The Middle Ordovician Contourite on the West Margin of Ordos
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摘要: 本文论述了在鄂尔多斯地区西缘中奥陶统中首次发现的碳酸盐等深流沉积及其堆积体－等深岩丘。其等深岩可划分为砂屑等深岩、粉属等深岩、灰泥等深岩和生物屑等深岩四种类型。识别出了完整的、不完整的和由单一的砂屑等深岩叠置组成的等深岩层序。砂屑等深岩大量发育是本区等深岩丘与其它已发现的各种等深岩丘的重要区别。古流向研究和等深岩的粒度、结构特征表明，在中奥陶世，沿鄂尔多斯地区西缘斜坡带存在较强的自南而北的等深流流动体系。Abstract: The Middle Ordovician on the west margin of Ordos is represented by a succession of deep-water sediments in which contourites are well developed. According to grain-size and component, the contourites can be classified into: ①calcarenitic contourite, ②calcisiltitic contourite, ③calcilutitic contourite and ④ bioclastic contourite. Calcarenitic contourite is mainly composed of sparite calcarenite and is characterized by coarser grain, very irregular boundary plane, development of parallel stratification and ripple bedding. The laminae in ripple bedding imply a palaeocurrent direction of northwest, which is coincident with the strike of the regional palaeoslopes. Calcisiltitic contourite consists mainly of micrite calcisiltite and is always interbedded with calcilutitic contourite or present both below and above calcarenitic contourite. Parallel stratification or small-scale ripple bedding are common in calcisiltitic contourite and the laminae imply a palaeocurrent direction of northwest. On the whole, calcilutitic contourite comprises (muddy) micritic limestone, which commonly contains a variable proportion of calcisiltite and bioclastic materials and is a common occurence of bioturbational structures. Sometimes, some horizontal stratifications can be found in this contourite. Bioclastic contourite almost consists of a succession of echinederm fragment limestone with a thickness of 2m～5m and the thickest is near to 7m, which is composed of a series of single beds with a thickness of 20cm ～ 50cm where large-scale cross-stratifications are well developed, whose laminae imply a direction of northwest. The first three types are well developed in Guanzhuang, Pingliang, Gansu Province, and the fourth type is mainly found in the Zhuozishan mountain, Inner Mongolia.In the study area, some sequences of contourite are complete and similiar to the typical sequence of Gonthier (1984), and some sequences are incomplete and others are special, i. e.,only consisting of calcarenitic contourite. Ripples, cross stratification and texture have generally revealed a strong northward palaeoflow direction along the palaeoslope on the west margin of Ordos. The distribution of all kinds of contourites in the section confirms the existence of the Pingliang contourite drift.
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