华北地台西缘中元古界蓟县系墩子沟群特征及其地质意义

华北地台西缘龙首山中元古界墩子沟群是一套基本没有变质的盖层性质的沉积,根据岩性可划分为三个组下组以砾岩为主,具底砾岩性质;中组是以灰岩为主的浅海碳酸盐沉积;上组是一套细粒的陆源碎屑岩.其下与古元古界龙首山岩群的角度不整合面代表吕梁运动,与华北地台主体相比缺失长城系和青白口系.墩子沟群分布特征是中组分布广,厚度和岩相都比较稳定,下组的陆源粗碎屑岩东部区域较厚,分选磨圆都比较好,岩相厚度稳定,粒度较细,如金昌的河西堡、永昌的韩母山一带.西部区域厚度薄分选磨圆均差,岩相厚度不稳定,有的部位缺失此组沉积或以很薄的底砾岩产出,如张掖东大山、山丹独峰顶一带.说明当时海侵自东向西侵进,反映出华北地台在蓟县纪中期形成海进,明显向西超覆的特点.墩子沟群虽然变质轻微但变形却十分强烈,其原因一方面内部有滑塌形成的同生角砾岩和层间小褶皱、揉皱等准同生构造,说明当时的地形有一定的坡度;另一方面后期又受到祁连运动的影响所致.

CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF THE DUNZIGOU GROUP OF THE MESOPROTEROZOIC IN THE WESTERN EDGE OF THE NORTH CHINA PLATFORM

The Dunzigou group of the Jixian system of Mesoproterozoic in the Longshoushan mountains at the western edge of the North China plateform is the sedimentary cover with slight metamorphism. According to lithology the Dunzigou group can be divided into three formations. The Lower formation is mainly composed of basal conglomerates; The Middle formation is mainly composed of limestones which are carbonate sediments of the shallow sea; The Upper formation is mainly composed of fine-grained terrigenous sediments. The unconformity between the Dunzigou group andthe underlying Lonshoushan group of Paleoproterozoic is the result of the Luliang movement. In contrast to the main area of the North China plateform, the sedimentary cover in the Long-shoushan mountains lacks the Changcheng and Qingbaikou systems of Mesoproterozoic. The Middle formation distributes widely, and its thickness and lithofacies are stable. The of Lower formation's conglomerates becomes thiker easternwards and both sorting and roundness become better. Besides, the grain size is also finer, in some eastern places like Hexipu, Jinchang city and Hanmushan, Yongchang county. In the western part of the lower formation's distribution the conglomerate is thinner or even absent, such as the situation in the districts of Dongdashan, Zhangye city and Dufengling, Shandan county. From these characteristics of the Dunzigou group, it can be concluded that the North China plateform started the marine transgression at the Jixian period from east to west, forming the obviously distinct overlap toward west. The Dunzigou formation was severely deformed although it rexperienced minor metamorphism. The Dunzigou group sediments were formed in an environment of slope characterized by slumping and corrugate. On the other hand, severely distortion is the result of the effect of the Qilian movement.