Research paperHow quiet was the epeiric sea when the Middle Cambrian Zhangxia Formation was deposited in SW Beijing, China?

- A limestone became brecciated by wave action in a Cambrian epeiric sea.
- The breccia layer became fragmented, probably by storm-wave overloading.
- A fragment of several metres slid down the very gently inclined sea floor.

The Middle Cambrian Zhangxia Formation, 65 km southwest of Beijing City, was deposited in an epeiric sea that extended over almost the entire North China palaeo-plate (including Korea). The depositional conditions under which the limestones were deposited are commonly considered to have been quiet, although oolites indicating current and wave activity occur fairly frequently. The presence of a large (6 m) lens of breccia, embedded in oolite, raises the question, however, how quiet the depositional conditions actually were. The lens is composed of a limestone breccia which occurs only rarely in the Zhangxia Formation, but that is common in the Late Cambrian Chaomidian Formation.It appears that the large lens was deposited as an entity. This does not fit the commonly presumed quiet depositional conditions of its host rock. Therefore it may be concluded that the lens was put in place by a special event. This event is interpreted to have been a form of mass movement, more particularly sliding of a consolidated but not yet completely lithified breccia mass.Only once such a feature has been described before from an epeiric sea, remarkably enough from the Chaomidian Formation, at a distance of several hundreds of kilometres from the present study site. Although this feature thus seems to be extremely rare, it is important as it indicates instable conditions. Such instable conditions in epeiric seas have not commonly been recognized thus far, which implies that our idea of the conditions in this environment need to be re-considered.