Study on Sedimentary Faciess Features of the Upper Triassic Yanchang Formation, in the Western Margin, Ordos Basin

Key words: 
• depositional characteristics /
• lithofacies association /
• fan delta /
• braided river delta /
• Yanchang Formation /
• Ordos Basin

Abstract: The study was based on detailed observations of the outcrop profiles in Rujigou and Yanjiazhuang, and drill cores from Huan52, Huan78 and Yan40 wells in the study area, and combined with previous research results. Analysis of the lithology and rock associations is discussed, as well as the sedimentary facies and their characteristics and the evolutionary process of the Yanchang Formation in the study area. The research suggests that 12 lithological types and seven rock association types are recognized in the Yanchang Formation at the western margin of the Ordos Basin. Within the study area, fan delta-lake, a braided river delta and an alluvial fan sedimentary system were the main developments in the Yanchang period. Cribriform deposition, water distributary channels, interdistributary channels, underwater distributary channels, mouth bar and distal bar were also substantially developed, together with other sedimentary microfacies and subfacies types. In the northern part of the study area in Rujigou, fan delta facies were mainly developed in the early Yanchang Formation, with lacustrine facies transitioning into fan delta facies in the middle stage. Deep-lake facies developed in the late stage. In the central area of the Yanchang Formation, in Huanxian-Yanwu, early braided river facies developed, and delta front subfacies and lacustrine facies were formed in the middle stage, with transition to delta facies subjected to uplift-denudation. In southern Yan Jia Zhuang, in the early Yanchang Formation the main developments were alluvial fan facies, then fan delta front facies in the middle stage, and uplifting and denudation in the late stage.