Dating the culmination of river aggradation at the end of the last glaciation using distal tephra compositions, eastern North Island, New Zealand

An extensive terrace (Waipaoa-1) that can be traced for about 29 km in the Waipaoa valley, eastern North Island, New Zealand, is underlain by at least 10 m of coarse, aggradational, river gravels. Terrace cover beds contain t ephras erupted from central North Island volcanoes and these provide minimu m ages for the underlying gravels. Tephra or tephric layers occurring in th e lower cover beds were investigated at five sites using;a combination of s tratigraphy, mineralogy. and the major clement composition of glass shards together with discriminant function analysis (DFA). The basal tephra is ide ntified as the ca. 14,700 C-14 years old (ca. 17,700 cal. years B.P.) Rerew hakaaitu Tephra, erupted from Okataina Volcanic Centre. Using the stratigra phic relationship of Rerewhakaaitu Tephra, the end of aggradation is dated at ca. 15,000 C-14 years (ca. 18,000 cal. years). Correlation with aggradat ional terraces elsewhere in North Island and northern South Island indicate s that aggradation ended at the same time over a wide area and confirms a c limatic origin for the terraces. Subsequent downcutting was apparently rapi d because Rerewhakaaitu Tephra also occurs at the base of cover beds on a c a. 15 m lower terrace. The downcutting represents a major change in river d ynamics and is most likely the response to climatic change and the resultan t upper catchment Landscape stability. (C) 2001 Elsevier Science B.V. All r ights reserved.