Volcanic ash layers facilitate high-resolution sequence stratigraphy at convergent plate margins: an example from the Plio-Pleistocene forearc basin fill in the Boso Peninsula, Japan

Abstract

In this paper a method is presented for the definition of high-resolution siliciclastic depositional sequences in onshore outcrop belts in a forearc basin setting, and the development of high-frequency depositional sequences in the infill of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan, in terms of the interaction between forearc tectonics and glacio-eustasy, is discussed. Volcanic ash layers are commonly intercalated in siliciclastic successions at convergent plate margins and provide physically defined chronostratigraphic markers. The mapping of lateral termination of volcanic ash layers, together with the interpretation of depositional facies discontinuities, permits the identification of onlap, offlap and downlap stratal termination patterns in onshore outcrop belts. In particular, this type of analysis is useful to identify bounding surfaces in offshore muddy sediments. The relative abundance of intercalated volcanic ash layers and their associated pyroclastic/volcaniclastic fragments in marine siliciclastic successions is interpreted to correspond to temporal variation in the relative rate of supply of coarse-grained terrigenous sediments to a marine environment during a relative sea-level cycle. In particular, transgressive stages are characterized by the reduction of active supply of coarse-grained terrigenous sediments to a marine elastic environment and this results in the increase of relative abundance of intercalated volcanic ash layers and their associated volcanogenic lithofacies. This kind of sediment is hierarchically identified in response to the development of marine flooding surfaces, transgressive systems tracts and transgressive sequence sets. In general, boundaries of depositional sequences and their component building blocks are interpreted to provide a chronostratigraphic framework for local and regional correlations of siliciclastic successions. The mapping of volcanic ash layers can identify diachronous evolution of bounding surfaces and component building blocks of a high-frequency depositional sequence, in relation to spatial variations in sedimentation rates and basin subsidence which are superimposed by eustatic sea-level changes.