New Zealand Journal of Geology and Geophysics abstracts
Dual sand sources on Farewell Spit intertidal sand flats, New
Zealand: partitioning during redistribution
Peter F. Ballance
Department of Geology
University of Auckland
Private Bag 92 019
Auckland, New Zealand
Present address: 279 Hampden Street, Nelson 7001, New Zealand.
ballance@clear.net.nz
Rob Schuckard
Ornithological Society of New Zealand
Taipari, RD 3 Rai Valley
Nelson, New Zealand
David S. Melville
Ornithological Society of New Zealand
Dovedale Road, Thorpe, Nelson, New Zealand
Philip F. Battley
Department of Mathematics and Statistics
University of Otago
P.O. Box 56
Dunedin, New Zealand
Abstract Extensive intertidal sand flats
(c.10 000 ha, c. 100 km2) on the
sheltered southern side of Farewell Spit, a 30 km long sand spit
located at the northern tip of South Island, New Zealand, extend for up
to 8 km into Golden Bay. They consist primarily of fine sand with
an upper size limit of 0.36 mm, blown from the spit during
northerly storms. In parts of the flats fine sand is supplemented by
significant but highly variable amounts of coarse sand, with rare
stones up to 40 cm long. Tree trunks with tangled root masses,
stranded on the flats, suggest that the coarse sediment is being
delivered in the root masses of trees which are washed out of rivers
discharging into Golden Bay from the mountainous southern hinterland.
The greatest concentrations of coarse sediment are located northeast of
the Aorere and Takaka Rivers, the two largest rivers discharging into
Golden Bay. We propose that some trees are blown by prevailing
southwesterly winds from the river mouths to the sand flat, and that a
clockwise tidal current gyre carries trees from all rivers onto the
flats in the northwestern corner of the bay. The patchy distribution of
coarse sand on the intertidal flats indicates that redistribution of
sand across the flats is partitioned: coarse sand (>0.5 mm) is
not widely mixed with fine sand by surface processes, while fine sand
is widely distributed. The sand flat extends subtidally to the
10 m bathymetric contour, giving the system a total area of
c. 200 km2. An estimated volume of Holocene age
sand in the spit and the sand flats of c. 5.7 km3
represents only c. 10% of the sand delivered to the northern tip
of the South Island by longshore movement up the West Coast. The bulk
of the sand may be accumulating on the continental shelf.
Keywords intertidal sand flat; Farewell Spit;
Golden Bay; New Zealand; mixed sand populations; sediment delivery by
floating trees; sand partitioning
G05037; Received 18 July 2005; accepted 5 October 2005; Online
publication date 28 February 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49:
91–99
0028–8306/06/4901–0091 © The Royal Society of New Zealand 2006
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