Lower Miocene geology and sedimentary history of the Muriwai-Te Waharoa coastline, North Auckland, New Zealand

Abstract

Lower Miocene (Altonian) Waitakere Group rocks of the Muriwai-Te Waharoa coastline are divided into three formations: (a) Piha Formation containing stratified cobble, pebble volcorudites; (b) Nihotupu Formation which comprises Maori Bay Member (new), consisting of muddy volcarenites together with two large pillow piles, and Wairere Member (new), consisting of clean, thinly laminated volcarenites; and (c) Tirikohua Formation (new) which is characterised by a high percentage of coarse volcorudite, pumice lapilli, and scoriaceous clasts, and is divided into South Maori Bay Conglomerate, Bartrum, Te Waharoa Conglomerate, and Otakamiro Members (all new). The pillow piles are inferred to have formed on the sea floor by internal production of additional pillows. Expansion of the pile culminated in the formation of giant intrusive “pillows” within it. Piha Formation accumulated on the outer neritic and upper bathyal slopes of a large andesite volcano while Nihotupu Formation was deposited near the foot of these submarine slopes at mid bathyal depths. Uplift, associated with eastwards advance of volcanic activity, together with erosion, produced submarine canyons in which Tirikohua Formation accumulated.