Tephra distribution and sedimentation rates m the Bay of Plenty, New Zealand

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Abstract
Sediment in the Bay of Plenty grades from dominantly sand near-shore to a green terrigenous sandy mud beyond the shelf edge. Cobbles, pebbles, and coarser sediments with considerable local textural variation occur on topographic highs. Nine rhyolitic tephras erupted from the Taupo Volcanic Zone are recorded from a series of piston cores taken from the Bay of Plenty. The tephras have been identified on the basis of stratigraphic position, dominant ferromagnesian mineralogy, and chemical analysis of their titanomagnetites. Radiocarbon dates of shell horizons found in some cores decrease in age with decreasing water depth. The shell beds and commonly associated pebble layers represent shallow water relict deposits which accumulated on the continental shelf during the late Pleistocene to early Holocene marine transgression. Rates of sedimentation on the continental shelf and slope have been estimated from the depth of burial of identified rhyolitic tephras and the radiocarbon ages of the shell beds as 8–45 cm/103 yr, and have not varied markedly over the past 30 000 years. Andesitic material erupted from White Island and possibly from nearby knolls is also preserved in some cores. The eruptions date back to at least 16000 yrs B.P. and two were large enough to be recorded as discrete tephra-layers in cores taken up to 66 km NE of White Island. The primary products of most andesitic activity are, however, preserved only within a radius of 12–15 km of White Island. Erosion of the eastern ranges, North Island, appears to constitute the dominant source of sediment in the Bay of Plenty.

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