西安凹陷 f12 地裂缝发育区第四系及裂缝沉降特征

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

在西安凹陷南缘清凉山地裂缝f12两侧钻取了两口超过400 m的全取芯井,笔者对DZ1井(420 m)和DZ2井(400.5 m)岩心作了沉积学分析,进行了古地磁、磁化率及粒度测量,分析了地裂缝沉降特征。结果表明:DZ1孔岩心包括0~93 m黄土–古土壤序列沉积L2~S11,缺失上部马兰黄土及S1,93 m以下为三门组河湖相沉积。DZ2钻孔岩心包括0~72 m黄土–古土壤S1~S9序列沉积,缺失上部黑垆土S0和马兰黄土L1,73 m以下为三门组河湖相沉积。DZ1孔岩心和DZ2孔岩心的黄土地层层序基本一致,包括中更新世离石黄土上部及早更新世离石黄土下部,未见午城黄土。河湖相三门组与午城黄土同时异相,在两口井都持续沉积至第四纪底界,并穿时至上新世顶部。两口井岩心均记录了B/M界限,J、O、R极性亚带及M/G界限,这些界线为两口井的地层对比提供了科学依据。其标志层粉砂质黄土层L9位置高差为5 m,古地磁B/M界线位置高差为4 m,J亚带位置高差为8~17 m,O亚带位置高差为31~22 m,R亚带位置高差为56~47 m,M/G界线位置高差为54 m。总体看来,DZ1孔地层界线和古地磁界线高程都较DZ2孔低,并且越靠下层差距越大。根据地层高差及古地磁界线高差判断,地裂缝f12上盘相对下降4~54 m。在DZ1孔373.6 m处发现次裂缝面,经计算裂缝面倾向南东,倾角为77°。研究成果为进一步认识西安凹陷沉积环境,地裂缝特征及对西安地区防控地裂缝地质灾害具有重要意义。

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 iden-tical. 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 depo-sitional 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.