An Inter-Arc Flysch Basin in Northern New Zealand: Waitemata Group (Upper Oligocene to Lower Miocene)

The Waitemata Group formed following major Mid-to Upper Oligocene block faulting in northern New Zealand. It consisted of a central flysch basin 130 km by 60 km, bounded to the north and south by basement horst on which shelf facies accumulated, to the west by an active andesitic/basaltic ridge, and to the east by a high basement ridge/active andesitic arc. Lithic sediment from the north mingled with contemporaneous volcanic debris to give poor-volcanic, rich-volcanic, and mixed flysch facies. Turbidity currents entered the basin from a western shelf, traveled an average distance of 30 km, and built a south-easterly palaeoslope oblique to the basin axis. The flysch is relatively proximal, overstepping eastwards across a steep and uneven basement surface with a thin basal shallow-water facies. Conglomerate moved south along the western shelf, entering the basin via submarine valleys to form thick lenses within a 4 km radius. Turbidity currents carried pebbles a further 20 km. Pebbly mudstones occur. Lahars from the western volcanic arc traveled up to 34 km into the basin and deposited graded beds up to 20 m thick. Slumping was frequent. Trace fossils belong to the Nereites facies. The western volcanic arc built up a thick, largely shallow-marine, fragmental pile, overstepping eastwards across the flysch basin. A basin model based on turbidity currents suggests depths up to 2,000 m, and requires substantial vertical movements.