A REVIEW OF THE GEOLOGICAL AND HYDROGEOLOGICAL REGIMES AT THE HIMALAYAN FOOTHILLS IN NORTHWEST PAKISTAN

The ongoing northward convergence of the Indian plate with greater Asia has resulted in the formation of four tectonomorphic terranes. From north to south, the major fault systems separating these terranes are the Main Karakoram thrust (MKT), which separates Asian plate metasedimentary rocks from the Kohistan island arc (KIA); the Main Mantle thrust (MMT), which separates KIA from rocks of the Indian continental margin; the Main Boundary thrust (MBT), which carries sedimentary and metasedimetary rocks on its hanging wall and folded Miocene foreland-basin deposits in its footwall; and the Salt Range thrust (SRT), separating rocks of the Indian continental shelf from the Quaternary alluvium of the Indus plain. The synorogenic upper Siwalik group in the proximal foreland of NW Pakistan records the imprints of the Pliocene-Pleistocene evolution of the Himalayas. During this time period the foreland basin was internally partitioned by Himalayan deformation giving rise to piggy-back basins such as Peshawar and Campbellpore Basins.

The Peshawar intermontane basin is a broad, oval shaped depression comprising of a thick sequence of lacustrine, deltaic and fluvial sediments overlain by loess and alluvial deposits, which are dated at 2.8 to 0.6 Ma. These sediments form well-productive aquifers and consist mainly of sand and gravel in the north and south of the basin. However, in the central part, the coarse sediments are interbedded with clay, silt and sandy silt attaining its maximum thickness and producing several semi-confined aquifers. Khyber, Attock-Cherat and Lower Swat-Bunner piedmont aquifers occur on the periphery of the basin, while flood plain and lacustrine aquifers occupy the central part of the basin. Depth to the water table is < 5 m, except on the margins of the basin and in the southeast where it ranges from 5 to > 30 m. The EC values range from 800 mS/cm near the margins to a maximum of 7800 mS/cm in the center of the basin. Similar is the case of mineralization. Majority of the groundwater has low TDS with dominant bicarbonate and sulfate types of the alkaline earths and alkalies. However, in the central part of the basin, there is a transition from the fresh bicarbonate to highly mineralized sulfate-chloride water. Such waters are either a product of evaporative process or a "squeeze out" from deeper evaporite formations.