Late quaternary constructional history of the southeastern Ruapehu ring plain, New Zealand

A late Quaternary constructional history of the southeastern Ruapehu ring plain is established from detailed mapping of volcaniclastic sequences preserved over a 200 km(2) area northwest of Waiouru. Lahar deposits dominate these sequences and form the major surfaces. Local andesitic tephras, and distal rhyolitic tephras from Taupo and Okataina Volcanic Centres, are interbedded with the lahar deposits, providing a framework radiocarbon chronology for these deposits dating back to c. 22 600 yr BP. Five formations are mapped across the southeastern Ruapehu ring plain with their respective constructional surfaces: Onetapu Formation (dated c. 1850 yr BP to Present), Manutahi Formation (dated c. 5370-3200 yr BP), Mangaio Formation (dated c. 4600 yr BP), Tangatu Formation (dated c. 14 700-5400 yr BP), and Te Heuheu Formation (dated c. >22 600-14 700 yr BP). Each formation records a period of ring-plain aggradation in response to increased sediment influx, principally by sediment flows associated with syn-eruptive and inter-eruptive activity. Sector-wide aggradation occurred between c. 22 600 and 14 700 yr BP, and during the Last Glacial Maximum (c. 23-13 ka). Movement along the Desert Road Fault post c. 25 000 yr BP led to more confined ring-plain aggradation west of the fault. Lithological and sedimentological characteristics of the lahar deposits, and their distributions, indicate changing rates of sediment supply and a progressive northwards migration of source areas and ring-plain sedimentation over time. This post-c. 22 600 yr BP record of sedimentation and ring-plain construction is important to deciphering the nature of eruptive and noneruptive activity at this volcano and the establishment of an integrated (tephra and lahar) record of the volcanic history.