Ashes, turbidites, and rates of sedimentation on the continental slope off Hawkes Bay

Abstract

Abstract Sediments sampled by piston and gravity corers from the continental slope off Hawkes Bay Land District, New Zealand, consist of mud with ash and turbidite layers. The ash layers can be correlated by mineralogical and geochemical methods with dated ash layers on the adjacent land. The 3400-year-old Waimihia ash is identified in all cores. In some cores a band of pumice in mud above the Waimihia ash is considered to represent the Taupo eruption of 1800 years ago, and deeply buried ashes are correlated with Oruanui Ash, Mangaone* Lapilli Formation member (c), and Rotoehu Ash, which were erupted about 20 500, 30 000 and 43 000 years ago respectively. Rates of sedimentation during the last 3400 years are estimated (from the thickness of sediment overlying the Waimihia ash) to range from 0 to 0·36m per thousand years. In synclinal depressions that are downslope from submarine channels, the sediment above and below the Waimihia ash consists of thick layers of mud separated by thin sandy layers, some of which contain shallow water foraminifera. The sandy layers are considered to have been deposited by turbidity currents, and a maximum of nine of them overlie the Waimihia ash.