Kuroshio Current-influenced sandy contourites from the Plio-Pleistocene Kazusa forearc basin, Boso Peninsula, Japan

Abstract

Abstract Lithofacies features of bottom current influenced deep-sea sandy deposits (sandy contourites) have been studied by detailed outcrop analyses of the Plio-Pleistocene infill of the Kazusa forearc basin on Boso Peninsula, Japan. Sandy contourites are characterized by traction structures, such as parallel lamination and ripple cross-lamination, together with minor inverse grading and wave ripple-lamination. Ripple cross-lamination is commonly associated with mud drapes and flaser and lenticular bedding. These sedimentary structures do not display any regular vertical sequences. Sandy contourites are commonly associated with turbidites and comprise turbidite-to-contourite continuums. In general, 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. The framework composition of sandy contourites, in contrast, is largely equivalent to that of the associated turbidites. Palaeocurrent directions of sandy contourites are variable in time and space and alongslope- and upslope-directed palaeocurrents are dominant. Turbidite-to-contourite continuums show spatial and temporal variation in bed thickness and lithofacies features. These variations suggest varying intensity of deep-sea bottom currents in the Kazusa forearc basin. Analogous flow conditions have been documented from the modern, deep-water mass of the Kuroshio Current. Thus, the observed variations in lithofacies features and palaeocurrents of turbidite-to-contourite continuums can best be interpreted in terms of fluctuating strength of the deep-water palaeo-Kuroshio bottom current during the Pliocene through Pleistocene