Distinctive erosional and depositional structures formed at a canyon mouth: A lower Pleistocene deep-water succession in the Kazusa forearc basin on the Boso Peninsula, Japan

摘要

The canyon mouth is an important component of submarine-fan systems and is thought to play a significant role in the transformation of turbidity currents. However, the depositional and erosional structures that characterize canyon mouths have received less attention than other components of submarine-fan systems. This study investigates the facies organization and geometry of turbidites that are interpreted to have developed at a canyon mouth in the early Pleistocene Kazusa forearc basin on the Boso Peninsula, Japan. The canyon-mouth deposits have the following distinctive features: (i) The turbidite succession is thinner than both the canyon-fill and submarine-fan successions and is represented by amalgamation of sandstones and pebbly sandstones as a result of bypassing of turbidity currents. (ii) Sandstone beds and bedsets show an overall lenticular geometry and are commonly overlain by mud drapes, which are massive and contain fewer bioturbation structures than do the hemipelagic muddy deposits. (iii) The mud drapes have a microstructure characterized by aggregates of clay particles, which show features similar to those of fluid-mud deposits, and are interpreted to represent deposition from fluid mud developed from turbidity current clouds. (iv) Large-scale erosional surfaces are infilled with thick-bedded to very thick-bedded turbidites, which show lithofacies quite similar to those of the surrounding deposits, and are considered to be equivalent to scours. (v) Concave-up erosional surfaces, some of which face in the upslope direction, are overlain by backset bedding, which is associated with many mud clasts. (vi) Tractional structures, some of which are equivalent to coarse-grained sediment waves, were also developed, and were overlain locally by mud drapes, in association with mud drape-filled scours, cut and fill structures and backset bedding. The combination of these outcrop-scale erosional and depositional structures, together with the microstructure of the mud drapes, can be used to identify canyon-mouth deposits in ancient deep-water successions.