Although the North Caspian depression has been explored for more than 80 years (as of 1989), its study remains far from complete. Earlier interpretations were that this feature was a simple saucer-like region of subsidence dating from Proterozoic time. It is interpreted here from the standpoint of plate tectonics involving the East European and Kazak continents and several micro-continents. The geologic evolution of the region took place in three cycles: ancient (Riphean- Cambrian), intermediate (Ordovician-Early Devonian), and late (Middle Devonian-Cenozoic).
Middle Riphean-Cambrian cycle. Extending from the north into the middle of the study area in Proterozoic time was the Pachelm rift. It is clearly a failed arm of a triple junction. Then two rifts developed along the margin of the study area, the Zilair on the east and the Emba on the south. These two rifts reached oceanic scale, the Zilair in the Riphean and the Emba in the Silurian. In Late Cambrian time the Zilair passive margin became the site of collision of the Mugodzhar plate from the east onto the East European continent. In contrast there was no Late Cambrian collision in the Emba area; it remained a passive margin. At the end of the Cambrian the central zone of the North Caspian basin consisted of rifts depressions and supra-rift downwarps. At this time the basin was not closed. The source of clastics was the orogen on the east and shield areas of the East European continent on the north.
Ordovician-Early Devonian cycle. Rifting was renewed in the Ordovician along the east and south margins of the study area, as indicated by clastics in the sections. On the east it reached the "Red Sea stage" in the Early Ordovician and in the south in the Late Ordovician. Beginning with the Silurian and on into the early Visean the Emba passive margin persisted along the margin of the East European continent. Sediments were deposited in open-sea conditions on the east on the Zilair passive margin during Silurian- Early Devonian time. Then an island arc developed to the east of this margin, and only in the Middle Devonian did this island arc collide with the East European continent with overthrusting. At this time the North Caspian depression was not yet closed off. Mountains were present only on the east.
Middle Devonian-Cenozoic cycle. This cycle began with rifting. At the beginning of the Middle Devonian a broad domal uplift formed in the eastern part of the depression, in the center of which linear grabens developed filled with sediments and volcanics. The shoulders of the grabens and arch highs appear to have been the source of the detritus deposited not only in the grabens but also along the periphery of the highs. The structures of this cycle developed during the time span Late Devonian-Early Permian. Divergence took place until the middle Visean, and then conversion from late Visean on. Barrier reefs formed along the margins of the depression, and deep-water depression facies collected in the central part of the depression. Continued in next News Letter.
Taken from Shein, Payrazyan, Pazmyshlyayev, and Solov'yev, 1989;
digested in Petroleum Geology, vol. 28, no. 5/6, 1994; one plate
tectonic map, one cross section, and one oil-gas map.
Copyright 2001 James Clarke. You are encouraged to print out this
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