北京西山寒武系滑塌构造的初步研究

北京西山地区的寒武系层序完整,剖面出露好,是中国地质学家研究寒武系的经典地区之一.前人对北京地区寒武系的研究集中于百花山北麓清水河永定河流域的军庄、丁家滩、下苇甸、青白口至军饷之间的露头.作者等在北京房山地区百花山南麓进行地质考察中,在上寒武统崮山组下部的中薄层条带状灰岩和砾屑灰岩中发现了典型的滑塌变形层,变形主要表现为发育大量的小型层间微型断裂或褶皱,大部分断裂表现为小型的低角度逆冲性质,断裂面呈舒缓波状,平均倾角10°左右,微型断裂面上盘还可见更次级的微型褶曲或断层和布丁.滑塌变形层之下还发现了与古地震有成因关系的楔状构造和特殊的砾岩.结合前人在寒武系中已经发现的震积岩,初步认为,这种典型的滑塌变形的直接触发机制很可能是伴随着华北古板块在中寒武世末与晚寒武世之间的“翘翘板式运动”而发生的古地震.

A preliminary research on the discovery of slump structures in Cambrian system in the Western Hills of Beijing

The Cambrian system in the Western Hills of Beijing outcropps mainly along the Yongding river valley in the north and along the Dazhihe river valley in the south. Famed for the complete sequence, unique rock types and good exposure, the Western Hills in Beijing has been a key location for the study of Cambrian system in China. However, nearly all the field work and publications have been concentrated on the northern part (along the Yongding river valley), for the convenient transportation. The slump structures are found about 5 m above the boundary between the Zhangxia Formation (Middle Cambrian oolite) and the Gushan Formation (Upper Cambrian) in the southern part of Wetsten Hillsalong a branch of the Dazhihe river valley). The slump is mainly composed of micro folds and faults. Axial planes of these folds are SN striking. The typically dip gently towards the east. The average dipping angle of the micro faults is about 10 degrees. The deformation pattern is very similar with that in the Late Pleistocene Lisan Formation near the Dead Sea. Two wedge shaped features are discovered just below the slump layer. Our preliminary research shows that the direct trigger for the newly discovered slump structures are probably the palaeoearthquake associated with the tilting movement of the North China palaeoplate in the late Cambrian.