Early Carboniferous age and lake genesis of the Tushama Horizon (Siberian Platform)

New data on the geologic structure, rock composition, and mode of occurrenc e of the Tushama Formation (Lower Carboniferous) are reported. It is demons trated that the formation deposited under limnic conditions. The ancient Tu shama lake (Early Tournaisian) is likely to have emerged as a result of an underthrust Fault of the ancient Bokson-Khubsugul-Dzabkhan microcontinent a nd the Ikat-Eravnoe island-arc system under the Baikalides of the southern margin of the Siberian Platform. In the Late Tournaisian, the Botuobiya-Mar kha uplift formed, resulting In a shift of the lake waters from northeast t o southwest and west. The rocks of the single Tushama assemblage include se quences of various ages: Tournaisian, in the northeast of the paleolake; La te Tournaisian-Visean, with an insignificant portion of Early Serpukhovian ones, in the south; and Serpukhovian, in the west.