Geology of the Ghudai Sar and Vicinity, Jamrud, Khyber Agency

Abstract

The area of investigation lying in the eastern part of the Khyber Pass, between Jamrud and Waran Ghundai, is covered by the formations of Lower and Upper Palaeozoic ages. They are represented by the Landikotal Slate (? Ordovician-Lower Silurian), the Shagai Limestone (?Middle to Upper Silurian) and Ali Masjid Formation (Upper Silurian to ? Lower Carboniferous).
All of these are marine, and clastic sedimentary rocks account for more
than two-thired of the total thickness of the strata exposed in the area. The
Lahdikotal Slate constitutes as much as 6000 feet (plus/minus) of clastic sedimentary rocks.
Of all the formations, the Ali Masjid Formation is the only fossiliferous formation. It is reefoid, and extensive reef depsits occur in it.
Thamnopora, Cladopora, and Favosites are the important tabulate corals. Other fossils present are stromatoporoids, orthoconic cephalopod, byrozoans, brachiopods, and abundant crinoidal stems and columnals. The fossil assemblage indicates an age betwean Upper Silurian and Lower Devonian. Large scale metamorphism and dolomitization has occurred, which has resulted in the obliteration of the details of fossils. This has rendered difficult to identify many fossils even at generic level. The effect of metamorphism increases towards north and the reef limestones are converted to marble.

Late Cretaceous and Early Tertiary igneous activity indicated by the dolerite, gabbro, and (?) microgranitic intrusive bodies. Most of these are sills. Dolerite and gabbro sills have mineralized the calcareous phyllites and reef limestones. Extensive and large deposits of soapstone occur within calcareous phyllites and reef limestone (converted to marble).

The area is traversed by NS and NW-SE striking thrust faults. In the northeastern part of the area the rocks are folded into a NE plunging syncline. Recent movement have occured in the area as indicated by elevated terraces and alluvial valleys.