Although more than 40 oil and gas fields have been discovered in the East Caucasus, and production has extended over more than a century, many questions remain as to the formation of the overthrusts. Since the oil-gas potential of the upper part of the sedimentary cover is practically exhausted, attention is turning to lower parts of the section.
The Greater Caucasus fold belt formed as a result of closure of Tethys and movement of the Arabian plate to the north-northeast. This led to underthrusting of the Trans-Caucasus micro-plate and volcanic arcs beneath the Scythian platform, the upthrust south margin of which is the Greater Caucasus uplift. The eastern part of the Caucasus experienced the greatest compression; it is located directly on the path of the Arabian "wedge". The Greater Caucasus uplift is widest here, and crustal thickness is great at 60 km. Marginal to it are two very deep basins, the Tersko-Caspian and Kura, where sedimentary thickness is more than 12 km.
The East Caucasus overthrust belt is located in the region of junction of the Greater Caucasus uplift and the Tersko-Caspian basin. It is about 500 km long and up to 100 km wide. At the surface are Jurassic-Cenozoic sediments, which dip regionally to the north.
One of the main characteristics of this region is the extensive occurrence of overthrusts of south vergence in the surface beds and a predominance of opposite, north vergence at depth. The seismic and drilling data show that this apparent paradoxical coexistance of overthrusts of opposite dips at different stratigraphic levels is the result of thrusting of wedges of allochthonous complexes into the sedimentary pile. These wedges consist largely of competent Jurassic-Eocene sedimentary rocks. They split the sedimentary cover of the Tersko-Caspian basin along plastic clays of the Maykop Series (Oligocene-upper Miocene), bounded beneath and above by faults of opposite dip. These faults merge with one another at the front of the wedge. The main evidence for this model is from seismic surveys, which show graphically the overthrusts of opposing dips.
The East Caucasus overthrust belt consists of two structural provinces, the Tersko-Sunzhen fold zone on the west and the Dagestan wedge on the east. Their sedimentary sections are substantially different. In the Dagestan wedge is a thick unit of Lower-Middle Jurassic shale, which pinches out to the north and west. The Tersko- Sunzhen zone contains Tithonian salt, which is absent in Dagestan.
The main result of this study is recognition that thrusting was along plastic horizons. In the Tersko-Caspian zone the main level of gliding was at the Upper Jurassic salt, whereas in the Dagestan wedge it was on Lower-Middle Jurassic shale. The structural discordance between the Neogene and the Cretaceous-Eocene complexes developed from wedging of rocks into Maykop clays.
This interpretation of the subsurface of the East Caucasus overthrust belt is a basis for expecting discovery of new oil and gas fields. In the western part of the Tersko-Sunzhen zone there is particular interest in the unexplored Paleozoic-Upper Jurassic rocks in the region of the Chernogor monocline, where folds are present. In Dagestan the outer periphery of the overthrust belt is favorable but has not been explored. Regionally oil-gas-bearing Upper Jurassic-Eocene rocks are present there.
Taken from Sobornov, 1995; digested in Petroleum Geology, vol 30,
no. 3, 1996, two maps, three cross sections, one strat column.
Copyright 2001 James Clarke. You are encouraged to print out this
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