Paleosols and their sedimentary setting in the Old Red succession of Podolia, Ukraine

Three types of paleosols have been recognized in the red siliciclastic Podolian Old Red succession in Ukraine – gleyed/calcic and calcic vertisols, and calcic protosols. Both types of vertisols are characterized by the occurrence of pedogenic nodules, an illuvial clay horizon and slickensides. They have a pseudo-anticlinal and gilgai microrelief, which document the intensive swelling and shrinking of soils in semi-arid to sub-humid (monsoonal) climates with seasonal precipitation. Moreover, the gleyed/calcic vertisols contain rhizoliths with yellow-brownish/gray depletion zones and barite accumulations in the topsoil, which suggest the development of gleying processes and reduction conditions in water-logged soils. Spectral Gamma-Ray (SGR) measurements document a high content of potassium in the vertisols. XRD analysis shows the monotonous composition of the soils with the domination of quartz, illite, chlorite, mica, and with some admixture of illite-smectite mixed layers. The SGR measurements have revealed Th-enrichment being the effect of rainwater-induced downward transport of small grains of Th-bearing heavy minerals into the cracks and micropores during wet seasons. The calcic protosols are characterized by the accumulation of calcite nodules, small and irregularly dispersed within the matrix. They represent less mature, multi-storey, stacked paleosols in contrast to the mature vertisols. The variability of paleosol maturity has allowed the recognition of the relative distance to the fluvial channel. Less mature calcic protosols indicate formation during short-term episodes of landscape stability, interrupted by rapid aggradation of mud after flood events. These vertically stacked paleosols are typical of proximal floodplains. By contrast, both types of vertisols were formed on distal floodplains and their occurrence is associated with a lower frequency and thickness of crevasse splays and sheetflow deposits. They document stable and long-term weathering processes. The sedimentological investigations have revealed the fluvial architecture of the pedogenized Old Red succession in Podolia.