香山群的解体及地层时代的重新厘定

DISINTEGRATION OF THE XIANGSHAN GROUP AND ITS AGE DETERMINATION

宁夏一带的香山群,由于各种原因,其时代和地层的划分自建立以来一直处于争论不休的状态。产生这种问题的原因最终聚集在两个问题上的分歧:香山群的地层构造和化石产出部位的沉积构造环境分析上。根据野外的实际观察及前人资料的综合分析,香山群的构造并非成简单的单斜排列,各个单元的沉积也各具特征。其中第一和第三亚群(包括徐家圈组第四段)为秦—祁—贺裂陷槽裂陷初期的产物,分布于盆地的东缘,时代为中寒武—晚寒武世;第二亚群(不含徐家圈组第四段)和第四亚群为裂陷槽裂陷中晚期的产物,主要分布于盆地的西北缘,时代为奥陶纪。结合区域地层的综合分析,提出了早古生代贺兰裂陷盆地的沉积充填模式及地层格架。

The age and stratigraphic sequence of the Xiangshan Group in the Ningxia region have been disputed. The debata centers on the stratigraphic structure and sedimentary tectonic environment of the Xiangshan Group. According to new field observation and analysis of previously published data, it is proposed that the Xiangshan Group does not occur as a monoclinal plunge as thought before. Instead, each stratigraphic subunit in the Xiangshan Group has different characteristics. The first and third subgroups （including the fourth member of the Xujiajuan Formation） were deposited in initial stage of the Helanshan Rift in middle-late Cambrian, and they were formed in the eastern part of the rift. The second subgroup （excluding the fourth member of Xujiajuan Formation） and the fourth subgroup were deposited in middle and late stage of the rift in the Ordovician, and they mainly occur in the northwestern part of the basin. Combined with regional stratigraphic analysis, the new data provide a stratigraphic framework and infilling model of the Helanshan Rift basin.