Petrological Analyses Research Articles

Petro-chemical characterization and depositional setting of a late Permian high ash coal deposit, Central Mongolia

Pan global Permian coal measures are unique in the evolution of the Earth, not matched in any period before or since. Middle and late Permian coal-bearing strata are widely distributed in Mongolia. In particular, a large concentration of transtensive coal-bearing sub-basins is located in southern Mongolia, some of which are well documented. However, the late Permian coal measures developed along the shores of the Mongol-Transbaikalian Seaway in central Mongolia, the focus of this contribution, has received very limited attention. This study focuses on the c. 420 m thick coal-bearing middle part of a c. 2,600 m thick Permo-Triassic succession in the Bayanjargalan district. The study draws on data from 38 drillholes, 3 km of trenches, mapping, petrological analysis of sandstone samples, analysis of macroflora, fauna and trace fossils, 82 coal quality samples as well as washability and ash XRD analysis from a 3t coal bulk sample. The unstable and wedge-shaped architecture of the coal seams strongly suggest a syn-tectonic influence on their development. Paleoclimatic indicators suggest the peat mire ecosystem developed during relatively cold - temperate climatic conditions. Peat-forming plants such as Cordaites, Rufloria and Koretrophyllites probably benefited from moist air currents along the seaway. Plant-arthropod interactions are reported from two sites, in particular DT228 and DT246 oviposition lesions, the latter being almost twice the size of a previous report from North America. Results from 82 proximate analyses returned consistent very high ash yields of 46.95% (db) and 43.45% (adb) from the 3t bulk sample, which are unusual for Permian coals in Mongolia.

Three-dimensional geological modeling applied to multiscale heterogeneity of a reservoir analog: Paleocoastal deposits from the Rio Bonito formation, Paraná Basin

Coastal sandstones have high permoporosity and contain significant hydrocarbon reserves, although they present variable flow heterogeneities. The goal of this paper is to understand the distribution of potential reservoir-flow units in the Rio Bonito Formation (early Permian, Paraná Basin) and to investigate heterogeneities on multiscale. We use stratigraphic, sedimentological, 3D model, and petrological data to evaluate reservoir heterogeneities. System tracts were modeled by sectioning the entire succession into five zones. Eleven facies associations were modeled separately within each zone, using geostatistical analysis and different algorithms. The 3D model was validated by jackknife procedure, and the distribution of facies associations adequately represented both depositional and stratigraphic frameworks. The results are adequate in a curve of accommodation changes similar to previous proposal for the study area. Tidal sand bar facies association presents as elongated (parallel to depositional dip) to sheetlike geobodies according to amalgamation grade, while upper and middle shoreface one is mostly elongated (parallel to depositional strike) to sheetlike. Besides, fluvial channel facies association occurs mainly elongated parallel to depositional dip, forming multilateral and multistory geobodies. Modeled depositional geometries were distinguished along the succession, showing similarities to Rio Bonito Formation outcrops, recent environments, and other deposits around the world, such as estuarine strata from Oriente Basin and shoreface ones from Gulf of Mexico Basin. Using a detailed petrological analysis, eleven reservoir petrofacies were defined and classified into good-, medium-, and low-quality reservoirs or non-reservoirs. Tidal sand bar, upper-middle shoreface, and fluvial channel deposits were categorized as the main reservoir-flow units. This study provides predictive scenarios for heterogeneities in reservoirs and may be applied in petroleum fields.

Source rocks of the Birrindudu Basin: perspectives from a new sampling program

The Paleo- to Mesoproterozoic Birrindudu Basin is an underexplored frontier basin straddling the Northern Territory and Western Australia, and is a region of focus for the second phase of Geoscience Australia’s Exploring for the Future (EFTF) program (2020–2024). Hydrocarbon exploration in the Birrindudu Basin has been limited and a thorough assessment of the basin’s petroleum potential is lacking due to the absence of data in the region. To bridge this data gap, a comprehensive analytical program including organic petrology, programmed pyrolysis and oil fluid inclusion analysis was undertaken on cores from six drill holes to improve the understanding of the basin’s source rock potential and assess petroleum migration. Organic petrological analyses reveal that the primary maceral identified in the cores is alginite mainly originating from filamentous cyanobacteria, while bitumen is the most common unstructured secondary organic matter. New reflectance data based on alginite and bitumen reflectance indicate the sampled sections have reached a thermal maturity suitable for hydrocarbon generation. Oil inclusion analyses provide evidence for oil generation and migration, and hence elements of a petroleum system are present in the basin.

Shallow mantle cycle of subducted sedimentary limestone indicated by carbonate xenoliths

In the subduction zone, carbon (C) release from the subducted plate into the mantle wedge takes place by various mechanisms, including metamorphic degassing, carbonate dissolution, and hydrous melting. Recent studies have highlighted the significant role of buoyant limestone diapirs in facilitating C recycling during subduction processes. Here, we report that the Hannuoba carbonate xenoliths of eastern China were introduced into the shallow mantle in the form of sedimentary limestone diapirs. We conducted a comprehensive study of the Hannuoba carbonate xenoliths using petrological analysis, major- and trace-element analysis, in situ zircon dating, and C-O isotopic analysis techniques. The whole-rock major and trace elements, along with high δ18OSMOW values (22.8‰−23.2‰), indicate that the protolith of Hannuoba carbonate xenoliths consists of sedimentary limestones. The spectral age peaks of detrital zircons (400−100 Ma, 500−450 Ma, 2.0−1.6 Ga, and 2.8−2.2 Ga) suggest that the primary source of the protolith is the North China Craton. When sedimentary limestone diapirs enter into the mantle wedge, they undergo partial melting under high-temperature conditions. This process results in the melting of low-melting point pelitic components, while leaving behind resistant minerals such as pure calcite. Due to its low density and viscosity, this pure limestone component will continue to migrate upward and assimilate with the mantle peridotite. In this process, the pure calcites are preserved in the shallow mantle in a solid-state form rather than as carbonatitic melts. The discovery of graphite and spinel in the Hannuoba carbonate xenoliths indicates that the depth of formation of the sedimentary limestone diapirs does not exceed 2.5 GPa (<70 km). The calcite in Hannuoba carbonate xenoliths exhibits extremely low δ13CVPDB (−11.8‰ to −11.0‰) and high δ18OSMOW (22.8‰−23.2‰) contents. This characteristic C-O isotopic signature can be explained by the Rayleigh decarbonation of sedimentary limestones under upper-mantle conditions (<3 GPa). Based on calculations, the Hannuoba carbonate xenoliths are a product of subducted sedimentary limestone experiencing 99% volume decarbonation. Through this metamorphic decarbonation process, limestone diapirs release a significant amount of CO2 into the arc magma system. During subduction processes, sedimentary carbonate rocks can be recycled to the mantle wedge of island arcs as solid limestone diapirs. The C flux released by this mechanism cannot be ignored.

Formation conditions of Jixian System cherts in the Qishan area, Ordos Basin: Implications for marine redox conditions and paleoecology

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Jixian System in the Ordos Basin. These cherts faithfully record the Jixian period oceanic conditions and paleoecology. However, the diagenetic mechanism of the cherts remains unclear and controversial. To understand the origin of these cherts, we performed a multitracer study by combining field reconnaissance, petrological analyses, Si isotope analysis, and major and trace element analysis of chert samples from the Qishan section. The results show that Jixian cherts had a high Al/(Al + Fe + Mn) value (~0.48), showed a flat distribution of rare earth elements in seawater, and most of the Fe/Ti values are <20. A weak negative Ce anomaly (~0.97) indicates the contribution of REY to weakly oxygenated seawater. The high isotopic composition of silicon (δ30Si = 0.74 ‰-1.35 ‰) and the average Siex value of 37.52 indicate that there is a source of biological silica, and the relationship between Eu/Eu* and Y/Ho and δ30Si shows that the hydrothermal and volcanic influences were less and correlated with seawater. The chert bands were closely related to biological activities. The chert nodules were influenced by hydrothermal activity. Secondary cherts were related to diagenesis, but the nature of siliceous fluids remains unchanged, still characterized by biogenic activity origins. The sea oxygen concentration in the Jixian period had increased, with periodic variations in the intensity of biological activities affecting the pH of the water body, or biological photosynthesis binding SiO2 colloid in water, affecting chert sediments, leading to frequent interlayering between dolomite and chert bands. Episodic hydrothermal fluids and late-stage diagenetic processes jointly influenced the formation of cherts. This study bears significant significance in enhancing our understanding of the sedimentary environment and the origin of cherts during the Jixian period in the Ordos Basin.

Pressure-temperature-time evolution of the Milin pelitic schists: Implications for the tectonic evolution of the eastern Indus-Yarlung suture zone, eastern Himalaya

We report an integrated comprehensive dataset composed of petrography, pressure-temperature (P-T) calculations, monazite U-Th-Pb ages, and trace-element data from pelitic schists in the eastern Indus-Yarlung suture zone in the Milin area of the eastern Himalaya. These rocks represent the exposure of subduction-related rocks within the eastern Indus-Yarlung suture zone accretionary complex. The dominant mineral assemblages of the pelitic schists are garnet + kyanite + staurolite + biotite + quartz and garnet + kyanite + staurolite + biotite + paragonite + sillimanite with quartz, plagioclase, and ilmenite assemblages. Phase equilibrium modeling of sillimanite-bearing pelitic schists yielded peak P-T conditions of ∼670−680 °C at ∼8.6 kbar, similar to that of kyanite-bearing schists (∼670 °C, ∼8.8 kbar). Monazite grains with complex internal structures retained variable ages ranging from 28 Ma to 15 Ma, which correlate systematically with changes in the concentrations of Y, Th, U, and heavy rare earth elements and ratios of Th/U. Combined with petrologic analysis, we conclude that the pelitic schists experienced a long-lived prograde metamorphism from ca. 28 Ma to ca. 22 Ma. Peak metamorphism occurred in the period 22−21 Ma, followed by quasi-isothermal decompression until 15 Ma. The discrepancies among metamorphic P-T-t paths in the eastern Indus-Yarlung suture zone indicate the presence of not only collision-related regional metamorphism at medium P-T conditions, but also subduction-related high-pressure−low-temperature terranes in the Milin region. These two domains experienced different P-T evolution and tectonic histories and were juxtaposed in the early Neogene during the India-Asia continental collision.

SEDIMENTATION CONDITIONS OF THE LOWER CRETACEOUS DEPOSITS OF THE NORTHERN CASPIAN

This research identifies sedimentation settings of Lower Cretaceous (Aptian-Albian) terrigenous sediments of the North Caspian, which are oil and gas reservoirs. These rocks are characterized by irregular distri- bution of pelitic and silty fractions, which leads to ambiguous results of petrophysical analyses. In order to correctly interpret the geophysical data, a detailed lithologic analysis of the rocks was carried out. In the studied area in the Early Cretaceous time marine conditions prevailed: coastal, shallow-marine, relatively shallow shelf lithofacies with signs of active hydrodynamics, as well as relatively deep-water shelf lithofacies of low hydrodynamics. The study of sections reveals a change from shallow-water sediments to deeper-water sediments and vice versa, which allows us to conclude that the sea level fluctuates periodically. Lithologic analysis also showed the predominance of shallow-water and coastal sediments in the Aptian time and deeper-water, shelf sediments in the Albian time. Consequently, we can assume the presence of local sea transgression in this study area during the Lower Cretaceous.

Discovery and petroleum geological significance of delta in the third member of Oligocene Lingshui Formation in southern Baodao Sag, Qiongdongnan Basin, South China Sea

Based on the 3D seismic data and the analysis and test data of lithology, electricity, thin sections and chronology obtained from drilling of the Qiongdongnan Basin, the characteristics and the quantitative analysis of the source-sink system are studied of the third member of the Upper Oligocene Lingshui Formation (Ling 3 Member) in the southern fault step zone of the Baodao Sag. First, the YL10 denudation area of the Ling 3 Member mainly developed two fluvial systems in the east and west, resulting in the formation of two dominant sand transport channels and two delta lobes in southern Baodao Sag, which are generally large in the west and small in the east. The evolution of the delta has experienced four stages: initiation, prosperity, intermittence and rejuvenation. Second, the source-sink coupled quantitative calculation is performed depending on the parameters of the delta sand bodies, including development phases, distribution area, flattening thickness, area of different parent rocks, and sand-forming coefficient, showing that the study area has the material basis for the formation of large-scale reservoir. Third, the drilling reveals that the delta of the Ling 3 Member is dominated by fine sandstone, with total sandstone thickness of 109–138 m, maximum single-layer sandstone thickness of 15.5–30.0 m, and sand-to-strata ratio of 43.7%–73.0%, but the physical properties are different among the fault steps. Fourth, the large delta development model of the small source area in the step fault zone with multi-stage uplift is established. It suggests that the episodic uplift provides sufficient sediments, the fluvial system and watershed area control the scale of the sand body, the multi-step active fault steps dominate the sand body transport channel, and local fault troughs decide the lateral propulsion direction of the sand body. The delta of the Ling 3 Member is coupled with fault blocks to form diverse traps, which are critical exploration targets in southern Baodao Sag.

Miocene evolution of the External Rif Zone (Morocco): Comparison with similar and lateral southern Mediterranean Tethyan margins

The Miocene evolution of the External Rif Zone (NW Africa Plate) was determined through multidisciplinary analysis of fourteen successions. The updated stratigraphic framework shows how Miocene sediments rest on the Cretaceous–Paleogene terrains through unconformity surfaces, whereas it rests with sedimentary continuity in two sectors. After recognition of lithofacies and three unconformities located near the Oligocene–Aquitanian, Aquitanian–Burdigalian and Serravallian–Tortonian boundaries, the Miocene sedimentary record was divided into three stratigraphic intervals representing deep to shallow marine deposits as Aquitanian–Burdigalian, Langhian and Upper Serravallian–Missinian. The two oldest unconformites are restricted to the central sector, while the upper one is generalized and probably related to the nappe tectonics registered in all sectors of the External Rif. Data from analysis of tectofacies, petrology, mineralogy, meaning and implications of unconformities, and subsidence indicate that: (i) mass flow deposits (turbidites, slumps, olistostromes) are common in all successions but more frequent during the Lower Miocene; (ii) petrology of the detrital components of the arenites indicates recycled orogen-derived sediments, with quartz coming from erosion of metamorphic rocks of the Atlas orogen and/or the African craton; (iii) mineralogy of mudstones suggests a complex erosional evolution of local emerged areas derived from a mixture of contributions coming from the erosion of Upper Jurassic to Paleogene suites, and especially from kaolinite-rich Albian–Cenomanian to Paleogene successions with absence of a clear unroofing. The conjunction of all these clues reinforces the idea of a synsedimentary tectonics affecting the margin/basin system during the Miocene. A thickness analysis of the studied sedimentary successions allows proposing the evolution of the orogenic front and main depozones (foredeep, bulges, wedge-top and intramontane sub-basins) integrated in a complex foreland system migrating from north to south with the Atlas-Mesetas area acting as foreland during Miocene. The orogenic front moved from the Internal Intrarif to Mesorif and later to Internal Prerif. The main wedge-top basin also migrated from the Internal Intrarif to External Intrarif. The foredeep migrated from the Mesorif to the Internal Prerif, while the main forebulge was located in the External Prerif and a secondary bulge developed in the External Intrarif. Intramontane basins developed behind the orogenic front in relative extensional conditions moving from the Internal Intrarif to External Intrarif. The reconstructed Miocene evolution was inserted into a 2D paleogeographic-geodynamic evolutionary model using GPlates software, and then compared to those reported in other external margins of the western Tethys (Betic Chain, Tunisian Tell, Sicilian Maghrebids and Apennines), revealing important similarities and local differences.

Structure and provenance of the Cretaceous Pingshanhu Basin in the Hexi Corridor: Implications for Mesozoic tectonics in the northern Tibetan Plateau

Abstract The construction of Earth’s largest highland, the Tibetan Plateau, is generally considered to have been generated by the Cenozoic India-Asia collision. However, the extent to which high topography existed prior to the Cenozoic remains unclear. The Hexi Corridor foreland basin of the northern Tibetan Plateau is an ideal region in which to investigate this history, given its widespread exposure of Early Cretaceous sedimentary sequences. In this study, we examined the Early Cretaceous strata in the northern Hexi Corridor to understand the relationships between pre-Cenozoic sedimentation and tectonic deformation and constrain the late Mesozoic tectonic setting of the adjacent Qilian Shan and Alxa blocks bordering the northern Tibetan Plateau. Results of sandstone petrology analyses, paleocurrent observations, and U-Pb geochronology suggest that the oldest Early Cretaceous sediments deposited in the northern Hexi Corridor were sourced from the southern Alxa block during the earliest Cretaceous. By the late Early Cretaceous, Hexi Corridor sediments were sourced from both the southern Alxa block to the north and the Qilian Shan to the south. Sandstone petrologic results indicate that the northern Hexi Corridor experienced a tectonic transition from contraction to extension during the Early Cretaceous. These findings suggest that the northern Tibetan Plateau region was partially uplifted to a high elevation during the late Mesozoic before the India-Asia collision.

Deep Learning for Refined Lithology Identification of Sandstone Microscopic Images

Refined lithology identification is an essential task, often constrained by the subjectivity and low efficiency of classical methods. Computer-aided automatic identification, while useful, has seldom been specifically geared toward refined lithology identification. In this study, we introduce Rock-ViT, an innovative machine learning approach. Its architecture, enhanced with supervised contrastive loss and rooted in visual Transformer principles, markedly improves accuracy in identifying complex lithological patterns. To this end, we have collected public datasets and implemented data augmentation, aiming to validate our method using sandstone as a focal point. The results demonstrate that Rock-ViT achieves superior accuracy and effectiveness in the refined lithology identification of sandstone. Rock-ViT presents a new perspective and a feasible approach for detailed lithological analysis, offering fresh insights and innovative solutions in geological analysis.

Late Paleozoic sedimentation recording back-arc basin evolution in response to Chinese Altai−East Junggar convergence in Central Asia

Sedimentological and geochronological data from late Paleozoic strata located between the East Junggar and Chinese Altai regions in NW China were examined, aiming to decipher the tectono-sedimentary evolution of this important tectonic boundary. Carboniferous sediments on the East Junggar side show arc-proximal depositional characteristics of the proximal Heishantou and Nanmingshui Formations and distal Beitashan and Yundukala Formations, while the Erqis complex on the Chinese Altai side is characterized by continental margin affinity. Lithological analysis revealed the dominant input of arc-related detritus for all these sequences and a uniform transition from volcaniclastic to siliciclastic components in their respective upper sections. The investigated East Junggar strata are dominated by Carboniferous zircons with positive εHf(t) values, sourced exclusively from the southerly Yemaquan-Jiangjunmiao arc domain, whereas the Erqis complex received detritus from the same arc domain but also evolved components from the northerly Chinese Altai. Combined with regional data, the examined strata are interpreted to have developed in a back-arc basin with regard to an arc that developed above the north-dipping Kalamaili subduction system. In contrast, the unmetamorphosed Lower Permian Tesibahan Formation, unconformably overlying the Erqis complex, received detritus mainly from the Chinese Altai. These sediments were deposited in an intracontinental piggyback or synformal basin following closure of the back-arc basin. The late Paleozoic sedimentation records support the interpretation that the Chinese Altai and East Junggar domains evolved from the same suprasubduction system prior to the Carboniferous rather than as independent terranes mutually juxtaposed during Permian lateral translation, as previously proposed.

Prokaryotic richness and diversity increased during Holocene glacier retreat and onset of an Antarctic Lake

Knowledge about biodiversity changes during transitions from glacial landscape to lake formation is limited to contemporary studies. Here, we combined analyses of lithology, chronology and geochemistry with sedimentary ancient DNA metabarcoding to assess such transition in maritime Antarctica. We inferred three paleoenvironmental stages covering the Holocene glacier retreat process. From 4900 to 3850 years before the present, we found the lowest prokaryotic richness/diversity, with bacterial taxa indicators associated to soil and terrestrial environments. From 3850 to 2650 years before the present, a higher carbon content, higher Carbon/Nitrogen variability, increased species richness/diversity, and prokaryotic taxa indicators of long-term energy starvation were detected. Finally, from 2650 to 1070 years before the present, we inferred the onset of a genuine lacustrine environment holding stable Carbon/Nitrogen ratios and the highest prokaryotic diversity, with known aquatic bacterial taxa. Our study unveils for the first time the evolution from a glacier-covered to a freshwater lake through a millennial scale.

Cenozoic tectonic transition within the western segment of the Longmenshan fault, southeast margin of the Tibetan Plateau: Insights from geological and geophysical data

Thrust fault zones around the Tibetan Plateau (TP) record the tectonic evolution between the Plateau and its surrounding terranes, which is helpful for understanding the uplift mechanism and deformational processes of the TP. The Longmenshan fault (LMSF) is the tectonic boundary (TB) between the Yangtze terrane (YT) and Songpan-Garze terrane (SGT), while the TB of its western segment, either the Lijiang-Xiaojinhe fault (LXF) or Jinhe-Qinghe fault (JQF), is controversial. Therefore, we conducted magnetotelluric (MT) imaging, surface structure surveys, and petrologic analysis to further determine the deep–shallow structural relationship of the western LMSF segment. Resistors R1 and R2 revealed by MT imaging may have originated from different magmatism. Among them, R2 may have originated from plume underplating, which is consistent with previous studies, while R1 may have originated from the residue of episodic mafic magma intrusion along the JQF over a broader period. Based on regional geophysics, surface structural patterns and petrologic mineralogy, it is suggested that the JQF may have deformed deep into the lower crust or upper mantle, accommodating the southeast expansion of the TP by thrusting and acting as the TB between the YT and SGT before ∼15 Ma. After ∼15 Ma, due to the activation of the large-scale strike-slip faults, the LXF gradually replaced the JQF to dominate the structural deformation of the western LMSF segment. Our results indicate that the above tectonic transition might be associated with the geodynamic process from centralized deformation to diffuse deformation within the southeast TP during the late Cenozoic.

Dolomite formation during penecontemporaneous subaerial diagenesis: evidence from modern dolomite crusts forming in lagoon Brejo do Espinho, Brazil

The early lithification of carbonate mud during the subaerial exposure stage under semiarid conditions has been proposed to facilitate dolomite formation. However, how the biogeochemical processes during subaerial diagenesis promote dolomite formation remains unclear. Here, we employ a multiproxy approach to investigate the process of dolomite formation by analysing modern dolomite crusts forming in lagoon Brejo do Espinho. Petrological analysis reveals that the crusts consist of coexisting high-Mg calcite and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, whereas a higher Sr concentration in well-lithified crust suggests primary bacterial-induced dolomite precipitation. The Mg isotopic composition of the crusts exhibits a lighter value than that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. The more negative δ 13 C values of the crusts than those of the unlithified carbonate mud in lagoon Brejo do Espinho indicate the incorporation of 13 C-depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg 2+ dehydration and enhances the dissolution of primary high-Mg calcite, ultimately triggering the transition of high-Mg calcite to dolomite and/or the direct precipitation of dolomite. The ancient ‘dolomite factory’ operated through the cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis. Thematic collection: This article is part of the Towards unravelling the ‘Dolomite Problem’: new approaches and novel perspectives collection available at: https://www.lyellcollection.org/topic/collections/towards-unravelling-the-dolomite-problem

Developing Characteristics of Shale Lamination and Their Impact on Reservoir Properties in the Deep Wufeng–Longmaxi Formation Shale of the Southern Sichuan Basin

The deep shale of the Wufeng–Longmaxi Formation in the southern Sichuan Basin has high gas potential. The development characteristics of lamination could significantly impact reservoir property. Samples were investigated using microscopic observation, element analysis, organic petrology analysis, mineralogy analysis, and pore structure analysis to determine the types of laminae and laminasets, clarify the formation conditions of argillaceous lamina and silty lamina as well as their relationships with the sedimentary environment, and explore the influence of laminae on shale reservoir property. Results indicate that the Wufeng Formation shale exhibits weak development of laminae due to bioturbation, while the Longmaxi Formation shale develops continuous, parallel, and plate-like laminae. Compared with light silty lamina-rich shale, dark argillaceous lamina-rich shale usually develops in an anoxic reduction environment, with higher total organic carbon content, porosity, pore volume, specific surface area, and more developed organic matter pores, which can provide greater space for shale gas adsorption and storage. Shales in the middle section of the Longmaxi Formation are characterized by the development of silty-argillaceous interbedded type laminaset, which have good reservoir performance, making them the primary target for deep shale gas exploration and development.

Organic geochemistry and organic petrology of Oligocene and Miocene lignites, subbituminous and high volatile bituminous coals and carbonaceous shales in northern and central Trinidad

The so far unstudied Oligocene and Miocene organic matter–rich deposits of the Cunapo and Nariva Formation of northern and central Trinidad, western tropical Atlantic Ocean have been investigated using organic geochemical and organic petrological techniques to assess the thermal maturity and paleoenvironmental conditions during peat accumulation. Twenty samples were collected along various sections within the Cunapo and Nariva formations. Evaluation based on organic petrological analyses and Rock-Eval pyrolysis reveals that the coals and carbonaceous shales are thermally immature in the lignite and subbituminous rank for Cunapo Formation and mature in the subbituminous to high-volatile bituminous rank for Nariva Formation. The respective maturities correspond to high volatile matter contents.Petrographically, the Cunapo Formation coals are dominated by huminite with minor amounts of inertinite and liptinite, and the Nariva Formation coals contain high amounts of vitrinite, with moderate to low liptinite and low inertinite contents. The detailed petrographic data for Cunapo and Nariva coals indicate predominantely limnic conditions and a herbaceous origin of organic matter. Further conclusions are deduced from petrographic ratios such as gelification-, tissue preservation-, vegetation- and groundwaterAC indices, ash- and sulfur contents, iso- and n-alkane distribution, and 17α(H)-homohopane ratio. In summary, the data supports that both the Cunapo and Nariva coals formed in a tropical peat environment, partly influenced by very high microbial activity and ± in case of Nariva coals - partly grading into ombrogenic mires. The Cunapo coals were influenced by the development of the Orinoco River system during the Miocene and exposure to marine water during transgressive phases in a tidal, coastal environment.

Late Pleistocene and Holocene sediments of Lake Chukhlomskoye

The paper discusses the structure of the bottom sediments of Lake Chukhlomskoye (Kostroma region, Russia). The results of complex lithological analysis and 14C AMS dating (TOC) of the sediments revealed in two boreholes located in different geomorphological positions of the bottom relief allowed to reconstruct stages and conditions of sedimentation in the lake. Mass accumulation rate (MAR) was calculated. High values of MAR correspond to 25.4–17.5 cal ka BP (up to 0.192 g/cm2 per year), a sharp decrease of MAR corresponds to 17.9–16.7 cal ka BP, and another stage of sediment influx occurred in Younger Dryas. Holocene MAR is low (0.001–0.053 g/cm2 per year).

Evolution from shallow‐water deltas to fluvial fans in lacustrine basins: A case study from the Middle Jurassic Shaximiao Formation in the central Sichuan Basin, China

AbstractDistinguishing between shallow‐water delta and fluvial fan deposits in the subsurface of lacustrine basins is challenging due to their similar depositional characteristics and distribution patterns. This study focuses on the Middle Jurassic Shaximiao Formation in the central Sichuan Basin using core observations, seismic analyses, petrology analyses, zircon analyses, palaeoclimate indicators and palaeocurrent analyses to address this issue. Fifteen sedimentary lithofacies and eight lithofacies associations were established, corresponding to channelized fluvial deposits, non‐channelized fluvial deposits and shallow‐water delta deposits. Shallow‐water deltas are dominated by channels and mouth‐bar complexes with grey, red and green interbedded mudstone. Shingled seismic reflection, green mud clasts, small burrows and wave ripples are common with occasional palaeosols. Fluvial fans are dominated by channels and crevasse‐splays with pink colour and accompanied by red coloured floodplain deposits. Bright spot seismic reflection, red mud clasts, big burrows, current ripples and palaeosols are common. The increased uplift of the Dabashan Mountains controls the palaeocurrent direction, promoting the evolution from a shallow‐water delta in relatively humid environments to a fluvial fan in relatively arid environments. The channel widths in shallow‐water deltas are wider than those in fluvial fans, whereas the opposite applies for the channel amalgamation rate. Highly frequent channel‐width variations are controlled by short‐cycle climate fluctuation, corresponding to chemical index of alteration value fluctuations in different sandstone groups. The channel width in the relatively humid environments is wider than that in arid environments for both shallow‐water deltas and fluvial fans. It is likely that the sedimentary evolution from shallow‐water deltas to fluvial fans is relatively common in lacustrine basins in relatively arid environments with wide and gentle slope landforms.

No significant provenance changes in the Hoh Xil Basin, central-northern Tibetan Plateau, from the Late Cretaceous to the Early Miocene

The construction and uplift of the Tibetan Plateau has played a critical role in Cenozoic global climatic cooling due to extensive chemical weathering and establishment of the Asian monsoons. However, the models, mechanisms, and timing of Tibet's uplift remain rigorously debated. Important outstanding questions include what were surface elevations of the Tibetan Plateau prior to the Paleogene India-Asia collision and what mechanisms were responsible for the Cenozoic uplift of the central Tibetan Plateau. In this study, we integrated new detrital zircon data from the Tuotuohe Basin, a sub-basin of the Hoh Xil Basin in the central Tibetan Plateau, with paleocurrent directions, petrological textures, sedimentary facies, and eastern and western Hoh Xil Basin isotopes to investigate the source-to-sink relationship between the Tanggula Range and the Hoh Xil Basin. Any notable provenance shifts would have implications for the topographic evolution of the central and northern Tibet and proposed plateau uplift mechanisms. Our detrital zircon data from the Tuotuohe sub-basin revealed no provenance changes between ca. 37 and ca. 20 Ma. Detrital zircon data from the Tuotuohe section and the eastern and western Hoh Xil Basins, paleocurrent directions, petrologic analysis, strata characteristics, and isotopic evidence suggest that there was a single expansive Hoh Xil Basin from the Late Cretaceous to the Early Miocene. Furthermore, the Tanggula Range has been the primary source region for the Hoh Xil Basin since the Late Cretaceous. However, the Fenghuo Range may have provided a significant secondary source of detritus starting in the Late Oligocene-Early Miocene. Our results imply that there was established topographic relief in central-northern Tibet between mountainous highlands in the south and the Hoh Xil Basin in the north since the Late Cretaceous. The Tanggula Range-Hoh Xil Basin experienced surface deformation and uplift from the Late Cretaceous to ca. 40 Ma, induced by the Lhasa-Qiangtang and India-Asia collisions, after which the deformation intensity ceased by ca. 27–24 Ma. Therefore, the additional ∼1.5 km surface uplift of the Hoh Xil Basin since the Middle Miocene cannot be the result of crustal shortening. Instead, surface uplift may have been caused by the convective removal of the mantle lithosphere or magmatic inflation, which explains the sub-horizontal attitudes of the Middle Miocene Wudaoliang Formation in the region.