Permian-Triassic Boundary in the Japanese Islands

Abstract

In the southern part of the Kitakami Mountainland, North-east Japan, the Permian-Triassic boundary is often well exposed, where the Upper Permian Toyoma Slate and the Lower Triassic basal member of the Inai Group are intimately associated. There, T. Kobayashi once inferred the Tate epeirogeny and later Y. Onuki the Otani orogeny at the Permian-Triassic interval. N. Kanbe who dealt with the gradual transition between the topmost Permian and basal Triassic limestones in Kyûusyû, stated, referring to the Permian-Triassic boundary in the Kitakami Mountainland, “the crustal movement occurred at the Permian-Triassic transition was no more than gentle undulation, although erosion took place at emerged part”.On the other hand, F. Ueda. after his many years' field studies. declared in his paper. “The geological structure of the Permian and Triassic in the Toyoma and Maiya district, southern Kitakami Massif, North-east Japan” (in Japanese), 1963, that he could trace a fairly large overthrust sheet of Permian formations in the Toyoma-Maiya district, Tome-gun, Miyagi Prefecture, along the east side of the Kitakami-gawa.The Permian system of the district, approximately trending N-S, consists of, in descending orderToyoma Slate, lacking fusulinids, Yamazaki Conglomerate (Usuginu Conglomerate), Yabeina zone, Tenzinnoki formation, Parafusulina matsubaishi zone (corresponding to the Neoschwagerina-Verbeekina zone of the other regions in Japan), Nisikori formation, upper part, Parafusulina zone, lower part, Pseudoschwagerina zone.His geological and tectonic maps (Pls. I, VII) show the overthrust sheet built essentially of Upper (eastern) layer of the Toyoma Slate, Middle (intermediate) layer of the Yamazaki Conglomerate, Lower (western) layer of the Yamazaki Conglomerate and Tenzinnoki formation closely folded together.The upper layer, the Toyoma Slate, is unconformably overlain by the Hineusi formation thought to be contemporaneous with the Lower Triassic Hiraiso formation of the Inai Group ; the boundary of the two formations is said to be strongly undulated, indicating an intense erosion interval.The boundaries on the maps, between the overthrust sheet and its superjacent Hineusi formation and subjacent foundation structure, as well as these between its three component tectonic layers are parallel to each other in broad sense, though variously undulated in themselves.The foundation of the overthrust sheet is built of the Nisikori, Tenzinnoki, and Yamazaki formations ; these formations are closely folded and in general overturned to east, but partly to the opposite direction, perhaps under the influence of the westward shift of the overthrust sheet.Now assuming Ueda's conception about his overthrust sheet as well established, there arises another question regarding to the geological age of this tectonic event. Ueda dated it at the Permian-Triassic interval, based on the fact that whereas the Permian system of the overthrust sheet and its foundation is extremely complicate in structure, the Triassic Inai Group is tranquil in its general behaviour. Furthermore, the tectonic lines separating six transverse segments of the overthrust sheet from each other do not extend into the overlying Triassic deposits, and there is also no trace of any significant relative shifting nor shearing between the Toyoma Slate and the Hineusi formation.There are, however, certain geological features incongruent with Ueda's view, mentioned below.