SEDIMENTARY BASINS OF PAKISTAN

Sedimentary basin is defined as “Geologically depressed area with thick sediments in the interior and thinner sediments at edges”. This definition is precisely applicable on present day configuration of Indus and Balochistan basins, where stratigraphic sequence is normal and stratigraphic record is not much influenced by tectonic activities. On the other hand, the intervening area between Indus and Balochistan basins affected by tectonism shows only a linear representation of different attenuated stratigraphic units of both the basins and is called Axial Belt. Similarly, 'Northern Montane Area' lacks any true form of sedimentary basin.With this situation in the later two areas, most workers do not use the word basin, but describe the stratigraphy, where applicable, under various terms like province, block, belt, domains, zones, island arc, folded belt, thrust belt, complexes, ridges, terrane, valleys and ranges. Very few workers used the term 'Basin' but probably as an informal term, as for example “Peshawar Basin”, “Hazara Basin” etc. Therefore, these areas with sedimentary fill do not conform to the strict sense of sedimentary basin, nevertheless, in one form or the other stratigraphy is there and workers have described it. Stated in other way, originally these haphazardly placed sediments were deposited in a true sedimentary basin but most of them away from their present day situation under different latitudes and longitudes. Now these units are
accumulated at one place with long history of tectonism.
              As for example, the filled up original basins of “Northern Montane Area”, West Himalaya was subjected to epeirogenic and orogenic movements. The orogenic movement triggered by plate tectonic activities brought in, interalia, heat and upheaval, which obliterated the original sedimentary basins beyond all repairs. Ultimately, the present day scenario presents the area as buckled, folded and faulted with nappe and thrust sheets and warped in a manner of alternating basins and swells. As for example, the southern part of the Himalayan Tectono stratigraphic basin contains, parautochthonous units characterized by folds and wedge structures directed towards south. Compressional forces, post-dating the nappe movements produced, steepening of the beds, steep reverse faults and thrusts, in this region, it is a characteristic of the nappe that their aerial extent shows much variation along the strike. Because of their sheet like form, nappe are eroded in axial culmination and arc preserved in depressions, imposing a prerequisite of tectono stratigraphic resolution before the stratigraphy of basin and ridge like structures is worked out. The
parautochthonous units override the Tertiary belt along a rather straight thrust line Main Boundary Thrust (MBT), also called Murree Thrust as noted by Fuchs (1982). Displacement in some of these nappes and other faults as measured or indicated, from the Indian side of the basin, is of the order of several kilometres.
           Further, north, alien stratigraphic sequences, belonging to different interacting continental plates form a collision zone. Here, presence of abnormal stratigraphic sequences is a common feature. In the entire northern area including Chitral, Gilgit and Hunza, large-scale allochthons are present, which thrust over the apparently normal stratigraphic section, at various places also pinching of the formations, crushing of the strata are seen on the ridges as well as in the valleys.Thick slabs, slices and slivers of lithological units arc assembled one above the other without any relation to adjacent strata. Here only palaeontology can come to the rescue. In fact, close coordination of structural and paleontological investigations becomes imperative for the construction of a correct stratigraphic section.

         To summarize the discussion it is obvious that some of the sequences arc less disturbed but lack age assignment due to non availability of fossils and most are tectonically much disturbed and the workers get be wildered as to what is stratigraphic and what is tectonic sequence specially when the units are non fossiliferous. Isolated exposures showing only a part of the original stratigraphic sequence, highly attenuated and repetition of strata arc there to deal with. Therefore, at the outset workers have to understand the tectonostratigraphic evolution of the area and its contents before the properstratigraphic knowledge is achieved.

         This makes it different from those of other areas, where sequence is normal, with little or no
diastrophism, having obvious and easily understandable stratigraphic set up with all the attributes of original sedimentary basin. This difference between the two areas is enough to designate them different domains with each having a characteristic mode of stratigraphic investigation. Therefore for the sake of convenience in the description of stratigraphy and for ready reference, the former domain is informally termed here as tectono stratigraphic basin and the later as stratigraphic basin.