Formation of Brahmani Natural Arch, Eastern India: A proposed geoheritage site of India

Abstract

Natural arch is one of the most fascinating geomorphological features formed by erosional process and many of them are formed within gently dipping sandstone. A natural arch (Location: 21°54′32.20″ N, 83°43′46.20″ E, RL: ∼370 m) of such type is present in the rocks of Upper Kamthi Formation of Middle Jurassic age (160–184 Ma), Upper Gondwana Group, Ib River Coalfield within dense Chhengapahar Reserve Forest near Brahmani Village, Sundargarh District, Odisha. This oval shaped natural arch with a maximum length and height of ∼30 m at the base and ∼12 m respectively. The alcove of the natural arch has maximum height and width of ∼7 m and ∼15 m respectively, is opened along NE-SW direction on the top of the hill. Flesh red colored, ripple topped, trough to planar cross stratified, coarse to gritty, texturally and mineralogically immature, ferruginous sandstone of Upper Kamthi Formation forms the hill. Kaolinisation of feldspathic members makes the sandstone loose and porous. Two orthogonal set of faults-joints (130°/60°W and 40°/60°E) initiated the formation of natural arch while the porosity of the immature, arkosic sandstone accelerate the process through subaerial weathering where surface/rain water percolation–temperature fluctuation played effective roles. Therefore, the formation of natural arch is a result of structurally triggered weathering and erosion. Efforts have already been taken to preserve this place as national geo-heritage site by Geological Survey of India, Government of India.