Revision of the age and stratigraphic relationships of Hinemaiaia Tephra and Whakatane Ash, North Island, New Zealand, using distal occurrences in organic deposits

Abstract

The stratigraphic and chronologic relationships of Hinemaiaia Tephra and Whakatane Ash are examined using distal tephras preserved in organic-rich deposits at five sites in eastern and northern North Island, New Zealand. A c. 10mm thick, unnamed white rhyolitic ash layer described at two of the sites (Tiniroto and Poukawa), and previously of disputed stratigraphic signillcance, also occurs at the other three sites (Kaipo, Rotomanuka, and Okoroire) as a primary airfall tephra. The tephra is derived from the Taupo Volcanic Centre and is correlated with Hinemaiaia Tephra (definition of Froggatt) using similarity of stratigraphic position, composition (ferromagnesian mineralogy and glass chemistry), and radiocarbon age. It stratigraphically overlies Whakatane Ash. The tephra underlying Whakatane Ash, and previously identified as Hinemaiaia Ash (definition of Vucetich & Pullar), is probably Motutere Tephra. Hinemaiaia Tephra has a mean age of old (T½) c. 4500 years, Whakatane Ash c. 4800 years. New ¹⁴C dates, obtained on peat or gyttja adjacent to these tephras, are (old T1/2, years B.P.): 4220 ± 60 (NZ316OA), 4490 ± 70 (Wk541)( above Hinemaiaia Tephra); 4470 ± 70 (Wk542) (below Hinemaiaia Tephra); 4800 ± 50 (NZ3161A), 4490 ±60 (Wk496), 4530 ± 60 (Wk497), 4260 ± 140 (Wk662) (below Hinemaiaia Tephra and above Whakatane Ash); 5210 ± 80 (NZ3162A), 4860 ±70 (Wk501), 4850 ± 80 (Wk660) (below Whakatane Ash). Based on the distal occurrences described here, the Hinemaiaia Tephra has a much more wide spread distribution than previously demonstrated, and may have been emplaced by a very powerful "above average" plinian eruption.