Study on Syngenetic Clast in Shuangqiaoshan Group,Northeast of Jiangxi

Key words: 
• syngenetic clast /
• Shuangqiaoshan Group /
• study /
• Northeast of jiangxi

Abstract: Syngenetic clast in sedimentary rock is a fragment which develops in the sedimentary pr0ocess where weakly consolidated argillaceous or silty sediments formed just a little earlier were re-eroded to fragments. It is a cepositional structural phenomenon representing a turbulent sedimentary environment. It is first time that some syngenetic clasts are discovered in the Dabeiwu area, Northeast Jiangxi,where eprmetamorphic classic rocks of Shuangqiaoshan Group of Middle Proterozoic erathem outcropped. The syngenetic clasts,mainly occur in the argillite,consist about 2-10 % of the sedimentary rocks. They have a well-rounded shape of circular or ellipsoid,and being as sand-size particle fragment with a diameter of 0.5-2.0mm. The inner fragments of the syngenetic clast are fine-silty feldspar and quartz,their size are a little more coarse than that of the sedimentary rocks around the syngenetic class. There is a closed conta:a relation and a well-defined boundary between syngenetic clast and the sedimentary rocks. Clearly,the discovery of the syngenetic clast provides a strong support to the view that the Shuangqiaoshan Group is a turbidite.Because of the closed contact relation and a little difference between syngenetic class and the sedimentary rocits, the syngenetic clast is a good strain-measuring marker. The defor:natione of syngenerir rlast in the area ttxe classified to three types.(laser deformation with Flinn coefficient K<0.5,shear deformation with 0.52.0. Each of them diatrihutes regularhr at certain place. Based on the deformation feature of the syngenetic chat,the authors believe that the Shuangqiaoahtan Group in this area has under-gone two phases Df deformation, (laser deformation and ductile shear deformation. The former is the regional fold deformation with a flaser rate among strata of 23.1%-45.3% calculated from the strain axial rate Kxz(1.58-2.51)of the syngenetic clast. According to distribution of ductilt shear deformed syngenetic clast and other shear deformaripn marker, Dabeiwu ductile shear zone developed in epi-metasnorphic elastic rncira is confirmed with,shear strain of 1.01-2.13 and the largest shear transposition of 0.6-0.8 km. As particles different from the sedimentary rocks with a little higher ponasity attd pertneability caused by their coarser inner-fragments,the syngenetic clasts play an important role in the dynamic differential materials in the course of rock deformation and become to he growth points of the ayntectonic siderite porphyroblasts.