Tephrostratigraphic studies at Tongariro volcanic centre New Zealand: An overview

Detailed tephra studies at Tongariro Volcanic Centre (TgVC), North Island, New Zealand, have followed several decades of tephrostratigraphic research, principally at the more northern rhyolitic Taupo and Okataina volcanic centres. The development of reliable fingerprinting techniques for correlating rhyolitic tephras sourced from these northern centres has permitted a volcanic stratigraphic framework to be established at TgVC, where distal rhyolitic tephras are found interbedded with local andesitic tephras and volcanic sediments. Field studies at TgVC in recent years have established a detailed late Quaternary stratigraphy (dating back to 22,600 BP) for both andesitic and rhyolitic tephra cover beds, and laharic deposits, of the Ruapehu and Tongariro ring plains, The eruptive history of TgVC in late Quaternary time is recorded in nine andesitic tephra formations. In order of increasing age these are Tufa Trig Formation, Ngauruhoe Formation, dated ca. 1850 BP to present; Mangatawai Tephra, 2500 +/- 200 BP; Papakai Formation, between ca. 9700 and 2500 BP; Mangamate Tephra, between ca. 9950 and 9700 BP; Pahoka Tephra, ca. 10,000 BP; Okupata Tephra, comprising units erupted between ca. 13,000 and 10,000 BP; Rotoaira Lapilli, ca. 13,800 BP; and Bullet Formation, between ca. 22,600 and 10,000 BP. Several of these formations (Mangamate Tephra, Pahoka Tephra, Okupata Tephra) are sufficiently distinct and widespread to be defined as regional marker beds, useful for the correlation and dating of local tephras and ring-plain-forming debris flow deposits of the Tongariro and Ruapehu ring plains, with which they are interbedded. Similarly, they are potentially important marker beds at sires distal to source where they are found interbedded with distal silicic tephra layers in non-volcanic Quaternary sediments. The potential value of TgVC tephras as chronostratigraphic marker beds, and thus a tool in Quaternary geomorphological studies in distal environments, is perhaps not fully appreciated, given the traditional focus on using rhyolitic tephras in correlation studies and the wealth of information now available on these. Copyright (C) 1996 INQUA/Elsevier Science Ltd