The paper treats over the results of lithological studies of the core dredg
ed from the borehole which was drilled under a 354 m thick water layer of L
ake Baikal.
The structural and geochemical specialization of clay minerals in bottom se
diments of Lake Baikal has been established for the first time to a depth o
f more than 100 m. The quantitative composition of mineral phases of a thin
-pelitic fraction is calculated with the help of computing procedures "Simp
lex". It is shown that smectites are represented by beidellite, montmorillo
nite, and, to a smaller degree, nontronite. They actually reflect the range
of mineral varieties of this group, which is characteristic for allochthon
ous and autochthonous genesis. Terrigene smectites in sediments are indicat
ive of semi-arid and semi-humid climate in the Miocene and, to a smaller de
gree, in the Pliocene. The acme of diatoms and maximum abundances of smecti
tes may be linked with the thawing of glaciers and ices whose regelation wa
ters are able to transport terrigene smectite and carry a great amount of d
issolved silica necessary for building a silica framework of this variety o
f phytoplankton. Also, the reserve of smectites considerably grew during au
thigenic mineralization. The proof is examination of samples on SEM.