蒙古国含煤盆地的地质背景

蒙古国含煤盆地的地质背景

E-mail: c_minjin@yahoo.com.

1 Carboniferous coal basins

Pennsylvanian coal basins developed in western Mongolia, within Mongolian Altai Mountains. Basement of these basins is the Lake island arc terrain,which developed during the Cambrian to Early Ordovician periods. Post accretion assemblages include Silurian, Devonian and Mississippian marine sedimentary and some volcanic rocks, and Ordovician and Devonian plutons (Badarch et al, 2002). Due to the closure of Carboniferous arc system, the Mongolian Altai basins were uplifted, consequently foreland structure was formed and thick coal seams with complicated seam splitting and low sulfur contents accumulated. Such type seam is typical in rapidly subsiding foreland setting. Low sulfur content is a main characteristic reflecting a scant hydrological connection to the sea (Diessel, 1992).

2 Permian coal basins

Permian coal deposits in Mongolia are known in the South Gobi area and the Southern Khangai Mnt.South Mongolia is geologically subdivided into several terranes, including island arc terranes, and fore/back arc basins. The Devonian and Carboniferous periods represent a time of continental growth and accretion within a long lived volcanic arc systems.Amalgamation was completed by the Permian period,except for the Sulankheer terrane, located in the south easternmost part of Mongolia (Lemb and Badarch,1997, 2001). In South Gobi coal-bearing upper Permian Tavantolgoi group deposits fill several sub-basins and are folded and faulted by normal and thrust faults. In the Permian, the Paleoasian Ocean between southern Mongolia and northern China closed(Lamb et al., 2008). Because of this collision predominant structures of the South Gobi basin were formed with peat accumulation in the foreland troughs.Southern Khangai Permian basins are situated in the northern part on the east-west oriented Valley of Lakes,which was formed as foreland settings during the Carboniferous, was influenced by the collision and accommodated Permian coal measures accumulation

3 Jurassic coal basins

Mongolia’s coal deposits of Jurassic age were formed in 2 stages and developed in several different basins, such as Basin of Great Lakes in Western Mongolia, Valley of Lakes in Central Mongolia, the Orkhon-Selenge rivers area in north Mongolia, and minor coal strata in east Mongolia.

In western Mongolia, coal-bearing Lower-Middle Jurassic Jargalant Formation in Basin of Great Lakes unconformably rests on the Vendian-Lower Cambrian rocks and is represented by conglomerate, interbedded with sandstone, siltstone and mudstone, with coal at the top of the formation. In Valley of Lakes in Central Mongolia and in south-southeast, Early-Middle Jurassic coal-bearing sediments are known in the rift-graben,which covered by upper Cretaceous and Cenozoic continental sediments.

The north-south directed contractile deformation in north and northwest China and south Mongolia occurred during the Triassic and Jurassic periods. This contractile deformation is known in coal-bearing sediments of above mentioned Jurassic basins.

In north Mongolia the basement consists of Precambrian metamorphic complexes overlain by Paleozoic sedimentary rocks with marine fossils and volcano-plutonic rocks of active continental margin. In this area many small graben are filled with Middle-Upper Jurassic coal-bearing sediments. The tectonic setting of this area is unclear due to lack of geological documentation. There are several possible mechanisms such as the closure of the Mongol-Okhotsk Ocean which had north-south directed regional contractile deformation, and extension in eastern Mongolia that started in the Late Jurassic.

4 Cretaceous coal basins

After closure of the Mongol-Okhotsk Ocean, terrestrial conditions dominated Mongolia. At the end of the Jurassic, in eastern and central Mongolia, tectonic condition changed from compressional to extensional.In eastern Mongolia, rifting began prior to 155 Ma and ended after 126 Ma (Graham et al., 2001). The extension may be proposed including back arc extension due to subduction of the paleo-Pacific plate (Watson et al., 1987), and the collapse of overthickened crust(Johnson et al., 2001). According to Graham et al.(2001), the Zuunbayan Group falls into rift valleys.Within the troughs, relatively large lakes formed where thick mud and oil shales accumulated. With decreasing subsidence rates, water levels decreased and paleoenvironmental condition became more suitable for peat accumulation. The shallow lakes were filled with Khukhteeg coal-bearing and Baruunbayan sandy-conglomerate formations.

Carboniferous (Mongolian-Altai basin), Permian(South Gobi and Southern Khangai basins) and Jurassic coal-bearing basins (West Mongolian and Valley of Lakes basins) are foreland basins formed under compressional tectonic conditions. Cretaceous coals (Eastern Mongolian) and may be Middle-Upper Jurassic coals (North Mongolia) accumulated within the rift valleys created by extensional tectonics.

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Geological Settings of Coal-bearing Basins of Mongolia

Minjin CHULUUN1), Batbayar MINJIN2)
1)Mongolian University of Science and Technology;
2)Genie Energy International Co.

Mongolia is located within the Central Asian Orogenic Belt. The belt is consist of numerous tectonic blocks or terranes, which resulted from collisions during the Early Paleozoic (Caledonian orogeny), Late Paleozoic(Hercynian orogeny) and partly in Early Mesozoic (Indosinian orogeny) (Sengör et al., 1993; Heubeck, 2001;Badarch et al., 2003). These collisions had a profound effect on the coal-bearing sedimentary basins in Carboniferous, Permian, Jurassic and Cretaceous periods of Mongolia. A total of more than 200 coal occurrences and deposits are known, of which about 70 have been explored.

Coal-bearing basin; Mongolia

10.3975/cagsb.2012.s1.23