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 Ian 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. Pa,lynomofPhs supported by magnetic polarity data suggest a latest PHocene 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 KaawaFortnation 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 Ian 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 theOruarangi 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.

Volcanic risk ranking for Auckland, New Zealand. I: Methodology and hazard investigation

“…Although the pyroclastic flow described by Wilson et al (1995) covers most of the Auckland Region, it was suggested by the same authors and by Nelson et al (1989) that topographic barriers south of Auckland, formed since 1 m.y. B.P., will prevent future pyroclastic flows inundating the Region to their previous extent.…”

“…In extreme cases it is possible that pyroclastic flows and co-ignimbrite ash may also reach the Region (Nelson et al 1989;Wilson et al 1995). Other potential hazards include poisonous gases and acid rain, mudflows and mudfills, flooding, climate variations and possibly earthquakes.…”

“…However, this is an overestimate, as it cannot be established whether pyroclastic flows originated from the Taupo volcanic zone. Also, the landscape between the sources and Auckland has changed greatly since these eruptions, further reducing the probability (Nelson et al 1989;Wilson et al 1995).…”

“…Of the centres considered in this study, the only likely sources of pyroclastic flows are Taupo or Okataina. Nelson et al (1989) described at least five silicic ignimbrites, dated between~2.5 and 1.8 m.y. B.P, situated at Port Waikato on the Southern edge of the Auckland Region, and suggested that their source were pyroclastic flows from the southern Coromandel Peninsula or the Taupo Volcanic Zone, which includes Okataina volcanic centre and Taupo Volcano.…”