The Oligocene Sedimentary System in the YA Area of the Songnan uplift in the Southeastern Hainan Basin

Key words: 
• Southeastern Hainan Basin /
• Songnan low uplift /
• Oligocene /
• sedimentary system /
• sedimentary pattern

Abstract: A systematic study was conducted on the sedimentary system and evolution of the Paleogene Oligocene in the YA area of the Songnan low uplift in the Qiongdongnan Basin, using comprehensive methods such as drilling core, thin section identification, paleontology, particle size analysis, logging, and seismic interpretation. The lithofacies and paleogeography of the Paleogene in the YA area were restored.Research has shown that the lithology of the middle and lower part of the Oligocene (Yacheng Formation) is mainly composed of medium to coarse sandstone and gravel bearing sandstone. The sandstone has poor sorting ability, and the mineral composition gradually tends to stabilize from bottom to top. The maturity gradually increases, and the probability accumulation curve of particle size mainly shows suspended and jumping characteristics. The fossil abundance is extremely low, and the sedimentary facies types are mainly fan delta, shore shallow lake, and lakeshore plain.The lithology of the upper part of the Oligocene (Lingshui Formation) is mainly composed of fine sandstone and mudstone, with a significant increase in fossil abundance. The sedimentary facies are mainly barrier coasts and shallow water continental shelves. The research area underwent a transition from marine terrestrial transitional sedimentation to shallow marine sedimentation during the Oligocene. The fault activity in the Early Oligocene (Yacheng Formation) area formed a structural pattern of Songnan low protrusions and alternating concave and convex edges. The water body was shallow, with shallow lake facies developed in the center of the depression, fan deltas developed in steep slopes, and braided river delta facies developed in gentle slopes. The seismic facies were mainly characterized by pre accumulation and chaotic reflection structures; The fault activity in the Late Oligocene (Lingshui Formation) area caused the deepening of sedimentary water bodies. The area mainly developed shallow sea shelf facies and deep sea shelf facies, and the seismic facies were mainly characterized by parallel sub parallel reflection structures. The sedimentary pattern of the Yacheng Formation is as follows: braided river deltas are developed in the gentle slope zone and shallow water areas of the depression, fan deltas are developed in the steep terrain and deep water areas, fan delta deposits are developed in the steep slope zone of the depression, and coastal shallow lake (sea) deposits are developed in the subsidence center. The sedimentary pattern of the Lingshui Formation is characterized by the development of coarse-grained thick layer sediments at the fault, and fine-grained thin layer wedge-shaped sediments at the center of the depression.