Sand and gravel deposits of the coast and inner shelf East Coast, Northland Peninsula, New Zealand (1979)
AuthorsGillie, R. D.show all
The east coast of Northland has a narrow and steep inner shelf which reaches depths of 50m within 1.5 km offshore. Mixed sand and gravel sediments occur at 30-50m depths as thin (1-10m), discontinuous, sheet deposits underlain by planed, bedrock surfaces. The deposits lie off rocky sections of the coast which form high standing cliffs between drowned embayments. Coastal deposits are predominantly situated in the embayments as localised pockets totalling only 15% of the shoreline length. The hinterland consists of a dissected block of greywackes which are deeply weathered to clay except for unweathered coastal cliff exposures. During the Pleistocene, the coast and inner shelf have evolved under conditions of relative tectonic stability in comparison to much larger sea level fluctuations. Coastal sand and gravel sediments were derived predominantly from erosion of coastal rock, rather than from hinterland fluvial sources. In addition, a high regional shell sand content of 40% varies from 2-82% between beaches depending upon contributions of sediment from other sources. Beach morpho-dynamics display a quasi-seasonal pattern with a tendency to erosion in winter, accretion in spring-early summer, and a variable response in summer-autumn, although episodes of erosion are possible at any time of the year. Short term beach changes of 20-40m in two years were greater than <10m historical changes determined from air photos for the last 30-40 years. A comparison of inner shelf sediment samples within and between five areas along the coast did not show either marked textural or geographic groupings. The gravel deposits were not interpreted as relict fluvial as has been previously proposed, and are now thought to have been derived directly from the erosion of adjacent coastal exposure s during sea level highs 20-40m below present in the last glaciations and since the post-glacial transgression. Shell presently forms 36-56% of the inner shelf gravel fraction and is derived from contemporary inner shelf communities. The shell is mixed within the deposits to a depth of at least 1.5m, and to the base of one of the deposits. Other sediment and core data suggest that the deposits are now at least palimpsest and may be modern. Detailed studies of sediment mobility in the shallowest (20m deep) deposit indicate that sediment up to 50mm diameter is actively involved in a seabed transport system within the deposit, and that the deposits are presently evolving in terms of their sediment characteristics and spatial distribution of facies. The major implication of these findings for a proposal to dredge inner shelf deposits is that coastal erosion is not likely at the five deposits located off cliffed, rocky coasts. This conclusion is less certain for the shallowest deposit which is adjacent to a long, sand beach, but it is thought that dredging would have very little adverse physical effect.