The North Caspian depression covers an area of some 600,000 sq km on the southeast margin of the East European craton. Sedimentary thickness in the central part of the depression according to geophysical data is 18-22 km. The base of the Lower Permian evaporite section is at depth of 1-10 km.
Three stages are recognized in the development of the basin before deposition of the Permian salt: 1) aulacogen (Riphean-Vendian, possibly Early Paleozoic also); 2) epi-continental (pre-Middle Devonian Paleozoic), 3) deep-water (Middle Devonian-Artinskian), when the basin experienced repeated intensive subsidence.
Composition and structure of the sub-salt sediments attest to the Early Permian basin having been a deep-water feature. On the west and north a border step had a difference in elevation of 1000-1500 m. On the east and southeast clastic material from the rising Ural fold system spread into the basin so that the increase in depth was gradual. Basin depth in the central part was several kilometers.
Large structures composed of Carboniferous and Lower Permian reefs formed along the north and west border steps of the depression. On the east and southeast they have a Carboniferous age. During the Early Permian conditions were not condusive to reef formation because of the debris apron shed from the rising Urals.
During Kungurian time late in the Early Permian the climate became arid, and intensive salt deposition ensued. In comparatively short time the deep-water basin was compensated by salt deposition. Stages of subsidence were accompanied by anhydrite deposition. This was due to a decrease in salinity in the basin as a result of introduction of fresher water from the Cis-Ural foredeep, which then connected the North Caspian basin with open ocean on the north in the Pechora depression. Potassium-magnesium salts were deposited under shallow-water conditions at the peak of salt deposition. The Kungurian was thus the fourth stage in the development of the North Caspian depression.
The Lower Permian evaporites are subdivided into the Filippov and overlying Iren Horizons. The Filippov consists of carbonate-sulfate rocks in the west and north parts of the basin, but contains much clastic material on the east and southeast. It is several hundred meters thick. The Iren is largely salt everywhere and is several kilometers thick.
Modern salt deposition differs in scale from that of ancient salt basins. In the geologic past the basins were broad and deep marine features, whereas now they are continental and depths are but a few tens of meters. Greatest depth of present salt basins is 400 m in the Dead Sea, and the greatest area is the 12,000 sq km of the Kara Bogaz Gol. (Taken from Komissarova (1986); digested in Petroleum Geology, vol. 23, no. 7/8.; one map and three cross sections)
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