Permian and Triassic Submarine Landslide Deposits in a Jurassic Accretionary Complex in Central Japan

摘要

Rocks of the Mino terrane, central Japan, are composed mainly of Permian basalts capped by shallow-marine limestone, Lower Permian to Early Jurassic deep-sea radiolarian chert, and post-Early Jurassic terrigenous rocks. Melanges incorporating blocks of these Permian to Jurassic rocks within a Middle to Upper Jurassic shale matrix are also major components of the terrane. The Mino terrane rocks are interpreted as Jurassic accretionary complexes on the basis of their lithology, age, and structure. The chert units contain two lines of evidence for submarine landslide episodes that occurred intermittently in mid-ocean regions during the Permian and Triassic. First, the upper Lower to middle Middle Permian chert unit incorporates resedimented carbonates, which were transported in submarine debris and turbidity flows from a shallow-marine buildup on the top of a seamount, down onto its lower flank, where the cherts were accumulating. Second, although most of the Mino terrane pelagic chert facies lack coarse land-derived clastic grains, the Middle to Upper Triassic chert contains thin beds and laminae of clastic debris. These grains are sand- to granule-sized and polymictic, consisting of chert, siliceous shale, basic or intermediate volcanic rocks, altered volcanic glass, polycrystalline quartz, and a glauconite-like mineral. These clastic sediments are interpreted as representing the distal facies of submarine landslide deposits, most likely derived from nearby oceanic islands. Comparable clastic sediment layers are also found in Middle Triassic deep-sea chert in a Jurassic accretionary complex in Far East Russia.