鄂尔多斯盆地西缘桌子山地区奥陶系深水条纹条带状泥岩等深流成因分析*

沉积形成的条纹和条带状构造多包含化学沉积和机械沉积2个沉积过程,在探讨其沉积机制时往往受到制约。而桌子山地区中、上奥陶统克里摩里组和乌拉力克组发育良好的条纹条带状泥岩,为单一的机械沉积作用所致,是研究条纹条带构造形成过程中沉积流体作用机制的理想层位。本次研究以详细的野外观察为基础,结合室内岩石薄片鉴定和近年来关于细粒沉积水槽实验的研究成果,探讨深水等深流在形成条纹条带构造中的作用。克里摩里组上段多为条纹状泥岩组成,呈极细的连续或断续透镜状,垂向上表现为细—粗—细序列;乌拉力克组多为条带状泥岩,与砾屑石灰岩伴生,垂向上发育窄—宽—窄序列。其成因则与等深流引起的黏土絮凝波和底载荷运动相关。在弱等深流作用期,絮凝波发育并存在长的尾迹,在等深流改造底载荷进行再沉积的同时,黏土絮凝波发生垂直降落沉积,形成条纹状泥岩;在强等深流作用期,絮凝波不发育,底载荷连续加积形成条带状泥岩。克里摩里组条纹状泥岩与乌拉力克组条带状泥岩的差别可能与水深相关,前者水体较深,后者水体较浅。研究结果说明鄂尔多斯盆地西缘中、晚奥陶世的最大海侵发生在克里摩里组上段,这对研究该地区的构造性海侵和大地构造环境具有重要意义。

Origin of deep-water stripped-and-banded mudstones related to contour currents in the Ordovician,Zhuozishan area,western margin of Ordos Basin

The research for the mechanism of stripped-and-banded sedimentary structures is usually restricted by its compound processes which include chemical precipitation and physical deposition. The stripped-and-banded mudstones in the Kelimoli and Wulalike Formations in the Zhuozishan area,which were formed by physical deposition,provide a good view of investigation for the sedimentary mechanism of currents. Based on detail observation of field and thin sections,the role of contour currents in the deposition of deep-water stripped-and-banded mudstones is deduced from recent advances in flume experiments of fine-grained sediments. The upper part of the Kelimoli Formation constitutes of stipped mudstones with continuous or discontinuous extremely-thin lenses and fine-coarse-fine vertical succession in grain size,while the Wulalike Formation constitutes of banded mudstones associated with calcirudites and has a narrow-wide-narrow succession vertically. The mechanism of stripped￣and￣banded mudstones is related to the accretion of clay flocculated ripples and the movements of bedloads which both induced by contour currents. The stripped sedimentary structures were formed by the isochronous deposition of bedloads and accretion of flocculated ripples(with long trails)in weak contour currents period,when the contour currents not only eroded,transited and redeposited,but also produce large clay flocculated ripples with long trails. And the banded sedimentary structures were formed by the continued aggradation of bedloads in strong contour currents period when the flocculated ripples were small. The differences between stripped mudstones in the Kelimoli Formation and banded mudstones in the Wulalike Formation perhaps are made by the depth of sea water;the stripped structures deposited in relatively deeper water than banded structures. The results show the greatest transgression of the Middle to Upper Ordovician is occurred in the upper part of Kelimoli Formation in the western Ordos Basin,which has an important significance for the tectonic transgression and environment in the study area.