Strontium isotope dating of the New Zealand Oligocene

Abstract

One of the least well resolved portions of the New Zealand Cenozoic time‐scale is that centred on and about the Oligocene Epoch, internationally regarded as spanning c. 10 m.y. from 33.7 to 23.8 Ma. We have determined the ⁸⁷Sr/⁸⁶Sr ratios and derived absolute ages for 77 macrofossil samples collected from several biostratigraphically dated mid‐Tertiary sections in the South Auckland (North Island) and North Otago/South Canterbury (South Island) regions. While the site‐specific stratigraphic significance of our ages remains to be assessed, we present them here to foster wider consideration and discussion in relation to evolving absolute age schemes for the New Zealand Oligocene biostratigraphic stages. Initial results suggest:

1. The approximate boundary ages for the mid‐Tertiary Stages are: Runangan/Whaingaroan, 34.8 Ma; early Whaingaroan/late Whaingaroan, 31.0 Ma; Whaingaroan/ Duntroonian, 28.5 Ma; Duntroonian/Waitakian, 25.5 Ma; Waitakian/Otaian, 22.2 Ma. These values are mainly older than ages assigned over the past decade.

2. The early Whaingaroan Stage, traditionally held to be entirely within the Oligocene, and to define its base, extends back across the Eocene/Oligocene boundary at 33.7 Ma into the late Eocene by up to 1.1 m.y., as previously suspected by Morgans et al. (1996) on biostratigraphic grounds.

3. There has been considerable uncertainty about placement of the Waitakian Stage over the past two decades, whether entirely in the Miocene, entirely in the Oligocene, or straddling both epochs. Our Sr dating shows that the Oligocene/Miocene boundary (23.8 Ma) lies about midway through the Waitakian Stage, in agreement with Graham et al. (2000).

4. The Whaingaroan/Duntroonian boundary approximates the international early‐late Oligocene one (28.5 Ma).

   Comparisons with recently published Oligocene stable oxygen isotope records suggest that δ¹⁸O maxima and attendant sea‐level lowering, with possibly significant unconformity development, may be anticipated on three occasions in the early Whaingaroan, two or three in the late Whaingaroan, two in the Duntroonian, and at least two in the Waitakian. The unconformities bounding formations and members in the Oligocene successions may relate to these sea‐level changes, and so be regionally correlatable, including the well publicised Marshall Paraconformity of latest Whaingaroan (c. 29 Ma) age.

Keywords:

• New Zealand
• Oligocene
• Whaingaroan Stage
• Duntroonian Stage
• Waitakian Stage
• absolute age
• strontium isotope dating
• fossils