Sedimentary boundary of southern basin

The southern boundary of Ordos basin has entered the present Qinling orogenic belt, which is located on the north side of Shangdan suture line. In today's North Qinling orogenic belt, the upper Triassic Yanchang Formation strata are distributed along Zhouzhi Banfangzi in Shaanxi, Yunjiashan in Luonan County, Nanzhao (Wang, 1993) in the south of western Henan, Wulichuan, Xuezi and Shuanghuaishu in Lushi, Henan Province, and the sediments are mainly conglomerate (Figure 2- 18

Figure 2- 18 Sedimentary Characteristics of Alluvial Fan in the Steep Slope Area on the Northern Edge of East Qinling Mountains

Biofossils are consistent with Yanchang Formation in Ordos Basin, mainly including Glossostrea Shaanxi, Orthodonta Shaanxi and Neocalamus. , (New Rubus) and Cladosporium. Stratigraphic characteristics and biological consistency show that the northern Qinling area in the Late Triassic, that is, the vast area on the north side of the Shangdan suture line, was once a part of the Great Ordos Basin, which was destroyed by Yanshanian tectonic activities and the southern boundary of the basin was steep (Yang Youyun, 2005).

Located at the southern boundary of the basin on the north side of Shangdan main suture line in East Qinling, the collision orogeny of the north and south blocks of Qinling orogenic belt formed the northward thrust fault edge. Before the closure of the ancient Qinling Sea, the northern part of Qinling orogenic belt was the North China ancient continent and its southern active continental margin, and the southern part was the Yangtze ancient continent and its northern passive continental margin. During Indosinian period, due to the disappearance of the ancient Qinling Ocean, the North China plate and the Yangtze plate merged and the Qinling orogenic belt began to form (Zhang Guowei, 1998). Due to the northward thrust of Qinling orogenic belt in the same period, the southern margin of the basin was unevenly and rapidly settled, forming a steep bank slope belt. In the south of the basin, underwater fan delta and turbidite deposits are developed in the area of Ningxian South-Xunyi-Yaoxian-Tongchuan, which are a combination of gray-green, dark gray mudstone, silty mudstone and thin silt-fine sandstone. Turbidite sand is multi-stage rhythmic deposition, with small single sand layer thickness, fine particle size, horizontal bedding and sand bedding.