235 / 2019-06-29 09:54:08

Sedimentary characteristics, distribution of gravity flow in Yanchang Formation，Ordos basin

Ordos basin; Chang 7 Formation; Sediment of gravity flow; Debris flows；Turbidity currents；

gravity flow deposition is a hot topic of deep water sedimentology field. According to outcrop profile around the basin, core and logging data , a large amount of gravity flow deposition was developed in the deep lake area of chang6-chang7 period in the Ordos basin. By comparing and analyzing the sedimentary characteristics of gravity flow in Huachi, Jiyuan, west of Huangling, Xunyi, tongchuan areas of the Ordos basin, five types of gravity flow types were developed: sliding rock mass, slump deposition, sandy clastic flow deposition, turbidity flow deposition, and mudstone clastic flow deposition.
Sandy clastic flow deposition consists of (1) gray and brown-gray massive fine sandstone facies, (2) bottom mud-gravel/sand-gravel massive fine sandstone facies, (3) positive and negative graded fine sandstone facies. Turbidite deposit is composed of (1) gray and dark gray with envelopment bedding, lenticular bedding siltstone facies, (2) wavy lamination fine siltstone facies, parallel bedding siltstone facies. Turbidite depositions develop complete or incomplete bauma sequence, and common groove mode structure at the bottom of sandstone. Sliding and slump depositions develop less, and composed with siltstone and silty mudstone which it develop slip surface at the bottom. Interface and lithologic difference is significant.
The distribution of gravity flow sediments has a certain regularity. The vertical development is mainly in the descending stage of the lake level. The scale of chang 6 member is larger than chang 7 member. The sand bodies of massive sandy clastic flow are superimposed and distributed especially in Huachi area. The sandy clastic flow deposition develop near source and the turbidity flow deposition is far source. The sediment changes in time and space from sandy clastic flow to turbidite flow in the direction of provenanc, which formed the mixing event layer of deep water gravity flow. By comparing the sedimentary characteristics of gravity flow under different provenance and sedimentary background in the Ordos basin, it is found that the single sand body deposited by gravity flow in the northeast provenance of the basin is thicker than in the southeast provenance. Moreover, hyperpycnal flow is develop in the Xunyi and Tongchuan area of Ordos basin.The main factors affecting the gravity flow deposition are related to the characteristics of paleogeomorphic units, the development of slope, distribution of paleotopography at the bottom of basin, the change of lake basin level and the supply of source.