鄂尔多斯盆地南部旬邑地区延长组长7段深水牵引流的发现及其意义

Discovery and Significance of Deep-Water Tractive Current Deposits in Member 7 of Yanchang Formation in Xunyi Area of Southern Ordos Basin

鄂尔多斯盆地南部地区延长组长7段主要发育以浊流及砂质碎屑流为主的深水重力流沉积。通过对盆地南部旬邑地区延长组长7段野外剖面露头进行详细考察时发现,该组砂岩中发育大量类似浅水环境的沉积构造,如交错层理、波状层理、平行层理、透镜状层理及波痕等。在综合研究区古构造、沉积背景的基础上,通过对野外露头进行详细的描述与刻画,认为盆地南部旬邑地区延长组长7段发育以等深流、内波、内潮汐为主要类型的深水牵引流沉积,上述类似浅水环境中的沉积构造为深水牵引流对先期形成的重力流沉积再作用的结果。总体上,研究区主要发育5种类型的深水牵引流岩相:(1)具双向交错层理粉细砂岩;(2)具单向交错层理粉砂岩;(3)具波状层理泥质粉砂岩、粉砂岩;(4)具平行层理细砂岩;(5)具大型交错层理中细砂岩。在此基础上探讨了研究区深水牵引流沉积模式。

Member 7 of Yanchang Formation in the south of Ordos Basin consisted of turbidity current and sandy debris flow of deep-water gravity flow deposits. Many similar sedimentary structures of the shallow water environment, such as cross-bedding, wavy bedding, parallel bedding, lenticular bedding and ripple marks, were discovered by field investigation of Member 7 of Yanchang Formation in Xunyi area located in southern Ordos Basin. Member 7 of Yanchang Formation in the south of Ordos Basin consists of deep-wa- ter tractive current deposits, mainly including the internal-wave deposit, the internal-tide deposit and the contourite based on the tectonic and sedimentary environment of the study area and the detailed description and analysis of the outcrop sedimentary characteristics. The sedimentary structures resulted from the deep- water gravity flow deposits are reworked by deep-water tractive current actually. There are mainly five types of deep-water tractive current rock microfacies formed in the study area： （1）silt-fine sandstone with bi-directional cross bedding structure; （2）siltstone with unidirectional cross bedding structure; （3） siltstone with wavy bedding structure; （4）fine sandstone with parallel bedding structure; （5）medium-fine sandstone with large-scaled cross beddings structure. The deep-water tractive current sedimentary model is discussed fully in the paper.