A LATE QUATERNARY STRATIGRAPHIC FRAMEWORK FOR THE NORTHEASTERN RUAPEHU AND EASTERN TONGARIRO RING PLAINS, NEW-ZEALAND

The northeastern Ruapehu and eastern Tongariro ring plains record a co mplex sequence of episodic lahar sedimentation. Andesitic and rhyoliti c tephrostratigraphy reveals 15 lahar episodes in the northeastern Rua pehu ring-plain record ranging in age from >65 to 5 ka, and five in th e Tongariro ring-plain record ranging in age from >23 to 14 ka. The mo st voluminous and widespread lahar deposition occurred during cool and stormy climatic periods equivalent in age to marine delta(18)O Stages 2 and 4. In these periods, accelerated physical weathering on the vol canoes supplied erosion debris, while large areas of snow and ice acte d as sources of water to form lahars triggered by a variety of mechani sms. Lahar distribution after c. 22.5 ka was affected by two landform changes in the area at about this time. First, a large lava flow was e mplaced along the boundary between Ruapehu and Tongariro ring plains s hortly before 22.5 ka, effectively separating the two ring plains sinc e. Second, Last Glacial moraines along the Whangaehu and Mangatoetoenu i Rivers have blocked direct drainage from the Ruapehu summit region t o a large sector of the northeastern ring plain, including the Upper W aikato Stream, formerly an important lahar path. These moraines have d irected subsequent lahars (after c. 15 ka) along their current routes (active in 1995) in the Whangaehu and Mangatoetoenui catchments. Lahar deposition in the study area during this time is linked to large, exp losive andesitic eruptions impacting on catchments where retreating gl aciers provided water for lahar generation.