Spatial Variation in Turbidite-To-Contourite Continuums of the Kiwada and Otadai Formations in the Boso Peninsula, Japan: An Unstable Bottom-Current System in a Plio-Pleistocene Forearc Basin

Abstract

ABSTRACT Spatial variations in winnowing, erosion, and redistribution of turbidites by deep-sea bottom currents have been recognized in basin-plain, submarine-fan, and slope deposits of the Kiwada and Otadai Formations of the lower Kazusa Group. This group represents the infill of the Plio-Pleistocene Kazusa forearc basin, now exposed on the Boso Peninsula, Japan. In general, bottom-current-influenced sandy deposits (sandy contourites) are better sorted than associated turbidites and have sharp or gradational basal contacts with underlying turbidites or hemipelagites, and sharp upper contacts with overlying hemipelagites. Ripple cross-lamination is the most common sedimentary structure in sandy contourites. These contourites are commonly associated with internal mud drapes, which indicate osci lating traction and suspension deposition from deep-sea bottom currents. By mapping of some sandstone beds and volcanic ash layers from the proximal to the northeastern downslope areas, spatial variations in the lithofacies characteristics of turbidite-to-contourite continuums and in paleocurrents of sandy contourites have been recognized. These variations indicate spatially varying intensity and directions of deep-sea bottom currents in the Kazusa forearc basin. Analogous flow conditions have been documented in the modern, deep water mass of the Kuroshio Current off the Japanese Islands. Thus, the observed spatial variations in lithofacies and paleocurrents can best be explained in terms of fluctuating strength of the deep-water part of the paleo-Kuroshio Current during the Pliocene through Pleistocene.