Paleobathymetry and structural and tectonic history of cenozoic drillhole sequences in Taranaki Basin

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Hayward, B.W. 1987 Paleobathymetry and structural and tectonic history of cenozoic drillhole sequences in Taranaki Basin. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report PAL 122. 63 p.

Abstract : This report is primarily based on the biostratigraphy and paleoenvironmental assessments of ca. 1200 foraminiferal faunas from 18 Cenozoic drillhole sequences, offshore and onshore Taranaki, that have been completed or revised in the last 7 years at NZ Geological Survey. The results are used to construct paleobathymetric curves for each sequence and paleogeographic maps for 12 time intervals. Most of the Taranaki Basin area was low lying land or paralic from late Cretaceous to late Eocene. Shallow marine conditions in the Paleocene in the north west, foundered to mid bathyal depths in the early Eocene as a southern extension of the New Caledonia Basin. A marine transgression, with rapid deepening to mid bathyal depths, advanced rapidly south over all the area from the late Oligocene to earliest Miocene, terminating in shelf depths at the north end of the South Island. Consequent to this rapid deepening, bottom currants appear to have swept the deep western part of the area resulting in an extensive early Miocene hiatus. A regressive phase began in the south during middle Miocene times and can bee followed through to the present as large volumes of sediment filled the bathyal basin and progressively advanced the continental shelf north westwards. With biostratigraphy, paleobathymetric curves and sediment thickness as a basis, it has been possible to calculate subsidence curves (corrected for sediment compaction and paleo-sea level change) and tectonic curves (corrected for loading) for all drillhole sequences. Taken together, these curves show rapid subsidence throughout the region during the late Cretaceous to middle Eocene, progressively slowing in the late Eocene and early Oligocene. This subsidence, partly in fault-controlled grabens, is probably due to crustal cooling following opening of the Tasman Sea. A pulse of rapid early Eocene subsidence in the northwest is linked to a southern tensional extension of New Caledonia Basin. A further pulse of rapid regional subsidence occurs in the late Oligocene and earliest Miocene. it is more substantial and long-lived in the south and east, and is bounded to the east by the Taranaki Fault. Most of the Western Platform was tectonically quiescent from early Miocene to the present, with major subsidence occurring in the Pliocene and Pleistocene as a result of sediment loading as the continental shelf (Giant Forests Formation) built out across it. A major compressional phase began in the east and south in the mid to early late Miocene producing most of the domal and thrust structures that have been drilled as potential hydrocarbon traps. This phase is possibly related to a major change in the direction of relative plate motion through central New Zealand at this time. In onshore Taranaki and northern Cook Strait compression ceased in the early late Miocene (mid Tongaporutuan) and rapidly switched to tension with major subsidence of the Taranaki Graben in the late Miocene and Pliocene. A further reversal to compressional uplift occurred during the Pleistocene, this being especially apparent in northern onshore Taranaki. In south western Cook Strait, however, compression appears to have continued into the Pliocene, followed by gentle Pleistocene subsidence.