Lithological indicators of discontinuities in mountain soils rich in calcium carbonate in the Polish Carpathians

Abstract

Six soils located within the Polish Carpathians, developed on calcium carbonate–rich sedimentary parent materials and representing various reference groups, were investigated in order to detect the lithic discontinuity. We propose using a multidirectional approach to assess the lithic discontinuity in these soils, one that includes grain size distribution, geochemical composition, heavy mineral content and micromorphology, supported by a traditional soil survey. A further aim of this process was to identify the possible admixture of allochthonous material of aeolian origin. The studied soils presented lithic discontinuities mostly at the contact of underlying calcium carbonate–rich coarsegrained slope deposits with the overlaying colluvium layer having a lower content of rock fragments. The significant changes in grain size distribution, especially in the silt and sand content, as well as high Uniformity Values and partially, high Lithological Discontinuity Index values, confirmed the occurrence of a lithic discontinuity in all studied soils. High heterogeneity in the soil profiles was also confirmed by the distribution of the major oxides; however, their distribution did not clearly indicate the lithic discontinuity. The most visible distinctions were noted from CaO content, which resulted from the deposition of carbonate-free materials (aeolian silts) and their mixing with the calcium carbonate–rich parent material. Furthermore, the analysis of heavy mineral content confirmed the allochthonous origin of the upper (and in some cases also the middle) parts of all profiles, which was manifested by the presence of highly weathering-resistant minerals such as zircon, epidote and various types of garnets. The micromorphological features of some of the studied soils showed distinctiveness within the soil profile, manifested by changes in b-fabric pattern, the occurrence and distribution of secondary carbonate and the coarse and fine coarse and fine ratio. Based on the high content of silt within the upper and middle parts of the soils, the content of Hf and Zr, as well as the higher content of weathering-resistant minerals, admixture of aeolian silt could be considered in some of the studied soils, yet with weak character. However, the dominance of minerals typical for metamorphic and igneous rocks suggested that the supply of aeolian silt was associated with loess covers rather than local sedimentary material.