Plane Sedimentary Facies Distribution
Sedimentary Facies Distribution for Period of E1+2
During E1+2, most area of the research region was composed of red and varicoloured fluvial facies clastic rock which was braided-river sedimentation and flood plain sedimentation. Grey microclasticrock could only be found in Xiaoliangshan, Nanyishan, Youquanzi area and Shizigou area. Those places were small-size lake and delta facies (Fig. 3).
Sedimentary Facies Distribution of Period E31
The research area developed braided-river sedimentation, braided-river delta sedimentation and lacustrine sedimentation in turn from northeast to southwest during period of E31, and the lake basin was obviously enlarged (Fig. 4). Braided-river sedimentation in this region mainly consisted of braided channel subfacies and floodplain subfacies. Delta facies included delta plain subfacies and delta front subfacies. Lake facies was divided into subfacies of bank lake, coasta-shallow lake and semi-deep lake. That basically inherited the river facies sedimentation features of period E1+2 and mainly developed braided channel sedimentation and floodplain sedimentation in Dafengshan and Heiliangzi area.
Grey-dark grey lacustrine facies microclasticrock developed in the west part of Xiaoliangshan. Delta facies was basically in the eastern area. Youquanzi area mainly developed delta front subfacies. It was mainly Delta plain subfacies that overspreaded in Jiandingshan, south part of Nanyishan, Huanggualiang and Youdunzi area.
According to the analysis above, the lake was mainly distributed in the west half region during period E31, and the area was significantly larger than that of E1+2 period. The depocenter was located in Xiaoliangshan area, and the delta lobes were mainly in Jiandingshan, Youquanzi and Youdunzi area. River-floodplain sedimentation was mainly situated in Dafengshan and Heiliangzi area. However, judging from the single well facies of Jian3 and Nan10, a small-scale orogeny happened in the northeast part of this region during early E31 period. It developed a set of alluvial fan conglomerate strata about 8 meters thick on the bottom of E31 Formation in Jian3 well. In early period of E31, a bottom-up progradational sequence of braided-river delta front subfacies and plain subfacies in Nan10 well were established, and there was a momentary lake regression. The sedimentation system of period E31 can be summarized as alluvial fan, braided river delta and shore-shallow lacus sedimentation.
Sedimentary Facies Distribution of Period E32
The overall sedimentary facies distribution did not change obviously when developed into period of E32 (Fig. 5). The area of lacustrine mudstone further extended and the sedimentary system of river and delta retreated to northeast. The amount of coarse fragments reduced in Jiandingshan, Heiliangzi, south of Dafengshan and Nanyishan area. Delta plain subfacies turned into bank lake subfacies in Jiandingshan area. Delta front subfacies turned into coastal-shallow lake subfacies in south of Nanyishan. And in Heiliangzi area, the Floodplain subfacies translated into delta plain subfacies.
According to the analysis above, E32 was the biggest lake flooding period when the area of lacus basin further expanded and the water depth further deepened. The semi-deep lake sedimentation obviously increased, and the deposit center located in Nanyishan and Xiaoliangshan areas. The sedimentary system of period E32 could be summarized as alluvial fan-braided-river delta plain-bank lake subfacies, coastal-shallow lake subfacies and semi-deep lake subfacies.