鄂尔多斯盆地下白垩统大型类碟状构造的发现及其地质意义*

在鄂尔多斯盆地北部下白垩统中发现一种特殊的软沉积物变形构造,其与碟状构造非常相似,主要出露于G109国道鄂尔多斯市至杭锦旗段100余千米长的公路两侧的露头剖面中。该类软沉积变形构造岩性为下白垩统的一套黄褐色或灰白色的中厚层砾岩或砂砾岩,在竖直面上其边缘翘起、中部平坦或下凹,故笔者把它称之为“类碟状构造”。类碟状构造特征为: (1)规模总体很大。小型者的宽度一般为1~3,m,高度多为0.5~1.5,m;大型者的宽度一般为5~6,m、最大可近10,m,高度多为1~1.5,m、少数可达2,m;(2)两侧翘起程度不同,包括倾角小于45°的倾斜边、倾角在45°~75°之间的陡斜边、倾角在75°~90°之间的近直立边和倾角大于90°的翻卷边;(3)类碟状构造之间为泄水形成的液化充填体,竖直面上多呈漏斗形和倒三角形;(4)类碟状构造边缘的砾石长轴优势排列方位平行于流动变形方向;(5)类碟状构造常与砂岩岩脉、液化脉、软变形层理或软变形褶皱等共生。(6)底部多为湖相棕红色(有时夹少量灰色)泥岩或砂质泥岩,其由于液化流动而多呈块状,极易破碎。根据上述特征,认为类碟状构造系泥泄水形成的,是受强烈地震扰动所致,地震强度可达8级、甚至9~10级。该地震活动可能与鄂尔多斯盆地北部中生代强烈的构造运动有关。

Discovery of large-scale dish-like structures of the Lower Cretaceous in Ordos Basin and its geological significance

A special soft-sediment deformation structure(SSDS),which looks very similar to the disk structure,was discovered in the Lower Cretaceous in the northern Ordos Basin. It mainly occurs in the roadside outcrops along the National Highway G109 from the Ordos City to Hangjinqi(over 100,km). It is a set of yellow-brown or gray-white medium-thick layered conglomerate or sandy conglomerate.Due to the concave-down shape in the vertical section,it is called “dish-like structure”here. Generally,it has the following features: (1)The scale is usually large. A small dish-like structure is about 1-3,m wide,and 0.5~1.5,m high;while a large one is about 5~6,m wide,with a maximum of nearly 10,m wide,and mostly 1~1.5,m high,some can be even 2,m high; (2)The edge tilting angle differs among 45°~90°,even reverses;(3)Laterally,the space among different dish-like structures is filled with liquefied sediments caused by escape water discharge,and shows a funnel or an inverse triangle shape in a vertical section;(4)The long axes of gravels along a dish-like structure edge tend to beparallelto the flow direction;(5)It is often associated with sandstone vein,liquefied vein,soft-deformation bedding,or soft-deformation fold,etc.;(6)Its bottom is mostly deposited with a lacustrine brownish red mudstone or sandy mudstone,which is normally massive and fragile due to liquefaction and flow activity.Based on the above-mentioned features,it is thought that the dish-like structure was formed by escape water,which was caused by intensive earthquake of magnitude of 8 or even 9-10. This earthquake is possibly related to the strong tectonic movements during the Mesozoic in the northern Ordos Basin.