Drilling Sedimentation and Subsidence of Ground Fissure (f12) in Xi’an, Shaanxi

Two fully coring wells over 400 m for each have been drilled respectively on both sides of the Qingliangshan ground fissure (f12) situated at the southern margin of the Xi’an depression. Sedimentological analysis and paleomagnetic dating, magnetic susceptibility and grain size measurements on DZ1 (420 m) and DZ2 (400.5 m) are made. The ground fissure subsidence characteristics are further analyzed. The results show that the DZ1 exposes loess–paleosol sequences of 93m, corresponding to L2～S11, without the upper Malan loess and S1. There are Sanmen Formation of river–lake phase below 93 m. DZ2 exposes 72m loess–paleosol sequences of S1～S9, missing the upper black loessial soil S0 and Malan loess L1. There is Sanmen Formation of river–lake phase below 73 m. The sequences of loess stratigraphy in both of DZ1 and DZ2 are basically identical. They both include the upper part and lower part of the Lishi loess, corresponding to Middle and Early Pleistocene, respectively. And the Wucheng loess has not been exposed. Sanmen Formation deposited until the bottom of Quaternary and upon the upper Pliocene. Both two cores successively recorded B/M boundaries, J, O and R polarity sub-bands and M/G boundaries, which provide a reliable evidence for stratigraphic comparison between the two cores. The elevation differences of several key layers is 5 m for the L9 layer, 4 m for the B/M boundary, 8～17 m for the J subband, 31～22 m for the O subband, 56～47 m for the R subband, and 54 m for the M/G boundary. In all, the elevation of the stratigraphic boundary and paleomagnetic boundary in DZ1 are generally lower than those in DZ2. The deeper the horizon is located, the higher the differences are. Based on the relative elevation difference of the stratigraphic and paleomagnetic boundaries between two cores, it is supposed that the upper plate of fault f12 is relatively lower by about 4～54 m. The sub-fracture plane was exposed at 373.6 m, with SE dip direction and 77˚ of dip angle. This study is important for further understanding the depositional environment of Xi’an depression, the characteristics of ground fissures and the prevention and control of the geological hazard related to ground fissures s in Xi’an area.