【摘要】 通过野外观察,发现贺兰山中段长城系黄旗口组中发育大量的软沉积变形构造,主要包括液化砂岩墙(脉)、液化水压破碎构造和液化角砾岩、负荷构造、砂岩球和砂岩枕构造、砂岩串珠状构造、包卷层理(液化卷曲变形)、阶梯状微断层等。室内显微镜下观察发现,石英砂岩成分成熟度相对较高,但结构成熟度较低,颗粒多呈次圆状—圆状,粒径大小不等,碎屑颗粒间有泥质混入,泥质多呈不规则脉状穿插在砂岩中,显微镜下也见微逆断层切过纹层。沉积环境分析结果显示,黄旗口组发育脊状波痕、小型交错层理、水平层理、泥裂等沉积构造,指示沉积时水体较浅,沉积环境为滨—浅海。认为黄旗口组中的软沉积变形构造不是由上覆重荷引起,可能为古地震成因。在黄旗口组中第一段下部识别出4个地震事件层,推测黄旗口组沉积时早期地壳运动相对活跃,晚期渐趋稳定。地震岩的出现可能与贺兰裂陷槽拉张—裂陷的构造背景有关。更多还原

【Abstract】 Through detailed field geological survey,various soft-sediment deformation structures are found in Huangqikou Formation in Changcheng System of the middle part of Helan Mountain,including liquefied sand dykes( veins),liquefied hydraulic fracture structures and liquefied breccias,load structures,ball-and-pillow structures,beaded structures,convolution lamination( liquefied curled deformation structures) and step microfaults. Relatively higher compositional maturity and lower texture maturity of quartz sandstones are observed under microscope. Sandstone grains are mostly subround-round,with various sizes. Muddy sediments mix with clastic particles,or intersperse in sandstones in irregular vein-like form. Micro thrust faults are found cutting laminae under microscope. The sedimentary environment of Huangqikou Formation is identified as littoral-neritic sea by analyses of environment and observation of sedimentary structures( such as ridged ripples,small cross-bedding,horizontal bedding and mud cracks).Soft-sediment deformation structures in Huangqikou Formation are most likely formed by seismic events,not gravitational load. Four seismic event layers are identified in middle-low part of the first member of Huangqikou Formation.We speculate that at early stage of deposition of Huangqikou Formation,crustal movement was relatively active,then gradually tended to a stable state,appearance of seismites may be related to tectonic setting of extension-rifting of Helan aulacogen.更多还原