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After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian-Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high-resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (δ13Corg) was carried out. Stratigraphic variation of the δ13Corg value shows large-amplitude fluctuations between −34.4 and −21.0‰ throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated δ13Corg records in the Inuyama area with high-resolution isotopic profiles of carbonate carbon (δ13Ccarb) from shallow-marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high-resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site. [ABSTRACT FROM AUTHOR]

The Yezo Group has a wide longitudinal distribution across Hokkaido, northern Japan. It represents a Cretaceous (Early Aptian-Late Maastrichtian) and Late Paleocene forearc basin-fill along the eastern margin of the paleo-Asian continent. In the Nakagawa area of northern Hokkaido, the uppermost part of the Yezo Group consists of the Hakobuchi Formation. Along the western margin of the Yezo basin, 24 sedimentary facies (F) represent 6 facies associations (FA), suggesting prevailing storm-dominated inner shelf to shoreface environments, subordinately associated with shoreface sand ridges, outer shelf, estuary and fluvial environments. The stacking patterns, thickness and facies trends of these associations allow the discrimination of six depositional sequences (DS). Inoceramids Sphenoceramus schmidti and Inoceramus balticus, and the ammonite Metaplacenticeras subtilistriatum, provide late Early to Late Campanian age constraints to this approximately 370-m thick final stage of deposition and uplift of the Yezo forearc basin. Six shallow-marine to subordinately non-marine sandstone-dominated depositional sequences include four 10 to 110-m thick upward-coarsening regressive successions (FS1), occasionally associated with thin, less than 10-m thick, upward-fining transgressive successions (FS2). The lower DS1-3, middle DS4-5 and upper DS6 represent three depositional sequential sets (DSS1-3). These eastward prograding and westward retrograding recurring shallow-marine depositional systems may reflect third- and fourth-order relative sealevel changes, in terms of sequence stratigraphy. [ABSTRACT FROM AUTHOR]