The P/T (Permian/Triassic) boundary is one of the most important boundary for phanerozoic geohistory which is said to be a drastic change of the lives between the Paleozoic and Mesozoic. The boundary rocks have been examined in the shallow marine sediments of the Tethyan regions and less knowledge has been obtained from the pelagic sediments.
Recent progress of the radiolarian biostratigraphy together with conodont zonation reveals the probable Latest Permian cherts of Southwest Japan，and possible P/T boundary lies in the overlying "Toishi"rocks (siliceous claystones).
Presumed succession of the Late Permian to Early Triassic pelagic sediments is as follows in descending order.
1 probable Spathian (Early Triassic) bedded cherts
2 alternating beds of bedded cherts and siliceous claystones
3 siliceous claystones (Toishi rocks) including black organic mudstones
4 black mudstones
5 Late Permian Meoalbaillella bedded cherts