A kaolinitic weathering crust in Tomsk, West Siberia: Interpretation in the context of weathering crusts in Russia and elsewhere

Abstract Two exposures of a thick kaolinitic weathering crust located in the banks of the River Tom, in Tomsk, West Siberia were investigated by X-ray diffraction, scanning electron microscopy and X-ray fluorescence analyses. The weathering crusts are developed upon a near vertical series of laminated fine-grained sandstones and siltstones of Carboniferous age that have been deformed by the Variscan orogeny and are overlain by horizontal seams of coals and bedded sediments of Palaeocene age. The geological evidence indicates that the likely age of the weathering crust is Mesozoic. However, there is only slight evidence of vertical mineralogical and chemical zonation of the weathering crust, and in the Blue Cliff exposure chlorite in the parent rock persists apparently unchanged into the upper parts of the profile. These features may be accounted for if the profiles examined are of the linear type, which typically develop upon tectonically affected features such as faults, fractures, veins and steeply dipping sediments, extend to great depths, lack well-developed mineralogical and chemical zonation and are significantly affected by lateral drainage and an unstable landscape. In contrast, areal weathering crusts are of limited thickness and are often truncated, extend over a wider area on stable sites and show well-developed mineralogical and chemical zonation. The distinction between these two types of weathering crust was described by Petrov (1958) , who further attributed the weathering to a period of virtually unprecedented tectonic quiescence during the late Triassic to early Jurassic. During this period the Pangea super-continent enjoyed a warm, humid and equable climate, with no polar ice caps, and well-vegetated soils were developed on a peneplained surface. The evidence supporting this concept is briefly reviewed and indicates that an initial period of early Jurassic weathering may well have affected other weathering profiles that have been attributed exclusively to much later periods.