Sankholi Fm

Himachal Pradesh-Uttarakhand, Sankholi village in Nigalidhar syncline (Bhargava et al., 1998). [Original Publication: Bhargava, O.N., Singh, I., Hans, S.K., Bassi, U.K. (1998). Early Cambrian trace and trilobite fossils in the NigaliDhar Syncline (Sirmaur district, Himachal Pradesh), lithostratigraphic correlation and fossil contents of the Tal Gr. Himalayan Geology, v. 19, pp. 89–108].

Sandy claystone. The Sankholi Fm is made up of alternate bands of grey to olive green, fine to medium grained sandy and clayey beds occurring as distinct members.

Greywacke Member: This member comprises quartz sandy claystone, siltstone and olive green to dark grey shale with sporadic limestone interbeds. These lithounits occur as alternating layers repeated in quick succession.

Banded Siltstone Member: Besides banded siltstone, it comprises carbonaceous siltstone, shale, 15 cm to 2 m thick quartzwacke and limestone lenses of 30 cm to 2.5 m.

Trilobites: Dolerolenus (Malungia) cf. M. laevigata, Drepanopyge gopeni, Protolenella cf. P. angustilimbata of early Cambrian from upper part of the Sankholi Fm in Nigalidhar syncline. A few trace fossils were also known from the Mussoorie and Nigalidhar synclines.

The greywacke unit displays a variety of sedimentary structures like graded bedding, convolute bedding, ball and pillow structure, flute casts, flaser and lenticular bedding and cross-bedding. The name greywacke used here is textural and does not meet the mineralogical requirement. The sedimentary structures in the order of abundance are horizontal lamination, graded bedding, small-scale cross-bedding, flame structures and hummocky cross-stratification. It shows deposition in deep to shallowing upward basin.

See Srikantia and Bhargava, 2021; Age taken from the correlation chart (fig.6) from (H. Xu, J.G. Meert and M.K. Pandit, Geoscience Frontiers 13 (2022)