Relating seafloor geomorphology to subsurface architecture: How mass-transport deposits and knickpoint-zones build the stratigraphy of the deep-water Hikurangi Channel

Abstract

Monitoring of modern deep-water channels has revealed how migrating channel-floor features generate and remove stratigraphy, improving understanding of how channel morphologies relate to their deposits. Here, seafloor and subsurface data are reconciled through an integrated study of high-resolution bathymetry and three-dimensional seismic data imaging a ca 150 km stretch of the trench-axial Hikurangi Channel, offshore New Zealand. On the seafloor, terraced channel-walls bound a flat, wide, channel-floor, ornamented with three scales of features that increase then decrease in longitudinal gradient downstream, and widen downstream: cyclic-steps, knickpoints and knickpoint-zones (in increasing size). Mass-transport deposits derived from channel-wall collapse, are bordered by wide and flat reaches of channel-floor upstream and by knickpoint-zones (reaches containing multiple knickpoints) downstream. In the subsurface, recognition of ten seismofacies and five types of surface enables identification of four depositional elements: channel-fill, sheet or terrace, levée, and mass-transport deposits. Integration of subsurface and seafloor interpretations reveals that knickpoint-zones initiate on the downstream margins of channel-damming mass-transport deposits; they migrate and incise through the mass-transport deposits and weakly-confined deposits formed upstream, as the channel tends towards equilibrium. Downstream of a knickpoint-zone, a flat channel-floor is bounded by newly-formed terraces. Knickpoints migrate by eroding upstream and depositing downstream, generating filled concave-up (cross-sectional) surfaces in their wake. Within knickpoint-zones, knickpoint-generated surfaces are re-incised by subsequently-passing knickpoints to produce a composite bounding surface; this surface does not delineate the morphology of any palaeo-conduit. The Hikurangi Channel’s subsurface architecture records the localized erosional response to mass-transport deposit emplacement via knickpoint-zone migration, showcasing how transient seafloor features can build channelized stratigraphy. This model provides an additional mechanism to conventional models of channel deposit formation through ‘cut-and-fill’ over long stretches of channel. These findings may aid subsurface interpretation in systems lacking a contemporary self-analogue or with poor data coverage.