Clay mineralogy of soils on hard carbonate substrates of the Lena-Angara plateau

The study objects are represented by calcareous and weakly differentiated soils underlain by the hard carbonate-silicate rocks (red-colored and without reddish pigment ones) that have formed in the cold, continental environment of the southern part of the Lena-Angara plateau, Eastern Siberia. The study aimed to investigate the autochthonous/ allochthonous nature of the loose calcareous substrate underlain by the hard rock using data of clay mineralogy. Intensive leaching of carbonates in the soil profiles led to decrease of the pH values simultaneously with increase of the proportions of dithionite extractable iron in the topsoils. The clay mineral association is the same in the red-colored soil and calcareous hard rock. It represented by chlorite and dioctahedral mica (illite), and a small proportion of the mixed-layer illite-smectite (vermiculite) structures with < 50% of smectite (vermiculite) layers. The latter increases slightly in the soil horizons compared to the hard rock. Oppositely, clay mineralogy in the profile on the non reddish colored rock differs from that in the silicate part of the hard rock. Dioctahedral smectite, talc, dioctahedral mica, chlorite, and mixed-layer chlorite-vermiculite (smectite) were identified in the silicate part of the calcareous hard rock. In the soil horizons the decrease of the proportions of talc and smectitic clay was occurred. Besides, appearance of kaolinite in the soil horizons that is absent in the hard rock can be an indicator of the allochthonous substrate occurrence. The studied soils are illustrated two variants of pedogenesis on the loose calcareous substrate underlain by hard carbonate-silicate rocks: (i) on the eluvium of hard rock (red-colored profile) and (ii) with admixture of allochthonous loose substrate in the profile underlain by hard calcareous rock.