LATE QUATERNARY CONTINENTAL SHELF SEDIMENTS OFF OTAGO PENINSULA, NEW ZEALAND

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Abstract
The bathymetry, surficial sediments, and benthic assemblages of approximately 1190 km2 of continental shelf and adjoining submarine canyons east of Otago Peninsula are investigated. Pebble and sand fraction composition shows that the detrital sediments have been derived predominantly from the Otago Schists. The sediments are supplied to the continental shelf largely by the Clutha River 72 km south of the area, and are transported north by the combined action of the Southland Current and longshore drift induced by southerly swell. The detrital sediments are polymodal, and by tracing the distribution of five prominent modes it is possible to determine the late Quaternary depositional history in the area. Sandy pebble gravel, which comprises a prominent and continuous band of gravel in mid-shelf position, extends from off the mouth of the Clutha to just north of Otago Peninsula, where it grades into sand. The distribution of sandy pebble gravel off the Clutha shows that it was supplied to the shelf during the early to middle phase of the post-glacial rise in sea level, and was distributed northward by the Southland Current. Probably as a consequence of a temporary halt in sea level rise 8–9000 years B.P., and of resulting aggradation of the lower reaches of the Clutha valley, supply of pebbles (Mode I) and medium sand (Mode II) ceased, and fine sand (Mode III) was furnished instead. Since sea level attained approximately its present position (c. 6000 years B.P.), only very fine sand and silt (Modes IV and V) has been supplied. Very fine sand and silt is accumulating over the inner shelf today, along with Mode III sand, which apparently is being reworked gradually northwards. Holocene sand and muddy sand predominate over the inner shelf. Middle and outer shelf sediments consist largely of organic skeletal debris, except where pebble gravel occurs, indicating that negligible Holocene sediment reaches these areas. Middle and outer shelf sediment is therefore largely relict from earlier and lower sea level situations.

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