| Abstrakt: |
Since oil shales in northern Qiangtang were found, the determination of the age of the Bailongbinghe Formation bearing the oil shales becomes particularly important economically. In general, the Jurassic rocks and fossils of northern Qiangtang demonstrate a vast shallow marine environment. Three sedimentary facies have been recognized, they are subtidal–lagoonal facies in the eastern area (e.g., the Yanshiping area), represented by siliciclastic depositional sequences, intercalated by two sets of calcareous deposition. The Jurassic rocks and fossils in the Bailongbinghe area demonstrate the shallow-water carbonate platform facies, in which ammonites, bivalves, brachiopods, corals and calcareous sponges, and patch reefs of oyster, are widely distributed, thus an open marine environment is implied. The oil shales, outcropping in a depression band near the northern margin of the middle rising ridge in the Bandaohu area, are considered as the restrict lagoonal facies. The Bailongbinghe Formation is subdivided into the Lower and Upper parts, the Lower Bailongbinghe Formation is capped by the oil shales of Bathonian to lower Oxfordian age, confirmed by brachiopods and bivalves. The Upper Bailongbinghe Formation has been regarded as upper Jurassic to lower Cretaceous age, consisting of evaporitic rocks with a considerable thickness. Two Jurassic marine–faunal dispersals are, respectively, recorded in lower Bathonian and lower Oxfordian strata. Occurrences of western Tethyan components such as Planisphinctes planilobus, Lobosphinctes intersertus, Procerites subprocerus, and P. cf. aurigera indicate a Bathonian eastward-dispersal. In the meanwhile, various species of Burmirhynchia attain a wide distribution over the northern Qiangtang region. The early Oxfordian transgressive in scale is considered smaller than the Bathonian one, represented only by bivalves, such as Radulopecten scarburgensis, R. variants, and Capillimya striata. Early Bathonian sea-level rise plays a significant role in the development of oil shales in the Bandaohu area. A depositional model of irregular-bottom topography is applicable to interpretation of the oil shales formation. The Himalayan Spiti shale fauna, utilized for age-evidence of the Bailongbinghe Formation, are mistaken and misunderstood. Cretaceous age of the Bailongbinghe Formation, derived from palynofossils and Re–Os dating is challenged by the present biostratigraphical succession in the light of facilitate accurate identification of Bailongbinghe fossils. Furthermore, so-called Barremian–Aptian negative excursions of carbon isotopes in the Bailongbinghe Formation is thereby questionable. [ABSTRACT FROM AUTHOR] |
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