Have local stages outlived their usefulness for the New Zealand Pliocene‐Pleistocene?

Abstract

Stratigraphic subdivision of the New ZealandPliocene-Pleistocene has its historical basis in biostratigraphy, especially in the shallow marine strata in the Wanganui and East Coast basins, North Island. Internationally, an astronomically calibrated, high resolution timescale is now well established for the PliocenePleistocene, based on predominantly precession-controlled sedimentary cycles in the Mediterranean region, and the mainly obliquity-controlled, high resolution, deep-sea, benthonic oxygen isotope record. Naish and others have recently provided a similar Milankovitch-frequency cyclostratigraphy for the Pliocene-Pleistocene of New Zealand (last 3.6 m.y.), based on the Wanganui Basin record. This cyclostratigraphic record allows numeric ages to be estimated for at least 114 stratigraphic horizons in Wanganui Basin, which are not otherwise dated. Based on this record, we propose new definitions for the boundaries between New Zealand Pliocene-Pleistocene stages. The new boundaries respect traditional biostratigraphic indicators, but are based upon objective markers, mainly magnetic polarity reversals. The two exceptions are the base-Marahauan (equated with the international Pliocene-Pleistocene boundary, at 1.81 Ma) and the base-Haweran (equated with the base of the Rangitawa Tephra, at 0.34 Ma). The revised positions and ages for the age/stage and subage/substage boundaries, from older to younger, are: base-Opoitian Stage, base-Thvera subchron, 5.25 Ma; base-Waitotaran Stage (= baseWaipipian Substage), Gauss/Gilbert boundary, 3.60 Ma; base-Mangapanian Substage, top-Kaena Subchron, 3.03 Ma; base-Nukumaruan Stage (= base-Hautawan Substage), Matuyama/Gauss boundary, 2.58 Ma; base-Marahauan Substage, Pliocene-Pleistocene boundary, 1.81 Ma; baseCastlecliffian Stage (= base-Okehuan Substage), baseJaramillo Subchron, 1.07 Ma; base-Putikian Substage, Brunhes/Matuyama boundary, 0.78 Ma; base-Haweran Stage, base-Rangitawa Tephra, 0.34 Ma.

Letter to the editor

“…While the paleomagnetic data are equivocal (see Edwards 1987 for another interpretation), the example indicates the inherent problems in using as chronostratigraphic proxies events that have been tested only for homotaxy. The semblance of a chronostratigraphy given by Carter & Naish's (1998) redefinition of stage boundaries is misleading when such data are used.…”

“…The extent of the dislocation is a function of the dispersal paths and transit times of the bioevents. Thus, summaries (e.g., Morgans et al 1996;Carter & Naish 1998) which purport to indicate a general relationship applicable throughout New Zealand, are likely to be inaccurate when applied to a particular section. The errors may not necessarily be significant at geological scales but the likelihood of their existence is high.…”

“…Carter & Naish (1998) draw attention to the revolution in stratigraphic techniques over the last decades and address the problem of incorporating the new data into stratigraphic classifications. For the New Zealand Pliocene-Pleistocene they propose that the biostratigraphic local stages be redefined using nine magnetic polarity reversals, the age of an international boundary, and the base of a widespread tephra.…”

“…Some data suggest that its performance as a chronostratigraphic event is suspect. In Wanganui Basin, Carter & Naish (1998) located the event in Chron C2An.ln at 2.90 Ma, c. 0.13 m.y. younger than their redefined boundary.…”

“…To illuminate the primary problem raised by Carter & Naish (1998), I summarise the taxonomy of bioevents used in New Zealand Cenozoic stages. 1.…”