The deep-water contourites are widely developed in the Middle Ordovician Kelimoli Formation in Zhuozishan area

western margin of Ordos Basin.The Shixiagu outcrop thereto is generally regarded as an ideal section for studying the contourites of the Darriwilian stage for its complete stratum and obvious lithological variations.Here we focus on the mechanism of limestone associations alternating with shale in the middle Kelimoli Formation based on outcrop observations

study of thin sections in labs

data from relevant geochemical tests and previous works.The results show that: ① the middle part of Kelimoli Formation consists of ten limestone associations which could be grouped into three cycles; ② the single limestone layer pinches out in shale with different forms and patterns of lens; ③ a succession of parallel layer-homogeneous layer-parallel layer and streaky structures in the shape of gentle ripples are developed in the limestone

displaying alternating zones of lime mud and very fine crystalline under microscope.In combination of previous research

we may conclude that the mixed sequences consisting of limestone and shale in the middle Kelimoli Formation can be divided into two types

one is the mixed sequence of terrigenous clastics with carbonate

and the other is the mixed sequence of carbonate(terrigenous clastics)with diamictite

and their genetic mechanism could be attributed to the periodic overflow of contour current controlled by astronomical cycles in deep-water slope environment.The discovery of mixed sequences caused by deep-water contour currents can be of great significance for the Ordovician shale gas exploration at the western margin of Ordos Basin.