北祁连石灰沟奥陶纪中堡群滑塌堆积特征及其古地理环境意义

Characteristics of olistostromes from the Ordovician Zhongbao Group in Shihuigou area, North Qilian orogenic belt and their palaeogeographic implications

北祁连永登县石灰沟作为奥陶纪中堡群命名地,历来是研究北祁连奥陶纪构造演化的理想场所。依据实测地层剖面,石灰沟奥陶纪中堡群可以划分为上、下两段:下段以中基性火山岩、火山碎屑岩为主;上段以出现大量碳酸盐岩、硅质岩、粉砂岩,夹中基性火山碎屑岩为特征。野外调查过程中发现,中堡群上段发育多层硅质岩,其层内发育强烈的构造变形。经岩石组合、地层序列、沉积相、火山喷发相、变形特征及空间组合关系研究,认为该套特殊的沉积层系为典型的滑塌堆积。根据滑塌堆积的内部结构特征及火山-沉积相序等分析,初步判断其形成于靠近岛弧的深水盆地环境,具多岛洋构造背景。这将为恢复和建立北祁连造山带奥陶纪沉积环境和古地理演化提供可靠依据,也为进一步研究北祁连奥陶纪沟-弧-盆体系空间格局提供了重要沉积学佐证。

As the first named location of the Ordovician Zhongbao Group, Shihuigou area in Yongdeng County of North Qilian Mountain is an ideal place for the study of the Ordovician tectonic evolution of North Qilian orogenic belt. On the basis of the measured stratigraphic section, the Zhongbao Group in Shihuigou area can be divided into upper and lower part respectively: thelower part mainly consists of mafic-intermediate volcanic rocks and pyroclastic rocks, whereas the characteristics of the upper part is the existence of carbonate, cherts and siltstone interbedded with intermediate-basic pyroclastic rocks. Field investigation shows that there are many layers of cherts and strong deformation in the upper part of the Ordovician Zhongbao Group. According to the study of the rock assemblage, stratigraphic sequence, sedimentary facies, volcanic eruption facies, deformation features and spatial combination relationship, this set of special sedimentary strata are considered to be the typical olistostromes. Considering its internal structure and volcanic-sedimentary succession, the authors hold that the olistostromes should be deposited in the deep-water basin near the island arc, which belonged to a tectonic background of archipelagic ocean. The discovery of olistostromes in the Zhongbao Group provides reliable evidence for recovering and establishing the Ordovician sedimentary environment and paleogeographic evolution of the North Qilian orogenic belt. It also offers important sedimentologic evidence for further studying the trench- arcbasin system spatial pattern of the North Qilian orogenic belt.