贺兰山中段长城系黄旗口组地震岩的发现及意义

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。

Discovery and Significance of Seismites in Huangqikou Formation of Changcheng System,Middle Helan Mountain

Through detailed field geological survey, various soft-sediment deformation structures are found in Huan-gqikou 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.Sand-stone 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 sedimen-tary environment of Huangqikou Formation is identified as littoral-neritic sea by analyses of environment and observa-tion 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 gravita-tional 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 He-lan aulacogen.