Impact of climate and related weathering processes on the authigenesis of clay minerals: Examples from circum-Baikal region, Siberia

The widely outcropping igneous and metamorphic rocks of the circum-Baikal region all contain illite and chlorite resulting from early alteration of primary minerals. These rocks were deeply weathered and leached, mainly to kaolinite, under the warm and mostly wet Upper-Cretaceous and Palaeogene climates. During the following severely dry and cold Pleistocene episodes, the glacier cover was restricted, allowing the formation of permafrost, which is still several hundred metres thick in the northern Baikal region, while the lowlands of the central and southern regions belong to the so-called “sporadic permafrost area”. The lack of glacial abrasion during the cryo-arid climate preserved somehow the weathering products of the previous warm periods that presently crop out next to the relics of the periglacial Pleistocene weathering. These relict deposits derived from the same substrate rocks as those weathered during the warm and wet climate, but they differ in the clay composition with the occurrence of a montmorillonite-type smectite that is dominant in some sites. ried weathering processes resulting in different clay assemblages emphasize the importance of the climate impact, especially the role cryogenic conditions have played on the clay distribution. A mineralogical study of the clay fraction extracted from various circum-Baikal Tertiary and Pleistocene weathering profiles and deposits confirms that: (1) chlorite and illite, which represent the initial minerals, are most sensitive to cryogenic weathering and were largely destroyed in the alluvial and colluvial deposits, (2) smectite formed under periglacial conditions, and (3) kaolinite, which is the reference clay mineral for the Palaeogene, formed in a warm and wet tropical climate, is still present without alteration features. The described smectite authigenesis confirms that geochemical processes are significant in cryogenic environments, suggesting that smectite of some present-day soils and continental deposits of temperate regions could be inherited, as it might have precipitated under periglacial conditions.