Application of Global Sea-Level and Sequence-Stratigraphic Models in Southern Hemisphere Neogene Strata from New Zealand

Summary

Current research on sedimentary facies development related to eustasy focuses on two models. The global sea-level model (GSM) comprises a best-estimate curve of Phanerozoic eustasy. Up to seven orders of change can be distinguished on the GSM, encompassing magnitudes of variation in sea level up to several hundred metres on wavelengths between tens of thousands (seventh-order cycles) and tens of millions of years (first-order cycles). The sequence-stratigraphic model (SSM) summarizes the stratigraphic architecture of sediments deposited during a single major third-order sea-level cycle, recognizing three major facies assemblages grouped into lowstand, transgressive, and highstand systems tracts. Passive margins adjacent to plate boundaries form ideal sites at which to test these sea-level models. Gentle uplift there exposes inner basin margins and may preserve the shoreline of the highstand systems tract on coastal terraces, as well as excellent sections through the transgressive and highstand systems tracts in coastal cliffs or ranges, and lowstand systems tracts near the shelf-edge offshore.

Analysis of seismic records of Neogene platform–clinoform shelf deposits from the Canterbury Basin indicates a lack of correspondence between sequence development in the southwest Pacific and the GSM. Maximum marine transgression occurred during the mid-Oligocene, when the GSM indicates the largest lowstand of the Phanerozoic. Nine seismic sequences occur between 9 and 13 Ma, a time when the GSM indicates only 2.5 third-order global sea-level cycles.

In contrast, tests of the SSM using data from the modern New Zealand shelf and from the Wanganui and Hawke's Bay Basins indicate a close match between the SSM and cyclothemic sediments deposited during the 40 000 and 100 000 year long, c. 80–130 m magnitude, fifth and sixth-order cycles of the Plio-Pleistocene. The cyclothems studied were deposited near the basin margins, and comprise an erosion surface (sequence boundary) overlain by shallow marine gravel–sand–mud with transported fossils (transgressive systems tract), an offshore facies shell-bed with in situ fossils (maximum flooding surface), and sometimes an upwards coarsening (shallowing) siltstone–sandstone section (highstand systems tract). In Hawke's Bay, the lower cyclothems contain fluvial facies of the lowstand systems tract preserved just above the sequence boundary. There is also a general correspondence between the SSM and the sedimentary architecture of the 80 Ma long, local relative sea-level cycle that controlled the development of the Canterbury Basin passive margin.

We conclude that, as tested in the Southern Hemisphere: (1) the mid-Oligocene and late Miocene parts of the GSM are at best imprecise, and at worst in error; (2) the SSM is a powerful and accurate descriptive model of sedimentary facies development that occurs under sea-level cycles as different in scale as 40 000 years (sixth order) and 80 Ma (first order); and (3) more precise tests of both the GSM and SSM require close transects of continuously cored Ocean Drilling Project sites across well-developed seismic sequence systems.