Internal-wave and internal-tide deposits of Middle Ordovician Xujiajuan Formation of Xiangshan Group in Ningxia, China

Xujiajuan Formation in Ningxia Autonomous Region, China is composed of gray-green, yollow-green fine- to medium-grained sandstone, calcareous sandstone, siltstone and mudstone, whose sedimentary environment was a slope environment. Cross laminations are well developed in very fine- to fine-grained sandstone, silty limestone, whose laminae are inclined not only bi-directionally up-and down-slope, but also multi-directionally which correspond to up-or down-slope directions. The characteristics indicated that these sedimentary rocks should be formed by alternating up- and down-slope currents produced by internal waves and internal tides.