Local stages to be used for the Wanganui Series (Pliocene‐Pleistocene), and their means of definition

Abstract

Suggestions to abandon New Zealand local stages, or to redefine their boundaries solely at physically defined horizons, confuse the two very distinct aims of a stage classification. These are objectively to order New Zealand rocks on the basis of New Zealand biostratigraphic or other chronostratigraphic criteria, and to allow correlation of the New Zealand time‐scale with the international one. For rapid, cost‐effective identification of stages in geological mapping and other frontier situations, their boundaries must be characterised by biostratigraphic criteria, supplemented where appropriate by physical stratigraphic horizons (magnetic polarity reversals, sedimentary cycle boundaries, and, in particular, tephras). Carter & Naish resurrected all Wanganui Series substages, but the original reasons for their proposal are outdated. Fleming's choice of subdivisions was governed by the “four glaciations” paradigm of the time, rather than the current Milankovitch time‐scale paradigm. New Zealand Pliocene‐Pleistocene stages need to be redefined at new stage‐base boundaries (standard section and point, or SSP), at horizons that allow them to be characterised by the criteria of greatest utility in New Zealand. Recommended stages and their SSPs (all sited in Wanganui Basin) are: Haweran Stage, base of Rangitawa Tephra (0.35 Ma), Rangitawa Stream, Rangitikei valley; Castlecliffian Stage, base of Ototoka tephra, Ototoka Beach, Wanganui; Nukumaruan Stage, base of Hautawa Shellbed, Hautawa Road, Rangitikei valley; Mangapanian Stage, base of Mangapani Shellbed, Mangapunipuni Stream, Waitotara valley. The SSPs for the Waipipian and Opoitian Stages need to be redefined using integrated molluscan, foraminiferal, and physical stratigraphic horizons in a continuous section in Wanganui Basin, preferably the Wanganui River section.