Late Quaternary Shoreline Movements in New Zealand

Abstract

Advance and retreat of the shoreline in New Zealand during the Late Quaternary and the dominant factors causing movements are discussed. From 20 ky to 6.5 ky ago eustatic sea level rose at about 10 mm•y^-1 outpacing almost any known tectonic movement in New Zealand, and causing widespread retreat of the shoreline. From 12 ky to 6.5 ky ago two stillstands of the rising sea occurred, the first between 8.8 ky and 8.3 ky ago at about -22 m and the second from 7.6 ky to 7.3 ky ago at about -10 m relative to the present sea level. Shoreline features formed during the stillstands. The Postglacial Transgression ceased at the present sea level about 6.5 ky ago and during the last 6.5 ky about one half of the 11,000 km of open exposed coastline has been extensively modified by coastal processes, especially longshore drift. A strong net northerly drift around all but the protected parts of the coastline is set up by the persistent southerly swell generated between Latitudes 40° South and 60° South. For the protected coastlines the direction of drift is counter or oscillatory and is set up either by wind-waves and swells from the northerly quadrant, or by refraction of the persistent southerly swell. Net drift directions appear to have been the same during the Quaternary at both maximum and minimum stillstands of eustatic sea level. Longshore drift takes place in the nearshore zone which includes an inner surf-zone extending to depths of 3 – 5 m and an outer wave-transport-zone extending to depths of 18 – 65 m. For sand, 70 – 90% is transported in the wave-transport-zone, whereas for gravel, 100% is transported in the surf-zone. Gravel transport is blocked by headlands and tidal inlets but sand bypasses both, including Banks Peninsula and the Kaipara Harbour Entrance. Where abundant sediment is supplied to the coast net longshore transport rates range up to 4,500 x 103 m^3•y^-1 for sand and up to 380 x 103 m3•y^-1 for gravel. Rates fluctuate by a factor of eight from year to year, apparently due to corresponding fluctuations in the rate of supply of material to the drift system from rivers and eroding sea-cliffs. During the past century about 56% of the open exposed part of the New Zealand coastline has remained static, 25% has eroded and 19% has accreted. Sea-cliffs cut in rocks ranging in age from Upper Cretaceous to Upper Pleistocene, are retreating from erosion at average net rates between 0.3 m•y-1 and 1.5 m•y^-1 with maximums of 2.3 m•y^-1 for mudstone, 2.0 m•y^-1 for conglomerate, 1.0 m•y^-1 for lahar and 0.4 m•y^-1 for sandstone. Holocene depositional shorelines are generally eroding or accreting at 0.02 – 2.0 m•y^-1 with maximum accretion of 68.9 m•y^-1 at Farewell Spit and maximum erosion of 24.4 m•y-1 at North Kaipara Head. Severe erosion is found to be episodic and not periodic and is therefore unpredictable. Horizontal shoreline movements of 40 – 180 m in periods as short as one year are likely to occur along depositional coasts regardless of long term advance or retreat. Since the Postglacial Transgression ceased eustatic sea level fluctuations have seldom exceeded 1 m. During the last 6.5 ky, these small fluctuations have not been the most important cause of advance or retreat of the shoreline. Fluctuations in sediment supply and resistance of coastal rocks to erosion appear to be the two most important factors. Although tide gauges at Wellington and Auckland indicate a 1 – 2 mm•y^-1 rise of sea level since 1903, about 20% of the New Zealand coastline has continued to advance from accretion. Sediment supply emerges as the dominant regional factor in shoreline movements in New Zealand during the last 6.5 ky, although vertical tectonic movements and man’s activities are of local importance. Tectonic uplift between 1 mm•y^-1 and 4 mm•y^-1 importance. Tectonic uplift between 1 mm•y^-1 and 4 mm•y-1 together with hardness of the rocks are the dominant factors resulting in the formation of the Holocene coastal plain along the North Island south and east coasts and the South Island east coast. Man’s activities have significantly interfered with sediment supply at many places along the coast resulting in rapid localised erosion or accretion.