四川冕宁昔格达组磁性地层学初步研究及意义  

MAGNETOSTRATIGRAPHIC DATING OF THE XIGEDA FORMATION IN MIANNING,SICHUAN AND ITS SIGNIFICANCE

作　　者：姚海涛[1] 赵志中[1] 乔彦松[1] 李朝柱[1] 王书兵[1] 王燕[1] 陈永生[1] 蒋复初[1] 

机构地区：[1]中国地质科学院地质力学研究所,北京100081

出　　处：《第四纪研究》2007年第1期74-84,共11页Quaternary Sciences

基　　金：中国地质调查局地质调查项目(批准号:200312300034)资助

摘　　要：文章首次对川西安宁河流域冕宁昔格达组河湖相沉积物进行岩石磁学和磁性地层学初步研究。结果表明沉积物具有正常的沉积组构,适合磁性地层学研究;赤铁矿和磁铁矿是主要载磁矿物;沉积物记录的古地磁场方向基本都为正极性,主要形成于高斯正极性时,约4.18～2.58Ma。综合对比表明,分布在大渡河、安宁河和金沙江干流的昔格达组以典型黄灰色岩性组合为特征,具有基本相同的沉积序列和磁极性序列,主要形成于高斯正极性时。The Xigeda Formation is renown for its well-developed Late Cenozoic fluvial-lacustrine sequence, which is distributed discontinuously in valleys of the Dadu River, the Anning River, the Jinsha River, and the Yalong River in the Panzhihua-Xichang region, Southwestern Sichuan. The sediments provide great potentials for understanding the formation process of the East-West geomorphological boundary zone of China between the Tibetan Plateau and the Yangtze Block. However, its lithostratigraphic and chronologic views in different areas remain contentious topics in the past 70 years. Here we contribute to this debate with detailed magnetostratigraphic result from the Mianning Section with a thickness of 300.7m at Zhoujiadagou in Mianning County. The 300.7-m section can be divided into two parts from top to bottom： the yellow coarse sand and thin silty clay alternating beddings （0 - 112.6m） and the grey leading silty clay layers （112.6 -300.7m）. Preliminary rock magentic and magnetostratigraphical studies on this new discovered section have been carried out. Anisotropy of magnetic susceptibility shows that the sediments have normal deposition fabrics with horizontal foliation planes and have not been subjected to physical disturbance since deposition, indicating a high potential for magnetostratigraphic research. Temperature-dependent susceptibility curves of representative samples illustrate that hematite and magnetite are the dominant magnetic carrier minerals. Stepwise thermal demagnetization successfully isolated 342 （96%） well-defined characteristic magnetization （ChRM） after removing one or two soft components of magnetization. Six magnetozones are recognized in the Mianning Section： three normal, N1 （0 -97.5m）, N2 （102.5 - 166m）, and N3 （168-297.2m）; and three reversed, R1 （97.5 - 102.5m）, R2 （166 - 168m）, and R3 （297. 2 - 300. 2m）. According to Electron Spin Resonance age of river terrace （1.29Ma） on the topmost of the Xigeda Formation and the Cande and Kent＇s pol

关 键 词：攀西地区 昔格达组 岩石地层学磁性地层学 岩石磁学 

分 类 号：P539.3[天文地球—古生物学与地层学]