Complex surface and subsurface conditions in China's northern Tarim Basin make it impossible to record good 3D seismic data with conventional acquisition geometry. In addition, the thickness of the low-velocity layer varies from 3 to 150 m, making 3D static corrections a challenge.
The Middle Permian black shales from the eastern Junggar Basin of northern Xinjiang, north‐west China are one of the richest and thickest lacustrine hydrocarbon source rocks in China. The palaeoenvironmental settings and controlling factors of organic matter accumulation along the margins of the Middle Permian lacustrine basin remain unclear, although the same works have been undertaken in the centre of the Middle Permian lacustrine basins. Here, we performed a combination of mineralogical and geochemical analyses on the black shales of the Middle Permian Pingdiquan Formation from the eastern margin areas of the Junggar Basin. The results show that the Pingdiquan Formation black shales in the study areas are organic‐rich, with total organic carbon (TOC) contents of 0.74–17.61 wt% (avg. 5.81 wt%). Salinity proxies including Sr/Ba and Rb/K ratios, combined with traces of gypsum and halite in a few samples, suggest brackish to saline water conditions. Palaeoclimate proxies indicate a warm‐humid environment during the deposition of the Pingdiquan Formation black shales. The redox proxies V/(V + Ni), V/Cr and U/Th ratios, and Eu and Ce anomalies together indicate dysoxic‐anoxic conditions of lake water. Strong positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and TOC contents imply that palaeoproductivity may have played an essential role in organic matter accumulation during the deposition of the black shales. In contrast, negative correlations between terrestrial detrital input proxies (Al and Ti) and TOC contents indicate that terrestrial detrital input may have acted as a diluent to organic matter accumulation. Additionally, although the correlation between redox proxies [V/(V + Ni) and U/Th] and TOC contents is not straightforward, dysoxic‐anoxic conditions of lake water are an essential prerequisite for organic matter accumulation in the Pingdiquan Formation black shales.
Multi‐dating on the same detrital grains allows for determining multiple different geo‐thermochronological ages simultaneously and thus could provide more details about regional tectonics. In this paper, we carried out detrital zircon fission‐track and U‐Pb double dating on the Permian‐Middle Triassic sediments from the southern Ordos Basin to decipher the tectonic information archived in the sediments of intracratonic basins. The detrital zircon U‐Pb ages and fission‐track ages, together with lag time analyses, indicate that the Permian‐Middle Triassic sediments in the southern Ordos Basin are characterized by multiple provenances. The crystalline basement of the North China Craton (NCC) and recycled materials from pre‐Permian sediments that were ultimately sourced from the basement of the NCC are the primary provenance, while the Permian magmatites in the northern margin of NCC and Early Paleozoic crystalline rocks in Qinling Orogenic Collage act as minor provenance. In addition, the detrital zircon fission‐track age peaks reveal four major tectonothermal events, including the Late Triassic‐Early Jurassic post‐depositional tectonothermal event and three other tectonothermal events associated with source terrains. The Late Triassic‐Early Jurassic (225–179 Ma) tectonothermal event was closely related to the upwelling of deep material and energy beneath the southwestern Ordos Basin due to the coeval northward subduction of the Yangze Block and the following collision of the Yangze Block and the NCC. The Mid‐Late Permian (275–263 Ma) tectonothermal event was associated with coeval denudation in the northern part of the NCC and North Qinling terrane, resulting from the subduction of the Paleo‐Asian Ocean and Tethys Ocean toward the NCC. The Late Devonian‐early Late Carboniferous (348±33 Ma) tectonothermal event corresponded the long‐term denudation in the hinterland and periphery of the NCC because of the arc‐continent collisions in the northern and southern margins of the NCC. The Late Neoproterozoic (813–565 Ma) tectonothermal event was associated with formation of the Great Unconformity within the NCC and may be causally related to the Rodinia supercontinent breakup driven by a large‐scale mantle upwelling.
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