Tectonic and paleoclimatic significance of Quaternary river terraces of the Waipaoa river, east coast, North Island, New Zealand

Abstract

Remnants of four aggradational terraces in the lower 45 km of the main branch of the Waipaoa River have been correlated with cold/cool climate episodes of the Otiran glaciation. The youngest of the aggradation levels—the Waipaoa‐1 terrace—has the c. 14.7 kaRerewhakaaituTephra as the oldest part of the coverbed sequence, indicating cessation of aggradation about 16 ka BP. This terrace is broadly correlated with Ohakean‐aged terraces in other parts of the North Island. The second most recent episode of aggradation—the Waipaoa‐2 terrace—is slightly older than the c. 28 ka Mangaone Tephra, and is broadly correlated with the Rata terrace. The third most recent aggradation episode— the Waipaoa‐3 terrace—is slightly older than the c. 55–57 ka Rotoehu Tephra (age estimate based on stratigraphic relationships in this study), indicating cessation of aggradation at c. 65 ka BP, and correlative with the Porewa terrace. The fourth, and oldest, aggradation episode we identify in the present landscape—the Waipaoa‐4 terrace—has poor age constraints, but is probably related to the cool period of late oxygen isotope stage 5 at c. 90 ka BP or the glacial period of oxygen isotope stage 6 at c. 140 ka BP. Tectonic deformation in the middle reaches of the Waipaoa catchment is deduced from the elevation difference of pairs of aggradation terraces, and takes the form of broad regional uplift in the range of 0.5–1.1 mm/yr. Uplift is probably driven by subduction processes in the middle part of the catchment and by a combination of deep‐seated subduction processes and local deformation associated with active faults and folds in the lower valley area. Downcutting rates of up to 7 mm/yr occur in upper reaches of the river. In the middle reaches of the valley, where there are both uplift and downcutting data, we find that downcutting is about four times faster than tectonic uplift. Thus, climate fluctuations are interpreted to be the primary control on formation of fluvial terrace landscapes in the region.