Analysis on Sedimentary Characteristics and Allochthonous Genesis of Extra-thick Seam in Jiergalangtu Depression, Saihantala Formation

Key words: 
• extra-thick coal seam /
• allochthonous coal /
• coal seam genetic /
• secondary accumulation /
• coal depositional models /
• Jiergalangtu depression

Abstract: The Saihantala Formation of Jiergalangtu depression deposited in basement subsidence deceleration-filling evolution stage. It was the product of the depresion in the early and middle stage of the lake's shrinking-stage. Its sequence stratigraphic units are parts of resgressive system tract. From bottom to top, it consists of shallow lacustrine facies, delta facies and meandering river facies. The extra-thick coal seam formed in the deep groove of the depression and symbiosed with a lot of thin layers of turbidity sandstone, its sedimentary environment should have been shallow lake and local deep lake.Traditional coal depositional model can hardly explain the cause of this extra-thick coal seam. Through the sedimentological study of gangue and the analysis of basin-fill sequence, a new model of extra-thick seam continental allochthonous genesis-"sub model of weak storms and underwater muddy and sandy gravity flow"-is established, abbreviated as Shengli basin sub model. The allochthonous coal may have deposited firstly in lake delta, and then transferred to shallow lake by slumping and turbidity, and after secondary differentiation, finally deposited as low ash coal seam.