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A mineralogical and textural study of the central North Island tephra, Okareka ash and its overlying tephric loess deposits : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University
In Central North Island, New Zealand, Post-Okareka tephric loess rests upon Okareka Ash (c.17,000 years B.P.). Tephric loess accumulation occurred under semiarid conditions which coincided with glacial advances in southern areas of New Zealand. Morphological and grain-size evidence indicates the tephric loess has been derived from a localised source, most probably that of Okareka Ash material, reworked and redeposited by aeolian processes. Optical and electron optical evidence reveals that Okareka Ash particles are angular and relatively unweathered, whereas tephric loess grains are subangular and more weathered. The sand and clay mineralogy of the tephra and tephric loess are similar. Sand fractions contain mainly rhyolitic volcanic glass, quartz, plagioclase feldspar, biotite, hypersthene, hornblende, titanomagnetite and traces of cristobalite, tridymite and augite, whereas clay fractions contain halloysite, allophane, imogolite and gibbsite in varying amounts. Grain-size analysis reveals Okareka Ash deposits show decreasing mean grain-size with increasing distance from source, are poorly-sorted, fine-skewed, and lepto/platykurtic. In contrast to tephra, tephric loess samples exhibit a narrow mean grain-size range, and are better sorted, but show similar skewness and kurtosis values to ash. Grain-size results also indicate that due to minimal weathering of Okareka Ash and Post-Okareka loess, the distinction between the two deposits is less well-defined than data from similar deposits reported by Fisher (1966). Furthermore, where ash deposits are thin, in distal areas from source, and under certain environmental conditions, textural and morphological characteristics of the tephra are similar to those of the tephric loess. Nevertheless, grain-size parameters may be used to differentiate airfall tephra and tephric loess deposits, although this differentiation is enhanced by post-depositional weathering. The contrasting clay mineralogies of tephra and tephric loess samples from sections of similar topography, altitude, drainage and rainfall, illustrates the problems of field sampling in weathering studies.