贺兰山地区中奥陶统樱桃沟组深水牵引流沉积的发现及其意义

Discovery of deep-water tractive current deposits in Middle Ordovician Yingtaogou Formation,Helan Mountain area,and its significance

鄂尔多斯盆地西缘贺兰山地区的中奧陶统樱桃沟组发育一套夹有碳酸盐滑塌重力流沉积的陆源碎屑浊积岩,研究程度颇高。笔者在野外调查过程中于樱桃沟组浊流沉积体发育的环境中发现了相当数量的交错层理,如大型交错层理,板状交错层理,波状、脉状、透镜状层理和波痕等,曾一度被前人解释为浅水沉积环境的识别标志。本文结合当时的古地理背景、沉积环境和区域构造演化特征,通过对樱桃沟组野外露头的详细描述和岩石特征的精细刻画以及对该组沉积序列的详细分析,认为上述疑似浅水沉积环境的交错层理实为底流(深水牵引流)对浊流改造的结果,并分别形成了内波、内潮汐沉积和等深流沉积,这在研究区为首次发现。据此将其归纳为7种岩石类型:(1)双向交错层理粉砂岩;(2)束状透镜体叠加的交错纹理粉砂岩;(3)具波状层理粉砂岩;(4)单向交错层理砂岩;(5)水流波痕细砂岩;(6)平行层理细-中砂岩;(7)大型交错层理细砂岩。其中,前5种岩石类型为内波、内潮汐沉积,后两种为等深流沉积。研究表明樱桃沟组为深水牵引流沉积(内波、内潮汐沉积、等深流沉积)、重力流沉积(碳酸盐滑塌角砾岩、浊积岩)和原地沉积的复合沉积,并建立了该区水道型内波、内潮汐沉积模式。

The Yingtaogou Formation in the Helan Mountain area located in western margin of the Ordos basin consists of a set of terrigenous-detrital turbidite with carbonate slump gravity flow deposits, and the study of the Yingtaogou Formation has reached a high level. The authors found lots of cross beddings, such as large scale cross-bedding, plate cross bedding, wavy bedding, flaser bedding, lenticular bedding and ripple marks, during the field investigation in the turbidity current deposits of the Yingtaogou Formation. Those cross beddings had been once interpreted as identification marks of shallow water sedimentary environment. Combined with the current graphic background, sedimentary environment and regional tectonic evolution, those above-mentioned cross beddings actually resulted from the turbidity current deposits reworked by bottom current （deep-water traction current） through describing the field outcrop in detail, fine depicting the petrology features and analyzing the sedimentary sequence in detail. Then, it came into being the internal-wave deposit, the internal-tide deposit and the contourite. This paper first discovered the internal-wave, internal-tide deposits and contourite in the Middle Ordovician Yingtaogou Formation in the Helan Mountain area. According to their depositional structures they can be divided into seven types of rocks, which are （1） siltstone with bi-directional cross bedding structure, （2） siltstone with fascicular lens superposed cross-lamination structure, （3） siltstone with wave, vein and lens like composite stratification, （4） sandstone with unidirectional cross bedding structure, （5） fine sandstone with current ripple structure, （6） fine-medium sandstone with parallel bedding structure and （7） fine sandstone with large-scaled cross beddings, respectively. In which, the first 5 types of rocks are internal-wave and internal-tide deposits, and the rest of 2 types are contourite. The present study shows that the Yingtaogou Formation is the composite deposits of deep-water tractive current deposits （internal-wave, internal-tide deposits and contourite）, gravity flow deposit （carbonate slump breccias deposits and turbidite） and autochthonous deposits. And the internal-wave and internal-tide sedimentary model of channels has been established on the basis of comprehensive analysis.