Thick carbonate and carbonate-salt units as well as Domanik-like bituminous clays were deposited on the Siberian craton during Late Vendian and Early Cambrian time. Three separate and distinct paleogeographic regions are recognized.
On the northeast is the Yudomo-Olenek region, where relatively deep-water Domanik-type bituminous sediments were deposited in open sea - part of the World Ocean.
Next to the southwest is the Anabar-Sin region, a broad belt of shallow-water limestones that extends across the craton from southeast to north and northwest. Archaeocyathid and algal bioherms are common, and barrier reefs appears to be present.
Then to the southeast of this biohermal belt is an enormous salt basin, the Turukhan-Irkutsk--Olekmin region, which is host to a large number of oil and gas fields.
The Upper Vendian Danilov and the Lower Cambrian Usol Formations are examined here. The upper Danilov and middle Usol are carbonates and are host to oil and gas pools. The lower and upper Usol are salt-bearing seals. The following descriptions are very generalized; in actuality facies changes are extensive.
The upper Danilov consists largely of limestone and dolomite and is in great degree phytogenetic, that is, composed on the whole of cyano-bacteria and beds of marl and shale. This indicates shallow-water marine conditions with normal or even below normal salinity. The section exhibits cyclicity. At the base of the cycles are dolomitic marl and limy shale; in the middle are phytogenic limestones; and in the upper part are fine-grained dolomite and some sulfate.
The rocks of the lower Usol are dolomite, dolomitic marl, salt, and lesser amounts of limestone. In the Anabar-Sin region are found small primitive organic buildups, fore-runners of the reef system that developed later. Then to its southwest is a zone of limestones characteristic of shallow-water marine deposition. A low-energy environment of deposition is indicated. Tectonic movements in the Turukhan-Irkutsk-Olekmin basin in early Usol time led to development of shallows, and this along with some drop in sea level resulted in salt deposition in the southwest of the Siberian craton.
The middle Usol marked the most extensive marine transgression of the Late Vendian-Early Cambrian. The Turukhan-Irkursk-Olekmin region had its greatest connection with the World Ocean. Salinity of this basin was normal or near normal for the ocean of that time and was favorable for deposition of limestone and dolomite with but little clastic material. An important feature of this stage was the appearance of organic buildups of the massive biohermal type. These were extensive areally but of low height. Most of these buildups are in the Anabar-Sin region with their steeper side toward the deep sea to the east.
The upper Uson consists largely of salt but with lesser amoiunts of dolomite, marl, and anhydrite. After the maximum transgression of the middle Usol there was a sharp regression with isolation of the East Siberian basin and an abrupt increase in salinity leading to maximum deposition of salt for the Vendian-Cambrian. The salt deposition was cyclic. This salt basin was bordered on the east and probably elsewhere by massive reef limestones.
This boundary between the Late Proterozoic Danilov and Early Cambrian Usol in important because within this interval the Vendian structures, which had been leveled off during Danilov time, were rejuvinated to become expressed again the early Usol time. This boundary also marked the appearance of organic buildups composed of living organisms. Such events indicate the importance of this boundary. (Taken from Kuznetsov and others, 1992; digested in Petroleum Geology, vol. 30, no. 1, four paleogeographic maps and one stratigraphic column) Copyright 2001 James Clarke. You are encouraged to print out this News Letter and forward it to others. Earlier News Letters are available at our web page: http://geocities.com/internetgeology/ This News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the mailing list please send your e-mail address to: jamesclarke@erols.com