.

О создании пунктов захоронения РАО в Европейской части России

The article dwells on feasibility and probability of designing and constructing disposal facilities for low and intermediate level waste in European part of the Russian Federation. Clay and mudstone deposits are considered as host rocks with regards of their widespread. Main reasons preventing the construction of disposal facilities being analyzed and the social reason is stated as a principal one.

О создании пунктов захоронения РАО в Европейской части России

The article dwells on feasibility and probability of designing and constructing disposal facilities for low and intermediate level waste in European part of the Russian Federation. Clay and mudstone deposits are considered as host rocks with regards of their widespread. Main reasons preventing the construction of disposal facilities being analyzed and the social reason is stated as a principal one.

Organic geochemistry of an Upper Jurassic – Lower Cretaceous mudstone succession in a narrow graben setting, Wollaston Forland Basin, North-East Greenland

The Oxfordian–Ryazanian was a period of widespread deposition of marine organic-rich mudstones in basins formed during the early phases of the rifting that heralded the formation of the present-day North Atlantic. Occasionally, uninterrupted deposition prevailed for 20 million years or more. Today, mudstones of this time interval are found on the shelves bordering the North Atlantic and adjacent areas from Siberia to the Netherlands. Here, we report data on two fully cored boreholes from Wollaston Forland (North-East Greenland, approx. 74° N), which represent an uninterrupted succession from the upper Kimmeridgian to the Hauterivian. The boreholes record basin development at two different positions within an evolving halfgraben, located at the margin of the main rift, and thus partially detached from it. Although the overall depositional environment remained an oxygen-restricted deep-shelf setting, rifting-related changes can be followed through the succession. The Kimmeridgian was a period of eustatic highstand and records the incipient rifting with a transgressive trend straddling the transition to the lower Volgian by a gradual change from deposits with high levels of total organic carbon (TOC) and kerogen rich in allochthonous organic matter to deposits with lower TOC and a higher proportion of autochthonous organic matter. This is followed by a slight regressive trend with lower TOC and increased proportions of allochthonous organic matter until rifting culminated in the middle Volgian–Ryazanian, indicated by increasing autochthonous organic matter and higher TOC, which prevailed until basin ventilation occurred towards the end of the Ryazanian. The properties of the reactive kerogen fraction remained rather stable irrespective of TOC, underlining the effect of terrigenous matter input for TOC. These variations are also captured by biological markers and stable carbon isotopes. The deposits are very similar to equivalent successions elsewhere in the proto-North Atlantic region, albeit the proportion of terrigenous kerogen is greater.

Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift

Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.

Research on the relationship between hydrocarbon source-reservoir assemblage system and lacustrine level changes in Triassic period of Lunnan area

The lacustrine level changes affects the sedimentary process of the basin, and has an important influence on the hydrocarbon source-Reservoir assemblage system. In this study, the method of wavelet transform and Fisher plot were used to identify 58 high-frequency cycles on drilling well, with an average cycle thickness of 8.21 m. On this basis, the cumulative offset value was calculated, and finally the lacustrine level changes curve was established during the Triassic period in Lunnan area. The results show that there were three obvious lacustrine level fluctuation cycles in the Triassic period in this area. In the descending stage of lacustrine level, the grain size of the sediments in the study area is large, mainly medium-fine sandstone. In the ascending stage of Lacustrine level, the sediment granularity is small, mainly fine-siltstone and mudstone deposits. Under the control of lacustrine level changes, three sets of “binary structure” lithologic assemblage were deposited in the study area, with large grain size lower and small grain size upper. It is found that the sandstone in the lower part of the “binary structure” has good porosity and permeability. The average porosity of the TⅢ oil formation of Karamay Formation is 15.80 %, and the maximum permeability is 3.785 μm2, which can be used as a good reservoir. The TOC content of the upper part of the “binary structure” is very high, and the average TOC is 3.8 % in Huangshanjie Formation, which is a very good source rock. Therefore, lacustrine level changes in this area has a very important control effect on the on the hydrocarbon source rock and reservoir.

Petrographic and geochemical characteristics of the Middle Devonian Eifelian Nabiao Formation in the Youjiang Basin, South China: Implications for paleoenvironment and organic matter accumulation mechanism

The Nabiao Formation of the Middle Devonian Eifelian period has been identified as a promising site for shale gas exploration in the Youjiang Basin, South China. And yet, little is known about the accumulation mechanism of organic matter under transgression in the archipelago paleogeography. In this study, the core samples were analyzed by thin section microscopy, field emission-scanning electron microscopy, pyrite morphology, X-ray diffraction, and geochemical analysis (total organic carbon (TOC) content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) to understand changes in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism of the Nabiao Formation. The study indicates that organic-rich sediments were deposited during the two transgressions of the Chotec Event and the Kacak Event. The deposition of carbonaceous mudstone occurred in water columns with weak upwelling, while siliceous mudstone and siliceous shale were developed in a strong upwelling water column. The correlations between multiple geochemical proxies (paleoredox, paleoproductivity, and terrestrial detrital influx) and TOC content indicate that the Chotec Event is more conducive to organic matter enrichment of the Nabiao Formation than the Kacak Event. In stage 1, TOC content is low due to oxic water columns and low productivity. In stage 2, the Chotec Event causes the relative sea-level of Member 2 to rise rapidly and the basin bottom water to quickly change to dysoxic conditions. This is the key to organic matter enrichment. In stage 3, the oceanic upwelling is crucial in organic matter enrichment by increasing paleoproductivity. Under the protection of dysoxic condition, organic matter is accumulated in stage 4. Organic matter of the Nabiao Formation is not significantly affected by the terrigenous detrital flux. By taking into account the changes in paleoproductivity, redox conditions, upwelling events, and terrigenous detrital flux, an evolution model is proposed in this paper to elucidate the paleoenvironmental change and the organic matter enrichment under transgression in the archipelago paleogeography.

Facies association analysis of a Toarcian siliciclastic‑carbonate lacustrine system, Sichuan Basin, China

Globally, considerable coverage has been devoted to marine environment affected by the Jenkyns Event, but little attention has been given to lacustrine system. Here, siliciclastic‑carbonate samples of the Da'anzhai Member (J1dn) were examined for petrology and geochemistry in order to evaluate lithofacies characteristics, depositional processes and environmental changes of the palaeo-Sichuan lacustrine basin contemporaneous with the Jenkyns Event. Unique sedimentological and petrological features were used to identify five principal facies associations (FA) and depositional environments, including the distal delta front, accretionary bioclastic shoals, proximal bioclastic shoal ramp, distal bioclastic shoal ramp and (semi-)deep lacustrine environment. The vertical transitions of FA reveal that J1dn forms a complete lacustrine transgressive-regressive sedimentary system during the Toarcian, which can be further divided into 3 units in combination with standard gamma logging. The lower unit is characterised by frequent intermixing of multiple FA interpreted as frequent lake-level fluctuations. The middle unit is characterised by the deposition of clay-rich mudstone and the demise of bioclastic shoals interpreted as intensive lacustrine transgression that were at times affected by storm-induced waves or oscillatory currents coinciding with the initiation of the Jenkyns Event. The upper unit is characterised by the thicker and higher proportion of biogenic carbonate-rich lithofacies interpreted as the lake regression, contemporaneous with the end of the Jenkyns Event. Overall, clay-rich lithofacies from suspension settling, turbidity current or slumping triggered by storm events were deposited during a high relative lacustrine-level, accompanied by organic carbon-rich deposition and preservation. In contrast, carbonate-rich lithofacies were mainly controlled by biogenic carbonate sedimentation, associated with a low relative lacustrine-level unconducive to organic matter accumulation. Finally, the proposed depositional evolution model at the lake-level highstand stage and lowstand stage of the palaeo-Sichuan lacustrine basin provides new insights and understanding on the changing palaeolimnological history of the Toarcian lacustrine system.

The environmental transformation mechanism of the coal- and oil shale-bearing interval in the Eocene Dalianhe Formation, Yilan Basin, Northeast China

Assemblages of multiple organic matter (OM)-related sedimentary energy systems are common in the Eocene nonmarine basins of Northeast China. During the Middle Eocene Climatic Optimum, oil shale developed in large quantities in the Yilan Basin. Numerous scientific papers have concentrated on the sedimentary cycle and industrial quality of the coal- and oil shale-bearing intervals, whereas few studies have studied the genetic environmental transformation mechanism of the coal and oil shale deposits. The Dalianhe Formation cover thick oil shale member and coal unit, which is a natural archive for studying the environmental evolution of lake swamp systems. In the present paper, the thermal maturity of organic matter (OM), oil shale industrial quality, coal rank, source of OM and paleoenvironmental conditions associated with OM accumulation in the Yilan Basin are investigated through bulk geochemical analyses, organic maceral analyses and biomarker composition analyses. The coal in the Yilan Basin is classified as subbituminous coal with high total organic carbon (TOC) values, high oil yields, low ash yields, and type II OM. The quality of the low-maturity oil shale in the Coal-Bearing Member is better than that in the Oil Shale Member. The oil shale in the Yilan Basin has type I-II OM and relatively high oil yields, which corresponds to the considerable content of liptinite with a predominantly alginite composition. Based on the higher Pr/Ph values, coal was deposited under oxic–dysoxic conditions, and the OM in the coal mainly originated from terrestrial plants, whereas the oil shale accumulated in a dysoxic environment with lower Pr/Ph values, and featured mixed OM sources, including terrestrial plants and low aquatic organisms. Affected by lake level fluctuations and wide peat swamping, the thickest and highest-quality coal and oil shale developed in swamps under a dysoxic–oxic water column in freshwater environments. Subsequently, the lake basin expanded rapidly, and the sedimentary environment changed to a dysoxic–oxic semideep–deep lake, in which extremely thick mudstone and a large amount of inferior oil shale with low oil yields accumulated under the influence of gravity flows. The basin continued to rapidly subside, the water column became anoxic–dysoxic, and the deposition of mudstone, siltstone and fine sandstone predominated.

Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

The Ocean Anoxic Event II (OAE2) was a significant global event associated with a positive carbon isotope excursion, occurring from the Late Cenomanian to the Early Turonian (C/T). It has been commonly associated with enhanced organic carbon preservation. Carbon isotopic analysis carried out over the C/T interval in the Agadir Basin, Morocco, integrated with previous biostratigraphic data, allows a refined correlation of the OAE2 interval. Detailed sedimentary facies analysis identifies ten lithofacies that record the transition from shallow marine environments in the Late Cenomanian to relatively deeper conditions in the Early Turonian. The variation in lithofacies can be correlated to relative sea level changes that show a correlation to the global eustatic curve.The OAE2 interval comprises dark grey mudstones beds that display low total organic carbon (TOC) values. Trace element and facies analysis suggest dilution from high detrital influx, along with oxic water conditions and low productivity. OM-rich black mudstones are identified in post-OAE2 Early Turonian strata. Trace element analysis suggests this increase in organic matter accumulation was related to increased sea surface productivity and oxygen-depleted bottom water conditions, which facilitated organic matter preservation. Deposition of OM-rich black mudstones is widely reported during the global Early Turonian marine transgression, suggesting the very high sea level was a major control on organic matter generation and preservation.

Quartz types and origins of the Upper Permian Dalong Formation shale of the Sichuan Basin: Implications for pore preservation in deep shale reservoirs

Quartz is critical to reservoir properties (e.g., rock mechanical behavior and porosity) of organic mudstones, exerting an important effect on shale gas production. However, the types and origins of quartz in deeply-buried Upper Permian mudstones, a potential target of shale gas exploration in South China, remain poorly understood. A suite of deeply buried (5636–5645 m) mudstone samples of the Upper Permian Dalong Formation (DL) in the Sichuan Basin (South China) was selected to characterize quartz minerals using petrological and geochemical approaches, including SEM-CL observation. The studied mudstone samples are relatively enriched in organic matter (OM) (total organic carbon (TOC) = 1.88–18.46 wt%) and have been thermally over-matured. The DL organic mudstones accumulated in a failed rift basin, far removed from provenance areas, are chiefly composed of intrabasinal quartz and carbonate (mostly calcite). Quartz is represented by six major forms, including detrital quartz grains, siliceous skeletal fragments, silica nanospheres, microquartz aggregates, grain-replacive quartz, and matrix-dispersed microquartz, of which the latter five are of authigenic origin. Detrital quartz grains are not common in the DL mudstones. Silica reprecipitated as authigenic quartz was largely sourced from dissolved siliceous skeletal fragments (mostly radiolarians), and some silica appeared to have been released from the alteration of volcanic debris (mainly plagioclase) to form matrix-dispersed microquartz grains. Authigenic quartz influenced pore development and preservation of shale reservoirs in two ways. A rigid framework formed from numerous authigenic quartz grains (e.g., microquartz aggregates, silica nanospheres, and siliceous skeletal fragments) shields organic pores during mechanical compaction, whereas authigenic quartz precipitation (e.g., cavity-filling quartz, grain-replacive quartz) reduces porosity by filling voids. A moderate abundance of authigenic quartz (Sixs = ∼20%) is most favorable for development and preservation of pores (especially organic pores) in deep mudstone. Organic pores, widely developed in the studied DL deep mudstones, seem to be largely preserved in the samples with TOC values as much as 18.46 wt%. In summary, the DL mudstone deposits enriched in OM and contained moderate contents of authigenic quartz are especially favorable targets of deep shale gas exploration.

Link between Lesser Himalayan Neoproterozoic Krol succession and the Adelaide rift complex of Australia ‒ evidence from the mudstone deposits of Krol succession, Himachal Pradesh, India

The mudstone deposits, a major constituent of the Lesser Himalayan Krol succession, represents the terminal phase of Marinoan snowball melting, an important part of the most significant events of the Proterozoic time, the Cryogenian Snowball Earth. The Facies analysis suggests that the sediments were supplied to the shelf by meltwater flows, and were later dispersed to the deeper part of the shelf by current/wave-enhanced bottom currents. The upper part of the deposit bears the signature of gradual shallowing of the basin caused due to glacioisostatic rebound. Petrographic study of the samples, randomly collected from different parts of the deposit, reveals that the mud deposit is composed of silt-sized particles of quartz and dolomite, and argillaceous material. Besides the variation in proportion of these constituent components, no other material was found within this deposit. This compositional homogeneity of this huge deposit primarily suggest derivation from some preexisting sediments. Mineralogical similarity and perfect match in trace and Rare Earth elements composition suggest that this mud was derived from the periglacial loess deposits of the Nuccaleena Formation of South Australia. This is in agreement with the provenance of the underlying Krol Sandstone. The mudstones, along with the underlying Krol Sandstone, thus suggest that the Lesser Himalayan Krol succession represent an extended part of the Adelaide rift system of South Australia. The relative position of India and Australia during Neoproterozoic, as per the reconstructed configuration of the Rodinia supercontinent, corroborates this inference.