The characterisation and correlation of late Cenozoic tuffs, south eastern, North Island, New Zealand

Abstract

Late Cenozoic (late Miocene to mid Quaternary) tuffs occur sparsely through marine and freshwater sediments in Wairarapa. Lower to mid Quaternary tuffs increase in abundance northwards to southern Hawkes Bay, where they represent a major lithology. Thirtyseven different tuff beds, ranging from centimetre scale laminations in marine nudstone to 10m plus pumiceous units in freshwater sequences, are examined. They display a range of sedimentary structures. Some deep marine tuffs are characterised by fine textures, partial Bouma sequences, scour and tool marks and a channel geometry indicative of sediment gravity flow deposits. Many non-marine tuffs display coarse textures, cross beds, ripple marks, convolute beds, water escape structures, rip-up clasts, fossil logs and charcoal suggesting rapid fluvial deposition associated with sediment laden floods. Charcoal in some tuffs indicates derivation from ignimbrites. All tuffs examined are calc-alkaline rhyolites with silica contents between 72 and 79 wt%. Thirtyeight percent consist of multiple glass populations resulting from sedimentary mixing. Most tuffs contain a small fraction of highly magnetic brown glass with rhyolitic, dacitic andesitic and. basaltic andesite compositions, which may represent accidental ejecta. Many tuffs can be correlated within basins but few between basins. Different tuffs of similar age occur in different basins suggesting separate transport routes. This distribution and the chemical and lithological features suggest most tuffs represent remobilised rather than primary airfall deposits. Lower Pliocene tuff correlation in the Managaopari - Hinakura area fit within a stratigraphic frame work provided by foraminiferal biozones, magnetostratigraphy and fission track ages. Correlations indicate at least one nannoplankton bioevent is diachronous or substantially reworked. The Rewa Pumice is recognised on the basis of mineralogy glass chemistry and magnetostratigraphic position in marine nearshore facies in Wanganui basin and lacustrine, fluvial loess and ox-bow lake facies in Wairarapa and southern Hawkes Bay. This widespread marker horizon represents a very voluminous airfall eruption, extensively reworked in places, which occured during a glacial interval (0.74 Ma ago), shortly before the Brunches - Matuyama reversal. Volcanogenic sedimentation in the study areas appears to have been dominated by remobilisation of pyroclastics resulting in localised deposition and thus limiting the correlation potential of tuffs. In the East Coast Basin the absence of the axial ranges until late Castlecliffian was a controlling factor in tuff transport and deposition.