New Zealand Journal of Geology and Geophysics abstracts

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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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