The area west of Cao Bang-Tien Yen fault, except for the Cao Bang basin, covers most of Nguyen Binh and Thach An districts. This area exhibits a mix of karst landforms and other denudation-erosion landforms on terrigenous rocks of the Song Hien formation and granites of the Phia Oac complex. The karst landforms are part of the Dong Mu karst massif 1,100-1,300m asl. It forms a plateau between the Nang River and Bang Giang River and has been termed “Binh Lang plateau” by Bourett (1922).
On the relatively flat plateau surface, the terrain is weakly intersected into mountain ranges extending mostly in a NWSE direction separating wide, flatbottomed inter-connected valleys. The area to the west of Cao Bang- Tien Yen fault has been identified as part of overthrust nappes “nappes préyunnanaises” (J. Deprat, 1915, 1917; Lepvrier et al., 2011). At the edge of the massif, the land surface has slipped down to 700- 1,000m, much lower than the ancient planation surface. This has favoured rapid karst evolution creating sharp pyramid-like towers separated widely by flat or undulating valley floors. Hogback topography is also conspicuous with cliffs facing east with piles of rock collapse at their foot, creating stepped erosional terraces of Nguyen Binh River and the so-called “self-covered” karst. In addition to the karst plateau west of the Cao Bang-Tien Yen fault, from Nguyen Binh district towards Thach An district there are large areas of non-karst rocks of the Song Hien terrigenous Formation (T1sh) and the Pia Oac granitic complex (γKpo), forming highly dissected structures. Along Provincial Route No.34 from Nguyen Binh to Cao Bang city, the mountain ranges are more than 1000m high, while to the west and east of Cao Bang city they are only 400-600m and 250-350m high, respectively.
These mountain ranges clearly demonstrate topographical steps which are remnants of planation surfaces at altitudes of 400-600m, 700-1000m and 1100-1300m. Thus, the following geoheritage features can be observed west of Cao Bang-Tien Yen fault in Nguyen Binh and Thach An districts (Annex 6.2.2 for more details):
– Planation surfaces 1100-1300m, 700-1000 and 400-600m asl; – Expressions of overthrust nappes (“nappes préyunnanaises”) with terrigenous rocks of the Song Hien Formation overlying Devonian and Carboniferous-Permian limestones;
– Residual karst landforms of tower and cone forms;
– Mountain divides with pyramid-shape peaks;
– Tinh Tuc basin, where placer tin ores are mined, and remnant of the ancient bottom of this basin and karst canyon at the eastern end of this basin.
Planation surface 700-1.000m
Pediment in Tĩnh Túc