Burnt Rocks Of Shenshangou In Dong Sheng Area

Coal self-ignition and the related burnt rocks formed by spontaneous combustion are common geological phenomena in the world.Middle Jurassic is one of the main coal-producing sequences in the Ordos Basin,northern China.The self-ignition of coal seams of middle Jurassic Yan'an and Zhiluo Formations was generally seen at the Shenshangou Valley,Dongsheng in the northeast Ordos Basin during the geological history,which resulted in the extensive distribution of burnt rocks.These coal spontaneous combustion events have been hardly studied and the paleoclimatic messages implied from these burnt rocks are still not clear.Four major sections,Shengshangou C,H,I and J were measured and sampled in detail.Based on the previous studies,the burnt rocks were divided into four categories in the northeastern Ordos Basin.Due to its outcrop and microscopic characteristics,only three types of burnt rocks,i.e.,melt rocks,grill rocks and baked rocks are distinguished in the Shenshangou Section.White and red sandstones,siltstones and mudstones are contained in the baked rocks.The microscopic analysis shows that the grain size of white baked sandstones is medium-fine,and amount of feldspar grains were kaolinized.It is indicated that the intergranular metasomatism occurring between the brown irony minerals and grains in the brown–red baked sandstones in which black organic matter is hardly found.The scanning electron microscopy(SEM)shows that kaolinite,illite and other clay minerals can be seen in the grill rocks and baked rocks.Straight tubular holes are seen among the clay minerals,which are interpreted as the vapour channels produced in the combustion process of burnt rocks.Geochemical tests of the burnt rocks from Shenshangou suggest an indium anomaly,which implies the enrichment of disperse elements during the calcined process.Besides,the rare earth element(REE)distribution of burnt rocks shows the enrichment of light rare earth element(LEE)and depletion of heavy rare earth element(HEE),and the total amount of REE from grill and baked rocks increases gradually with the decrease of burning degree,indicating the element migration during the combustion process of coal seams.For other trace elements in the Shenshangou H Section,the concentration of Co in sandstones is relatively high,the change of Ni,Cu,Pb is relatively distinct,Sr is relatively deficient,and Zr and Hf are relatively enriched,indicating a certain reduction environment.The concentration of Mo,As,Fe and Th shows that the sandstones experienced the process of various degrees of high temperature,which led to the weak migration of some elements.The formation of burnt rocks in the Ordos Basin experienced multiple periods.In the present study,however,the spontaneous combustion of coal seams at Shenshangou is deduced to occur in the Quaternary according to the estimated weathering rate of overlying stratum.Paleoclimate and the exposure degree might affect the burning process of coal seams at Shenshangou during the geological history.The development of burnt rocks and overlying layers show that the Quaternary paleoclimate change from drought to humid,leading to the rise and fall of water table,contemporaneous to and then after the coal self-ignition event.Therefore,the change pattern of water table caused by coal combustion and paleoclimate is put forward.