北京西山及邻区寒武系顶部微晶丘特征及层序地层学意义

The Characteristics and Sequence Stratigraphic Significance of the Micrite Mound at the Top of Cambrian in Western Hills,Beijing and Neighboring Areas

北京西山、河北涞源及山西浑源等地位于寒武系—奥陶系界面处的厚层块状“纯灰岩”实际是由微晶方解石、丛状或丝状蓝绿藻、碎屑等共同营造而成的大型微晶丘,具明显的丘状外貌及侧向地层向微晶丘顶部的上超;地球化学特征也显示出与周围同期地层的不同及与上覆冶里组中层纹状叠层石灰岩的相似性;内部岩石微相有:藻粘结灰岩相、具不规则孔洞藻粘结灰岩相、碎屑藻粘结灰岩相(藻间为黄褐色白云岩砾块、砾屑灰岩及砂屑灰岩)。内部有7个粘土层和2个白云岩砾石层,它们是由邻区搬运物堆积形成的,代表微晶丘内9个生长间断面——9个次级水体变浅过程。微晶丘形成于寒武纪末较稳定低海平面时期,代表冶里层序低水位体系域,层位可能与三山子白云岩相当。

' Pure limestone' , located at the boundry of Cambrian and Ordovician in the Western Hills of Beijing and Hunyuan county of Shanxi province,was found to be a big'micrite mound' built with micrite limestone, blue-green algae and detritus. It is onlapped by lateral beds and shows a mound appearance. Its geochemical characteristics are different from those of surrounding beds but similar to lamellar stromatolites of the Yeli Fm. It contains mainly 3 micrite facies:algal bindstone,algal bindstone with irregular caves, clastic algal bindstone (in the interstices of the algae are yellowish brown dolomite fragments, calcirudite and calcarenite facies). The micrite mound represents the LST of the Yeli sequence, formed during the shrinkage of the sea area at the end of the Cambrian and stratigraphically corresponds with a part of the Sanshanzi dolomite. Within the micrite mound there are 7 clay beds and 2 dolomitic gravel beds,which were formed by accumulation of materials transported from neighboring areas, representing 9 surfaces of growth discontinuity of the mound,i. e. 9 secondary water-shallowing events.