Discovery of Internal-tide Deposits from the Third Member of Pingliang Formation in Longxian Area,Shaanxi Province

The middle Ordocivian Pingliang Formation in Longxian Area,Shaanxi province contained mainly of dark shale with interbeds of thin siltstone,sandstone,limestone and tuff,it could be divided into three members.And the third member was consisted mainly of dark-green,gray black shale,siltly shale and sandy shale with interbeds of dark gray thin sandstone,siltstone,grained limestone and cyan gray crystal stuff,and so on.The analyses indicate that sedimentary environment of Pingliang Formation was mainly from slope to basin margin environment.And it consists mainly of deepwater in-situ deposits,subsequently debris deposit and turbidity deposits.Bi-directional cross lamination and unidirectional bedding are well developed in some thin sandstone and siltstone in the third member.The laminae are inclined not only bi-directionally up and down the slope,but also unidirectionally which correspond to up or down slope directions,but most of them incline bi-directionally up the slope.So they should not be contour current deposits or turbidity deposits,but should be formed by the alternating up and down slope currents produced by internal waves and internal tides.The preliminary result indicates that they should be internal tides deposits in submarine channels.