Sedimentary Characteristics of Lower-Middle Ordovician Yingshan Formation Carbonate intra-platform Shoals in Keping area, Tarim Basin

Key words: 
• Yingshan Formation /
• carbonate shoal /
• peloids /
• sedimentary microfacies /
• facies successions /
• facies belt

Abstract: Three intervals were identified in the Penglaiba Section of Keping area, and every interval has different facies associations and sedimentary characteristics:the lower interval is coarse grains intraclast wackestone or packstone limestones with a little of terrigenous mud; the middle interval is the association of sparry silt and fine grains intraclast grainstone limestones and laminites limestones; the upper interval is medium thickness peloids micrite limestones. And the intra-platform shoals are found in the lower and middle interval. The Keping cement plant cross-section also can be divided into three intervals, but the boundary is not clear, the sparry intraclast limestones and micrite intraclast limestones appearing in rotation overall. According to outcrop observation, thin section identification and sedimentary microfacies analysis to the Penglaiba cross-section and Keping Cement Plant cross-section, we clarified the types of rock and facies association and built up the sediment models of Yingshan Formation:According to the water depth and energy, the Keping area developed four types facies belts-high energy shoals belt, middle-low energy shoals belt, inter-shoals despressions sediment belt, and open shallow marine sediment belt. The high energy shoals belt developed sparry fine-middle grains intraclast shoals and the middle-lower energy shoals belts developed micrite middle-coarse grains intraclast shoal, and every shoal can be divided into three subfacies-shoal body, shoal wing and inner-shoal depression; The Keping area was located in shallow marine environment with extensive microbial peloids and it was a special stage of Ordovician that the carbonate rock transformed from microbial constructions to metazoan constructions in Tarim Basin, so the formation and reform of sediment was affected by microbial activity. On the one hand, microbial action of the Ordovician Yingshan Formation based the material condition for the shoals. On the other hand, it controlled the carbonate sedimentary characteristics of Yingshan Formation.