乌拉特后旗测老庙坳陷早白垩世古流向分析

摘要

位于狼山以北的测老庙地区是内蒙古西部巴音戈壁盆地群东部边缘的次级坳陷,面积850 km2,沉积主体为白垩纪陆相碎屑岩.在目前能够搜集到的所有资料(包括内部资料)中,前人在该坳陷内的沉积相模式分析以及水文地质研究等工作均认为:测老庙坳陷下白垩统巴音戈壁组的物源区主要是坳陷东南侧的狼山,这意味着狼山在早白垩世就已经隆起.几十年内,这个模式一直作为指导思想影响着该地区找矿勘探方向,但找矿效果均不佳,造成了极大的财力、物力和人力资源的浪费.笔者等认为是这一模型存在问题,因为早白垩世巴音戈壁期狼山尚未隆起,无法充当测老庙坳陷的主要物源区.在沉积盆地分析中,利用古流向判别沉积岩的物源区方位是一种非常简便、有效的研究方法,然而笔者等未找到前人在本研究区进行该项工作的记录.笔者等在测老庙坳陷露头区对巴音戈壁组碎屑岩进行了大规模的古流向测量工作:寻找发育波痕和前积纹层等沉积构造为主的露头48处,并依46组前积纹层和5组波痕测得共计51组数据,利用野外现场恢复和室内赤平投影的方法恢复其原始古流向,最终用玫瑰花图和古流向分布图展示测老庙坳陷早白垩世古流向格局.分析结果表明,测老庙坳陷在早白垩世巴音戈壁期的古流向以自北向南为主,向SE、SW两个方向对称递减,集中分布在140°～210°的范围内,甚至在紧靠狼山的区域,古流向垂直指向狼山山脉延伸方向.据此可以判断:狼山不是巴音戈壁组碎屑岩的主要物源区,狼山最早也应在早白垩世巴音戈壁末期之后才可能隆起.这一研究结果为测老庙地区早白垩世沉积岩相关矿产的勘探开发提供了重要的科学依据,为狼山隆起时代的研究提供了重要的突破点,同时也为沉积岩的古流向测量、恢复与物源区分析提供了成功的范例.%Objectives:Celaomiao Depression,Urad Rear Banner (Wulate Houqi),located to the north of Mount Langshan,in the west of Inner Mongolia,is filled with Cretaceous clastic rocks,covering 850 km2.According to the former studies on the sedimentary facies and paleogeography in the Celaomiao Depression,it was believed that the clastic rocks of the Early Cretaceous Bayingebi Formation were mainly supplied by the neighboring orogenic belt,Mount Langshan,to its southeastern margin.It means that Mount Langshan had already been uplifted in the Early Cretaceous.However,there is no evidence to support this model.This paper's research indicates that the source area of the Early Cretaceous clastics in the Celaomiao Depression was a fossil orogenic belt situated to the north and Mount Langshan had not been uplifted yet.Methods:The authors of this paper conducted field expeditions in the Celaomiao Depression and the Mount Langshan.Using the foreset beddings/laminations and ripple marks as the major indicators of the waterflow direction,the authors of this paper measured the attitudes of the sedimentary beds and the sedimentary structures on the outcrops of the Early Cretaceous Bayingebi Formation,recovering the original paleocurrent directions of the depression during the Early Cretaceous through stereographic projection.In this paper,the authors provided the data of field attitude measurements,stereographic projection results and rose diagrams of paleocurrent directions in the Celaomiao Depression,analyzing its paleocurrent regime and the provenance direction.Results:The authors of this paper got 46 sets of foreset beddings/laminations and 5 sets of wave-induced asymmetric ripple marks measured on the outcrops of the Early Cretaceous clastic rocks over the Celaomiao Depression (Table 1).All the measurements indicate that the paleoflow directions were mainly from north to the southeast with a range of 140° ～210°,perpendicularly pointing to the extension trending of Mount Langshan (Figure 4 and 5).Conclusions:This paper's investigation shows that the Early Cretaceous paleocurrent directions were mainly from north to the southeast in the Celaomiao Depression.This means that the provenance of the Early Cretaceous Bayingebi Formation was a fossil orogenic belt situated to the north of the Depression.Obviously,Mount Langshan had not been uplifted by the end of the Early Cretaceous and could not have been the source area of the Celaomiao Depression.