贵州剑河八郎地区寒武系“清虚洞组”沉积环境初探——以剑河八郎地区松山剖面为例

PRELIMINARY ANALYSIS OF SEDIMENTARY ENVIRONMENT FROM CAMBRIAN “TSINGHSUTUNG FORMATION” IN BALANG AREA,JIANHE COUNTY,GUIZHOU PROVINCE——CASE STUDY OF SONGSHAN SECTION FROM BALANG VILLAGE,JIANHE COUNTY

贵州剑河八郎地区松山剖面的寒武系"清虚洞组"出露完整,总厚272m,主要由灰岩、泥岩及白云岩组成。文中自下而上将"清虚洞组"分为4个岩性段:Ⅰ段,灰色薄-中厚层含丰富生物碎屑泥质微晶灰岩;Ⅱ段,灰白色薄-中厚层含泥质微晶灰岩;Ⅲ段,灰色薄-中层含完整生物碎屑泥质微晶灰岩;Ⅳ段,灰色薄-中厚层含藻屑泥质微晶灰岩和细晶白云岩。Ⅲ段泥岩中产出包括软躯体化石在内的多门类化石,即剑河生物群。在前人工作的基础上,文中通过野外剖面观察、生物化石采集和微相分析等研究方法,对"清虚洞组"四个岩性段的沉积环境及其对应的海平面变化趋势进行了初步分析,认为"清虚洞组"沉积于含氧量较充沛、盐度正常、低能的较深水陆棚-陆棚边缘环境。中上部的特异埋藏化石群指示一个快速的沉积事件。分析结果表明,"清虚洞组"沉积期水体经历了3次变浅和2次变深的交替变化情况。其中,Ⅲ段沉积期,海底位于氧化还原界面附近;Ⅰ、Ⅱ和Ⅳ段沉积期海底均位于氧化还原界面之上。

The Cambrian ＂Tsinghsutung Formation＂crops out extensively in eastern Guizhou,it is perfectly exposed in the Songshan section at the Balang Village,Jianhe County. ＂Tsinghsutung Formation＂is 272 m thick.Lithologically,the＂Tsinghsutung Formation ＂, dominated by limestones, mudstones and dolomites can be subdivided into four members in ascending order：Thin-bedded to medium-bedded biodetritus pelmicrite（Member I）;Thin-bedded to mediumbedded pelmicrite（Member Ⅱ）;Thin-bedded to medium-bedded biodetritus pelmicrite（MemberⅢ）; Thin-bedded to medium-bedded pelmicrite with algae underlaying fine-crystal dolostones（Member Ⅳ）.Whats more,silt mudstone from the upper part of the ＂Tsinghsutung Formation＂contains a Burgess shale-type Lagersta¨tte,known as the Jianhe biota. Based on detailed field investigation, microfacial analysis and palaeontological evidence,we proposed that the limestone-dominated ＂Tsinghsutung Formation＂was deposited at the edge of deep shelf or deep shelf, which refers to a low water energy environment with high oxygen,normal salinity and good light conditions.Specifically,the Lagersta¨tte in the mid-upper part of the ＂Tsinghsutung Formation＂indicates a rapid-burial event.In general, the sea level regarding the entire Tsinghsutung succession changes corresponding to three shallowing periods and two deepening periods,respectively.Our study also indicates that the MemberⅢ may have been deposited nearby the redox surface,whereas the members I,ⅡandⅣ were probably deposited above the redox surface.