by
Richard I. Gibson
Gibson Consulting
Presented at Society of Exploration Geophysicists meeting in Moscow, July 1992
The Turgay Basin occupies a basement terrane on the western side of the Kazakhstan Craton, east of the southern part of the Ural Orogen. The magnetic map suggests that this area is one of cratonic (probably Precambrian, but possibly Paleozoic) basement. In fact, it is a terrane of size, shape, and character similar to the East Caspian Craton, which lies in an analogous position on the west side of the Ural Orogen. The Turgay terrane has undergone a different tectonic history from areas to the west, however, and this difference can be inferred from interpretation of the magnetic map.
Triassic-Jurassic normal faulting affects the basement in the Turgay Basin. Differences in depth to magnetic basement are evident across linear breaks inferred to be faults. Some, but not all, of these interpreted faults exploit pre-existing lines of weakness in the basement. Most of those weak zones are lithologic contacts that probably originated during the accretionary history of the Kazakhstan Craton, and may have been rejuvenated during the late Paleozoic collision between Kazakhstan and the East European Craton that produced the Urals.
The Triassic-Jurassic extension in the Turgay Basin may be analogous to extension in the West Siberian Basin at about the same time, but the grabens in the Turgay area are not inferred to contain basalt as are those in the West Siberian Basin. Deposits in the grabens include excellent Upper Jurassic source rocks with as much as 23% organic matter (Kryukov et al., 1987). Cretaceous rocks overlying the graben deposits resemble the "sag" stage deposits in other extensional settings, such as the North Sea and southern Sudan.
Structural analogs to production within the basin at Kumkol' Field, as well as to Basin-and-Range production in Nevada, USA, can be mapped using the interpretation of the magnetic data as a guide.
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