Conditions for the formation and distribution of black shale deposits in the carboniferous period

the article presents an analysis of the geological conditions for the accumulation and distribution of black shale deposits in the Carboniferous period. Each epoch is characterized in geological-structural, tectonic and paleogeographical aspects, the sedimentation regimes on the platforms and in geosynclinal areas are considered. The geological and geochemical characteristics of some of the most common black shale deposits of this period are given. The most favorable epochs of formation of black shales in the Carboniferous period are named.


No 163
Compared with the Upper Devonian, the areas and volumes of the red-colored, marine clastic and submarine-volcanogenic formations have sharply decreased. The intensity of terrestrial effusive volcanism decreased, however, there was an increase in intrusive magmatic activity. Significantly increased carbonate sedimentation. If in the Upper Devonian the accumulation of limestones and dolomites was 30 and 24%, then in the Lower Carboniferous it was 50 and 41%, respectively. In the Middle and Upper Carboniferous, the accumulation of limestones and dolomites again decreased to 32 and 28%, respectively [3]. Changes in the nature of lithogenesis were due to climate, Hercynian uplifts, which reached their full development in the Sudetenian and Asturian phases of tectogenesis. According to [4], the zones of maximum subsidence in the Lower Carboniferous were the residual troughs of the Caledonian (Central Kazakhstan, Northern Appalachians -more than 8-8,5 km) and Hercynian (North American Cordillera -up to 5 km, Tasmanian -up to 4 km) geosynclines. In Eurasia, significant subsidences occurred in the North-Eastern Balkhash region (~ 8.8 km), Donetsk, Karaganda, Silesian basins (~ 3 km), where coal-bearing deposits accumulated. At the same time, the carbonate formation of the Hercynian geosynclines of Eurasia has a thickness of no more than 1,5-2 km (Urals, Taimyr, South Tien Shan). In Africa, in the Atlas fold zone, the thickness of the Lower Carboniferous (marine clastic and carbonate deposits) does not exceed several hundred meters.
In part of the Siberian platform experienced the maximum subsidence. The East European platform sagged less: Kuibyshev Trans-Volga region -1,6 km, the North American (Williston basin -1,5 km), the Australian (North-Western basin -1,2 km), the African (Cape Province -1,2 km) and South American (Parana basin -0,7 km). The maximum thickness of the deposits of the Pennsylvanian system was noted in the mountain systems of North America (Northern Appalachians, Cordilleras, etc.) -from 3,7 to 7,5 km, in South America (Central Andes) -up to 2,5 km [2]. During the Carboniferous period, a warm and humid maritime climate prevailed, as evidenced by the predominance of carbonate rocks and the widespread occurrence of coals and bauxites.
Black shale deposits. Sedimentation of the black shale type of the late Devonian -Early Carboniferous (Tournaisian age) was very significant in terms of volume and distribution area. In addition to the Tournaisian age, black shale strata with organic matter of sapropel and mixed humus-sapropel types were formed in the Early and Middle Visean epochs mainly in large areas of subsidence, for example, the Caspian depression, foredeep Urals, Pre-Dobrudzha trough, paleorifts -Dnieper-Donetsk, Pripyat, in basins Midcontinent (Arcom, Illinois, etc.), in the Rhine shale mountains.
In the EEP black shale deposits of the Lower Carboniferous are widely developed on its eastern margin, which is characterized by subsidence throughout the entire Carboniferous period. Here, the main structural element was the Kama-Kinel trough system, where uncompensated accumulation of relatively deep-sea Domanic-type took place.
In the Tournaisian, carbonate and carbonate-argillaceous sediments enriched in organic matter up to 5-10% Corg formed under normal marine conditions in a sharply reducing geochemical environment (sulfide facies). The sapropelic nature of OM is evidenced by an increased value of the hydrocarbon coefficient (2,9-5,0%). According to the hydrocarbon characteristics, Tournaisian black shale are diagnosed as favorable and good source rocks.
In the Visean, black shale, in the form of clays, clayey marls, and calcareous silts, accumulated in a shallow marine Clay rocks and mudstones are characterized by a fairly high amount of C org (1,6-6%, less often 9,3-11%), but a significant admixture of humus material in the original OM is reflected in a low hydrocarbon coefficient (0,30-0,99%). Humus-sapropel type OM is enriched only in clayey limestones and marls. Only clays and mudstones of the Tula horizon have an increased hydrocarbon coefficient (up to 1,56%).
In general, the Visean black shale in the Kama-Kinel system of troughs are characterized as gas source and as favorable oil source rocks [5].
Lower Carboniferous black shales of siliceousargillaceous composition were accumulated at Pai-Khoya and in the Lemva zone of the Polar Urals, on the island of Novaya Zemlya. The Corg content in them ranges from 3,45 to 8,15% [6].
I the Late Carboniferous, a gradual rise in EEP began and the uncompensated mode of sedimentation practically did not occur. In the Middle and Upper Carboniferous, the accumulation of oil shale (OS) and coals predominated. OS of marine or lagoonal origin occur among coal-bearing suites, interbedded with terrigenous rock layers. An example is the shales of the Kenderlyk deposit in Kazakhstan (42,4 km from the city of Zaisan) of the Upper Carboniferous or transitional C 3 -P 1 age.
The Kenderlyk shale deposit is a flat basin with a dissected relief and a strongly eroded surface. The basin is surrounded by mountain ranges on both sides. The section of the Titovskaya mine is composed of coal shale with sand interbeds and tuffaceous sandstone with lenses of shell limestones and OS. In OS, fossils of fish (genus Acrolepis), pelecypods, ostracods, and plant sediments were recorded. Inside the sequence, a layer of black bituminous shale with a calcareous interlayer, 1 m thick, was noted. Individual sheets of shale are flexible bituminous and burn when ignited with a match. The length of the OS is 21,2 km, and the reserves are ~ 8,15 million tons. The Kenderlyk OS are rich in volatile substances and can be used to produce liquid fuels and lubricating oils [7].
On the North American continent, black shale rocks of  around a mineralized "pipe" (shown by arrows) according to [8] In the south of the Midcontinent, in the northeastern and central parts of New Mexico, the oil and gas potential of the system of basins (Tucumcari, Estancia, Vaughn, Carrizozo) under the general name "Elevator" is associated with the Pennsylvanian black shales. These basins began to form in the early Pennsylvanian epokh, and their tectonic development  continued until the early Permian. They were formed along the edges of the Late Paleozoic Sierra Grande uplift, which is complicated by faults. The width of the Elevator system basins varies from 3,5 to 18 km. The Pennsylvania formations contain oil source shale containing up to 9% C org ; shale kerogentypes II and III. The graben-like type of these basins, the great depth of burial of oil source rocks contributed to their thermal maturation [9]. According to [10], the largest Pennsylvanian oil reserves are in the Permian and Anadarko basins. In these basins, source rocks include the Atoka shale with an average content of 8,6% C org and Canyon in Missouri with a content of 5,3-16% C org . In general, the Pennsylvanian source shale has become an important source of hydrocarbons.

Conclusions.
The most favorable epoch for the accumulation of black shales in the Carboniferous period was the Early Carboniferous (Early Tournaisian and Early-Middle Visean). The wide distribution of depression facies in the Middle and Late Carboniferous was not facilitated by abrupt changes in the nature of lithogenesis, due to major structural changes in connection with the Hercynian orogeny and its main tectonic phases (Sudetian, Asturian, Palatinate), as well climate. The latter, to a greater extent, was favorable for the formation of oil shale and coal-bearing deposits.