Four Precambrian reef deposits are described here: two in East Siberia, one in the South Urals, and one in Timan-Pechora province.
The Late Precambrian Chenchin reef body is traced more than 600 km in a large downwarp between the Aldan Shield on the southeast and the Angara platform on the northwest. The lower part of the Chenchin Formation consists largely of crystalline limestone, and the upper part is mostly stromatolitic limestone. Thickness ranges from 360 to 890 m. This reef deposit collected on the boundary between shallow-water and relatively deep-water zones, occupying a position characteristic of a barrier reef. No well expressed debris train is present, however.
The Uluntuy reef formation is in the upper carbonate portion of the Uluntuy Formation of the Baykal Complex of Middle-Late Riphean age. This unit extends northeastward immediately northwest of Lake Baykal. The reef rock is largely stromatolitic and granular carbonate 350-480 m thick. This reef deposit probably formed on a narrow (20-30 km) carbonate plateau, which sloped to the east toward an open marine basin. Biomorphic carbonates were deposited as lenses in a shallow-water zone with an active hydrodynamic regime. The stromatolites contributed to marked paleo-relief, entrapping finer material. Clastic material filled low places between carbonate lenses and extended thence into the deep-water zone.
A thick clastic-carbonate unit of Late Precambrian age is present in the South Urals within the Bashkir mega-anticlinorium. These Riphean and Vendian rocks extend as a broad belt some 400 km. Carbonate stromatolite deposits are present at various stratigraphic intervals in the Riphean section, the thickest being in the Min'yar and Uk Formations. These units are largely stromatolitic, granular, and crystalline limestones and dolomites. Clastics are subordinate. Thickness ranges from 500 to 1300 m. The Min'yar-Uk reef deposits collected in a miogeosynclinal zone along the eastern margin the the East European craton. Min'yar time ended with formation of a carbonate plateau with steep slope toward the open sea on the east. During Uk time biohermal complexes were deposited in a single linear zone, which graded eastward into deep-water limestones.
The Nizven-Bystrin reef formation of Late Riphean age extends as a single carbonate unit along the western margin of Timan-Pechora province. Total thickness is 1000-1500 m. It is traced along a series of outcrops of Precambrian rocks. The lower part of these reef rocks consists of dolomites and limestones; biohermal stromatolites are rare. The middle is largely stromatolitic dolomites, and the upper is crystralline and granular dolomite. These rocks are represented on the east by deep-water clastics and on the west by carbonates and clastics.
The composition, structure, and spatial distribution of the Late Precambrian platform-margin reef formations suggest that they can be targets for oil and gas exploration. Associated with these reef rocks are source beds that have high contents of organic matter. Some of these carbonates may have good reservoir properties if they have not experiences much uplift and have remained buried beneath younger sediments.
Taken from Khabarov, 1985; digested in Petroleum Geology, vol. 24, no. 1/2,
p. 48-64, Nineteen illutrations including maps, cross sections, and strat
columns.
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