The Stratigraphy of the Siwalik Series in the Northern Potwar, Punjab, Pakistan

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Summary Detailed mapping of the Jhamat area in the west and of the Soan syncline in the east forms the basis of revision of the stratigraphy of the Siwalik Series in the northern Potwar region. The succession is here affected by facies changes of regional significance :— 1. Near the Indus River the uppermost 3000 feet of the Middle Siwalik sequence (Upper Nagri and Dhok Pathan stages) contain thick beds of conglomerate which die out eastwards and south-eastwards into sandstones and clays. Farther eastwards, across the Soan syncline, a clay facies develops at progressively lower horizons in the Nagri Stage, replacing a considerable portion of the massive sandstones of the type area. The facies change is accompanied by a reduction in thickness. The facies variation in the Middle Siwalik rocks is indicative of a channel environment in the west at Jhamat, clearly coinciding with the line of the present Indus River and grading eastwards into deposits of “ flood-plain ” type. 2. The Lower Siwalik rocks show a development of coarser facies in the opposite direction to that noted in the Middle Siwalik. At the eastern end of the Soan syncline the lowest 1500 feet of the Chinji Stage, which in the west is almost entirely of clay-shale facies, contain thick beds of hard sandstone and the sequence is very similar to that of the underlying Kamlial Stage. In the Soan area, the complete Siwalik succession from the Kamlial Stage to the Pinjor Stage (Villafranchian) is free from any marked unconformities but is overlain with strong unconformity by post-Siwalik Pleistocene beds—the Lei Conglomerates. This interpretation differs greatly from that of de Terra (1936, 1939), who placed the first great unconformity of the area at the base of the Tatrot Stage. The revision explains Pilgrim's reluctance to include the Villafranchian in the Pleistocene, since in the Indian Siwalik belt this stage forms the highest member of a conformable succession which was intensely folded and subjected to peneplanation before the next deposition took place. The decision at the International Congress in 1948 to adopt the Villafranchian as the base of the Pleistocene places the Siwalik phase of Himalayan orogenesis within the Pleistocene System.