11:00 〜 13:00
[BCG05-P02] 美濃帯中部三畳系層状チャートにおける放散虫化石層序および化学層序の検討
キーワード：チャート、三畳紀、放散虫、美濃帯、パンサラッサ海
The Ladinian stage of the Middle Triassic is subdivided into the Fassanian (early Ladinian) and Longobardian (late Ladinian) substages. The radiolarian faunal turnover is known to have occurred during the Longobardian. Although this biotic change might be related to climatic event, such as a large-scale volcanism (Italian Volcanic Center event) and the occurrence of humid events during the Longobardian, the cause of the radiolarian faunal turnover remains unclear. To investigate the relationship between the extinction and humid events during the Longobardian, we examined radiolarian biostratigraphy and obtained geochemical profiles from the Middle Triassic bedded chert succession (Section O) in the Inuyama area, central Japan.
Section O is mainly composed of rhythmical brick-red bedded cherts with a thickness of 21 m, which accumulated in a pelagic, open ocean setting within a low-latitude zone of the Panthalassa Ocean. We obtained total 56 chert samples and 33 shale samples from Section O. The biostratigraphic study reveals that the four Sugiyama's radiolarian subzones were recognized in the study section: the TR 3B (Yeharaia elegans) to TR 5A (Capnuchosphaera) zones, which can be compared to the late Anisian to early Carnian. In addition, our high resolution biostratigraphy suggests that radiolarian faunal changes occurred within the Longobardian interval.
A geochemical analysis reveals that no significant marine redox changes were observed in the study section. In contrast, changes in biogenic apatite productivity, that originated from marine vertebrates (e.g., conodonts and fish) decreased in the Longobardian. Furthermore, the continental weathering proxies such as CIA (Chemical Index of Alteration) and the WIP (Weathering Index of Parker) suggest that chemical weathering of hinterland rocks intensified in the Longobardian. Our results suggest that the humid events in Logobardian may have triggered a decrease in pelagic vertebrate productivity and the radiolarian faunal turnover in a pelagic ream of the Panthalassa Ocean.
Section O is mainly composed of rhythmical brick-red bedded cherts with a thickness of 21 m, which accumulated in a pelagic, open ocean setting within a low-latitude zone of the Panthalassa Ocean. We obtained total 56 chert samples and 33 shale samples from Section O. The biostratigraphic study reveals that the four Sugiyama's radiolarian subzones were recognized in the study section: the TR 3B (Yeharaia elegans) to TR 5A (Capnuchosphaera) zones, which can be compared to the late Anisian to early Carnian. In addition, our high resolution biostratigraphy suggests that radiolarian faunal changes occurred within the Longobardian interval.
A geochemical analysis reveals that no significant marine redox changes were observed in the study section. In contrast, changes in biogenic apatite productivity, that originated from marine vertebrates (e.g., conodonts and fish) decreased in the Longobardian. Furthermore, the continental weathering proxies such as CIA (Chemical Index of Alteration) and the WIP (Weathering Index of Parker) suggest that chemical weathering of hinterland rocks intensified in the Longobardian. Our results suggest that the humid events in Logobardian may have triggered a decrease in pelagic vertebrate productivity and the radiolarian faunal turnover in a pelagic ream of the Panthalassa Ocean.