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

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 Peninsula and/or Taupo Volcanic Zone, both over 100 km east of Oruarangi. Fluvial reworking of these and other silicic pyroclastics in the hinterland across a broad west-facing plain provided the bulk of the pumiceous sediments in the coeval Late Pliocene deposits. Significant glacio-eustatic sea-level oscillations in the Hautawan may be registered in the Oruarangi sequence by formation of paleosols or erosion surfaces during the glacials, and sedimentation during the interglacials. Stratigraphic relations indicate basalts of the Ngatuturu Volcanics at Kaawa had erupted by 1.8 m.y. B.P. (late Hautawan) and therefore were coeval with the arc-related silicic volcanics further east. The Pliocene-Pleistocene boundary, now aged as c. 1.6 m.y. B.P., probably lies within the basal part of the Awhitu Formation, stratigraphically higher than previously suggested.

Keywords:

• Pliocene
• KaawaFormation
• Kaihu Group
• Port Waikato
• Pliocene-Pleistocene boundary
• ignimbrites
• pumiceous sediments
• palynology
• correlation
• tectonics
• sea-level changes