An In Situ-Transgressive Barrier Model for the Nelson Boulder Bank, New Zealand

Abstract

The Nelson Boulder Bank, a 13 km long gravel barrier, is located in a relatively low energy environment on the southeast side of Tasman Bay, New Zealand. The barrier, composed of well rounded clasts of a single lithology, has two distinct gravel units: 1) a gravel ridge which consists of actively moving and well-sorted pebbles and cobbles, and 2) a gravel platform which is exposed only at low tide and contains poorly sorted cobbles and boulders. Ridge gravels are actively transported by waves, but boulders on the platform surface are immobile and occur as a thin erosional lag overlying smaller, clast -supported polymictic gravels. This erosional surface, the immobility of the boulders, and the extent of the platform gravels cannot be explained by longshore drift. An in situ-transgressive barrier model suggests that platform gravels of the Boulder Bank derive from a buried ridge of granodiorite that lies seaward of the Boulder Bank. As sea level rose prior to 6.5 ka, former beach gravels buried the ridge and transgressed over estuarine muds which were accumulating in the valley behind the ridge. This has resulted in a long, shore-parallel gravel barrier that has previously been interpreted as a spit formed by longshore drift.