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How mass-transport deposits and knickpoint-zones build the stratigraphy of the deep-water Hikurangi Channel

Daniel Tek 1 , Adam McArthur 1 , Miquel Poyatos-Moré 2 , Luca Colombera 1 , Marco Patacci 1 , Ben Craven 1 , and Bill McCaffrey 1
1 School of Earth and Environment, University of Leeds, Leeds, UK
2 Department of Geosciences, University of Oslo, Oslo 0371, Norway.

Bridging the gap between seafloor geomorphology and subsurface architecture is important for understanding the evolution of deep-water sedimentary systems over a variety of timescales. Monitoring of modern deep-water channel systems has revealed how geologically transient, migrating channel-floor features can generate channelised deposits. However, their manifestation at larger architectural scales is less well understood. This study integrates high-resolution bathymetric and three-
dimensional seismic data from the Hikurangi Channel, offshore New Zealand, in order to reconcile observations from the seafloor and the subsurface. Features identified on the modern seafloor that sit within the active channel include channel-damming mass-transport deposits (MTDs), knickpoints and knickpoint-zones. The subsurface deposits are categorised into ten seismic facies, which are grouped to form four depositional elements. Observations the subsurface immediately beneath prominent seafloor features are used to inform a novel conceptual model of channel evolution and deposit formation: (1) the walls of a palaeo-conduit collapse, forming an MTD that partially blocks the conduit; (2) a decrease
in longitudinal gradient on the upstream margin of the MTD causes deposition, whereas on the downstream margin, a gradient increase causes the formation of a knickpoint-zone; (3) though time, the knickpoint-zone migrates upstream, sequentially incising through the MTD, then the deposits formed upstream of it; (4) eventually, the channel re-attains its equilibrium profile, and a new conduit is formed. This provides a new mechanism for channel-deposit formation, that complements existing models of
‘cut and fill’.