干旱区生态环境知识资源中心

The Shan 2 Member, Shan 1 Member and He 8 Member of the Mid-Late Permian Shanxi and lower Xiashihezi formations, in the southeastern Ordos Basin, together comprise approximate to 150 m of deltaic deposits. This sequence records an overall evolution from deep marine environment to shallow lake associated with braided river, braided river delta and meandering river delta. Core description, well log interpretation, and stable isotope analysis, including carbon, oxygen and strontium, were conducted to understand the sedimentary evolution of Shan 2 to He 8 Member. The Shanxi Formation, which consists of the Shan 2 and Shan 1 members, is characterized by a tidal-influenced meandering river delta environment and a higher C-13 value and Sr-87/Sr-86 ratio and a lower O-18 value. The He 8 Member, the basal part of the Xiashihezi Formation, is featured by a braided river to braided river delta system and a lower C-13 value, Sr-87/Sr-86 ratio, and a higher O-18 value. Four third-order depositional sequences separated by five sequence boundaries are determined. Coarsening upward sequences of the Shan 2 Member-He 8 Member indicate a general regression trend, which can be correlated to global sea-level fall occurring during the Roadian-Wuchiapingian, as also evidenced by previous published zircon U-Pb results. The coal-bearing sequence (Shanxi Formation) to non-coal-bearing sequence (He 8 Member), as well as a decrease of Sr-87/Sr-86, suggest a trend from humid to arid climates. A combined effect of sea-level drop and a small uplift at the end of Shanxi Formation are proposed.