Sedimentary process of the bedded chert in the Mino Belt, central Japan: Three problems

Abstract

Triassic to Jurassic bedded chert sequences that were deposited in the pelagic Panthalassa Ocean occur within Jurassic accretionary complexes throughout Japan. The bedded chert consists of rhythmic alternations of chert and shale, which are considered to have resulted from cyclic changes in the accumulation rate of biogenic (radiolarian) silica against a background of slow accumulation of aeolian dust. Although the cyclic change of biogenic silica may have been related to paleoclimatic and paleoceanographic changes, factors controlling its cyclic sedimentation are unclear. To understand the origin of chert-shale alternations, this review summarizes three unresolved problems concerning the formation of bedded chert: 1) sedimentation rates of the chert and shale beds; 2) paleoenvironmental factors controlling their cyclic sedimentation; and 3) the primary cause of thickness variations in chert beds.