Rift structures formed in West Siberia in five successive phases, largely in the following time segments: 1) Late Permian-Early Triassic (Krasnosel'kup Series), 2) Triassic (Turin Series), 3) Late Triassic- Early Lias (Chelyabinsk Series), 4) Early-Middle Jurassic (Ubagan Series), and 5) Middle Jurassic (Glushin Series). These grabens have been found by drilling and seismic survays. They number more than 100. The rifts that are filled by the Krasnosel'kup and Turin Series formed during times of active igneous activity. The Krasnosel'kup contains tholeiitic basalt and dolerite. The Turin consists largely of calc-alkaline basalt and some rhyolite. Conglomerate and fanglomerate are present locally. Bochkarev and others (2000) show no overlap in age of the Krasnosel'kup and Turin Series. No igneous rocks are present in the younger grabens. They are filled by coal-bearing and conglomeratic units. The earliest grabens to develop are those filled by basalt, dolerite, tuffs, and clastics of the Krasnosel'kup Series. All of those filled by Krasnosel'kup rocks are located in the east in the Yenisey part of the West Siberian platform. A similar lithologic section is found also in the south of the platform in the Kuznetsov and Minusin downwarps, where they are localized in an area of Salair (Cambrian) basement at the boundary with a Hercynid terrane. Their formation was in the 260-245 m. y. time span, suggesting their association with cratonization. Basaltic rocks of the Kasratoikhin downwarp of the Cis-Urals are also tholeiitic and occupy a similar structural position. This activity was at the boundary between the Permian and Triassic. As a rule the rifts of this group have not been subsequently rejuvinated. The grabens filled with deposits of Turin Series occur in the central and southwest parts of the West Siberian platform. Weathering beneath the Turin rocks indicates that these grabens began to form immediately after peneplanation of the Hercynian folded mountains. The largest of these grabens are on basement anticlinoria. Gentle arches are present above these features, however, in the overlying Jurassic and Cretaceous sedimentary cover. These grabens are not expressed structurally in the post-Triassic rocks. Grabens of the three youngest groups formed beyond the margins of the West Siberian sedimentary basin of the Jurassic and Cretaceous. The grabens filled with Chelyabinsk deposits occur in the southwest of the West Siberian platform. The Jurassic grabens are located farther south than the Triassic and Triassic-Jurassic grabens. They are filled with coal-bearing and conglomeratic deposits that are 800-1500 m thick. They form isolated depressions disposed on domal highs. Later, when the crust began to downwarp, these grabens, just as the Triassic ones, experienced block compression and were transformed into ramp grabens with overthrusting along their borders and development of gentle folds in the Lower Mesozoic sediments. There is an angular discordance between them and the younger sedimentary cover (Taken largely from Vochkarev and Nesterov, 1994; digested in PETROLEUM GEOLOGY, vol. 29, no, 7/8, 1994, one map and one cross section). Copyright 2000 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at http://geocities.com/internetgeology/ Many are receiving this News Letter for the first time on the recommendation of one of the present recipients. I hope you find it worthwhile. Please visit our website for more information on the reasoning behind Internet Geology. Thank you for your time. James Clarke