Some Basic Problems in Tephrochronology

Abstract

So much tephra deposits which are popularly called “Loam” or “Brown ash”, obscure discrimination of particular tephra layer from many others within the Quaternary section in Japan. Detailed description in petrography and in lithology is therefore needed for characterizing any particular tephra layers. Our attentions should first be called to petrographic feature of such essential vesiculated materials as pumice and scoria beds which usually prove to be the marker bed in the field. The marker bed may be an early product ejected during a series of many eruptions, of what is called “an eruption cycle”, and individual marker bed represents a lower part of a “tephra member” (Table 1). Recognition of each tephra member becomes more difficult with increasing distance from the source vent, and only pumice or scoria bed unmistakably marks the existence of a member within a tephra formation or merely within a tephra section in far distance. Buried soil or palaeosol can be used for detection of appreciably long interval of ash showering to establish a “member” or a “formation” in the field, but the strict application may not always be easy, as in Japan the accumulation of humic soil through Holocene time is said to have got a much greater rate than that during Pleistocene times.
Through sequence of several tephra members, some of which being marked respectively by a pumice or scoria bed, are often recognized systematic variations in the type of eruption and in chemical composition of their products (Momose et al., 1968; Kobayashi et al., 1971).
The problem on the homogeneity of vesiculating magma was discussed according to informations from samples at every horizon closely spaced in a single marker bed. As to a single marker bed maybe consisting of single shower beds or fall-units (Nakamura, 1960, 1964), sometimes orthopyroxene in the upper part somewhat contains more amount of Mg than those in the lower part, seemi ngly indicating that the magma chamber or column had a zonation due to crystallization differentiation (Arai, 1970, 1971, 1972, this number). Additionally, slight decrease in Curie temperature and increase in TiO₂-content of ferromagnetic minerals also correspond, from lower to upper, well with the coexisting mineral assemblage mentioned above (Kobayashi et al., 1971). In most cases, however, the inhomogeneity of petrographic feature of the marker beds falls within a small range of variation, so that the marker bed can usually be represented by samples from comparatively a few horizons of a single bed.