Soft-Sediment Deformation Structures Related to Slumping in Lower Cretaceous Turbidite in Lingshan Island, Shandong Province
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摘要: 灵山岛上出露的软沉积物变形构造其成因具有多样性,而灯塔剖面底部滑塌层内的变形构造的触发因素尚不明确.结合野外观察和极射赤平投影方法,研究了滑塌层和内部变形构造的形成过程、触发机制和地质意义,结果表明:灯塔剖面主要由浊积岩沉积序列组成.滑塌层夹在未变形层之间,由地震触发形成,内部发育4个滑脱面,将滑塌层划分为5个变形单元,缩短率和变形程度各不相同.软沉积物变形构造主要为褶皱,形态特征表现为砂岩厚度在枢纽部位大大增加,也可见一些砂岩增厚和减薄现象,两者均是液化的砂岩在驱动力作用下的结果.滑塌过程中,滑塌层中的砂泥岩与海底沉积物之间的孔隙流体自由交换被切断,产生了暂时性的超压,导致了液化的发生.褶皱的轴线延伸方向为SSW-NNE,轴面倾斜方向主要为SEE(120°),指示古水流主要来自SEE方向,与浊积岩内底痕指示的古水流方向一致,说明斜坡沉积系统上发育的滑塌褶皱能够指示古水流方向.Abstract: The origins of the soft-sediment deformation structures outcropped in Lingshan Island are diverse, and that the trigger of the deformation structures in the lowermost part of the Dengta section is in dispute. This contribution studied the forming processes, trigger agents and geological significance of the slump and the included structures combining yield observation and stereographic projection methods. The results show that Dengta section mainly consists of turbidite sequence. The deformed horizon is sandwiched between undeformed layers and triggered by earthquake. 4 decollement surfaces occurred internally, dividing the slump into 5 units which were different from each other in shortening and deformation degree. The main type of soft-sediment deformation is fold which display thickened fold hinge in sandstone, and layer thickening and thinning occur occasionally, both are deformations of liquefied sand under the action of driving forces. The folded sand layer and mud layer became isolated from pore-fluid communication with the sea-floor during slumping, in consequence they became temporarily overpressured, allowing the liquefaction of sand to take place. The fold axes are mainly oriented SSW-NNE, and the axial planes mainly dip to SEE (120°), both indicating the flows coming mainly from SEE. This result is consistent with the regional paleocurrent measurements from the sole marks in turbidites. Therefore, the folds in slump that develop in the subaqueous slope system have significance in indicating paleocurrent direction.
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Key words:
- slump fold /
- soft-sediment deformation structure /
- liquefaction /
- turbidite /
- Lingshan Island /
- sedimentology /
- tectonic geology
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图 6 变形的沉积岩中2种不同类型褶皱理想图
图 6修改自Waldron and Gagnon(2011);黄色代表砂岩;黑色代表泥岩.a.低变质程度沉积岩层中典型褶皱的几何形态,砂岩层内弧曲率更大,而泥岩层外弧曲率更大;b.砂岩液化时形成的褶皱,几何形态特征与a相反(红色直线代表等倾斜线,连接连续层面上倾向倾角相同的点)
Fig. 6. Idealized diagrams of two different types of folds in deformed sedimentary rocks
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