THE GEOCHEMISTRY OF BASALTIC MATERIALS AND AUSTRALIA ASSOCIATED SOILS OF SOUTH‐EASTERN SOUTH AUSTRALIA

Summary Four profiles in the Mt. Burr forest area of south-eastern South Australia have been examined in detail. Two profiles were podzolized deep aeolian sands overlying buried soils which had developed on basaltic tuff, a third profile was developed on olivine basalt while the fourth was a composite profile developed on basaltic tuff and containing a buried profile formed on an earlier ash shower. The main factors operative in influencing the geochemical distribution of the elements in these profiles were the translocation of elements by plant growth and their relative mobility on weathering, association with the clay fraction, and restricted drainage. In the aeolian sands Ca, Mn, and V with lesser amounts of K, P, Mo, Cu, Mg, Zr, and Al, were the elements which generally accumulated in the surface horizon as the result of plant action. In the surface soils of volcanic origin Ca, Mn, P, and Mg with lesser amounts of Cu and K accumulated. The highest concentrations in the clay fraction were, in general, for the aeolian sands Al, Fe, Ga, V, and Ni, and in the basaltic soils Na, Al, Ga with, to a lesser extent, K, V, P, and Ni. In the soils developed on basaltic material the elements most readily lost from the weathering zones during soil formation were in decreasing order of magnitude, Ca, P > Na, Mg, Co, Zn > Cu, Mn > Ni. K, V, Ga, Mo, Fe, Al, Ti, and Zr were in general much less mobile and tended t remain incorporated in the weathering products and concentrated in the upper weathering zones. P, Zn, and Cu and to a lesser extent Mn, Fe, Ni, Co, and Ca were found to be more readily lost on weathering under conditions of restricted drainage. The elements most concentrated in the hardpan of a ground-water podzol were, in decreasing order of magnitude, Fe > Al ≫ V, Zn, Ni > P, K, Mo > Na, Mn, Cu, and Ti. When the more strongly cemented nodules were separated for analysis it was found that relative to the matrix Fe > Al, V, and P, in that order, were most concentrated. Zn alone was lower in the nodules than the sandy matrix. Mn and Mo were most concentrated in the upper part of the hardpan, and Zn, V, Ni, Cu, Ti, and P in the lower part. Some of the mineralogical changes which have taken place during the weathering of the basaltic material have been studied by examination of sand and clay fractions of the soil and thin rock sections. In the olivine basalt, the plagioclase and, to a lesser extent, olivine have weathered most readily. The main weathering products of the soil developed on the olivine basalt were kaolin and haematite. In the tuff the first important weathering product was montmorillonite, formed by the breakdown of the volcanic glass. This montmorillonite was found to break down in the upper weathering zones of the soils formed on tuff to give kaolin and goethite.