SEISMIC STRATIGRAPHY AND STRUCTURAL HISTORY OF THE REINGA BASIN AND ITS MARGINS, SOUTHERN NORFOLK RIDGE SYSTEM

The Reinga Basin northwest of the North Island of New Zealand was init ially formed by crustal extension in Cretaceous time. Gravity models s uggest up to 35-40% crustal thinning. The seismic stratigraphy of the basin is continuous with that of the offshore western North Island, wh ere reflectors are well constrained by oil exploration data. In the Re inga Basin, there are two Cretaceous sequences above an older Mesozoic basement. The lower sequence is apparently terrestrial and may includ e both pre-rift and synrift subsequences; the upper is a rift-filling marine sequence. These are overlain by Paleocene and Eocene blanket se quences that were laid down during a period of relative tectonic quies cence consistent with cooling subsidence, continued submergence, a nor theast-facing continental shelf, and absence of a significant active p late boundary. A strong regional reflector, caused by a combined uncon formity and Oligocene condensed sequence, separates the Paleogene and Neogene sequences. The Neogene sequences record sedimentary infill fro m several source directions, not only from the New Zealand landmass, b ut from the north and west as well. Near the Northland coast, sediment accumulated in elastic wedges and ponded sub-basins from the Miocene to the present day. Along the flanking ridges to the northwest, simila r deposition occurred in the Early and Middle Miocene but changed in t he Late Miocene to sedimentation in drifts flanked by scours. This cha nge reflects the end of tectonism, a diminishing elastic sediment supp ly and the establishment of a throughgoing oceanic current regime as t he marginal ridges submerged. This pattern of sedimentation persists t oday. Post-Cretaceous volcanism occurred in two parts of the basin. In the central southeastern part, volcanic bodies in the ?Oligocene to E arly Miocene sequences could be a northwestern extension of the Northl and volcanic are. In the western part, small intrusive and extrusive b odies appear to be of Pliocene intraplate origin. Compression (or tran spression) had an important role in developing the basin's present for m. Miocene compressional structures-asymmetric anticlines, reverse fau lts, everted basins, and pop-ups-are present everywhere but at the sou theastern end. The present marginal ridges have structurally complex o rigins. The Reinga Ridge which forms the northeastern margin is a tran sform boundary with the Norfolk backarc basin. Deformation thought to be caused by the action of the transform is recorded in folded and fau lted Cretaceous-Paleogene sequences and syntectonic Early and Middle M iocene sequences along its length. The southwestern margin of the basi n is a double ridge comprising the Wanganella Ridge, an early Middle t o early Late Miocene, compressional uplift, and the older, eroded West Norfolk Ridge, which contains Cretaceous half-grabens. The northern h alf 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 o f folded sedimentary rocks of probable Cretaceous or older age. Paleog eographic reconstructions show that Oligocene uplift of the Norfolk Ri dge 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 estimate d to be 3.5-5.5 km. Potential source, seal and reservoir rocks are pre sent, and there is an abundance of suitable structures. The potential for petroleum occurrences in the basin is good.