PROVENANCE STUDY OF THE NAGRI FORMATION SIWALIK GROUP, KALLAR SYEDAN SECTION, RAWALPINDI, PAKISTAN

Abstract

The Miocene-Pliocene Nagri Formation of the Siwalik Group is molasse deposit exposed in the Defence housing Authority (DHA) Valley, Kallar Syedan, Rawalpindi. Here the formation comprises sandstone facies (N1), and sandstone-siltstone-mudstone facies (N2). The major lithology of the formation is green, fine- to medium -grained, and thin- to thick sandstone beds with intercalation of siltstone and maroon mudstone. Graded and cross-bedding, spheroidal and differential weathering in the sandstones are the common features observed. In thin section study, the rocks contain monocrystalline and polycrystalline quartz, alkali feldspar, plagioclase, rock fragments, biotite, muscovite, zircon and epidote. Groundmass is calcite and at places mixed calcite and clayey material. Alkali feldspar is microcline and perthite whereas anorthite content in plagioclase ranges up to 10%. Both the feldspars are partly altered into sericite, calcite and epidote. Rock fragments constitute volcanic and metasedimentary rocks. On the basis of petrography, the sandstones of the Nagri Formation of the Siwalik Group fall into feldspathic and feldspathic litharenites type domains. The dominance of alkali feldspar over plagioclase suggests granitic composition of the eroded rocks, probably derived from Lesser and Higher Himalayas and/or the Kohistan Island Arc. The presence of mica, epidote, amphiboles and illite may suggest low-medium grade metamorphism encountered in the area before the uplift and erosion. Q-F-RF ternary plot distinguish the sandstone and siltstone of the Nagri Formation as recycled orogen and dissected arc provenance. The medium to coarse-grained sandstone may signify it as sandy bedload deposit due to river action. The intercalation of sandstone, siltstone and mudstone may indicate, the deposition in a relatively calm depositional environment.