Preservation of a drowned gravel barrier complex: A landscape evolution study from the north-eastern English Channel

Abstract Landscape response to post-glacial relative sea level during the Quaternary is documented using an integrated dataset of multibeam bathymetry and 2D seismic reflection profiles from the Hastings Bank area in the north-eastern English Channel. Mapping of nine seismic stratigraphic units calibrated to lithological information from multiple vibrocores has enabled the interpretation of fluvial, shoreface, barrier, washover fan, back-barrier and tidal environments of deposition. The interpreted landscape evolution is as follows: (i) fluvial incision of bedrock during sea-level lowstand; (ii) progradation of a shoreline and then development of a barrier complex as sea-level rose; (iii) recycling and breaching of the barrier; (iv) rapid drowning of the barrier complex; (v) landward migration of the shoreline through continued sea-level rise; and (vi) complete abandonment and submarine preservation of the barrier complex during sea-level highstand. The previously undocumented, yet exceptionally well preserved, drowned barrier complex at Hastings Bank records phases of barrier initiation, breakdown and retreat, and documents coastal response to high rates of relative sea-level rise. Initial development of the barrier complex required a sufficient supply of sediment, maintained by offshore sources, to keep pace with rising sea level, which permitted progradation of a shoreline and development of a barrier complex. Inherited topography in the north-eastern English Channel is an important factor in the development of the barrier complex. Phases of barrier breakdown occur when sediment supply is outpaced by a rapid increase in accommodation controlled by existing basement morphology and rising sea levels. Subsequently, the barrier responds through internal reorganisation by breaching and reworking of existing sediment bodies. Barrier retreat is characterised by a phase of ‘sediment surplus’ overstepping under rapid rates of sea-level rise where increased water depths limit wave reworking, followed by a phase of discontinuous retreat where the shoreline steps back through ‘sediment deficit’ overstepping. Hastings Bank presents a rare opportunity to examine the conditions and processes that control barrier response to sea-level rise and, to assess the preservation potential of barrier deposits as a function of the style of retreat.