Abstract：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.
. A study of sedimentary facies and sequence stratigraphy of the Changlongshan Formation(Qingbaikouan)in Xiaweidian area, West Mountain of Beijing[J]. JOPC, 2017, 19(6): 955-964.
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