陕西陇县地区平凉组三段发现内潮汐沉积

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

鄂尔多斯盆地西南缘陕西陇县中奥陶统平凉组主要为一套暗色页岩夹薄层粉砂岩、砂岩、灰岩及凝灰岩,可分为3个岩性段。其中第三段以灰绿、灰黑色页岩、粉砂质页岩、砂质页岩为主,夹深灰色薄层砂岩、粉砂岩、粒屑灰岩及青灰色灰化晶屑凝灰岩等。沉积环境分析表明,平凉组主要为斜坡至盆地边缘沉积环境,以深水原地沉积为主,次为碎屑流和浊流沉积。第三段中部薄层砂岩、粉砂岩中发育双向交错纹理和单向交错层理,纹层倾向既有沿斜坡向上和向下双向倾斜的,也有沿斜坡向下或沿斜坡向上单向倾斜的,但以沿斜坡向上单向倾斜的为主,它们不应是等深流沉积或浊流沉积,而应为内波、内潮汐产生的沿斜坡上下交替的流动所形成的,初步认为应属水道内潮汐沉积。

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.