Evolution of a mixed sand and gravel barrier system in North Canterbury, New Zealand, during Holocene sea-level rise and still-stand

Abstract

The morphology, stratigraphy and transgressive barrier history of a sand and gravel barrier system between the Ashley and Kowai Rivers in North Canterbury, New Zealand is described. A Holocene transgressive and regressive sand and gravel barrier system overlies a complex series of lagoonal and fluvial sediments. The marine deposits comprise a basal gravel associated with the transgressive barrier, overlain by a shallow water marine sand. These are capped by a regressive gravel beach sequence. The morphological and sedimentological relationships between the transgressive barrier deposits and underlying fluvio-lagoonal deposits suggest that marine transgression in this area was accompanied by the impoundment of the Ashley and Kowai Rivers between the transgressive barrier and a coastal cliff. This situation mirrors modern river—beach interactions on the erosional South Canterbury Bight and elsewhere in New Zealand. Coastal erosion in this system during marine transgression was dominantly driven by fluvial undercutting of coastal cliffs and non-cliffed shores ahead of an advancing gravel barrier rather than by direct marine erosion. This paramarine process appears to be characteristic of erosional sand and gravel barrier beach systems in eastern New Zealand. It has a distinct facies association and it may be both significant and identifiable from other sand and gravel systems.