Erosion and deposition on a tilting continental shelf during quaternary oscillations of sea level

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Abstract
Continuous seismic profiles of high resolution were obtained of the continental shelf and upper slope between Napier and Castlepoint, New Zealand. They illustrate the topographic and stratigraphic effects of sea level changes on a tectonically active continental shelf. The profiles show a maximum of four, seaward-tilted unconformities which are used to define four named formations and “basement”. The formations onlap onto a relatively steeply dipping “basement” of middle Quaternary age and are, therefore, late Quaternary in age. They are constructed of prisms of sediment that parallel the coast. The prisms built the continental shelf upwards during (interglacial) periods of high sea level and outwards during (glacial) periods of low sea level. Waves and currents erode rock platforms near the present shore during periods of high sea level and cut planar surfaces in soft sediments on the middle and outer continental shelf during periods of low sea level. The rock platforms have been elevated to form “raised beaches” and the planar surfaces have been downwarped and buried by the prisms of sediment to form the unconformities. During and since the last period of low sea level, the thickest part of each prism has been deposited at rates ranging from 1.5 to 3 m per thousand years, at distances from shore ranging from 4 to 20 km and at depths ranging from 30 to 150 m. The shelf break was formed at the end of the last period of low sea level at depths between 30 and 70 m, being either at the outer limit of the top planar surface, or formed by deposition beyond the surface. The shelf break has subsequently been submerged and downwarped to depths of between 150 and 200 m.

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