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 tephras erupted from central North Island volcanoes and these provide minimum ages for the underlying gravels. Tephra or tephric layers occurring in the lower cover beds were investigated at five sites using a combination of stratigraphy, mineralogy, and the major element composition of glass shards together with discriminant function analysis (DFA). The basal tephra is identified as the ca. 14,700 14C years old (ca. 17,700 cal. years B.P.) Rerewhakaaitu Tephra, erupted from Okataina Volcanic Centre. Using the stratigraphic relationship of Rerewhakaaitu Tephra, the end of aggradation is dated at ca. 15,000 14C years (ca. 18,000 cal. years). Correlation with aggradational terraces elsewhere in North Island and northern South Island indicates that aggradation ended at the same time over a wide area and confirms a climatic origin for the terraces. Subsequent downcutting was apparently rapid because Rerewhakaaitu Tephra also occurs at the base of cover beds on a ca. 15 m lower terrace. The downcutting represents a major change in river dynamics and is most likely the response to climatic change and the resultant upper catchment landscape stability.