Late Pliocene distal silicic ignimbrites, Port Waikato, New Zealand: Implications for volcanism, tectonics, and sea-level changes in South Auckland

Abstract

Abstract At least five distal silicic ignimbrites occur in coastal alluvial plain pumiceous sandstone, mudstone, and peat of the basal Kaihu Group at Oruarangi, 5 km south of Port Waikato on the southwest Auckland coastline. The ignimbrites are 0.1-3.0 m thick, include rip-up paleosol clasts, carbonised logs, and gas-escape pipes, and are intimately associated with synignimbrite sedimentary wash deposits. The ignimbrite-bearing succession rests on Jurassic Huriwai Group and is unconformably overlain by thick dune-sand deposits of the mainly Pleistocene Awhitu Formation. Palynomorphs supported by magnetic polarity data suggest a latest Pliocene age for the succession, from late Mangapanian(?) to Hautawan (c. 2.5-1.8 m.y. B.P.). Correlative units include the Ohuka Carbonaceous Sandstone Member of the Kaawa Formation at the nearby Kaawa-Ohuka section, and the widespread Puketoka Formation of inland South Auckland. The eruptive source(s) for the ignimbrites may have included sites in southern Coromandel Penin...