The Outer Sedimentary Belt:
The Outer Sedimentary Belt is delineated in the south by the MBF (= Krol Thrust) and in the north by the Ramgarh Thrust. It includes a relatively less thick pile of sediments correlatable with the Krol Belt of the Garhwal-Simla Himalaya. It occurs as a detached outlier, which widens westwards and extends upto Himachal Pradesh through Garhwal. Bhattacharya (1980) considers the Outer Sedimentary Belt to rest conformably over the Inner Sedimentary Belt, and is hence considered the younger of the two.
The lowermost horizon of the Outer Sedimentary Belt is the Nagthat Formation, which appears to correlate well with the Berinag Formation (Valdiya, 1978). It consists of conglomeratic and pebbly protoquartzites, shales, slates, with some basic rocks.
The Blaini Formation which overlies the Nagthat Formation consists of two typical rock types; one is the boulder beds, the other the limestones, overlying the former. The rock association is not constant and often the boulder beds or the limestones can occur alone, or several horizons of boulder beds are found together with the limestones. These complications may be due to tectonic imbrication (Gansser, 1964).
Conformably overlying the Blainis is the Infra-Krol Formation consisting of gray to black slates and shales with siltstone and quartzite bands. Rarely blue limestone beds are seen. The Krol Formation, which has been divided into three sub units viz., the Lower, Middle, and Upper Krols overlie the Infra-Krols. The Lower Krols consist of shales and slates with dolomitic intercalations, gray shales with intercalations of limestone, dolomites, and minor red shale. The Middle Krols have green to red, medium to fine grained sandstone with shaly and silty intercalations. The Upper Krols include massive and thickly bedded dolomites, cherty and oolitic dolomite, and subordinate shale and quartzite interbeds.
The strata of the Outer Sedimentary Belt have been folded into a syncline on an E-W axis, the northern limb of which is open as compared with the southern limb. The northern limb exposes all the rock units of the Outer Sedimentary Belt and is best exposed near Nainital (Bhattacharya, 1980). The Outer Sedimentary Belt has been affected by a number of tear and normal faults of varying dimensions.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)