GENESIS OF EARLY MIOCENE COOL-WATER LIMESTONES AND IMPLICATIONS FOR RAPID YOYO TECTONICS ON EASTERN TARANAKI BASIN MARGIN, NEW ZEALAND

A section of coastal western North Island, New Zealand, exposes two cool-water shelf limestone units in unconformable contact. Both limestones are earliest Miocene in age and yield Sr isotope dates on fossils of about 22 Ma. The lower unit is a tightly cemented bryozoan-echinoderm-planktic foraminiferal grainstone (Otorohanga Limestone of Te Kuiti Group), while the upper one is a well cemented rudstone (Papakura Limestone of Waitemata Group) in which the dominant framework material comprises large clasts (typically 5-30 cm) of the underlying Otorohanga Limestone. The calcite cement in the parent Otorohanga Limestone was sourced from burial-induced pressure-dissolution, but burial of the limestone was interrupted by uplift, subaerial exposure, erosion of overlying sediment, and local development of karst topography and possible paleosols, all prior to Papakura Limestone deposition. Pebble- to boulder-sized limestone clasts eroded from paleocoastal escarpments in the Otorohanga Limestone were abraded, bored, and partially dissolved within a high energy marine environment. These clasts were episodically redeposited offshore during storm/seismic events as debris flows that incorporated coeval shelf skeletal sediment as matrix within the Papakura Limestone rudstones. Several episodes of scour and fill occurred within the stacked debrites that retain an eroded thickness of at least 7 m. A feature of the Papakura debrites is the close 3-d fitting of the Otorohanga Limestone clasts, possibly a consequence of prolonged in situ abrasion during frequent seismic shocks. Shallow burial lithification of the Papakura Limestone was then interrupted by another cycle of uplift, subaerial exposure and development of local karst-like stacks upon the Papakura rudstone. Subsequently both limestones were buried by rapidly accumulating Early Miocene Waitemata Group siliciclastic sediments. The suggested evidence for two yoyo tectonic events in rapid succession (<1 m.y.) in the earliest Miocene on the eastern margin of Taranaki Basin, New Zealand's only producing hydrocarbon basin, attests to tectonic mobility previously undocumented elsewhere at this time for the basin, and coincides with a period of oblique compression associated with inception of the Pacific-Australian plate boundary through New Zealand.