Charley, Douglas T.

Cronin, Shane J.

Nemeth, Karoly

Stewart, Robert B.

English

Massey Research Online

GEOLOGICAL SOCIETY OF NEW ZEALAND  & NEW ZEALAND GEOPHYSICAL SOCIETY JOINT ANNUAL CONFERENCE  Launching International Year of Planet Earth 26-29 November 2007 Baycourt Community and Arts Centre, Tauranga  New Zealand Geophysical Society Inc. Programme and Abstracts  Edited by Nick Mortimer and Laura Wallace  ORGANISING COMMITTEE Keith Lewis, Susan Ellis (Convenors), Janet Simes, David Skinner, Dan Barker, Annie Cervelli, Ursula Cochran, Bryan Davy,  Nick Mortimer, Laura Wallace  WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass  Symposium Organisers Joel Baker, Willem de Lange, Geoffroy Lamarche, Graham Leonard, Kathrin Otrel-Cass, Martin Reyners, Julie Rowland, Wendy Saunders, Marcus Vandergoes,  Pilar Villamor, Richard Wysoczanski  Fieldtrip Leaders Willem de Lange, Geoff Kilgour, Mauri McSaveney, David Milner, Ian Nairn, Mike Rosenberg, Brad Scott, Hannu Seebeck, Pilar Villamor, Colin Wilson  Technical Support Kate Bodger, Daniel Clarke, John Creech, Claudine Curran, Sharryn Duggan, Sara Ewan, Alison Graettinger, Janine Krippner, Rochelle Hanson, Fiona Sanders, Tom Whittaker   Geological Society of New Zealand Miscellaneous Publication 123A ISBN 0-908678-08-8 A LARGE-SCALE, NEAR-SEA LEVEL, SILICIC CALDERA-FORMING ERUPTION IN EFATE? AN ALTERNATIVE MODEL FOR THE 1 MA EFATE PUMICE FORMATION, VANUATU, SW-PACIFIC  1R.B. Stewart, 1K. Németh, 1S.J. Cronin, 2D Charley 1Volcanic Risk Solutions, CS-INR, Massey University, Palmerston North, NZ 2Department of Water Resources, Geology and Mines, Port Vila, Vanuatu R.B.Stewart@massey.ac.nz  The Efate Pumice Formation (EPF) is a rhyodacitic volcaniclastic succession widespread in the central part of Efate Island, and on small islands to the north. The volcanic succession was inferred to result from a major, entirely subaqueous explosive event north of Efate Island. The accumulated pumice-rich units were previously interpreted to be subaqueous pyroclastic density current deposits on the basis of their bedding, componentry and stratigraphy characteristics. Here we suggest an alternative eruptive scenario of this widespread succession based on recent field work at key locations. The major part of the EPF is distributed in central Efate, where pumiceous pyroclastic rock units several hundred meters thick are found within fault scarp cliffs elevated about 800 m above present day sea level. The basal 200 m of the pumiceous succession is composed of massive to weakly bedded pumiceous lapilli tuff units, each 2-3 m thick. This succession is interbedded with wavy, undulatory and dune bedded pumiceous tuff units, characteristic of co-ignimbrite surges and ground surges. Hence, the surge beds imply that the intervening units comprise a subaerial ignimbrite succession. There are no sedimentary indicators in the basal units that are consistent with water-supported transportation and/or deposition. Upwards the basal facies grade toward 200 m of massive to finely bedded, fossil-rich pumiceous volcaniclastic sand and silt interbedded with pumice gravels. Cross-lamination and convolute-bedding in this upper unit, accompanied by dish-structures and dewatering pipes, suggest traction and suspension deposition from water-supported density currents followed by loading deformation. This unit is interpreted to be deposited from reworking of the earlier volcaniclastic pumiceous units in a wet environment, such as a lagoon. The pumiceous succession is topped by reef limestone, which presumably preserved the entire EPF against erosion.   During formation of the EPF, we now propose a combination of initial subaerial ignimbrite-forming eruptions, followed by caldera subsidence. The basal part of the EPF, commonly referred to as Efate Pumice Breccia, is considered to be a normal subaerial rhyodacitic ignimbrite sequence, rather than a succession of subaqueously formed pyroclastic density currents. The upper volcaniclastic successions in our model represent intra-caldera pumiceous volcaniclastic deposits accumulated in a shallow marine environment in the resultant caldera. The present day elevated position of the succession is a result of a combination of potential caldera resurgence and ongoing arc-related uplift in the region. TALK – Tue pm  165MASSEY UNIVERSITYMASSEY RESEARCH ONLINE http://mro.massey.ac.nz/Massey Documents by Type Journal ArticlesA large-scale, near-sea level, siliciccaldera-forming eruption in Efate? Analternative model for the 1 Ma EfatePumice Formation, Vanuatu, SW-PacificStewart, Robert B.2007-01-01http://hdl.handle.net/10179/965620/01/2020 - Downloaded from MASSEY RESEARCH ONLINE

A large-scale, near-sea level, silicic caldera-forming eruption in Efate? An alternative model for the 1 Ma Efate Pumice Formation, Vanuatu, SW-Pacific

https://mro.massey.ac.nz/bitstream/10179/9656/1/content.pdf

A large-scale, near-sea level, silicic caldera-forming eruption in Efate? An alternative model for the 1 Ma Efate Pumice Formation, Vanuatu, SW-Pacific

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