Plio-Pleistocene cyclothems from Wanganui Basin, New Zealand: type locality for an astrochronologic time-scale, or template for recognizing ancient glacio-eustacy?

Abstract

Wanganui Basin is a 200 200 km 2 ovoid sedimentary basin of Plio-Pleistocene age situated in a back-arc position in western North Island, New Zealand. The eastern edge of the basin has been subjected to smooth tectonic uplift since about 350 000 years BP, resulting in the exposure onland of young sediments which elsewhere are more usually located beneath the modern continental shelf. The ll of the basin comprises several kilometres of markedly cyclothemic sediment of Plio-Pleistocene age. Accurate environmental interpretation of the succession is greatly enhanced by the presence of abundant sedimentary structures, and fossil invertebrates whose conspecic descendants live in modern New Zealand seas. The maximum water depth attained during the deposition of most cyclothems is less than 100 m, i.e. less than the known 100{130 m magnitude of most Late Pleistocene sea-level fluctuations. The magnetostratigraphic boundaries and abundant tephra which occur within the section allow accurate international correlation. The 47 Wanganui cyclothems which correspond to the last 2.4 Ma (oxygen isotope stages 100{1) each correlate with the highstand part only of each glacial-interglacial isotope stage couplet, i.e. with oddnumbered isotope stages. Even-numbered glacial stages are represented by the unconformities which separate stratigraphically adjacent cyclothems, and which mark sealevel withdrawal and subaerial exposure of the inner part of the basin. Therefore, only about half of elapsed geological time is represented by sediment within shallowwater basin lls deposited during times of high-amplitude glacio-eustatic sea-level fluctuation. Such strata, including those at Wanganui, are unsuitable for use as type sections for time-scale intervals. Nonetheless, the cyclothemic sedimentary motifs described from Wanganui, and especially the known relationships there between shellbed type and systems tracts, provide invaluable insights for the interpretation of other Phanerozoic glacio-eustatic successions such as those of Permo-Carboniferous and Ordovician age.