New Zealand Journal of Geology and Geophysics abstracts

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A widespread, early Pleistocene tephra (Potaka tephra, 1 Ma) in New Zealand: character, distribution, and implications

PHILIP A. R. SHANE

Department of Geology
Research School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand*

*Present address: Department of Geology, University of Toronto, Earth Sciences Center, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada.

Abstract  About 1 Ma ago, a large eruption in the Taupo Volcanic Zone of North Island, New Zealand, produced an extensive silicic ignimbrite now found up to 200 km from the source, and a fallout ash up to 600 km distant. The eruptive products, collectively referred to as Potaka tephra, constitute an important marker horizon and temporal control point in many early Pleistocene sequences, which otherwise lack numerical age control. The tephra is identified at many localities using multiple criteria including lithology, mineralogy, glass and mineral chemistry, magnetostratigraphy, and isotopic ages. It shows a variety of facies including a primary nonwelded ignimbrite, ash fall, and reworked volcaniclastic deposits. Potaka tephra occurs near the top of the Jaramillo Subchron and is immediately overlain by a normal to reversed transition. The tephra is found in both marine and terrestrial facies, and on both major islands of New Zealand. Its identification allows correlation of (1) mid-shelf, glacioeustatic cyclothems on the western side of North Island with fluvial and lacustrine facies on the eastern side of North Island, and (2) the only known middle Pleistocene marine sequence in South Island with extensive sequences in North Island. Stratigraphic position and chronology suggest Potaka tephra was erupted during oxygen isotope stage 27. The distribution of Potaka tephra indicates rapid uplift of the main axial ranges in central North Island and progradation of coastlines in the last 1 Ma.

Keywords  Potaka tephra; tephrostratigraphy; early Pleistocene; magnetostratigraphy; geochronology; tephra; correlation; Castlecliffian; glass chemistry; ignimbrite; volcaniclastic

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