MUD SEDIMENTATION ON THE CONTINENTAL-SHELF AT AN ACCRETIONARY MARGIN - POVERTY BAY, NEW-ZEALAND

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 tectoni sm, meteorological extremes, and, in historical times, changing land u se. Off Poverty Bay, mud is supplied by the Waipaoa River, New Zealand 's fourth largest river in terms of sediment supply. Under normal cond itions, suspended sediment is dispersed as surface or hypopycnal plume s 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 suspe nded sediment concentrations are probably high enough to form subsurfa ce or hyperpycnal plumes that move and disperse under gravity and shel f currents. After the 100 year Cyclone Bola event of 1988, reef commun ities of the inner shelf were temporarily inundated by a fluid mud lay er. Surficial sediments and 3.5 kHz seismic reflection profiles reveal that mud accumulates in a subsiding synclinal basin occupying the mid dle shelf. Offshelf dispersal is hindered by the growing Lachlan and A riel Anticlines along the outer shelf. As a result, 20 km(3) of mud ha s been deposited since c. 18 ka, of which 8 km(3) accumulated since c. 8 ka. This late Holocene rate is nearly five times lower than the mod ern rate of mud supply, which equates with the marked increase in terr estrial erosion following European deforestation in the late nineteent h century.