The use of physical and chemical techniques in the identification of Tephera (volcanic ash) in the North Island, New Zealand
Field evidence is no longer adequate to resolve the problems of tephrostratigraphy, particularly in areas far from the likely volcanic source. In such areas the deposits are thin, and consequently more strongly weathered, as they are more or less continually involved in soil development processes. Under such conditions lithologic resemblance of a given ash to the same material closer to source may be lacking. The identification and correlation of tephra in the Waikato Basin of the North Island of New Zealand is one such example. These tephra form the parent materials of the agricultural soils of the region, and are at considerable distances from the likely sources, these being the Egmont volcanic centre in Taranaki, the Okataina volcanic centre in the Bay of Plenty, and the Tongariro and Taupo volcanic centres in the Central North Island. In particular, this thesis sought to resolve the problem of the identity of the Tirau and Mairoa Ash beds; whether they merely represent the products of pedogenesis of the same parent materials under differing weathering conditions. In the resolution of this problem, many laboratory techniques were exhaustively and systematically investigated, the methods being reviewed and extended in their applicability and their sensitivity being improved where possible. Physical properties of the rhyolitic glass associated with the samples that were found to provide useful data were refractive index, density, and magnetic properties. (a) Refractive index: This classical method of determination of a parameter indicative of bulk composition was improved an order of magnitude over that typical for petrologic investigation by the development of a thermal variation method. The increased sensitivity so obtained enabled the use of this parameter in the correlation of glass shards. The use of refractive index for this purpose had previously been abandoned because of its lack of sensitivity. (b) Specific gravity: The determination of density was investigated by gradient techniques using solutions of acetone in bromoform. The natural variation in density exceeded the precision of the technique, but in spite of this the method was shown to have some application in the solution of tephrostratigraphic problems. (c) Magnetic properties: Simple methods based on the Guoy magneto-balance were found to provide useful information on the magnetic susceptibility of glass, and were found to be particularly useful in the attempted correlation of the Aokautere and Teviotdale Ashes. More generally, the magnetic susceptibility was found to be a ready method of assessing the ferrous contents of glasses. In addition to these physical methods the glasses were analysed chemically. As well as these techniques, use was made of mineral assemblages, particularly of felsic minerals; and of the relative proportions of rhyolitic and andesitic glass. The former of these two glasses is easily extracted physically, but the existence and quantitative estimation of andesitic glass, being difficult to determine directly, was determined indirectly by infra-red spectroscopic techniques. The relative proportions of these two glasses was itself found to be a parameter of some importance in correlation and has been used in conjunction with a first-order kinetics model for the weathering of glass to allophane to provide approximate dates for the samples. While the object of this thesis was to review and develop physical, chemical, and mineralogical techniques of use in tephrostratigraphy, the techniques were used to solve some specific tephrostratigraphic problems, in particular, problems involving thin beds in the Waikato region, and some problems in long-range tephra correlation. These specific problems investigated were: (a) The Tirau-Mairoa Problem: There has been a long-standing debate over whether the ash beds that make up the Tirau Ash and Mairoa Ash sequences are the same or whether the parent materials differ. From the information derived by the use of the techniques described above, the two ash sequences were found to differ: the Mairoa Ash sequence being both older and more andesitic in character than the Tirau Ash sequence. The Mairoa Ash sequence was considered, as a result of this work to underlie and possibly interdigitate with the Tirau Ash. Samples from sites between the type localities of the two sequences shoved a composite character. Chemical analysis of the rhyolitic glass and refractive index data suggested, that at least in some localities, the Mairoa Ash was contaminated with material from the underlying Oruanui Formation. (b) Oruanui-Aokautere Correlation: The Oruanui Formation is a widely distributed bed or sequence of beds in the central North Island and had been previously tentatively correlated with the Aokautere Ash in the southern part of the North Island. Application of the techniques developed in this thesis confirm this correlation specifically between the Oruanui Ash and the Aokautere Ash. (c) Oruanui-Teviotdale Correlation: The Teviotdale Ash of North Canterbury is believed by some to be correlative with the Oruanui and Aokautere Ashes. The magnetic and chemical properties, however, suggested that the Teviotdale Ash represents a separate eruption. The soils developed on Kaharoa and Taupo Ashes show elemental deficiencies when exploited for agriculture. Soils developed on similar rhyolitic tephra of greater age show no such deficiency. Accordingly, the relative merits of such deficiencies being caused either by pedogenetic factors or by initially low amounts of the microelements in the parent materials was investigated. In the case of selenium, most of this element potentially available is to be found in the glass. Analysis of the glasses from the various ashes showed little variation in selenium content, and the results obtained bore no relationship to the agricultural selenium status of the derived soils. From this it was inferred that the selenium deficiencies are pedogenetic in origin, being caused principally by the short length of time available for the leaching of selenium from the glass. In that cobalt tends to be enriched in the more mafic minerals rather than in the glass, deficiencies of this element in the soils derived from the Kaharoa and Taupo Ashes may be rather more inherent, since the parent tephra concerned contain but small amounts of these minerals.
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