GEOLOGY OF THE ANGARA REGION

Industrial development of the Angara region has necessitated geological exploration for available resources. The Angara River flows north from Lake Baikal, intersecting the mountains surrounding it; passes through the 'Irkutsk Amphitheater, ' part of the central Siberian platform; and, near Bratsk, flows across a diabase intrusion, forming the Bratsk rapids (approximately 300 kilometers long). The Angara river basin is underlain by crystalline basement (the Siberian craton) composed of schists, gneisses, marble, and Archean and Proterozoic granites. These rocks dip sharply from the Sayan and Baikal ranges, where they outcrop, toward the Irkutsk Amphitheater, where they reach 3000 meters depth. The craton is covered by sediments ranging in age from Cambrian to Quaternary. Lower Cambrian rocks over 2500 meters thick are overlain by Middle Cambrian strata which are generally eroded. In the Irkutsk coal basin, north of Lake Baikal, Mesozoic rocks 600 meters thick are covered unconformably by Tertiary sediments. Quaternary deposits are known to occur; the Angara River terraces are probably pre-Quaternary. Regional tectonics involved fracturing within the Irkutsk Amphitheature and in the surrounding mountains. Geophysical survey and drilling revealed a wide horizontal protrusion (the 'Angara swell') in the Siberian craton; this protrusion divides the Irkutsk Amphitheater in the Pre-Baikal and Pre-Sayan depressions. Overlying Cambrian sediments are folded in conformity with these basement-complex dislocations. Jurassic deposits, generally horizontal, are disturbed only near the younger uplifts of Sayan and Baikal. The southwestern part of a large trap-rock intrusion crosses the Irkutsk Amphitheater; concordant intrusions, e. g. sills, entered lower Paleozoic sediments along with dikes, during late Permian and, principally, Triassic times. Mineral deposits are rich and varied: Precambrian rocks contain magnetic iron ore (of the Krivoy Rog type); talc; magnesite; pure crystalline limestone; and, possibly, phosphates. Paleozoic rocks contain large marine and lacustrine salt deposits, gypsum, phosphatized shell rock, and, possibly, oil: as well as carbonaceous rocks with lead and zinc minerals. Hydrogen-sulfide -saturated and saline mineral waters as well as subsurface water with high potassium content occur in Cambrian rocks. Siberian trap rocks are rich in magnesium and iron; magnesium-magnetite ores occur in volcanic necks as large veins of pure ore, associated with tuffs and aureoles. Trap rock (diabase) may be used in the new stone-melting industry. Jurassic deposits include saprolitic-bog and humus coals as well as extensive fire-clay and high-quality kaolin deposits. Cambrian fossils include trilobite and brachiopod remains, reefs, and molluscs; fish, insects, ostracods, and numerous fossil plant traces, are representative of the Mesozoic. The Angara River terraces, 25 to 30 meters thick, contain mammal remains, brackish-water molluscs, and, from the Middle Paleolithic, Azilian and Solutrean artifacts. -- D. D. Fisher