IUGG 2003 Abstract

V05b-Posters Volcanic Flows: Observation, Experiment, and Theory  Friday, July 4 PM Location: Site D Presiding Chair:T. Koyaguchi

TIME [ 1400-224 ] [ V05b/04P/D-003 ] [ Poster ]

THE GIANT DUNE BED FACIES OF THE OHAKURI IGNIMBRITE (TAUPO VOLCANIC ZONE, NEW ZEALAND), AS REVEALED BY GROUND PENETRATING RADAR

Darren McClurg GRAVLEY(Department of Geological Sciences, University of Canterbury, PO Box 4800, Christchurch, New Zealand)

Matt I. WATSON(Scantec, PO Box 35-180, Browns Bay, Auckland, New Zealand) James W. COLE ( Department of Geological Sciences, University of Canterbury, PO Box 4800, Christchurch, New Zealand ) Colin J.N. WILSON ( Institute of Geological and Nuclear Sciences, Gracefield Research Centre, PO Box 30368, Lower Hutt, New Zealand )

Ground Penetrating Radar (GPR) has been successfully utilised worldwide to imageshallow subsurface bedforms in a variety of volcaniclastic deposits. Here we present results from a GPR survey of a dune-bedded facies of the ~235 ka Ohakuri ignimbrite in the Taupo Volcanic Zone, New Zealand. The study area is a ~2 km stretch of highway where 5-20-m-high benched roadcuts were established, but have subsequently been obscured by grass. GPR profiles were established on the topsof roadcut benches and are subparallel to the south-to-north transport direction of the parental pyroclastic density currents. A 400MHz frequency antenna was used to provide optimum resolution and depth penetration in these conditions. A very high sampling density of 100 scans per metre was used to enable high image resolution and assist in the interpretation of smaller scale features. The profiles clarify and extend to the subsurface the previously visible bedforms, revealing giant dune structures with wavelengths up to 35 metres and amplitudes>5 metres. Onlap relationships between successive dunes are consistent with the transport directions of the pyroclastic density current. In addition the profiles reveal internal structures within the mega dunes that provide insightsinto the depositional mechanisms of these deposits. The giant dune-bedded deposits represent an aerially-confined facies of the Ohakuri ignimbrite and are correlated with more-conventional massive ignimbrite that was being coevally deposited in other sectors at similar distances from source. The dune forms show low angles of deposition on both stoss and lee sides, and the deposits are fines-rich and lack any indication for the presence of liquid water on deposition. Grain size characteristics of laminae within the dunes change mostly with variable amounts of lapilli-grade material and, overall, grainsize distributions for the dune beds are very similar to those in the massive Ohakuri ignimbrite facies. We conclude that the parental currents for the dune-bedded facies were highly turbulent, but heavily loaded with material. Their closest analogues are some of the dune-bedded proximal deposits in the 1.8 ka Taupo ignimbrite, except that the Ohakuri ignimbrite examples are not simply a proximal variant but occur in a particular geographic sector.