Terrace systems and coverbeds: an investigation of the depositional and erosional history, Makahu Valley, Taranaki

Abstract

Fluvial terraces have much to offer in investigations of past environments, as they often represent the most stable sites for sediment storage within valleys. Makahu Valley (about 50 km inland from Stratford) is part of inland Taranaki's highly dissected hill country. It is a narrow, steep, headwater valley, in which the fluvial terraces constitute a small, complex system of sloping, interlocking spur ends. Assessment of the terrace sequence is based on both cross valley and long valley profiles and stratigraphic investigation of the terrace material and cover bed deposits. The terraces are mapped by both field survey, using an electronic distance meter, and aerial photograph interpretation, using a photogrammetric plotter. These two data sources are integrated using Techbase (a relational database for use with any type of geologic, engineering or environmental data). The Techbase programs enabled modelling of the study area within a 5m by 5m grid. Once modelled, the data are contoured and represented in a three-dimensional perspective view, giving good representation of the valley's morphology. The stratigraphy of the terraces has been examined by the extraction of cores from representative terrace sites, field observation of road cuts, and hand auguring. The stratigraphic investigation is kept simple, concern is with the facies (their presence or absence) and time control such as tephra marker beds and datable wood material. The typical sequence comprises cover beds of tephra and Ohakean age tephric loess, over alluvium overlying the bedrock. Correlation of the cover bed deposits and elevations of the alluvial surface enable identification of the different terrace sequences. Four levels of terrace have been identified within the study area. These include two aggradational terraces attributed to major climate change, a complex response terrace and a contemporary floodplain. The highest aggradational terrace (the Ford/Manning terrace) is interpreted to have formed during the Moerangi interstadial and is correlated with Milne's Rata terrace. Radio carbon dating of wood material gives this terrace a maximum age of >40ka, and the presence of the Kawakawa tephra (22.6ka) dates the alluvial surface at c.23ka. The complex response terrace (the Jury terrace) is interpreted to be the result of slump material temporarily clogging the valley system. This terrace has a maximum age of C.23ka and an it's alluvial surface dates at <12ka. The lower aggradational terrace (the Onaero terrace) is correlated with Milne's Ohakean terrace, and has a maximum age of c.23ka.