During Early and Middle Cambrian time an epicontinental basin was present on the Siberian craton. It was divided into two parts. On the east and northeast was open sea of normal salinity, an area of accumulation of the organic-rich Kuonam deposits. To the west and southwest of this region was an extensive Archaeocyathid algal barrier, which separated the open sea from an enormous salt basin in the southwest of the craton. The area of accumulation of these Kuonam sediments extends farther to the east but is now covered by Late Cretaceous (Laramide) westward overthrusting of the Verkhoyansk Range.
The belt in which the Kuonam complex is present extends some 2500 km from the Igarka region on the northwest to the Aldan Shield on the southeast of the Siberian craton. Width of this belt ranges from 150 to 600 km, and its area is 700,000 sq km. Thickness is quite constant at 40 to 70 m. The complex is composed of clayey-carbonate sediments enriched in organic matter and authigenic quartz. The rock is generally dark brown or black and is shaly, although light-colored material is also common. The organic matter is present in rock-forming quantities - up to tens percent in the Olenek region, where combustible shales are up to several meters thick. In other regions the content of organic carbon does not exceed 1-3 percent.
Three groups of trace elements are recognized: 1) those that are deposited with the organic matter (Mo, Cd, V, Bi, and others); 2) those indifferent to the organic matter (Sc, Sn, Be, Zr, and others); those antagonistic to the organic matter (Nb, Sr, Ba, B, Y, and others).
Three paleogeographic zones are recognized. The first is on the eastern margin of the Siberian craton and is characterized by a pre- dominance of siliceous, carbonate, clayey sediments. The second is to its west adjacent to the barrier reef, and carbonate rocks form a large part of the section. The third zone is along the southeastern margin of the Siberian craton, and along with siliceous, carbonate clayey sediments are clastics indicating proximity of a source area.
In the first zone the rock appears to be from planktonic and nektonic skeletal remains. Clay particles were brought in by eolian processes or marine currents. The light-colored carbonates may have resulted from gravity sliding from the barrier reef on the west. Thin, uninterrupted beds and great persistence of individaul horizons suggest deposition in a calm, low-energy environment.
The second zone is in direct proximity to the barrier reef and is not the same everywhere. In the north the transition is gradual: Black argillites and chert are replaced by clayey and bioclastic limestone, which in turn passes into organic carbonates. In the southwest the transition is sharper, and in places bituminous Kuonam sediments are in direct contact with bioherms of the barrier reef. These rocks have a rhythmic character, possibly reflecting eustatic changes in sea level.
The third zone in the southeast is like the first except that beds of sandstone and siltstone are present. The source area may have been a group of islands.
The high content of organic matter in the Kuonam appears to have resulted from its position between the open sea and the salt basin. There was a continuous flow of marine water toward the evaporite basin, and the shallow-water areas on the seaward side of the barrier reef would have been very favorable for abundant life. The Kuonam has a high source-rock potential for oil.
Taken from Nakhturov, Yevtushenko, and Pereladov (1988);
digested in Petroleum Geology, vol. 24, no, 3/4, four cross
sections, one map, and one stratigraphic column.
Copyright 2001 James Clarke. You are encouraged to print out
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