New Zealand Journal of Geology and Geophysics abstracts
Mud sedimentation on the continental shelf at an accretionary
margin--Poverty Bay, New Zealand
GREG FOSTER
LIONEL CARTER
New Zealand Oceanographic Institute
National Institute of Water & Atmospheric Research (NIWA)
P.O. Box 14-901, Kilbirnie
Wellington, New Zealand
Abstract Sediments on the continental shelf, atop the
accretionary prism of the eastern North Island, are dominated by mud. This
situation reflects a highly erodible provenance of soft Tertiary sediments,
active tectonism, meteorological extremes, and, in historical times, changing
land use.
Off Poverty Bay, mud is supplied by the Waipaoa River, New Zealand's fourth
largest river in terms of sediment supply. Under normal conditions, suspended
sediment is dispersed as surface or hypopycnal plumes that have a net
northeastward or southward dispersal along the shelf, mainly in response to the
prevailing wind-driven circulation. During extreme floods with return periods
of 10 years or more, fluvial suspended sediment concentrations are probably
high enough to form subsurface or hyperpycnal plumes that move and disperse
under gravity and shelf currents. After the 100 year Cyclone Bola event of
1988, reef communities of the inner shelf were temporarily inundated by a fluid
mud layer.
Surficial sediments and 3.5 kHz seismic reflection profiles reveal that
mud accumulates in a subsiding synclinal basin occupying the middle shelf.
Offshelf dispersal is hindered by the growing Lachlan and Ariel Anticlines
along the outer shelf. As a result, 20 km3 of mud has been
deposited since c. 18 ka, of which 8 km3 accumulated
since c. 8 ka. This late Holocene rate is nearly five times lower
than the modern rate of mud supply, which equates with the marked increase in
terrestrial erosion following European deforestation in the late nineteenth
century.
Keywords continental shelf; mud sedimentation; accretionary
margin; Poverty Bay
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 157-173
0028-8306/97/4002-0157 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (4222K); (scanned from paper original: notes about this process)
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