Petrography and geochemistry of the kamlial formation, southwestern Kohat Plateau, Pakistan: Implications for paleoclimate of the western Himalayas

Abstract

The Middle to Late Miocene Kamlial Formation that largely consists of sandstone and interbedded clay/mudstone sequences is exposed in the southwestern part of the Kohat plateau, which constitutes the westernmost extension of the Himalayan Foreland Basin. Whereas the sandstone is gray to brownish gray, fine- to medium-grained, and mostly thick-bedded, the interbedded clay/mudstone sequence is brownish gray to maroon red and occurs as continuous beds as well as lenses. Some mudstone/clay beds are bioturbated and seem to be pedogenic surfaces. Results of geochemical investigation of fresh (unaltered) representative sandstone and mudstone samples from three different sections of the formation in the southwestern Kohat plateau are presented and discussed. The average chemical index of alteration (CIA) values of both the sandstone (70-86) and mudstone (71-85) suggest moderate to slightly intense weathering in the source area. However, the high CIA values may also be due to the presence of abundant sedimentary rock fragments, which occur in the studied sandstone, rather than a result of severe weathering. Furthermore, the possibility of intensive chemical weathering in the Himalayas orogenic belt is highly unlikely, as it requires tectonic quiescence for a long period, higher temperature, and humidity. Accordingly, the range of the index of chemical variability values (0.6-2.1) of mudstone and low contents of Rb and Cs in both the mudstone and sandstone indicate somewhat moderate weathering. Furthermore, the Th/U and Rb/Sr ratios of the Kamlial Formation are lower than the corresponding average values for the upper continental crust and post-Archean average Australian shale, which shows that these sediments are first-cycled in origin. However, the Zr/Sc ratio indicates minor contributions from recycled sedimentary sources. The values of authigenic U and the U/Th, V/Cr, Cu/Zn, and Ni/Co ratios all suggest that the Kamlial sediments were deposited under oxidizing conditions.