Late Holocene Sedimentation on the Southern Kāpiti Coast

The Kāpiti Coast is a broad low lying coastal plain on the western coast of New Zealand’s North Island. The coastal plain has formed over the last 6500 years through rapid progradation of coastal sediment, developing a distinct cuspate foreland. With numerous coastal communities across the low coastal plain, recent coastal erosion of the southern coastal plain combined with forecast sea level rise has drawn attention to coastal hazards. However, understanding these hazards has been hampered by a lack of information on the Holocene tectonic and sedimentary development of the coastal plain.  This study focuses on the southern portion of the Kāpiti Coast using a geological approach to document coastal outcrops and drillcores. Using detailed sedimentological analysis including description, grainsize, composition and shape, in addition to observation of the modern environment, a detailed facies scheme and depositional model for the southern Kāpiti Coast are produced.  Combining the interpreted depositional environments and age control provided by C14, OSL and well-dated pumice deposition, progressive coastal progradation and a transition from marine to terrestrial environments is reconstructed for the southern Kāpiti Coast. Records from this study reveal rapid sedimentation, at rates of up to 12.6m/1000 years within this southern limb, slowing dramatically with coastal retreat beginning within the last 400 years.  Recognising the vertical offset of the beach/dune boundary as a marker of past sea level recorded in the cores and outcrops, a 1m uplift is recognised at the southernmost point of the coastal plain. In addition to constraining the penultimate movement of the Ohariu Fault, it contrasts with the tectonic stability of the central part of the coastal plain and subsidence further north. Such insight into vertical base level change across the coastal plain has implications for future coastal hazard identification.