New Zealand Journal of Geology and Geophysics abstracts
Nelson Boulder Bank, New Zealand
M. R. JOHNSTON
395 Trafalgar Street
Nelson, 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 BP. 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.
Keywords Nelson Boulder Bank; Nelson Haven; Cable
Granodiorite; longshore drift; coastal boulder barrier; interglacial sea
levels; Holocene; late Quaternary
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44: 79-88
0028-8306/01/4401-0079 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (4889K); (scanned from paper original: notes about this process)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
