Abstract

Bottom-current-influenced sandy sediments (sandy contourites) were identified in outer-fan and basin-plain deposits of the Kazusa Group, the infill of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. These are characterized by traction structures, such as parallel lamination and ripple cross-lamination, together with minor inverse grading and wave-ripple lamination. In particular, ripple cross-lamination is commonly associated with mud drapes and flaser and lenticular bedding. In general, sedimentary structures of these sandy contourites do not display any regular vertical sequences, such as the Bouma sequence in turbidites. These lithofacies indicate oscillating traction and suspension deposition in a deep-sea environment and are interpreted to document long-term fluctuation in bottom currents. Additionally, internal erosion surfaces in some of the sandy contourites similarly record fluctuations between periods of erosion versus deposition. In general, paleocurrent directions of sandy contourites are variable compared to those of associated turbidites; the latter indicate northeastward to southeastward downslope transport in the Kazusa forearc basin. In contrast, some paleocurrents of sandy contourites fall in a range of directions from north to northwest, and other paleocurrents fall in a range of directions from south to southwest. These indicate largely alongslope bottom currents, although some upslope- and downslope-directed paleocurrents were also recognized. Within the sequence stratigraphic framework for the Kazusa Group, no distinct relationship between paleocurrent directions of sandy contourites and associated systems tracts was evident. Variations in speed and direction of deep-water currents under the modern Kuroshio Current and its associated deep-sea environments have been documented off the Japanese Islands. Thus, analogous processes appear to have been responsible for the deposition of sandy contourites in the Kazusa forearc basin during the Pliocene through Pleistocene.

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