Basement structure and sediment thickness beneath the continental shelf of the Hauraki Gulf and offshore Coromandel region, New Zealand

Abstract

The basement structure and sedimentary history of the continental shelf east of the northern section of North Island, New Zealand is interpreted from geophysical data collected by petroleum companies. Interpretation of magnetic data shows the basement to be composed of both igneous and nonigneous rocks in the vicinity of the Coromandel Peninsula and Great Barrier Island; further north the basement is almost entirely nonigneous. From seismic reflection data, basement in the region can be seen to have undergone extension, being faulted into a series of fault-angle depressions and grabens. Sea-floor expression indicating continued development of the fault-angle depressions is found locally, especially in the Tauranga Harbour-Cuvier Island region.Outside the Hauraki Gulf, the sedimentary section overlying basement is separated into two units by an unconformity of unknown age. The lower unit is probably late Tertiary -Quaternary sediments and the upper unit is probably late Quaternary sediments. Within the Hauraki Gulf, the poor quality of seismic reflection data precludes analysis of the sedimentary section.

Geophysical characterisation of large silicic volcanic structures in the Coromandel Peninsula, New Zealand

“…Nevertheless, andesite forms the base of the southeastern island, and andesite dikes intrude this and the overlying rhyolite and andesite conglomerates (Skinner 1995). Furthermore, seismic pre-Cenozoic basement lies at the shelf edge some 10 km to the east (Thrasher 1986). …”

Anatomy and origin of autochthonous late Pleistocene forced regression deposits, east Coromandel inner shelf, New Zealand: Implications for the development and definition of the regressive systems tract

“…Overall, the seismic character and superposition of unit 2 are similar to an early Pliocene-Pleistocene sequence identified by Thrasher (1986) from multi-channel seismic observations which extends across the east Coromandel shelf and slope from subsurface depths of a few hundred metres to >2 km. Thrasher (1986) described syndepositional growth within this sequence, which appears to be tilted to the east.…”

“…3). The syndepositional growth and regional eastward tilting described by Thrasher (1986), and deformation apparent in high-resolution seismic sections from the Whiritoa shelf region (Fig. 4B), indicate that unit 2 was associated with a regional tectonic event.…”

“…Back-arc extension was initiated off the east Coromandel coast c. 4 Ma and continued until c. 1 Ma (Table 1) (Wright 1992). This period of back-arc extension produced a series of basins flanking the Coromandel Peninsula that infilled with up to 1 km of volcaniclastic sediments (Thrasher 1986;Wright 1992). Onshore equivalents of these Pliocene-Pleistocene back-arc deposits are represented by coastal terrace deposits t1 and t2 near Waihi Beach, dated at 1.5-1.7 Ma, that are part of the Tauranga Group (Fig.…”

“…Farther south the volcanic basement descends steeply beneath a thick sedimentary section. Interpretations of deeper multi-channel seismic data by Thrasher (1986) indicate that volcanic basement is c. 200 m beneath the shelf surface south of Onemana. The sudden appearance of unit 1 off Onemana coincides with an igneous platform which extends from the coast (Thrasher 1986), and defines the northwestern boundary of an early Pliocene-age back-arc basin complex on the east Coromandel shelf.…”