TEPHROSTRATIGRAPHIC STUDIES AT TONGARIRO VOLCANIC CENTER NEW-ZEALAND - AN OVERVIEW

Detailed tephra studies at Tongariro Volcanic Centre (TgVC), North Isl and, New Zealand, have followed several decades of tephrostratigraphic research, principally at the more northern rhyolitic Taupo and Okatai na volcanic centres. The development of reliable fingerprinting techni ques for correlating rhyolitic tephras sourced from these northern cen tres has permitted a volcanic stratigraphic framework to be establishe d at TgVC, where distal rhyolitic tephras are found interbedded with l ocal andesitic tephras and volcanic sediments. Field studies at TgVC i n recent years have established a detailed late Quaternary stratigraph y (dating back to 22,600 BP) for both andesitic and rhyolitic tephra c over beds, and laharic deposits, of the Ruapehu and Tongariro ring pla ins, 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 pre sent; Mangatawai Tephra, 2500 +/- 200 BP; Papakai Formation, between c a. 9700 and 2500 BP; Mangamate Tephra, between ca. 9950 and 9700 BP; P ahoka 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 suffi ciently 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 imp ortant marker beds at sires distal to source where they are found inte rbedded with distal silicic tephra layers in non-volcanic Quaternary s ediments. The potential value of TgVC tephras as chronostratigraphic m arker beds, and thus a tool in Quaternary geomorphological studies in distal environments, is perhaps not fully appreciated, given the tradi tional focus on using rhyolitic tephras in correlation studies and the wealth of information now available on these. Copyright (C) 1996 INQU A/Elsevier Science Ltd