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[SGL36-05] Stratigraphy and geological structure of Goto Group in the Narujima Island, Goto Islands, Nagasaki Prefecture, West Japan
Stratigraphy of this sequence is divided 3 formations in ascending order, as follows: A formation (40m thick) is characterized by alternating beds of pyroclastic rock and tuffaceous sandstone. Pyroclastic rock in A formation contains 1-3 cm elongated lapilli oriented with their long axes parallel to bedding. B formation (350m thick) is composed of mudstone-dominant alternating beds of sandstone and mudstone, and subdivided 3 members. Lower Member (90m thick) is characterized by mudstone-dominant alternating beds of sandstone and mudstone, Middle Member (140m thick) by sandstone-dominant alternating beds sandstone and mudstone, and Upper Member (110m thick) by mudstone-dominant alternating beds of sandstone and mudstone. Sandstones of B formation exhibit a variety of depositional structures. Sandstones of Lower Member are normally graded and of Middle Member sometimes show internally climbing ripple lamination and trough and planar cross bedding. Fossils of mud snails occur in Middle Member. C formation (500m + thick) consists of thick sandstone and thin mudstone. Sandstone of C formation exhibits internally thick cross bedding.
It is thought that pyroclastic rocks of A formation are deposited as volcanogenic mud flow sediments, normal graded sandstones of the B formation are turbidity current deposits, and thick sandstones which exhibit cross bedding in the C formation are sediments near estuary. And In B formation, limnetic fossils occur. Therefore this stratigraphic change indicates rifted volcanic event, lake sediment and river-delta sequence at the rift zone of Japan sea. Type 1 deformation may be related by opening face of Japan sea. Northwest to southeast trend Type 2 deformation might be related rifting of north Okinawa Trough.