MIOCENE BOULDER TSUNAMITE IN NEW ZEALAND

From Cape Rodney to Kawau Island, on the Pacific coast of the North Island of New Zealand about 100 km north of Auckland, the Lower Miocene Waitemata Group is regionally sub-horizontal and unconformable on the Permian to Jurassic Waipapa Group. The Waipapa Group (basement) is a strongly-deformed subduction-accretion complex of manganiferous grey-green silicified greywackes and argillites with tectonic mafic pillow lava lenses, The Waitemata Group was deposited in the 60 km-wide, 120km-long intra-arc Waitemata Basin east of the early Waiketere Volcanic Arc and west of a west-dipping subduction zone. The lower part of the Waitemata Group is superbly-exposed on the rocky foreshore from Daniels Reef to south Matheson's Bay. Deposition of the lowest Waitemata Group was on a complex rocky shoreline with a relief of up to 140m, which was buried, at a subsidence rate of about 1 mma-1, by Waitemata sediments, a sequence from littoral/neritic inner shelf sediments of the Kawau Subgroup (4-70 m) to the bathyal argillites/ turbidites of the Warkworth Subgroup (70 m), which onlap without facies change onto basement, A mega-boulder (angular to sub-angular boulders up to 120 tons) and rhodolith breccia unit, containing shallow-water foraminiferal assemblages and shell-hash, in the upper part of the Wawau Subgroup, was deposited by a mega-tsunami surge and retreat, generated by a large subduction-zone earthquake, rather than by neritic/littoral processes involving gravity slides. Judging from a recent similar imbricated and angular boulder field with angular boulders to thirty tons on the Annagh Head coast of County Mayo, Ireland in western Ireland, the New Zealand mega-boulder unit could have been generated by a freak wave sequence of the kind that caused the tragic death of David Johnston, a great Irish geologist, on Annagh Head on October 2nd 1995. Tsunamites and freak wave deposits have to be much more common than hitherto supposed in the geologic record.