Channel-lobe transition zone development in tectonically active settings: Implications for hybrid bed development

Channel-lobe transition zones are dynamic areas located between deepwater channels and lobes. Presented here is a rare example of an exhumed channel-lobe transition zone from an active-margin setting, in the Kazusa forearc Basin, Boso Peninsula, Japan. This Plio-Pleistocene outcrop exposes a thick (tens of metres) channel-lobe transition zone succession with excellent dating control, in contrast to existing poorly dated studies of thinner (metres) deposits in tectonically quiescent settings. This high-resolution outcrop permits the roles of climate and associated relative sea-level changes on stratigraphic architecture to be assessed. Three development stages are recognised with an overall coarsening-upward then fining-upwards trend. Each stage is interpreted to record one obliquity-driven glacioeustatic sea-level fall-then-rise cycle, based on comparison with published data. Deposition of the thickest and coarsest strata, Stage 2, is interpreted to record the end of a period of relative sea-level fall. The thinner and finer strata of Stages 1 and 3 formed during interglacial periods where the stronger Kuroshio Oceanic Current, coupled to increased monsoonally driven tropical cyclone frequency and intensity, likely resulted in inhibited downslope sediment transfer. A key aspect of channel-lobe transition zone deposits in this case is the presence of a diverse range of hybrid beds, in contrast to previous work where they have primarily been associated with lobe fringes. Here hybrid bed characteristics, and grain-size variations, are used to assess the relative importance of longitudinal and vertical segregation processes, and compared to existing models. Compared to channel-lobe transition zones in tectonically quiescent basin-fills, this channel-lobe transition zone shows less evidence of bypassing flows (i.e. thicker stratigraphy, more isolated scour-fills, fewer bypass lags) and has significantly more hybrid beds. These features may be common in active basin channel-lobe transition zones due to: high subsidence rates; high sedimentation rates; and disequilibrium of tectonically active slopes. This disequilibrium could rejuvenate erodible mud-rich substrate, leading to mud-rich flows arriving at the channel-lobe transition zone, and decelerating rapidly, forming hybrid beds.