Petrography and Geochemistry of late Cretaceous Pab Sandstone, Laki range, Southern Indus Basin, Pakistan: implications for Provenance and Paleoclimate
Keywords:
Pab Sandstone, Laki Range, Provenance, PaleoweatheringAbstract
Geochemistry and petrography of the late Cretaceous Pab sandstone exposed at the Laki Range, Southern Indus basin has been carried out in order to acquire better understanding of source rock, paleoclimate and the tectonics. The unit in the study area dominantly comprises of sandstone with some conglomeratic sandstone and mudstone beds. The petrographic studies of Pab sandstone reveal the quartz as the major mineral constituent in almost all analyzed samples. The monocrystalline variety of quartz was seen dominant than the polycrystalline; with dominant non-undulatory extinction, the undulation was also noticed. In addition, studied samples reflect medium to coarse size grains, angular to subangular to subrounded shape characterized with the grain to grain contacts as point to straight and sutured. Iron oxide is observed to be leading cement with calcareous as the second most abundant cementing material, the matrix was also observed in minor amount. The monocrystalline quartz abundance in Pab sandstone indicate the existence of granitic rocks in the source region. The lesser fragments of feldspar and lithics suggest of their alteration to various clay minerals. The Pab sandstone is composed mainly of monocrystalline quartz derived dominantly from craton interior as revealed through plots of model composition on Quartz -Feldspar- Lithics (QFL) and Quartz Monocrystalline-Feldspar-Lithics (QmFLt) plots as per the petrographic characterization. The major element geochemistry of Pab sandstone indicates that the samples comprised of abundant SiO2 (56.21 to 69.93 wt.%), Al2O3 (12.56 to 22.98 wt.%), CaO (1.68 to 6.38 wt.%) and Fe2O3 (3.87 to 14.12
wt.%), the rest of the major oxides are <2 wt.%. Additionally, major element compositions suggest the sediment derivation from a granitic source. The Chemical Index of Alteration (CIA) values ranging from
61.12 to 83.75 and the values of Plagioclase Index of Alteration (PIA) ranging from 59.94 to 89.44 suggests that studied sediments underwent the weathering conditions being moderate to intense in warm and humid climate regime.
The present petrographic findings and geochemical data suggest the sediment derivation from huge Cratonic landmass (Indian Shield). The similar rock types in the form of Nagar Parkar Igneous Complex are still exposed in the southeast of study area in Pakistan and Malani Igneous Suite in India.
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