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AAPG Bulletin

Abstract


Volume: 65 (1981)

Issue: 9. (September)

First Page: 1586

Last Page: 1601

Title: New Zealand and Australian Cenozoic Sedimentary Cycles and Global Sea-Level Changes

Author(s): Tom S. Loutit, James P. Kennett (2)

Abstract:

A global sea-level history determined by Vail et al consists of 23 sea-level cycles between the base of the Cenozoic and the Pliocene-Pleistocene boundary (65 to 1.8 Ma). We have examined the relation between this global sea-level history and the New Zealand and Australian continental margin shallow-marine sedimentary record.

Sedimentary cycles in the Australian Cenozoic marginal-marine sequences are bounded by unconformities of various durations. The tectonic stability and aridity of the western and southwestern margin of Australia during the Cenozoic produced a sedimentary record dominated by hiatuses. This contrasts with the relatively complete sequences of the tectonically more active southeastern marginal basins. The four major sedimentary cycles in the Australian Cenozoic (Paleocene to early Eocene; middle to late Eocene; latest Oligocene to late middle Miocene; and latest Miocene to Quaternary) correlate with the supercycles Ta, Tb, Tc, Td, and Te of Vail et al.

In New Zealand, most of the Cenozoic stages represent classic sedimentary cycles bounded by unconformities or correlative conformities formed as a result of large, rapid eustatic sea-level changes. The marine Tertiary sequence in New Zealand consists of 23 stages. Of the 18 stage boundaries between the end of the Paleocene (53 Ma) and the end of the Miocene (5 Ma), 16 appear to correlate with the boundaries of eustatic sea-level cycles.

Eustatic sea-level lowstands are well recorded (as unconformities) in New Zealand as a result of its unique tectonic setting during the Cenozoic. For most of the early Cenozoic until the middle Oligocene (~ 30 Ma) New Zealand was generally subsiding. After the middle Oligocene this region began to be uplifted as the Pacific-Australian plate boundary migrated onto New Zealand, climaxing in the latest Cenozoic (late Pliocene-Quaternary), thus exposing a nearly complete sequence of marine Cenozoic strata well suited for the study of sedimentary cycles.

Unconformities, which form as a result of eustatic sea-level changes, represent a very useful correlation tool, especially on continental margins with a sufficiently high terrigenous sediment supply, and can be used to supplement paleontologic correlations. Their ultimate usefulness for correlation depends on the rapidity of the sea-level changes or the speed at which sedimentary facies change in response to sea-level changes. If sea level changes rapidly (10 m/1,000 years), stratigraphic resolution will be high (±105 years) but resolution decreases as the speed of sea-level change decreases. Unconformities may well prove to be a vital tool linking the classic land-based sections around the world with deep-sea sections.

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