北京西山及邻区下古生界白云石化与层序地层

Lower Paleozoic Dolomitization in Western Hills of Beijing and Its Adjacent Areas and Its Relationship with Sequence Stratigraphy

近十年来 ,笔者在对北京西山及邻区下古生界进行层序地层学研究的过程中 ,对白云石化及其与沉积环境的关系和层序地层意义进行了探讨 ,获得了如下新认识 :①根据泥质条带灰岩、泥晶灰岩和颗粒灰岩中白云石化发生的规律 ,证实粘土矿物 (泥质混入物 )的存在对白云石化有重要的催化作用 ;②北京西山下寒武统昌平组下部“疙瘩状”灰岩的形成与白云石化有关 ;③寒武纪和奥陶纪广泛发育的“生物扰动灰岩”已受到强烈的白云石化改造 ;④中寒武统徐庄组和张夏组中“砾屑状”鱼而粒灰岩的砾状构造和断续条带状构造并非原生沉积构造 ,有些“砾屑”是准同生白云石化的结果。根据白云石的形成条件 ,笔者认为 :①在奥陶系马家沟组的厚层藻纹层灰岩 -生物扰动灰岩 -白云质灰岩 -白云岩旋回中 ,白云岩是这类副层序的顶面 ;②三山子白云岩 (上部 )是寒武纪末期高水位体系域晚期的产物 ,而人们将其与三山子白云岩对比置于北京 -涞源 -浑源一带的微晶丘为低水位体系域的沉积 ,即寒武纪末期的海水仅退出了三山子白云岩分布区 ,而没有退出微晶丘分布区。

During the study of the Lower Paleozoic sequence stratigraphy in Western Hills of Beijing and its adjacent areas in the last decade,the authors studied dolomitization,its relation to sedimentary environments and sequence stratigraphy.Four new considerations are obtained as follows: ①According to the dolomitization in mud_stripe limestone, micrite limestone and grainstone,the existence of clay minerals(muds) was an important accelerant in dolomitization.②The formation of the “abnormal debric like limestone” in the lower part of Changping Formation in Western Hills of Beijing is related to dolomitization.③Trace fossils in bioturbited limestone have been heavily reformed by dolomitization.④Rudaceous structure and discontinuous ribbon strutucres in rudaceous oolite limestone are not necessarily primary structures and some of the “gravels” are products of penecontemporaneous dolomitization. According to the formation condition of dolomite,the authors have reached the following conclusions:①In the cycle of thick bedded algae lamellar limestone bioturbited limestone dolomitic limestone dolomite in Majiagou Formation,dolomite constitutes the top of this kind of parasequence.②The upper part of Sanshanzi dolomite was formed during late HST at the end of Cambrian,and its correlated stratum,the micrite mound located along Beijing Laiyuan Hunyuan,is the sediment of LST.This suggests that when regression took place at the end of Cambrian, the areas where micrite mound is now located were sea areas,whereas the areas where Sanshanzi dolomite is now located were not sea areas.These viewpoints are valuable in studying sediment environments and sequence stratigraphy.