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

Wanganui Basin is a 200 x 200 km(2) ovoid sedimentary basin of Plio-Pleisto cene age situated in a back-are position in western North Island, New Zeala nd. The eastern edge of the basin has been subjected to smooth tectonic upl ift since about 350 000 years BP, resulting in the exposure onland of young sediments which elsewhere are more usually located beneath the modern cont inental shell. The fill of the basin comprises several kilometres of marked ly cyclothemic sediment of Plio-Pleistocene age. Accurate environmental int erpretation of the succession is greatly enhanced by the presence of abunda nt sedimentary structures, and fossil invertebrates whose conspecific desce ndants live in modern New Zealand seas. The maximum water depth attained du ring the deposition of most cyclothems is less than 100 m, i.e. less than t he known 100-130 m magnitude of most Late Pleistocene sea-level fluctuation s. The magnetostratigraphic boundaries and abundant tephra which occur with in the section allow accurate international correlation. The 47 Wanganui cy clothems which correspond to the last 2.4 Ma (oxygen isotope stages 100-1) each correlate with the highstand part only of each glacial-interglacial is otope stage couplet, i.e. with odd-numbered isotope stages. Even-numbered g lacial stages are represented by the unconformities which separate stratigr aphically adjacent cyclothems, and which mark sealevel withdrawal and subae rial exposure of the inner part of the basin. Therefore,. only about half o f elapsed geological time is represented by sediment within shallow-water b asin fills deposited during times of high-amplitude glacio-eustatic sea-lev el fluctuation. Such strata, including those at Wanganui, are unsuitable fo r use as type sections for time-scale intervals. Nonetheless, the cyclothem ic sedimentary motifs described from Wanganui, and especially the known rel ationships there between shellbed type and systems tracts, provide invaluab le insights for the interpretation of other Phanerozoic glacio-eustatic suc cessions such as those of Permo-Carboniferous and Ordovician age.