Abstract

The Canterbury Plains, New Zealand, bounded by the Southern Alps and the Pacific Ocean, are traversed by four large gravel rivers. The coastline is wave dominated and microtidal (2 m) with high rates of longshore sediment transport. The Canterbury coast is subdivided into southern and northern portions separated by the Banks Peninsula. The southern coastline, which is subjected to large oceanic swell originating as far away as 2000 km, is retrogradational and has wave-cut cliffs up to 25 m high. Coastal erosion at about 1 m/yr steepens the gradient, causing the rivers to incise 1.5-4.2 mm/yr during the present sea level highstand and accompanying transgression. Coastal erosion is caused by the extreme wave energy and efficient longshore sediment transport. River mouths are incising into the regional flood plain, with the amount of incision decreasing inland from the coast. The fluvial head-waters in the Southern Alps are rising tectonically and isostatically, causing incision that decreases seaward. Thus, fluvial incision takes place in the west due to mountain uplift and in the east, along the coast, due to a retreating shoreline during marine transgression. A zone of minimal valley incision occurs 8-15 km from the coast. In contrast to this observation, sequence stratigraphic models suggest that downcutting should occur during falling sea level, not during transgression. The northern coastline progrades about 1 m/yr and is largely sandy in its southern reaches. Thus, the Canterbury coastline is at one locale progradational and elsewhere retrogradational. The thickness of material being removed by erosional shoreface retreat during the present transgression is about 40 m. Although sea level plays a role, more important controls on progradation, retrogradation, and valley incision on the Canterbury Plains are the extreme wave energy and longshore drift.

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