Sedimentary Types of Internal-wave and Internal-tide Deposits of Middle Ordovician, Xujiajuan Formation, Xiangshan Group, Ningxia Autonomous Region, China

Abstract

The study of internal-wave and internal-tide deposits is a very young research field with twenty years of history in the deep-water deposition,and there is no study on its sedimentary types.Here,we forcus on the internal-wave and internal-tide deposits developed in Xujiajuan Formation,Xiangshan Group,Ningxia Autonomous Region and describe related sedimentary structures in detail.We interpret combined-flow-ripple lamination and the cross beddings with undulatory laminations,offshooting laminations and cross-laminated lenses as combined-flow and oscillatory flow generated by short-period internal waves respectively.According to these new interpretations and the interpretations of deep-water bidirectional cross-beddings and unidirectional cross-beddings in literatures,we attribute internal-wave and internal-tide deposits to short-period internal-wave deposits,internal-tide deposits and interaction internal-wave deposits.Here,we use internal-wave and internal-tide deposits instead of internal-wave deposits because of the special origin and research status of internal tides.