Unconformity boundary on Khau Khoang pass

Khau Khoang pass Unconformity is located on the left side of the National Highway 4A in the direction from Cao Bang city to Dong Khe town, in Thai Cuong commune, Thach An district. An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer. The unconformable relationship here is clearly observed between limestone of the Na Quan formation (D1-2 nq) having mottled color and a 270 50 bedding, being medium-bedded, brecciated and ductile deformed into boudinage with dimensions of up to tens of centimeters along the bedding and the dark grey siliceous-clayish shale of the Song Hien formation (Tish), inter-bedded with purple sandy siltstone medium-bedded, having a 27050 bedding and being ductile deformed into scabbard folds.

Surrounding is peak cluster-depressions karst topography developed on limestone of Na Quan formation (D_1-2nq) and eroded topography in the form of hills on shale, basalt... This ttopography is cut by intrusive gullies and saddles on fractured zones resulted from faulting zone.

Currently, the direct unconformable relationship between limestone of the Na Quan formation (D1-2 nq) and siliceous-clayish shale of the Song Hien formation (Tish) is unable to be observed as it is cut by the newly opened road to the village in the north and north west. Now, only the indirect unconformable relationship can be observed through the road: medium-bedded limestone about 20m high on one side and black shale with mottled yellow and brown surface of weathering and covered by thick vegetation on the other side. The unconformable boundary between Devonian and Triassic rocks represents intra-continental rift tectonic activity with bed is the Palaeo-Tethys ancient ocean limestone in the Northeast Vietnam - South China region, causing sedimentary discontinuities. Boudinage, folded forms represent the ductile deformation while scratches, slickensides, fractures and brecciation represent ductile deformation in tectonic phases. Pillow basalts are the result of ocean floor spreading.

Karst landforms such as peak cluster-depressions, blind karst valleys, saddles, etc. represents the weathering and erosion processes occurring in fractured zones due to the paragenetic NW-SE and NE-SW fault system. Some facets on limestone are evidence of NW-SE faulting activities during the Neotectonic period, which is of high geoscience significance, reflecting the evolution of the region and the formation and development of the Earth's crust in Northern Vietnam. The existence of unconformity is evidence that the transition from oceanic crust to continental crust is not continuous, but cyclical.