北京西山下苇甸地区青白口系长龙山组沉积相及层序地层研究*

北京西山下苇甸地区出露良好的青白口系长龙山组,笔者通过实测野外露头剖面以及岩石薄片镜下鉴定,对该组下部沉积相及层序地层进行研究,并对沉积环境演化进行分析。识别出长龙山组下部8种岩石类型有含砾砂岩、羽状交错层理砂岩、丘状交错层理砂岩、波状层理粉砂岩、脉状层理粉砂岩、透镜状层理粉砂岩、碳质泥岩以及水平层理泥岩;并识别出辫状河道、潮坪(潮道)、潮下浅水及潮下深水等沉积相类型,建立该区辫状河—滨岸潮坪沉积模式。进而通过识别长龙山组与其下伏地层之间的区域不整合面和下切谷河道充填砂砾岩底面确定层序界面。其中,初始海泛面以每个砂体之上覆盖的细粒沉积的底面为代表,最大海泛面以厚层碳质泥岩及水平层理泥岩的底面为代表。依据这些关键层序地层界面,将该区长龙山组下部划分为3个层序,每个层序内部进一步划分为低位体系域、海侵体系域以及高位体系域。综合分析表明,京西的长龙山组发育于由燕辽裂陷槽转为华北稳定克拉通的过渡期。

A study of sedimentary facies and sequence stratigraphy of the Changlongshan Formation(Qingbaikouan)in Xiaweidian area,West Mountain of Beijing

The Changlongshan Formation of Qingbaikouan is well exposed in the Xiaweidian area,West Mountain of Beijing. Based on the analysis of the lower part of Changlongshan Formation and the thin sections of the sandstones examined by the optical microscope,sedimentary facies,sequence stratigraphy,and environmental evolution of the Changlongshan Formation were determined. A total number of 8 rock types were identified,including gravelly sandstones,Herringbone cross-bedded sandstones,hummocky cross-bedded sandstones,wavy-laminated siltstones,flaser-laminated siltstones,lenticular-laminated siltstones,carbonaceous mudstones,and horizontal-bedded mudstones. Based on the lithofacies analyses,four main sedimentary facies were recognized,including braided channel,tidal flat(tidal channel),shallow subtidal,and deep subtidal facies,and a braided channel-shoreline tidal flat depositional model was proposed. The regional unconformity between the Changlongshan Formation and its underlying strata,and the bottom of glutenite filling the incised valley were identified as sequence boundaries. The initial flooding surfaces are characterized by basal fine-grained deposits overlying on the major incised valley filled sandstones,while the maximum flooding surfaces are typified by the bases of a thick succession of the carbonaceous mudstones and horizontal-laminated mudstones. Three sequences were recognized in the lower part of the Changlongshan Formation in the Xiaweidian area,with each further subdivided into LST,TST and HST by the initial flooding surface and maximum flooding surface. The comprehensive analyses indicate that the Changlongshan Formation in western Beijing was developed during a period characterized by the transition from the Yan-Liao rifted trough stage to the North China stable craton stage.