MIOCENE DEEP-WATER DEPOSITION BY A SUBMARINE VOLCANIC ARC: MOHAKATINO FORMATION, TARANAKI BASIN, NEW ZEALAND

The mid-Miocene Mohakatino Formation of the Taranaki Basin, New Zealand, is a submarine fan succession sourced dominantly from an offshore, submarine andesitic arc. The ~100 m-thick formation, exposed over ~30 km of coastline, is strongly tabular, with minimal evidence for erosion and cm- to m-scale beds persisting in thickness for many kilometers laterally. This submarine fan is unique in the Taranaki Basin for its combination of deposition by sediment gravity flows and ash-fall settling, the dominance of coarse-grained sand to gravel-sized grains, and the minimal clay content. This stands in contrast to the interfingering and overlying, fine-grained siliciclastic Mount Messenger Formation, which is a well-documented example of deep-water channels and lobes.

In 165 m of section, measured at the cm scale, we document five lithofacies. LF1 is cm-bedded, poorly sorted sandstone, with subset LF1a showing coarse-tail grading and LF1b showing normal grading. LF1a is interpreted as lag deposits from bypassing, high-density turbidity currents, while LF1b represents passive settling of ash through the water column. LF2 is thick-bedded, normally graded, dewatered, massive sandstone, deposited by collapsing, unconfined high-density turbidity currents. Coarse-grained dune sets (LF3) are interpreted as elements of a small deep-water channel. LF4 and LF5 are quartz-rich siltstone and fine sandstone, suggesting a terrigenous source, and are attributed to the Mount Messenger Formation.

The character of these lithofacies and the non-erosive nature of the deposits indicate a lobe environment, where flows lose confinement and are dominantly depositional. The spatial distribution of the lithofacies suggests a fan system sourced from the north and oriented axial to an elongate basin, confined by a growing volcanic arc to the north and west and a fault-controlled topographic high to the east. This is supported by dating of the volcanic arc showing dominant activity north of the deposits at 12-8 Ma (Giba et al., 2013), and by paleocurrent indicators that suggest largely W- or SW-directed flows. Characterizing this unique formation in the context of the well-studied Mount Messenger Fm. provides an opportunity to document the interaction of distinct, coeval depositional systems in a deep-water basin.