Distribution of shallow water delta sand bodies and the genesis of thick layer sand bodies of the Triassic Yanchang Formation, Longdong Area, Ordos Basin

LIU Hanlin,QIU Zhen,XU Liming,WANG Fengqin,TONG Qiang,LIN Jiahao,YIN Shuai,WANG Wenqiang

Table 1 Division basis of shallow water delta shoreline of Chang 82 sub-member in Longdong area, Ordos Basin (revised according to Reference [10]).

Development position Sedimentary facies zone Environmental characteristics Biomarkers Sedimentary structure Logging curve Above the average high water level Shallow delta plain Above water for a long time Stem of large plant Large and medium-sized trough cross bedding, plate- shaped cross bedding, and erosion mud gravels Bell shape, box shape Between the average high water level and the average low water level Inner front of shallow water delta Sometimes above the water, sometimes under the water Coal vein, vertical wormhole, plant roots and large leaves Large cross bedding and parallel bedding Box shape, weak serration Below the average low water level Outer front of shallow water delta Under water for long term Oblique wormhole, horizontal fibrous roots, plant debris and shells Small sand grain and wavy bedding Box-bell shape, bell-funnel shape, with stronger serration