Study on the recognition marks of seismites in Upper Triassic Yanchang Formation,Ordos Basin,China

Seismites are identified mainly by soft-sediment deformation structures.Many earthquake-related seismites have been discovered in the Chang 6-Chang 8 of the Upper Triassic Yanchang Formation in the south of Ordos Basin.Having surveyed the cores of wells,many typical deformation structures are discovered,such as syn-sedimentary micro factures,stone veins,vibrational liquefaction,deformation structure,earthquake breccia and load-casts and balls-and pillows-types,etc.They develop in const-rained horizons bounded by undeformed strata,suggesting that they are syndepositional.The complete vertical sequence of seismite of Well Ta 17,includes a shattered rock bed,ladder-shaped faulted bed,vibrational liquefaction deformation structure bed,liquefied sandstone vein bed, rubble bed and homogenized bed.The discovery of seismite can be used to interpret the tectonic evolution of the basin.In Late Triassic,with the closing of Qinling and the sea slot of the southern Qilian,strong activities occurred in the south-north thrust belt.They are the direct inducing factors of the seismo-deposition in the Yanchang Formation and provide the evidence for the turbidite of Chang 6-Chang 8 of the Yanchang Formation in the southwest of Ordos Basin,that is,the outside trigger mechanism of the turbidite is earthquake activity.