Tephrostratigraphy arid chronology of the kaipo lagoon, an 11, 500 year-old montane peat bog in urewera national Park, New Zealand

Abstract

Eleven well-preserved Holocene tephras occur interbedded with peat in the Kaipo Lagoon bog in Urewera National Park, North Island. They are identified chiefly by their field appearance, stratigraphy, and ferromagnesian mineralogy. Glass shards from one tephra (Hinemaiaia) were analysed by electron microprobe.The tephras originate from the Taupo, Okataina, and Tongariro Volcanic Centres and, from youngest to oldest, are: Kaharoa Ash, Taupo Pumice, Mapara Tephra, Waimihia Lapilli, Hinemaiaia Tephra, Whakatane Ash, Rotoma Ash, Opepe Tephra, Poronui Tephra, Karapiti Tephra, and Okupata Tephra. Thirteen new radiocarbon ages were obtained on six of the tephras (old TY2, years B.P.): Waimihia, 3, 250 ± 70 (Wk498), 2, 910 ± 60 (Wk499), 3, 040 ± 50 (Wk500); Hinemaiaia and Whakatane, 4, 490 ± 60 (Wk496), 4, 530 ± 60 (Wk497); Whakatane, 4, 860 ± 70 (Wk501); Rotoma, 5, 440 ± 170 (Wk493), 7, 380 ± 80 (Wk494), 7, 560 ± 100 (Wk495) (Wk493 -495 are all considered anomalously young); Opepe, 8, 7lO ± 80 (Wk492); Poronui, 10, 160 ± 130 (Wk351), 9, 960 ± 90 (Wk352), 9, 560 ± 80 (Wk491). Estimated ages, assuming constant sedimentation rates, for Karapiti Tephra and Okupata Tephra are 10, 100 and 10, 300 years, respectively. Two peat horizons below the Okupata Tephra were dated at (old TY2) 10, 600 ± 90 years B.P. (Wk263) and 11, 500 ± 80 years B.P. (Wk264) and date the initial growth of the Kaipo Lagoon bog. Peat accumulation rates have been slow (average 0.19 mm/year) but variable.The identification and dating of the tephras at Kaipo extends their known distribution in eastern North Island, and improves their potential usefulness as isochronous stratigraphic marker units.