Lower Siwalik Fm
Type Locality and Naming
Pilgrim (1910, 1913, 1934) classified the Siwalik Group into Lower, Middle and Upper divisions. The Lower Siwalik is further subdivided in Pakistan (type sections on Potwar Plateau in easternmost Pakistan) into a lower Kamlial Fm and upper Chinji Fm.
Lithology and Thickness
Lower Siwalik Fm is characterized by red mudstones. In Ganga Basin, this formation consists of alternations of sandstone, variegated claystone and siltstone. In Gandak-1 the formation dominantly comprises of claystone with few interbeds of sandstone in the upper part. The claystones are various shades of brown and grey and are calcareous. The sandstone are fine to very fine grained. The calcareous matter is fairly common. In Matera-1 this formation consists of alternations of sandstone, siltstone and claystone. In Puranpur depression this formations is predominantly composed of dirty white to light grey, medium to fine grained sandstone with subordinate earthy brown to grey, silty, sticky claystone. In Ganga Basin, the thickness varies from 276 to 1825 m with maximum development of this formation seen in the Gandak depression.
Relationships and Distribution
Lower contact
Depends on the basin. In Punjab, it is conformable on the Upper Dharamshala Fm (early Miocene). In Ganga basin, it is conformable on the Matera Fm (Olig-?Miocene) or unconformable on Karnapur Fm (Cambrian). In Purnea, the group unconformably overlies the Lahil Fm of Late Jurassic of the Upper Gondwana subgroup.
Upper contact
It has a gradational contact with the overlying Middle Siwalik Fm in some regions; but a ca. 0.7 Myr gap in others (see Siwalik Gr age summary). Petrographically Kyanite appears at about the Lower and Middle Siwalik boundary.
Regional extent
It extends from Indus in the west to Brahmaputra in the east except for a small break near Sikkim where the Lesser Himalaya comes in direct contact with the Indo-Gangetic plains. Tectonically, it is the least affected sequence of the Himalaya. The northernmost boundary of the Siwaliks Group is marked by the Main Boundary Thrust (MBT), over which the low-grade metasedimentary rocks of the Lesser Himalaya overlie..
GeoJSON
Fossils
Generally unfossiliferous. However, in Ganga Basin well Puranpur-2 from CC-10 (Intervals 2946-2952 m) and CC-11 (Intervals 3065-3074 m) plenty of microfossils have been recorded from the Lower Siwalik Formation. They belong to the foraminifera, nannoplankton, marine diatoms, dinoflagellate acritarchs and calcareous algae groups. These microfossils are important indictors for determination of age and environment. Besides, Angiospermic fossil leaves (CC-5, Gandak-1) are also present in this Formation
Age
Depositional setting
Lower Siwalik sediments were deposited in braided fluvial system. Deposition took place in a tectonically quiescent setting, following a thrust event at 20 Ma.
Additional Information