Recurrent deposition of organic-rich sediments in Early Triassic pelagic Panthalassa and its relationship with global oceanic anoxia: New data from Kyoto,Southwest Japan

摘要

The Early Triassic experienced one of the most extensive episodes of oceanic oxygen depletion in the Phanerozoic, which was considered to be reflected in the deposition of black organic-rich claystone and general absence of radiolarian chert in pelagic deep-water areas of Panthalassa. However, while studies of peri-continental sections demonstrate that oceanic anoxia repeatedly occurred throughout the Early Triassic, black claystone deposition was thought to be restricted to earliest Triassic times. In this paper, the lithostratigraphy, conodont biostratigraphy and chemical composition are investigated at two Olenekian-Anisian sections in Japan, deposited in pelagic deep-water areas of Panthalassa. Sedimentation rates and major element compositions support published findings that the absence of chert is a consequence of increased clastic input and not radiolarian extinction due to oceanic anoxia. Biostratigraphic correlation with previously described sections reveals that the deposition of extensive black claystone occurred in the Induan, early Spathian (early late Olenekian) and across the Olenekian-Anisian boundary. The three black claystone lithofacies are associated with sedimentary and geochemical evidence of anoxia in the water column and bottom waters. Organic-rich deposition is generally coeval with episodes of expanded anoxic seafloor as indicated by uranium isotopes, although they are decoupled in Smithian (early Olenekian) times. Black claystone of end-Olenekian to early Anisian age represents the first finding of a laterally extensive deposit associated with oceanic anoxia during this time interval. Results imply that the deposition of black organic-rich claystone in pelagic deep Panthalassa reflects global trends in the spatial extent of bottom water anoxia during the Early Triassic, probably controlled mainly by nutrient input to the oceans.