Journal of the Royal Society of New Zealand abstracts

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Silicic tephras in Pleistocene shallow-marine sediments of Wanganui Basin, New Zealand

Brad Pillans¹, Brent Alloway², Tim Naish³, John Westgate⁴, Steve Abbott⁵, and Alan Palmer⁶

¹Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. Email: brad.pillans@anu.edu.au
²Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand.
³Institute of Geological and Nuclear Sciences, Gracefield Research Centre, P.O. Box 30 368, Lower Hutt, New Zealand.
⁴Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada.
⁵School of Environmental Science and Management, Southern Cross University, P.O. Box 157, Lismore NSW 2480, Australia.
⁶Soil and Earth Sciences, Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North, New Zealand.

Abstract  Vitric-rich volcaniclastic horizons are important for correlation of glacio-eustatic sedimentary cycles, both within the well known shallow-marine record of Wanganui Basin, and other New Zealand terrestrial and deep marine records. They also record distal major rhyolitic eruptions from the Taupo (TVZ) and Coromandel (CVZ) Volcanic Zones that are lacking in proximal source areas. Twenty-eight volcaniclastic horizons are recognised in the Castlecliffian and late Nukumaruan strata of Wanganui Basin from glass shard major element geochemistry and stratigraphic position, and are dated using magnetostratigraphy, orbitally tuned cyclostratigraphy and isothermal plateau fission track (ITPFT) ages. The major named volcaniclastic horizons (with ITPFT and/or astronomical ages, respectively) are: Onepuhi (0.57 Ma), Kupe (0.63 ± 0.08 Ma; 0.65 Ma), Kaukatea (0.86 ± 0.08 Ma; 0.90 Ma), Potaka (1.00 ± 0.03 Ma; 0.99 Ma), Rewa (1.20 ± 0.14 Ma; 1.19 Ma), Mangapipi (1.51 ± 0.16 Ma, 1.54 Ma), Ridge (1.56 Ma), Pakihikura (1.58 ± 0.08 Ma; 1.58 Ma), Birdgrove (1.60 Ma), Mangahou (1.63 Ma), Maranoa (1.63 Ma), Ototoka (1.72 ± 0.32 Ma; 1.64 Ma), Table Flat (1.71 ± 0.12 Ma; 1.65 Ma), Vinegar Hill (1.75 ± 0.20 Ma; 1.75 Ma), and Waipuru (1.79 ± 0.15 Ma; 1.83 Ma). The ITPFT ages are consistent with the astronomically tuned Geomagnetic Polarity Timescale. Volcaniclastic horizons in Wanganui Basin have been emplaced through a variety of primary and secondary processes, including direct tephra-fall as well as transitional water supported mass flow through to hyperconcentrated flow. No gas supported flow deposits have yet been recognised. Only some horizons from Wanganui Basin can be chemically and chronologically linked to known TVZ eruptions, while others remain uncorrelated owing to proximal source area erosion and/or burial as well as vapour phase alteration and devitrification within near-source welded ignimbrites. Nevertheless, many volcaniclastic deposits in Wanganui Basin can be reliably correlated to distal sedimentary successions in Auckland Region, Hawke’s Bay and in Ocean Drilling Program (ODP) cores 1123 and 1124, to the east of New Zealand. The orbitally tuned chronology for ODP cores, which is calibrated by numeric ages on tephras and magnetostratigraphy, enhances inter-regional correlation, providing an important framework for future palaeoenvironmental reconstructions.

Keywords  Wanganui Basin; tephras; fission track dating; silicic glass; Pleistocene

R04021 Received 17 November 2004; accepted 13 May 2005; Online publication date 27 July 2005
Journal of the Royal Society of New Zealand
Volume 35, Numbers 1 & 2, March/June, 2005, pp 43–90

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