Abstract The Xiangshan Group of the Middle Ordovician in Ningxia is dominated by deep water turbidity current deposits which are mainly a set of lightly metamorphic terrigenous clastic rocks with lower content of carbonate rocks and chert interbeds. The Xujiajuan Formation of the Lower Xiangshan Group is composed of grayish green， yellowish green， fine- to medium-grained lightly metamorphic sandstone， calcareous sandstone， siltstone and shale(slate)， and thin bedded limestone at the top. Cross laminations including bi-directional cross bedding with abundant shapes and clear lamina and unidirectional ones with dipping up-slope or large angle between them are found in thin- to medium-bedded and very fine- to fine-grained sandstones， silty micrite and silty shale. Those lamina of bi-directional cross bedding are also inclined bi-directionally up- and down-slope. These cross beddings in deep water environment may not be formed by contour currents or turbidity currents， but be formed by alternating up- and down-slope currents produced by internal waves and internal tides. This paper has made detailed researches on the internal-wave and internal-tide deposits in the Xujiajuan Formation from sedimentary background， sedimentary structures and paleocurrents. A deep-water slope environment was suggested for the internal-wave and internal-tide deposits and an interpretation was made for the deletion of flaser bedding， wavy bedding and lenticular bedding in this case.
About author: Li Xiangdong， born in 1973 and is a Ph.D.candidate in Yangtze University. He is engaged in researches of sedimentary basin analysis and reservoir sedimentology. E-mail: lixiangdong614@163.com.
Cite this article:
Li Xiangdong,He Youbin,Wang Dan et al. Internal-wave and internal-tide deposits of the Middle Ordovician Xiangshan Group Xujiajuan Formation， Ningxia[J]. JOURNAL OF PALAEOGEOGRAPHY, 2009, 11(5): 513-523.
Li Xiangdong,He Youbin,Wang Dan et al. Internal-wave and internal-tide deposits of the Middle Ordovician Xiangshan Group Xujiajuan Formation， Ningxia[J]. JOPC, 2009, 11(5): 513-523.
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