09:00 〜 09:15
[SGL22-01] 北部北上帯中のパンサラッサ遠洋深海地層における石炭系－ペルム系境界
キーワード：付加体、コノドント、国際模式層序断面および地点、チャート
The Carboniferous and Permian periods in the late Palaeozoic encompass a time in Earth’s history of the penultimate ice age and the assembly of the Supercontinent Pangea. The Carboniferous–Permian boundary (CPB) is therefore an important reference datum in studies on these topics. The Global Stratotype Section and Point (GSSP) of the boundary was ratified at Aidaralash Creek in northern Kazakhstan (Davydov et al., 1998) with the first appearance datum of the conodont Streptognathodus isolatus as its definition. Conodont biostratigraphic studies around the CPB were concentrated mainly around the Eastern European Platform (e.g., Chernykh and Ritter, 1997) and Midcontinent USA (e.g., Ritter, 1995). Biostratigraphic schemes were established separately making inter-regional correlation difficult. In the following years, revision on taxonomy and biostratigraphy in these and other regions revealed that some species were indeed useful for international correlation, while others were restricted to certain areas or environments. In this study, we document conodont biostratigraphy across the CPB from a unique geological setting: pelagic deep-sea sedimentary rocks deposited in Panthalassa. This contribution from a unique setting provides important information on the role of conodonts in correlation around the CPB interval.
The studied section is in Okoshi Stream (Okoshizawa), Iwaizumi Town, Iwate, Northeast Japan, and belongs to the Jurassic accretionary complex of the North Kitakami Belt. Carboniferous and Permian conodonts were reported from this locality, but no illustrations were provided (Ehiro et al., 2008). Conodonts in the sedimentary rocks are altered and cracked, making them generally impossible to extract using chemicals. We performed X-ray computed microtomography on specimens found on rocks pieces in order to observe these specimens.
As a result, we obtained conodonts belonging to six genera (Idiognathoides, Idiognathodus, Gondolella, Streptognathodus, Hindeodus, Mesogondolella and Sweetognathus). Conodonts indicate a Moscovian (middle Upper Carboniferous) to Artinskian (middle Lower Permian) age for the studied section. The CPB interval was correlated based on occurrences of species belonging to the genus Streptognathodus and Mesogondolella. The CPB was placed between the occurrence of the Gzheian conodont St. bellus and the Asselian conodont St. constrictus. Asselian species of Mesogondolella were confirmed in pelagic Panthalassa for the first time, strengthening their role in regional correlation. An interesting change in the faunal composition of conodonts from a Streptognathodus-dominated fauna to a Mesogondolella-dominated fauna occurred in the middle Asselian. This transition likely records an ecological takeover from Streptognathodus by Mesogondolella in deep waters.
The studied section is in Okoshi Stream (Okoshizawa), Iwaizumi Town, Iwate, Northeast Japan, and belongs to the Jurassic accretionary complex of the North Kitakami Belt. Carboniferous and Permian conodonts were reported from this locality, but no illustrations were provided (Ehiro et al., 2008). Conodonts in the sedimentary rocks are altered and cracked, making them generally impossible to extract using chemicals. We performed X-ray computed microtomography on specimens found on rocks pieces in order to observe these specimens.
As a result, we obtained conodonts belonging to six genera (Idiognathoides, Idiognathodus, Gondolella, Streptognathodus, Hindeodus, Mesogondolella and Sweetognathus). Conodonts indicate a Moscovian (middle Upper Carboniferous) to Artinskian (middle Lower Permian) age for the studied section. The CPB interval was correlated based on occurrences of species belonging to the genus Streptognathodus and Mesogondolella. The CPB was placed between the occurrence of the Gzheian conodont St. bellus and the Asselian conodont St. constrictus. Asselian species of Mesogondolella were confirmed in pelagic Panthalassa for the first time, strengthening their role in regional correlation. An interesting change in the faunal composition of conodonts from a Streptognathodus-dominated fauna to a Mesogondolella-dominated fauna occurred in the middle Asselian. This transition likely records an ecological takeover from Streptognathodus by Mesogondolella in deep waters.