准噶尔西北缘前陆冲断带三叠纪-侏罗纪逆冲断裂活动的沉积响应

笔者从冲断活动的产物———各种成因扇体(冲积扇、水下扇、扇三角洲)出发,由160口单井剖面、15条联井剖面、8个层位平面渐次展开了准噶尔西北缘前陆冲断带三叠—侏罗纪逆冲断裂活动的沉积响应研究。三叠纪扇体在乌尔禾—夏子街地区发育叠置程度最好,T1b到T3b,其由盆内向盆缘老山方向退缩迁移明显,并具T1b到T2k1由盆缘向盆内、T2k2到T3b由盆内向盆缘迁移的2个进退波动变化。侏罗纪扇体在八道湾组最为发育,总体叠置关系较差,J1b到J2t,均呈由盆内向盆缘老山退缩沉积的退覆式迁移特征。三叠纪到侏罗纪,总体为由强到弱的退覆式冲断活动及扇体迁移模式,即随主要同生控扇断裂分布由盆缘向老山方向退缩迁移,冲断活动强度由盆缘向盆地方向逐渐减弱直至停息;相应地,各期扇体平面上分布规模渐小,总体呈由盆内向盆缘老山退缩迁移的沉积响应,两者耦合性良好。各类扇体的沉积分布受不同时期同生断裂活动的严格控制,其时空叠置及迁移规律的差异是红山嘴—车排子、克拉玛依—百口泉及乌尔禾—夏子街各构造带冲断作用地域性及作用强度差异性的沉积响应。进而引入“活动性指数”的概念与方法,对前陆冲断带同生断裂的冲断活动强度进行了定量化统计分析,并根据冲断推覆事件的地层、沉积标识划分出T1b-T3b、J1b-J2x、J2t三套构造层序,识别出三叠—侏罗纪的3个逆冲推覆幕、6次逆冲推覆事件。

Sedimentary response to Triassic-Jurassic thrust faulting in the foreland thrust belt of the northwestern Junggar Basin

For the first time,the authors have systematically studied the sedimentary response to Triassic-Jurassic thrust faulting in the foreland thrust belt of northwestern Junggar Basin by using the theory and method of tectono-sedimentology and foreland basin analysis. The study was conducted in terms of “points” (sections of 160 wells on 1∶500 to 1∶(2 000) scale and 70 core samples),“lines” (15 well-tied sections,52 seismic lines and 13 interpreted profiles) and “planes” (8 structural-sedimentary environment horizons from the T_(1)b to J_(2)t stages).This research indicates that Triassic fan bodies overall are moderately superposed,mostly in the Wuerhe-Xiazijie area ,and poorly superposed in the Chepaizi-Zhongguai area. The main types are large-scale underwater fans at the T_(1)b stage,and alluvial fans and fan deltas at the T_(2)k to T_(3)b stages. Triassic synsedimentary faulting was sectional,fluctuant,and successive. The Wuerhe-Xiazijie fault zone had the strongest thrust faulting,with wave property, and endured the longest time. The Jurassic fan bodies are poorly superposed;most developed during the J_(1)b stage. Jurassic thrust faulting was uniform,and with only small differences in movement intensity.On the whole,the Triassic-Jurassic is characterized by a model of backward thrusting movement and gradual shrink-back migration of fan bodies from stronger to weaker. That is to say,from T_1b to J_2t,with the backward thrusting movement of controlling-fan synsedimentary faults from the edge of the Junggar basin to mountainous region,the intensity of thrusting movement gradually weakened until vanished from the edge to the interior of the basin. Accordingly the fan bodies distributed in all fault belts became smaller,and showed an obvious sedimentary response of gradual retrograde migration from the edge of the basin to mountainous area. There is an excellent coupling between them. So synsedimentary faults active in different periods controlled the sedimentation and distribution of various fan bodies.The movement manner and intensity of thrust faulting determined the migration rate and growth scale of fan bodies respectively. A concept and method of “activity index” is introduced, so that the authors can quantitatively analyze the thrusting intensity of synsedimentary faults in the study area. By using the sedimentary and stratigraphic markers of thrusting, three thrust-overriding tectonic phases and six thrust-overriding tectonic events are identified. In addition, three tectonic sequences, consisting of T_(1)b-T_(3)b, J_(1)b-J_(2)x, J_(2)t are distinguished. Finally, the thrusting intensity of the synsedimentary faults are semi-quantitatively determined and analyzed.