The quaternary geology of the area east of Carterton, Wairarapa, New Zealand

Abstract

A map is presented, showing the Quaternary geology of the Carterton East District, in central Wairarapa Valley, New Zealand. A flight of terraces ranging in age from Ohakean to the Last Interglacial Period was mapped and correlated with terraces in other parts of southern North Island. The height differences between terraces and their degree of tilting were determined by levelling. Uplift rates were found to be relatively low, being up to 0.5m per 1,000 years. The terraces represent intervals of alluvial aggradation that occured in cold climatic periods. Loesses formed at about the same time as the alluvial terraces. The most recent loess is Ohakean in age and contains the Kawakawa Tephra (identity confirmed by the chemistry of the volcanic glass shards). It is not halfway to two thirds the way down the loess profile as expected, because the top of the loess has been removed by wind erosion. Other loesses are Ratan and Porewan in age. Their ages were determined by their weathering and correlation with other districts in the lower North Island. Warm periods are represented by paleosols but they are not always present. The Francis Line Formation, consisting of clay, silt, sand and gravels is described for the first time. It represents a period of fine sediment deposition during the Last Interglacial period. Most of it is interpreted as overbank deposits; laminated silts at its base are lacustrine. Variations of magnetic susceptibility in loess sequences are inferred to reflect changes in accumulation rates and climate. Clay mineralogy analysed using X-ray diffraction, backed up by scanning electron microscope examination, showed a degradation of micas into vermiculite in the loesses, but much less degradation in the Francis Line Formation. A previous course of a major river (the Waingawa or the Waipoua River) ran between Carterton and Tiffin Hill and probably changed to its present course, joining the Ruamahunga River near Masterton, some time during the build up of the Waiohine Surface.