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Abstract

In north Taranaki, New Zealand, spectacular examples of deep-water mass-transport deposits (MTDs) are exposed in coastal cliffs and imaged in nearby offshore seismic reflection data. The MTDs are Late Miocene (Tortonian) in age, and lie within successively overlying successions of volcaniclastic sandstone-dominated basin-floor turbidites (Mohakatino Formation), epiclastic sandstone- and siltstone-dominated basin-floor turbidites (Mount Messenger Formation), and siltstone-dominated slope deposits (Urenui Formation).

Seismic-scale MTDs are exposed north of Awakino River and also between the Mohakatino and Tongaporutu river mouths. These MTDs incorporate a range of original deep-water lithofacies that subsequently have been dramatically deformed into highly chaotic packages during re-emplacement in mid-bathyal to lower bathyal water depths near to or beyond the base of slope. There does not appear to be any stratigraphic or lithologic control on the types of beds incorporated into these particular MTDs, and the deformed strata are potentially derived from several original submarine-fan depositional units.

These seismic-scale MTDs are in places at least 50 m thick and appear to extend for a few kilometers, although lateral stratigraphic correlation is made difficult by the complex MTD deformation and by postdepositional normal faulting. It is possible that two different MTDs are represented in the outcrop section between Mohakatino and Tongaporutu, and together they may form a larger composite MTD or mass-transport complex.

At least two large MTD successions are revealed on seismic reflection profiles located offshore, relatively near to the coastal outcrop section. They extend for several tens of kilometers in length, and are up to 330 m (250 ms two-way travel time (TWTT) thick. These MTDs are in approximately the same stratigraphic interval as those in outcrop (Mohakatino and lower Mount Messenger Formations). At least one of these MTDs extrapolates up structural dip to where an MTD is exposed onshore, but an absolute correlation between the two MTDs is precluded by the lack of seismic data in the intervening coastal transition zone. MTD intervals equivalent to those exposed are also imaged in high-resolution behind-outcrop seismic reflection lines, and are recorded by anomalous biostratigraphic signatures in the nearby Pukearuhe-1 exploration well.

Siltstone-dominated MTDs of subseismic scale (up to 15 m thick) are evident in central parts of the outcrop section, within fine-grained intervals near the tops of inner-fan depositional cycles. They are also present in southern parts of the section (retrogradational and progradational slope), within fine-grained intervals that lie stratigraphically close beneath incised slope channels infilled with sandstone. Whilst these MTDs are only 5-15 m thick, they could thicken away from the outcrop transect.

The magnitude and styles of deformation within the various exposed MTDs may reflect differing transportation processes or triggering mechanisms. The larger, seismic-scale MTDs appear to be the product of massive slope failure and downslope translation of strata, or relatively local tectonic movement of the sea bed. The ordered stratigraphic position of the sub-seismic-scale MTDs near the tops of inferred depositional cycles suggests that changes in relative base level may have controlled their development.

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