The Storm Deposits in Danfeng Group in Lantian, Shaanxi

By means of the study on the blasto-sedimentary structures in metamorphosed sedimentary rocks of the upper member of Danfeng Group in Lantian area, Shaanxi Province, the auhtors found and determined some sedimentary structures of storm origion, such as hummocky cross stratification, erosion surfaces, graded bed.ect. The sectional textures of the storm deposits are classified into three types: ABC type. EBCD type. and ABCD type. The tuff at the bottom of the upper member of Danfeng Group deposited on the basic pillow lava at the top of the middle member. Upwards, come the normal marine deposits, which is mainly composed of clay, silty clay, and calcilutite. This phenomenon means that the volcanic activity decreased obviously. The lack of the cross bedding, ripples, which reflect stronger current acting, indicates that the deposition took place below the normal wave base. In the middle part of the upper member of Danfeng Group, there is obvious rhythmic sedimentation which shows the character of flysch. At the top of the upper member, there are thin bedded quartz sandstones, conglomeratic sandstones and dolomites(marbles), additionally, cross bedding, erosion surface can also be seen, therefore, the flow energy is higher and the corresponding sedimentation took place approximately over the normal wave base. Small scale erosion troughes, quasi-flate-gentle wavelike erosion surfaces, and the pebbles with the conponent different from that of the underlying rocks; indicate that it belongs to the distal source type; according to the characters of the storm-formed turbidity current. As a whole, the sedimentary formation of the upper member of Danfeng Group is a shoaling-upwards sequence, which formed approximately at the upper part of the slop to the shelf facies. According to the previous researchers' studies on the geotectonic environment of Danfeng Group, the most of geochemical components of the basic volcanic rocks of Danfeng Group drop into the area of island arc basalt. To the north of Danfeng Group, located Xieyuguan Group of Lower Palaeozoic; while Qinling Group(Pti qn) located between them two. Danfeng Group, the authors propose, to be formed in a Palaeoz forearc basin; and Xieyuguan Group in the backarc basin, while the arc between these two basins was posed of Qinling Group which undergone Proterozoic structural deformation.