Chapter 23 Environment of Deposition of Cretaceous Chert from the Shimanto Belt, Kii Peninsula, Southwest Japan

Abstract

Chert from the Shimanto Belt in Kii Peninsula, Southwest Japan is mostly red, bedded, radiolarian chert. These rocks are commonly associated with greenstone. Besides chert-greenstone, flysch units occur repeatedly in the area studied. Radiolarians date the chert as Early Cretaceous, older than the surrounding argillaceous rocks or flysch unit which are of Late Cretaceous age. Ages and field occurrences indicate a slump origin for chert and greenstone, presumably related to subduction. However, some of these rocks formed at the same sites where argillaceous units occur. Sedimentary structures in the bedded cherts suggest formation by transportation and accumulation of siliceous materials, mostly radiolarian tests, on or near a submarine volcanic seamount by low density currents something like a bottom nepheloid layer. the frequent inflow of volcanic materials is indicated by intercalated black tuffaceous partings.