Reef Characteristics of Yijianfang Formation in Bachu Area of Tarim Basin

There exist 6 characteristics of reef features which include individual reefs are small, but numerous; ②the reef-bearing in the middle member of the Yijiangfang Formation is composed of a lower main reef unit and an upper less well developed reef unit; ③the reefs have various shapes and exhibit sharp contacts with surrounding strata; ④faunal and floral reef components are relatively simple, and their microfacies are relatively monotonous; ⑤calathium occurs in great abundance, but low diversity, and has autochthonous occurrences as both living and buried elements; ⑥Channel fills may be developed within the reef cores. Among the various reef growing-controlling factors are the hydrodynamic conditions that affect both reef construction and destruction. Local shallw-water sand bars is served as reef bases, is beneficial for the establishment and growth of Calathium, and strengthening the binding and baffling actions that modified reef growth. Subsequent currents erode the reefs, wash out and destroy their original outlines and shapes, and affect completeness of preservation of reef interiors. Erosional processes modify the growth of reef-building organisms, and restrict the scales and shapes of the developing reefs. The interactions between reef-building and reef-destruction process; therefore, it results in three main stages of development of these middle Ordovician reefs: such as foundation stage, reef construction stage and destruction stage.