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The geochronology of tuffaceous sandstone and elemental geochemistry of mudstones in the Lower Permian Zhanjin Formation (well QZ‐5) in the Central Qiangtang Basin were studied to determine their depositional age, provenance, tectonic setting, and depositional conditions. Twenty‐six synchronous igneous zircons from tuffaceous sandstone at 596.5 m have a weighted average U–Pb age of 292.5 ± 2.5 Ma, tentatively constraining the sedimentary age of the Zhanjin Formation to the Early Permian. Low SiO2/Al2O3 and Th/Sc ratios indicate a low sedimentary recycling and compositional maturity for the source area of the Zhanjin Formation mudstones. The low Al2O3/TiO2, moderate‐high Fe2O3/Al2O3, high TiO2/Fe2O3, low‐moderate TiO2/Zr, and moderate Co/Th ratios combined with post‐Archean Australian shale‐normalized rare earth element distributions imply that the provenance of the Zhanjin Formation mudstones was mainly from the mixed mafic and felsic sources that is composed of basalt and rhyolite exposed in the Central Qiangtang Basin. The tectonic discrimination plots, low Fe2O3/TiO2, (La/Ce)N, and high Al2O3/(Al2O3 + Fe2O3) ratios reveal that the studied mudstones mostly formed in rift settings, consistent with the geologic evolutionary history of the Early Permian Qiangtang Basin. Trace element ratios (V/Cr and Cu/Zn), enrichment factors (MoEF), and δCe values combined with sedimentary features indicate that almost all the studied mudstones were deposited in an open marine environment along the continental margin, and were associated with oxic and dysoxic water columns. [ABSTRACT FROM AUTHOR]

The Lower Jurassic clastic rocks in the Qiangtang Basin, northern Tibet, are generally regarded as the early sediments of the new Mesozoic Qiangtang Basin (Late Triassic to Early Cretaceous). However, little attention has been paid to the palaeoenvironment, provenance, and tectonic setting of this unit. The QZ‐16 well, located in the east of the northern Qiangtang Basin, firstly recovered the Lower Jurassic strata (Quemo Co Formation) in the Qiangtang Basin. In this paper, the whole‐rock major‐trace elemental characteristics of 34 mudstone samples and detrital zircon geochronology of one sandstone, collected from the Quemo Co Formation of the QZ‐16 well, are reported in order to infer the palaeoenvironment, provenance, and tectonic setting. Quemo Co mudstones are dominated by SiO2 and Al2O3, while Fe2O3, CaO, K2O, and MgO are the second most abundant oxides. They are generally characterized by high REE concentrations, enrichment in light rare earth elements (LREEs), marked negative Eu and Ce anomalies. The REE contents of the mudstones are mainly controlled by terrigenous detrital minerals input. The Lower Jurassic mudstones have relatively high chemical index of alteration (79.1–83.6) values, indicating moderate to high degree of chemical weathering conditions of the source area. Sr/Ba and Rb/K ratios reflect that the marine transgression impact was limited and the mudstones were generally deposited in a brackish water environment. Major, trace, and REE contents indicate that the mudstones were mainly derived from intermediate igneous rocks with subordinate felsic igneous rocks. Prominent age peaks at ca. 200 ~ 400 Ma, 700 ~ 970 Ma, 1,700 ~ 1,900 Ma, and 2,100 ~ 2,700 Ma of the detrital zircon grains suggest that the Lower Jurassic clastic rocks were derived from the Central Qiangtang and Songpan–Ganzi complex. Moreover, tectonic discrimination diagrams based on major and trace elements show that the source of the Lower Jurassic sediments were most likely derived from a collision tectonic setting. [ABSTRACT FROM AUTHOR]

Many studies have been focused on the development of carbonate platforms, however, the controlling factors of their evolution are still unclear. The QTKT1 well, drilled in 2016 by the Chengdu Center of the China Geological Survey at the Northern Qiangtang Depression, has allowed further studies of Jurassic environments and resources in the Eastern Tethys. In this study, we investigate variations in spectral gamma‐ray logs and CaCO3 content and perform a cyclostratigraphic analysis in the Xiali mudstones and Buqu limestones and Xiali mudstone to analyse causes for the demise of the Middle Jurassic carbonate platform in the Qiangtang Basin. The concentrations of Th, K, U, and Th/U and Th/K ratios increase suddenly from the Buqu Formation to the Xiali Formation, suggesting increased terrigenous input and/or decreased carbonate production rates. Cyclostratigraphic analysis detects the existence of astronomical cycles and an abrupt change in sediment accumulation rates within the target intervals. Combined with sedimentary facies analysis, our research suggests that the demise of the Middle Jurassic carbonate platform may have been caused by terrigenous detrital inputs into the Qiangtang Basin. High‐frequency sea‐level fluctuations increased terrestrial inputs and carbonate production rates at the orbital scale. The regression happened before the facies change and controlled terrestrial inputs at a larger scale, which resulted in the demise of the carbonate platforms. Contrary to the Western Tethys, a regression took place from the Bathonian to Callovian ages in the Qiangtang Basin, Eastern Tethys, which increased the exposure levels. The large exposure led to significant erosion, accelerated detrital input from the land to the Northern Qiangtang Depression, reduced carbonate production as well as resulted in the demise of the Buqu carbonate platform. The relative sea‐level fall is mainly the result of the regional tectonism during this period. [ABSTRACT FROM AUTHOR]

The early Jurassic black shales are the most important source rocks in the Qiangtang Basin, Tibet. These black shales may provide a useful example for understanding the sedimentary conditions during the early Jurassic in the eastern Tethys. Thirty-two black-shale samples were collected from the Qiangtang Basin to reconstruct the depositional environments during the early Jurassic in the eastern Tethys. The palaeoclimate index Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na) varies between 0.58 and 1.14, together with low Sr/Ba (0.20-0.60) and Sr/Cu (1.68-7.88) ratios, indicating a warm humid to semi-humid climatic condition during black-shale deposition. The early Jurassic black shales exhibit slight Ce anomalies (0.89-0.94) and Mn enrichment (EF = 1.2), suggesting a slightly oxic depositional environment. The conditions of the sedimentary water system during black-shale deposition were apparently brackish as evidenced by middle B/Ga (3.96-5.73) and Th/U (3.96-6.51) ratios. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

[Display omitted] • The minimum paleoelevation of the Qiangtang terrane was ∼2830 m in the Eocene-Oligocene. • Pollen analyses indicate subtropical-temperate vegetation in the Eocene- Oligocene. As the largest and highest continental plateau on the Earth, the Tibetan Plateau has attracted worldwide attention to its formation and its remarkable paleoenvironment changes. Here we used the new stable isotope results of the lacustrine carbonate cement and pollen assemblages from the middle Eocene Suonahu Formation that was drilled from the QD17 well to infer the middle Eocene paleoclimate and paleoelevation of the northern Qiangtang basin in the central Tibetan Plateau. According to geological logging of the QD17 well, the sedimentary succession in the Suonahu Formation consists of purple-red calcareous mudstone, marlstone, siltstone, white gypsum, and gray-green mudstone. Analyses of the lithofacies indicate that the Suonahu Formation was deposited in lacustrine-evaporation environments. Both petrographic observations and isotopic data suggest that the lacustrine carbonate cement likely experienced an early-stage digenesis. The results of pollen records show that palynological taxa are mainly dominated by Ephedripites-Betulaceoipollenites-Pterisisporites and Ephedripites-Nitrariadites- Taxodiaceaepollenites assemblage, which suggest the vegetation types of the Suonahu Formation during the depositional period were mainly subtropical-temperate. The δ 18O psw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ 18O c values (−10 ‰) of the weakly altered carbonate cement are used to make the minimum estimation of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as about 2830 m by the middle Eocene-early Oligocene, approximately 2200 m lower than the present-day elevation. [ABSTRACT FROM AUTHOR]

The Late Barremian (Early Cretaceous) is a global/regional warming interval. Deciphering the mechanism of organic matter (OM) accumulation in this interval could cast lights on OM accumulation mechanism under global/regional warming. This study presented high-resolution geochemical data of the Late Barremian marls and calcareous shales in the Qiangtang Basin to unravel the effect of paleoclimate and hydrothermal activity on OM enrichment, and to further discuss OM enrichment mechanism under warming climates. Our work revealed that the Late Barremian calcareous shales in the Qiangtang Basin showed high total organic carbon contents and were deposited under an anoxic-euxinic condition, high bioproductivity, and relatively stable high-salinity water mass. Hydrothermal activity was intense during this calcareous shale interval deposition. Hydrothermal fluids not only released reducing gas to increase the reducibility of water column, but also provided sufficient nutrients to enhance bioproductivity, thus facilitating the preservation and accumulation of OM. A climate transition from cold-arid to warm-humid occurred at the onset of calcareous shale deposition. A warming climate evidences in the late Barremian succession in the Qiangtang Basin suggested at least a regional warming event. The OM enrichment in the Late Barremian calcareous shales in the Qiangtang Basin was caused by the combined effect of regional (warming climate) and local (hydrothermal activity) factors. Accelerated hydrological cycle under warming climates promoted fluvial delivery of nutrients to sedimentary basins, and thus flourished bioproductivity. Local factors (e.g., hydrothermal activity and water-column stratification) facilitated OM accumulation and preservation under a warming setting. [ABSTRACT FROM AUTHOR]

The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian oceanic anoxic event (T-OAE). This event is recorded by a large negative carbon-isotope excursion (CIE) in marine carbonates and marine organic matter, as well as fossil wood. Here, we present new high-resolution carbonate (δ 13 C carb ) and organic (δ 13 C org ) carbon-isotope records and total organic content from the Bilong Co section in the Qiangtang Basin, the eastern Tethys. This section is high in organic carbon, and black shale facies are also identified. The Bilong Co section shows heavy δ 13 C org values compared to other reported marine organic carbon-isotope values for the T-OAE. This is explained by variable mixing of terrestrial and marine organic matter. Compared to European sections, the magnitude of the negative CIE in the Bilong Co section is slightly different, which could be explained by different local paleoenvironmental conditions and strata missing. Although these, the carbon-isotope profile from the Bilong Co area displays a similar trend to those in contemporaneous strata in the western Tethyan and Boreal realms of NW Europe, which is characterized by a distinct negative excursion in the Early Toarcian. Our new data from the Bilong Co section in N Tibet provide the first high-resolution record of the Early Toarcian Oceanic Anoxic Event in the eastern Tethys. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Late Triassic volcanic event in the Qiangtang Basin was episodic, not continuous. • About four volcanic eruption phases were recorded during the Tr-J transition in the Qiangtang Basin. • A significant paleoenvironment shift occurred during the Tr-J transition in the eastern Tethys. • An extreme arid climate and ocean acidification may lead to the end-Triassic biotic crises. During the Triassic-Jurassic (Tr-J) transition, a negative basin inversion, massive volcanic eruption and biological crises occurred in the Qiangtang Basin (eastern Tethys), while the volcanic eruption phases and the paleoenvironment characteristics during this transition were paid little attention. Here we present petrology, whole-rock geochemistry and zircon U-Pb isotopes data from the continuous Triassic-Jurassic sedimentary strata in the northern Qiangtang Basin, in order to reveal the volcanic eruption episodes, paleoenvironment characteristics during this transition, and to discuss the possible linkage with the biological extinction event. Our results show that the Late Triassic volcanic event displays episodic eruption process and contains about four phases. Zircon U-Pb ages of four tuff samples from the Quemo Co south section are, from bottom to top, 220.4 ± 1.1 Ma, 210.9 ± 0.9 Ma, 208.3 ± 0.8 Ma, and 202.9 ± 1.1 Ma, respectively. Statistical analysis of the zircon ages of the Late Triassic suggests that the age of the Late Triassic magmatic event ranges from 227 Ma to 201 Ma and shows about five age peaks. The Tr-J boundary is suggested to be located between the E'erlongba and Quemo Co formations in the eastern Qiangtang Basin. Geochemical indicators such as CIA corr , C-value, Sr/Cu, and Rb/K ratios indicate that a moderate to high chemical weathering condition, semi-arid to semi-humid climate and normal marine water condition before the Tr-J transition, and suddenly changed to be low chemical weathering, arid to extreme arid climate, and freshwater water environment during the Tr-J transition, and then rebounded to be moderate to high chemical weathering and semi-humid to humid climate after the Tr-J transition. These evidences suggest that the basin inversion, significant paleoenvironment shift and regional regression occurred nearly coincidentally during the Tr-J transition. The extreme arid climate (probably cool) and ocean acidification may play an important role in the end-Triassic mass extinction in the eastern Tethys. [ABSTRACT FROM AUTHOR]

The organic-rich Changliang Mountain oil shale, located in the North Qiangtang depression, northern Tibet, is considered to be excellent mineral resource in China. The Changliang Mountain oil shale reported here was deposited in a tidal flat-lagoon environment and is characterized by black thin-layered oil shales intercalated with dark-gray marls. Here, we present geochemical data from the Changliang Mountain oil shale profile, in order to investigate the mechanism of organic matter (OM) accumulation and to establish the formation model for the marine oil shale deposition. Total organic carbon (TOC) values range from 2.96 to 23.47% in the oil shale samples, while the marl samples contains low TOC contents, ranging from 0.06 to 0.21%. In the organic-rich oil shale sediments, many redox indicators, including Mo/Al ratios, V/Cr ratios, Th/U ratios, Ni/Co ratios, and the relationship of Mo to TOC suggest a deposition under dysoxic to anoxic environments. Subsequently, the bottom water evolved into an oxic water body when the organic-poor marls were deposited. However, the negative and/or weak relationship between TOC and productivity indices P/Ti and Ba/Al indicates that OM accumulation was not controlled mainly by primary productivity, but dysoxic/anoxic bottom water environment. A stratified water column may be initiated by the supply of fresh water from the continent nearby, combining with warm and humid climate, which is beneficial to the reproduction of marine organisms. The death and bury of these organisms could lead to the formation of dysoxic/anoxic bottom waters and enhance the preservation of OM. In this study, a preservation model of the Changliang Mountain oil shale was established. The model implies that excellent preservation is the major controlling factor for OM enrichment in the oil shale layer. In addition, factors such as mixed deposition with clay minerals, and detrital matter input cannot be ignored for their influence on OM enrichment. [ABSTRACT FROM AUTHOR]

Organic-rich mudstones in the Lower Permian Zhanjin Formation may be primary source rocks in the Qiangtang Basin. However, the factors controlling organic matter accumulation remain unclear. Based on total organic carbon (TOC), geochemical, and mineral data, the paleoclimate, redox conditions, paleosalinity, hydrothermal activity, and primary productivity of the Zhanjin Formation mudstones (well QZ-5) were studied to discuss the organic matter accumulation mechanism. The Zhanjin Formation mudstones at 688-675 m and 672–622.8 m have higher TOC contents (avg. 1.52%) than those (avg. 0.94%) at 674-673 m and 662–659.5 m. Medium chemical index of alteration (CIA) values of samples at 688-675 m and 672-668 m indicate a warm and humid climate, whereas those at 674-673 m and 666–659.5 m indicate rapid cooling due to low CIA values. High Sr/Ba and S/TOC ratios indicate a high-salinity water column during the Zhanjin Formation mudstones deposition. Fluctuating U EF and Mo EF values and low to medium Th/U, V/Cr, and C org /P ratios suggest that these studied mudstones formed in an oxic to dysoxic environment. Relatively low Al/(Al + Fe + Mn) ratios, positive Eu anomalies, and Fe–Mn-(Cu + Co + Ni) × 10 and Co–Zn–Ni ternary diagrams indicate that these mudstones were influenced by atypical (weak) hydrothermal activity associated with relatively high femic terrigenous detrital influx. Relatively high P/Al ratios reveal that the level of primary productivity in the surficial water column was relatively high, but high Ni/Al and Cu/Al ratios may be influenced by paleoclimate and hydrothermal activity. Good correlations among the TOC concentrations and paleoclimate, detrital influx, and hydrothermal activity indicators combined with poor correlations between TOC and primary productivity, paleosalinity and redox proxies demonstrate that organic matter accumulation in the Zhanjin Formation mudstones was mainly controlled by detrital influx (adsorption of clay minerals) and hydrothermal activity, but not by primary productivity levels and preservation conditions such as paleosalinity and redox conditions. [Display omitted] • ►Zhanjin Formation mudstones formed in an oxic-dysoxic environment with high salinity. • ►Atypical hydrothermal activity was involved during the deposition of these mudstones. • ►OM accumulation was mainly controlled by hydrothermal activity and detrital influx. • ►Primary productivity, paleosalinity and redox conditions on OM accumulation are limited. [ABSTRACT FROM AUTHOR]

The Upper Triassic successions in the Qiangtang Basin are important targets for petroleum exploration, although they are poorly understood at present. The black mudstones collected from well QZ-16 are the deepest Upper Triassic source rocks to date and have over-mature organic matter (OM) (R o > 2). In this study, the total organic carbon (TOC) and systematic inorganic geochemistry were determined to reconstruct depositional conditions (palaeoclimate, palaeosalinity, primary productivity, redox conditions, sedimentation rate, and detrital influx) and to assess the OM accumulation mechanism. The TOC contents of the Bagong Fm. mudstones first decrease and then increase from bottom to top (Unit A to Unit D), and Unit C has the lowest TOC contents. These mudstones have relatively obvious correlations between TOC content and the palaeosalinity proxy (Sr/Ba), redox indexes (U/Al, V/Al, Mo/Al, U/Th, and V/Cr) and sedimentation rate indicator ((La/Yb) N), which indicate that OM accumulation was dominated by both high salinity and a slow sedimentation rate in a dysoxic environment. The high Al 2 O 3 /TiO 2 ratios indicate that the provenance of these mudstones was primarily from felsic igneous rocks. High detrital influx of felsic igneous rock provenance would have diluted the OM and inhibited OM preservation, as evidenced by the negative correlations between TOC content and detrital influx parameters, including SiO 2 , Al 2 O 3 , TiO 2 , and K 2 O. The primary productivity (P/Al and Ba/Al) is poorly correlated with TOC for these mudstones, suggesting that its effect on OM accumulation was negligible or limited. The relatively high TOC contents of mudstones from Units A and B are mainly related to the interaction of dysoxic conditions, a high-salinity water column, slow sedimentation rates, and a low detrital influx. An oxidizing water column with low salinity and a relatively high detrital influx and sedimentation rate upward in Unit C led to greater OM degradation and dilution and the lowest TOC contents. The significant TOC content fluctuations in Unit D may be related to vertical variations in the depositional conditions. [Display omitted] • OM accumulation mechanism of the over-mature source rocks are presented in detail. • The effect of primary productivity on OM accumulation was limited in these mudstones. • Preservation conditions were the main factors controlling the OM accumulation. [ABSTRACT FROM AUTHOR]

The Carnian Pluvial Event (CPE) is recorded widely across the Tethys region. Here we present high-resolution carbonate (δ13C carb) and organic (δ13C org) carbon-isotope records and mineral data from the Boli La to lower Bagong formations in the Quemo Co area of the Qiangtang Basin, Qinghai-Tibet Plateau. In this studied section, a calcareous fine-grained sandstone bed that yields a detrital zircon maximum depositional age of 232.5 ± 3.3 Ma, together with ammonoid species and bivalves, allows identification of the Carnian Stage. An intensification of siliciclastic supply and a large drop in calcite and illite is recorded in the Carnian section, suggesting a palaeoclimatic shift from arid to warm and humid during the CPE. A negative carbon-isotope excursion (CIE) in both δ13C org and δ13C carb is also reported, which could be explained by a global perturbation of the carbon cycle and the release of CO 2 enriched in 12C. The magnitude of the negative CIE in the Quemo Co area displays a significant shift towards lighter values compared to other reported carbonate carbon-isotope values across the onset of the CPE. A combination of both volcanic emissions from the synchronous Wrangellia large igneous province and release of methane clathrate hydrate is the most likely explanation for the significant negative excursion in δ13C. • The Carnian Pluvial Event (CPE) is investigated in Qinghai-Tibet Plateau. • Negative carbon isotope (organic, carbonate) excursion occurs across CPE. • Detrital zircon age indicate onset of CPE is 232.5 ± 3.3 Ma. • This age falls in the range of the ages of the Wrangellia LIP. • The CPE may be related to LIP and release of methane clathrate hydrate. [ABSTRACT FROM AUTHOR]