Middle Siwalik Fm
Type Locality and Naming
Pilgrim (1910, 1913, 1934) classified the Siwalik Group into Lower, Middle and Upper divisions. The Middle Siwalik is further subdivided in Pakistan (type sections on Potwar Plateau in easternmost Pakistan) into a lower Nagri Fm and upper Dhok Pathan Fm.
Lithology and Thickness
Middle Siwalik Fm is characterized by multistoried, grey, medium to coarse grained, salt and pepper sandstones. A major regional lithological change occurs at the beginning of the Middle Siwalik. The multistoried vertical accretion, fine-grained lithology of the Lower Siwalik changes to a dominantly multistoried lateral accretion sandstone lithology in the Middle Siwalik. The interbedded clays are thinner and dull red, grey, bluish grey types compared to the dark red beds of the Upper Dharmsala Fm and Lowe Siwalik Fm. In the Ganga Basin, the formation consists of sandstone grading from subgreywacke to arkose with calcareous matter occurring as segregation rather than disseminated throughout the rock mass. Grain size varies from coarse to medium and the sorting is poor. Sandstones are dull to steel grey in color. These are interbedded with pebbly sandstone bands especially towards the top. These sandy bands are alternated with dull colored claystone. In Ganga Basin, this formation attains maximum thick in the eastern part of the basin. In Madhubani and Gandak depressions up to 1800 m of this formation is encountered in the subsurface. In Puranpur depression more than 1500 m section is encountered in the northern part and there is a gradual decrease in thick in southerly direction and in Kasganj well the thick reduced to 250 m In Saharanpur-1 about 1000 m of this section is penetrated.
Relationships and Distribution
Lower contact
It has a gradational contact with the underlying Lower 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.
Upper contact
In the outcrop, the upper contact with the Upper Siwalik is taken at the appearance of massive conglomerate beds. Away from the foothill area, in the subsurface of Ganga basin, the upper contact is gradational.
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
The palynomorph assemblage is characterized by Graminidites sp. Pinuspollenites spp., Laevigatosporites sp., Cicatricosisporites spp., Polypodisporinites sp., Compositoipollenites sp.,
Age
Depositional setting
The Middle Siwalik represents widespread braided fluvial system of deposits.
Additional Information