Plio-Pleistocene cyclothems from Wanganui Basin, New Zealand: type locality for an astrochronologic time-scale, or template for recognizing ancient glacio-eustasy?
Rm. Carter et al., Plio-Pleistocene cyclothems from Wanganui Basin, New Zealand: type locality for an astrochronologic time-scale, or template for recognizing ancient glacio-eustasy?, PHI T ROY A, 357(1757), 1999, pp. 1861-1872
Citations number
75
Categorie Soggetti
Multidisciplinary
Journal title
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
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.