| Abstract: | A mixed siliciclastic-carbonate system that responds to changes in Permian climate and subsequent carbonate platform evolution is investigated using microscopic details of the Middle Permian Amb Formation (Fm.), in Saiyiduwali section, Khisor Range, northern Pakistan. Thin sections were made from rocks throughout the stratigraphic section of the Amb Fm. and analyzed with an emphasis on carbonate and clastic microfacies, and the latter interpreted within the existing chronostratigraphic framework. Outcrop observations reveal that the units comprise coarse-grained, channelized, ripple-marked, and burrowed sandstone and sandy, fossiliferous limestone with minor marls and shale intercalations, suggesting deposition in a subaqueous tide-dominated delta to beach barrier. Based on the determined seven microfacies coupled with outcrop observation, the Amb Fm. was deposited in a tide-influenced subaqueous delta to middle shelf environment under fluctuating sea level. The deposition of compositionally mature sandstone in the lower part of the formation suggests reworking of detritus from the rift shoulders and an adjacent source area with an ambient warm and humid climate. The stratal mixing of carbonates and compositionally mature siliciclastic units in the middle part suggest deposition under tectonic and climate-induced terrigenous and carbonate fluxes to the basin. Thus the deposition shows a perfect transition from clastic-dominated deltaic to pure carbonate platform settings as a result of warm climate and tectonics. This Middle Permian warming is confirmed by sea-level rise and the presence of a temperature-sensitive fusulinid fauna in association with photozoan-based ooids. Deposition of the Amb Fm. and establishment of a carbonate platform are envisaged to be associated with major rifting of northern Gondwana, which subsequently resulted in the development of a rift basin at the passive margin of the NW Indian Plate then in northern Pakistan. |
