DISCOVERY OF PERMIAN VOLCANICLASTIC ROCK IN SHANDAN COALFIELD, GANSU AND ITS MASTER CHARACTER

Shandan County is situated in the passageway of Hexi, Gansu Province, at the foot of the Qilian Mountains. In this region outcrops of Permian System are very common with a distinct appearance and their geological structure is uncomplicated. Volcanic-lastic rock is discovered in the lower series of Permian System, namely the Dahuang-gou Group, in Chaihuoguo of Shandan and it is described as follows: It is brick red, rough and interlayered in the continental sedimentary rocks of the lower series of Permian System with a thickness of 6-10 meters and its horizontal bedding is indistinct. Beneath the layer is a volcanic breccia which has 1.2 meters in thickness and consists of lava breccia and crystal fragments. It has a volcanic breccis texture. Tuff is mainly composed of vitric fragments accounting for more than 50 percent. Diameter of the fragment is 0.25-0.50 mm and shapes are mainly bow-curved, angulat, archedand triangular porous. The rock can be named as vitric tuff according to the clasificatory principles of volcaniclastic rock summed up by F. J. Pettijohn. and determined to be the attribute of limealkali series on the basis of analytical data. The discovery of volcaniclastic rock in Chaihuoguo of Shandan is of significance in three aspects. (1) The volcaniclastic rock is characterized: by. K2O/Na2O 1 , Fe2O3FeO, (FeO + Fe2O3)MgO a little higher and characteristic of grain-size and discoverd in continental clastic rocks of lower series of Permian System. All of these have shown with the surroundings of shallow water environment. It is a sediment of fall ash. (2) The layer of pyroclastic rock which has a certain thickness and a special colour can be distinctly regarded as a marker bed and used in the respect of areal stratigraphical contrast. (3) The first discovery of volcaniclastic rock has been confirmed the fact that there was a volcanic activity present in the area of Shandan and its neighbourhood in the Early Permian, and this will provide a series of the most useful geological information for the researches of areal geology.