Abstract

Coastal cliffs in north Taranaki, western North Island, New Zealand, provide excellent exposures of a deep-water clastic depositional system of Miocene age, within which a wide range of reservoir facies and sedimentological features are present. The overall succession is progradational. It comprises heterolithic but generally sandstone-dominated basin-floor to base-of-slope strata deposited in middle to lower bathyal water depths (Mount Messenger Formation), which are overlain by siltstone-dominated slope strata deposited in middle to uppermost bathyal water depths (Urenui Formation). Urenui siltstones are intermittently cut by channels infilled with a variety of lithologies. These Miocene rocks are generally uncemented and only moderately indurated, and exposures are relatively unweathered and clean because of washing by waves during high tides.

The shoreline cliffs are mostly 20–40 m (65–131 ft) high, consisting of Miocene deposits that are unconformably overlain by Quaternary sediments. At one location (Whitecliffs), the Miocene exposure is more than 200 m (656 ft) high.

The outcrop transect extends for approximately 50 km (31 mi) along the gently curving, west- to northwest-facing modern coastline. Miocene depositional dip was approximately to the northwest, but uplifted Miocene strata today have a gentle structural dip towards the southwest. These orientations allow progressive examination of older fan deposits towards the north and/or younger slope deposits towards the south. This paper presents outcrops in order from north to south (older to younger). The hinterland is steeply dissected and commonly heavily vegetated. However, sandstone-dominated intervals commonly form prominent bluffs, making it possible to trace some fan bodies inland for a few kilometers (>1 mi). In addition, petroleum exploration wells and seismic reflection profiles in the region allow extrapolation of the outcrops into the subsurface, and also provide a wider context for interpreting the overall Mount Messenger/Urenui depositional system.

The Mount Messenger Formation is petroleum-productive in the onshore Taranaki Peninsula and remains a key exploration target. Producible hydrocarbons have also been discovered in inferred channel-fill sandstones in the Urenui Formation.

The upper Miocene Mount Messenger/Urenui stratigraphic interval has a composite thickness of approximately 1800 m (5905 ft) along the coastal outcrop section, but is generally 800–1200 m (2624–3937 ft) thick in the subsurface to the southwest. It forms part of a thick, mainly regressive, clastic Neogene succession that evolved in the Taranaki basin in the earliest Miocene, upon the inception of the Australian-Pacific convergent plate boundary in the New Zealand region.

The Taranaki basin has a Cretaceous-rift and Paleogene-passive-margin origin. However, in the late Miocene it became an active-margin basin, with a complex fold-thrust belt and intra-arc morphology. Mount Messenger/Urenui sediments in the outcrop study area were deposited in a foredeep trough sandwiched between a relatively steep, fault-controlled shelf margin to the east, and an active, mostly submarine, andesitic, volcanic arc to the west. The width of the trough depocenter was approximately 50 km (31 mi). Sediment transport directions were generally towards the northwest.

The Mount Messenger/Urenui interval spans approximately 2 million years (~10–8 Ma; Tortonian). In conventional sequence stratigraphic terms, it primarily consists of lowstand systems tract elements (specifically basin-floor fan, channel-levee complex or slope fan, and prograding complex) of a third-order depositional cycle. In outcrop, the third-order succession is subdivided into several fourth-order or fifth-order cycles (~100–25 ky duration, respectively, based on micropaleontology data). Exploration well logs and seismic reflection profiles in the region show similar higher frequency cyclicity.

Cycles generally fine upwards, from either conglomerates or thick-bedded sandstones at the base, to thin-bedded sandstones, then capping siltstones. Bed geometries, facies types, lithologic dominance, and stacking patterns systematically change along the outcrops, allowing prediction of likely updip or downdip reservoir facies at any individual location. The degree of sequence-boundary incision and sandstone-body confinement becomes progressively greater upsection.

Intervals of synsedimentary slumping or mass-transport complexes are present throughout the succession. Collectively these indicate one or more of the following: 1) the sea bed was relatively steep, 2) proximity to a steep margin, 3) intermittent earthquake activity, and/or 4) high sediment influx and slope oversteepening. Notably, slump intervals are present beneath the basal coarse-grained deposits of several individual fourth- or fifth-order cycles, indicating a genetic relationship between seabed instability and triggering of sand influx.