Depositional surfaces in the upper Mangawhara catchment and adjacent areas, South Auckland, New Zealand

Abstract In the upper Mangawhara Stream catchment and adjacent areas (N52, N56), 13 surfaces are recognised and grouped into 9 named depositional complexes on the basis of degree of weathering, degree of soil development, characteristics of the sediment body, overlying tephra deposits, and topographic position relative to the other surfaces. Sediments deposited during two catastrophic floods in 1966 and 1967 seem distinctive; two new radiocarbon dates indicate the only comparable deposits were laid down c. 5000 years ago. Other depositional surfaces, younger than three new radiocarbon dates c. 10 000 years ago, may have resulted only from very minor aggradation/ degradation and lateral migration of the channel. Aroronga sediments indicate that, in the upper Mangawhara catchment, extensive aggradation was completed prior to deposition to Rotoehu Ash ( c. 40 000 years ago) and that the stream downcut sufficiently far by 20 000 years ago that even major floods did not reach as high as the ash‐mantled surfaces. Earlier depositional surfaces occurred prior to H4 Hamilton Ash and prior to H1, the basal member of the Hamilton Ash Formation. The earliest surface identified predates the last major period(?) of red weathering and at least some ashes of the Kauroa Ash Formation. Comparison of the depositional surfaces described here with those observed by other authors reveals no direct correspondence. The difficulties of correlation are compounded by previous attempts to relate terraces to sea levels.