Seismic stratigraphy of the Plio-Pleistocene Giant Foresets, Western Platform, Taranaki Basin

Beggs, J.M.

New Zealand Oil Exploration Conference 1989, Queenstown, NZ 1: 201-207


Rapid uplift along the Alpine/Hikurangi plate boundary since the Miocene has resulted in an abundant supply of clastic sediment to adjacent basins. Within Taranaki Basin, the southern graben was filled by the end of the Miocene, since when the continental shelf has prograded westward across the Western Platform. The northern West Coast of South Island has been the primary source of sediment for the Plio-Pleistocene construction of the continental shelf on the Western Platform. Sedimentation at the shelf margin has been largely confined to sea-level lowstands, whereas highstands are characterised by shelf aggradation. The Pliocene and Pleistocene section on the Western Platform is up to 2.2 km thick and comprises a spectacular seismic-stratigraphic unit, the Giant Foresets. Seismic character and stratal patterns in the Giant Foresets define the following facies: Topset: subparallel continuous reflectors; Progradational foresets: coherent offlapping moderate amplitude reflectors; Degradational foresets: chaotic offlapping low-amplitude reflectors; Bottomset: moderate amplitude, variable continuity subhorizontal reflectors. Typical transects include varying proportions of these facies, depending mainly on antecedent physiography. The Giant Foresets comprise over half the stratigraphic thickness of much of the Western Platform and the attending loading must exert a critical impact on the thermal history of the underlying sequence. Additionally, an understanding of the Plio-Pleistocene depositional style can be applied to similar units, mainly of Miocene and younger age, in several New Zealand basins.