Occurrence of tephra / volcanic tuff in the tertiary sediments of Himachal Himalaya from Tileli area,Mandi district,H.P.: Implication for stratigraphy and uranium mineralization

Presence of Early Tertiary pyroclastic material (tephra) has been documented petrographically, for the first time, in the Mandi-Bilaspur Sector from Tileli area, Dharamsala basin of Himachal Pradesh. The tephra is reported from the red shale, identified as tuffaceous siltstone belonging to lower Dharamsala Formation that lies above the uraniferous sandstone body and occurs as thin layers of over 300m along the strike, close to the contact of lower and upper Dharamsala formations. The tuffaceous material shows crude but preferred orientation of minerals like biotite, muscovite, chlorite, clay, hematite and specularite. Various features indicating presence of tephra are, glass shards altered to clay but retaining ‘U’ shaped outline, spindle-shaped hematite with preferred orientation, spherical to sub-spherical clay and altered Feoxide rich balls, clay groundmass with flow pattern, flaky minerals in association with clast depicting asymmetrical ramp structure. A zone of approximately 300 m length containing tuffaceous material has been established at Tileli overlying the uraniferous sandstone body. Identification of tephra at Tileli has significant implications as it enabled in demarcating the boundary between the upper and lower Dharamsala formations in central part of the basin in Bilaspur-Mandi Sector of HP Himalaya and also in guiding the uranium exploration programme in the lower Dharamsala Formation.