Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China |
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Authors: | You-Bin He Jin-Xiong Luo Xiang-Dong Li Zhen-Zhong Gao Zhan Wen |
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Institution: | 1. School of Geosciences, Yangtze University, Nanhuan Road 1, Jingzhou, 434023, Hubei, People??s Republic of China 2. Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Road 26, Beijing, 100037, People??s Republic of China
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Abstract: | The Xujiajuan Formation of the Lower Xiangshan Group in Ningxia, China, is composed of grayish-green to yellowish-green, fine- to medium-grained sandstone, calcareous sandstone, siltstone, and shale. The upper part is thin-bedded limestone. At the top of the second and third members of the formation, a number of beds intercalated between turbidites and deep-water shale show well-developed cross-bedding. These beds are composed mainly of thin- to medium-bedded calcareous siltstone, fine-grained sandstone, fine-grained calcisiltitic limestone, and silty shale. All bedding types reflect traction-current action. The laminae of the bidirectional and unidirectional cross-bedded units tend to dip either opposite to or at a large angle to the regional slope. The units vary in shape and orientation in both upslope and downslope directions. A comprehensive evaluation of the sedimentary structures and inferred paleocurrents suggests that the cross-bedded intervals were not formed by contour currents or turbidity currents, but most probably represent internal-wave and internal-tide deposits. |
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