Geological Complexities of Rawana Landslide, Sirmaur District, Himachal

The Rawana Landslide is unique as it is dynamic in nature, associated with emission of hot water vapors (Temp 45–60 °C), change in chemical quality and a foul smell emitting from some of the vents. The active landslide area is increasing in its aerial extent as observed from LISS III, the present dimension shows a length of 420 m and variable width ranging 190–270 m. The toe of the landslide is in the Giri River Channel and continuous erosion by the river is leading to reactivation of the slide. Geologically, the landslide material is composed of carbonaceous shale/slatey shale with nodules of pyrite minerals, occurrence of quartzite of Infra-Krols and carbonate rocks of Upper Krols formation. Thus the landslide is in a geologically complex area where lithology, structural features, seepage from springs and so on are contributing to the recurrence of landslide activity. This activity is associated with the NW-SE trending linear Giri Thrust, and is marked by a prominent fault trending NNW-SSE. The other prominent structural feature is the presence of a NE-SW trending planar lineament, orthogonal to the Giri Thrust, which dissects the landslide mass on its SE extreme and rendering the western portion to move down compared with the eastern part. This has resulted in the formation of a prominent scarp on the western side facilitating the slide. The landslide area has a close network of drainage pattern and presence of two springs which continuously drain through the landslide, producing an exothermic reaction converting seepage water into acidic water with low pH (2.4) and high salinity at EC 6320 mhos. These chemical reactions have altered the basic composition of rock producing new mineral assemblages.