深水重力流类型、沉积特征及成因机制——以济阳坳陷沙河街组三段中亚段为例

以济阳坳陷沙河街组三段中亚段(沙三中亚段)深水重力流沉积为研究对象,结合其沉积物成分结构、浓度、搬运沉降方式及流体流变学特征的差异,对该深水重力流类型、沉积特征及成因机制进行了研究。济阳坳陷沙三中亚段深水重力流包含碎屑流和浊流2种类型,碎屑流沉积根据杂基含量和结构差异,分为起结构杂基作用的泥质碎屑流和起成分杂基作用的砂质碎屑流;浊流根据其触发机制、沉积相序特征,分为沉积物滑塌成因的似涌浪浊流和洪水成因的准稳态浊流(异重流)。泥质碎屑流沉积物多为厚层块状杂基支撑砾岩、含碎屑砂质泥岩等,顶、底面与泥岩突变接触;砂质碎屑流沉积多为厚层块状砂岩、含砾砂岩,顶、底面与泥岩突变接触,可见逆粒序,泥岩撕裂屑和漂浮泥砾多在沉积相序中上部发育。似涌浪浊流沉积以中细砂岩、粉砂岩为主,发育完整或不完整的鲍马序列,可见冲刷充填构造;准稳态浊流沉积以含砾粗砂岩、中细砂岩、粉砂岩为主,逆正粒序、正粒序沉积发育,相序内部可发育突变接触面或侵蚀界面,上攀层理、炭质纹层发育。滑塌成因深水重力流沉积以碎屑流和似涌浪浊流沉积组合为主,包含部分的滑动滑塌伴生沉积;洪水成因深水重力流沉积以碎屑流和准稳态浊流沉积组合为主。

Types,sedimentary characteristics and genetic mechanisms of deep-water gravity flows:a case study of the middle submember in Member 3 of Shahejie Formation in Jiyang depression

The deep-water gravity flow in the middle submember of Member 3, Shanghejie Formation, Jiyang depression was taken as the research object in this study. Based on the differences in sediment composition structure, concentration, transportation-sedimentation mode and fluid rheological characteristics, a study was carried out on the types, sedimentary characteristics and genetic mechanisms of deep-water gravity flow. In the middle submember of Member 3, Shanghejie Formation, Jiyang depression, deep-water gravity flow was classified into debris flow and turbidity current. The former was primarily divided into muddy debris flow with structural matrix effect and sandy debris flow with composition matrix effect based on the matrix content and structure difference in debris-flow sediments; according to trigger mechanism and sedimentary facies sequence characteristics, the latter was divided into two types, i.e., surge-like turbidity current resulting from sediment slump, and quasi-steady turbidity current (density flow) resulting from flood. The sediments of muddy debris flow were mostly composed of thick-massive matrix-supported conglomerate and clastics-bearing sandy mudstone, where there is abrupt contact with top and bottom surface. The sediments of sandy debris flow also mainly consisted of thick-massive sandstones and gravel-bearing sandstones, showing abrupt contact with top and bottom surface and reverse graded bed sequence. Moreover, mudstone tearing clastics and floating boulder clays were mostly developed at the middle and top of sedimentary facies sequence. The sediments of surge-like turbidity current primarily consisted of medium-fine sandstones and siltstones, where complete or incomplete Bouma sequence was developed with scouring-filling structure. The sediments of quasi-steady turbidity current were dominated by gravel-bearing coarse sandstones, medium-fine sandstones and siltstones, and developed in reversed normal graded bed sequence and normal graded bed sequence. In the inner facies sequence, abrupt contact surface or erosion interface was developed with climbing beddings and carbonaceous laminae. The sediments of deep-water gravity flow caused by slumping were dominated by an assemblage of debris flow and surge-like turbidity current, including partial sliding and slumping associated sediments. The sediments of deep-water gravity flow resulting from flood were dominated by an assemblage of debris flow and quasi-steady turbidity current.