Sixteen stratigraphic levels are recognized in the Upper Precambrian and Lower Paleozoic sections of the Sibrian craton that are in some measure enriched in organic matter. These carbonaceous units are subdivided according to their content of organic carbon into Domanik-type (more than 0.5 percent organic carbon) and sub-Domanik-type (0.1-0.5 percent).
The area in which these units were deposited was not constant in time. For example, areas in which both types were deposited increased from Early to Middle Riphean and then decreased in Late Riphean time. During the Vendian the area of deposition of carbonaceous units again increased and reached a maximum in Early Cambrian time. They then covered almost all of the Siberian craton. Subsequently, however, their area of distribution was reduced significantly. In the Late Cambrian they were deposited in the northern part of the craton, and in the Ordovician and Silurian only in the western part.
Thickness of the carbonaceous units ranges from several meters to hundreds of meters. As a result the content of organic carbon is inversely proportional to thickness. There are exceptions, however. For example, the Nizhnetunguska Formation and its analog the Shuntar Formation of the Middle Riphean of the western margin of the Siberian craton have thickness of 800-1000 m and an average concentration of organic carbon of 0.6-0.8 percent.
Epochs of deposition of Domanik-type and sub-Domanik-type sediments were largely transgressive stages and rarely regressive stages, whereas both inundational (maximum transgression) and emergent (maximum regression) stages were unfavorable for deposition of organic matter in the sediments.
It is notable that deposition of almost all units that are enriched in organic matter, and all without exception of the Domanik-type horizons, followed in time the deposition of redbeds or variegated units regardless of the stage of the cycle. An example is the Ust'-Il'in Formation of the northwest flank of the Anabar massif, where combustible shales (5-20 percent organic carbon) rest on redbeds of the Mukum Formation.
Units are found from the Riphean to the Silurian that are enriched in "secondary carbon". These are oil-bearing or saturated with viscous or solid bitumens. These horizons have no special sedimentational association except that they are generally in a transgressive part of a cycle. Most are sandstones, rarely carbonates. Content of organic carbon may reach several percent.
The organic matter of the Domanik-type and sub-Domanik-type of sediments is sapropelic. The source was phyto-plankton and to a lesser degree blue-green algae. These oil-source sediments generated an enormous quantity of liquid and gas hydrocarbons, part of which accumulated in traps. Taken from Bazhenova and others; digested in Petroleum Geology, Vol. 18, No. 12, one stratigraphic chart.
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