Nelson Boulder Bank, New Zealand

Abstract Nelson Boulder Bank, a 13 km long barrier, separates Nelson Haven from Tasman Bay in the northern South Island. Its boulders, up to 1.2 m across, are of Cable Granodiorite, identical to rocks at Mackay Bluff at the northern end of the bank. Smaller clasts predominate in the top course and form recurved beach ridges, which mark successive ends of the bank, and are being replaced by longitudinal ridges. The clasts, which show a general decrease in size and increase in rounding away from MacKay Bluff, are moved southwest by longshore drift during northerly storm generated waves in the higher part of the tidal cycle. Smaller clasts tend to raft the larger boulders. The base course, of poorly sorted clasts in a soft, fine‐grained matrix, is protected by a layer of closely packed loose boulders. As the bank erodes and migrates shoreward, the smaller clasts are reworked and carried southwest, leaving large boulders seaward as a lag deposit. Radiocarbon dating indicates that the base course accumulated as the sea rose to its present level 6000 yr BR The top course spread along the base course during the past 6000 yr. Older boulder banks, and terrestrial gravel that accumulated during the Last Glaciation at the toe of Mackay Bluff, are an inferred major source of the boulders in the Boulder Bank. A submerged bank, of possible Last Interglacial age, off the end of the Boulder Bank may form the substrate to the northern part of the Boulder Bank.