KrolB Fm

Simla (Shali Basin), Himachal Pradesh-Uttarakhand, the name Krol Series was first given by Medlicott (1864) to a series of limestone, red and green shales and sandstones seen in the Krol mountain near Solan in Simla area. [Original Publication: Auden, J.B.,1934. The geology of the Krol Belt. Rec. Geol. Surv. Indi. 67, 357-454.]

Claystone. It is characterised by thinner laminated purple to red shales with blotches and intercalations of green shale. There are interbedded wedges of thin dolostone and cherty limestone. Ripple marks are sporadically seen. On top of this unit, parallel bedded calcilutite, similar to those seen in the Lower Krol Limestone, is present.

[Figure: Krol Gr lithology table (provided by O.N. Bhargava & Birendra Singh)]

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It is a deposition in a protected shallow lagoon/embayment/mudflat where due to increased supply of terrigenous material of clay, deposition of carbonate mud became subordinate. Detailed sequence stratigraphic studies identified several disconformable surfaces (Jiang et al. 2002) indicating interruptions in largely tidal to intertidal (Singh 1980) sedimentation in a shallow NW-sloping marine Krol Basin, with minor phases of deep-water environment (Jiang et al. 2003).

Red shales of the KrolB constitute nearly 90% of the lithofacies. It has a thickness of about 100 m and is underlain and overlain by carbonate rocks of shallow marine origin. It comprises laminated purple-red shales of uniform colour with patches of green shales of variable extent. In thin section red shales show a fine-grained and homogeneous groundmass composed of terrigenous quartz of silt grade with subordinate amounts of microcrystalline calcite and accessories (feldspar, sericite and opaques). The red pigment of the shales is identified as haematite. It occurs in a finely diffused state permeating the entire rock mass; rarely it is organised into irregular patches or rings. The fine fraction of the red shales is characterised by a constant clay mineral composition of illite and iron-rich chlorite. The green shales are texturally and mineralogically similar to the red shales, but they lack in finely distributed haematite. Gypsum pockets are sporadically found in this Fm, the one at Rajpur was economically exploited.