Seismic stratigraphy and structural history of the Reinga Basin and its margins, southern Norfolk Ridge system

Abstract

The Reinga Basin northwest of the North Island of New Zealand was initially formed by crustal extension in Cretaceous time. Gravity models suggest up to 35—40% crustal thinning. The seismic stratigraphy of the basin is continuous with that of the offshore western North Island, where reflectors are well constrained by oil exploration data. In the Reinga Basin, there are two Cretaceous sequences above an older Mesozoic basement. The lower sequence is apparently terrestrial and may include both pre‐rift and syn‐rift subsequences; the upper is a rift‐filling marine sequence. These are overlain by Paleocene and Eocene blanket sequences that were laid down during a period of relative tectonic quiescence consistent with cooling subsidence, continued submergence, a northeast‐facing continental shelf, and absence of a significant active plate boundary. A strong regional reflector, caused by a combined unconformity and Oligocene condensed sequence, separates the Paleogene and Neogene sequences. The Neogene sequences record sedimentary infill from several source directions, not only from the New Zealand landmass, but from the north and west as well. Near the Northland coast, sediment accumulated in clastic wedges and ponded sub‐basins from the Miocene to the present day. Along the flanking ridges to the northwest, similar deposition occurred in the Early and Middle Miocene but changed in the Late Miocene to sedimentation in drifts flanked by scours. This change reflects the end of tectonism, a diminishing clastic sediment supply, and the establishment of a throughgoing oceanic current regime as the marginal ridges submerged. This pattern of sedimentation persists today. Post‐Cretaceous volcanism occurred in two parts of the basin. In the central southeastern part, volcanic bodies in the ?Oligocene to Early Miocene sequences could be a northwestern extension of the Northland volcanic arc. In the western part, small intrusive and extrusive bodies appear to be of Pliocene intraplate origin. Compression (or transpression) had an important role in developing the basin's present form. Miocene compressional structures—asymmetric anticlines, reverse faults, everted basins, and pop‐ups—are present everywhere but at the southeastern end. The present marginal ridges have structurally complex origins. The Reinga Ridge which forms the northeastern margin is a transform boundary with the Norfolk backarc basin. Deformation thought to be caused by the action of the transform is recorded in folded and faulted Cretaceous‐Paleogene sequences and syntectonic Early and Middle Miocene sequences along its length. The southwestern margin of the basin is a double ridge comprising the Wanganella Ridge, an early Middle to early Late Miocene, compressional uplift, and the older, eroded West Norfolk Ridge, which contains Cretaceous half‐grabens. The northern half of the Wanganella Ridge is an everted ?Oligocene to Early Miocene aulacogen in which slivers of basement rock were thrust up along with the sedimentary fill, whereas the southern half is an uplifted block of folded sedimentary rocks of probable Cretaceous or older age. Paleogeographic reconstructions show that Oligocene uplift of the Norfolk Ridge and Miocene uplift of the Reinga Ridge could have provided a means for terrestrial biota of New Caledonian affinities to spread into New Zealand. The total sediment thickness in the Reinga Basin is estimated to be 3.5–5.5 km. Potential source, seal and reservoir rocks are present, and there is an abundance of suitable structures. The potential for petroleum occurrences in the basin is good.