Sedimentary And Diagenetic Evolution Of The Middle To Late Permian Carbonate Platform On The Northwestern Passive Margin Of Indian Plate,Pakistan

The present study aims to investigate the evolution of Middle to Late Permian carbonate platform in response to ambient climate and tectonics at the northwestern margin of the Indian Plate,Pakistan.The carbonate platform is archived within the Zaluch Group(Amb,Wargal,and Chhidru formations)in the Indus Basin,Pakistan.The field investigations revealed that the Amb Formation(Wordian)is predominantly comprised of siliciclastic rich carbonate rocks,followed by monotonous carbonate succession of the Wargal Formation(Capitanian to Wuchiapingian).The gradual enrichment of shallow marine biota from the basal part of the Amb Formation to the carbonate rich Wargal Formation argue for an evolving carbonate platform.The increasing bed thickness of sand dominated carbonates within the overlying Chhidru Formation(Changhsingian)hints towards the gradual demise of carbonate platform.Moreover,the pinching of the Zaluch Group towards eastern Salt Range indicates that the shoreline lies to the east while the facies ocean water are progressively deepening from Central to Western Salt Range.The petrographic attributes of the Zaluch Group further suggest the gradual replacement of regressive deltaic facies of the Amb Formation by middle shelf facies of the Wargal Formation with the establishment of full-fledged carbonate platform during Capitanian to Wuchiapingian.The gradual demise of carbonate platform is evident from the siliciclastic rich carbonate and deltaic microfacies of the Changhsingian Chhidru Formation.The evolution and demise of carbonate platform is usually the outcome of interplay between ambient climate and tectonics.The shale geochemistry of Amb Formation(Wordian)is utilized to underpin paleoenvironment,paleoclimate,paleoweathering,and tectonic conditions on the northwestern margin of the Indian Plate.Different parameters indicate intense chemical weathering of the protoliths under warm humid subtropical climate and the deposition of shale under oxidizing environment with fresh water settings.In addition,the presence of abundant mineralogically stable quartz grains,detrital kaolinite and montmorillonite while complete absence of chlorite and illite further supports warm and humid conditions during the deposition of the Amb Formation.The consistent higher values of Al augmented by detrital proxy as well as Si/Al and Ti/Al ratios indicate enhanced fine grained sediments.Among the different possible sources there are an igneous source derived from the post-Archean fractionated upper continental crust and a felsic source.The discriminants diagrams based on major oxides also suggest felsic origin of igneous and quartzose sedimentary recycled orogen.The geochemical attributes in association with the provenance analyses of underlying and overlying rocks indicate the derivation of sediments from Aravalli and Malani Ranges in the southeast.The Permian tectonics and climate have played a pivotal role in the establishment of marine carbonate platform at the northwestern margin of the Indian Plate which is characterized by rift flank basin in Pakistan.The onset of platform is recorded as transgressive episode during Wordian postdating the Gondwana rift cessation followed by thermal cooling of basin margins.Such tectonic package progressed till Capitanian resulting in the establishment of carbonate platform within the Neo-Tethys rim basin.The climatic warming during the Middle Permian(Wordian)is further supported by the sea-level rise,formation of ooids,and preservation of temperature sensitive fusulinid fossils.The evolution of platform can be classified into three stages: i)Wordian marginal marine stage of evolution followed by ii)carbonate sediments of Capitanian age in the middle part of the Zaluch Group.And iii)demise of the carbonate platform in Changhsingian.The delta dominated shelf were established in response to regression which has deposited the clastic dominated carbonates in Changhsingian.Thus the initial phase of the evolving platform is recorded by mixed clastic and carbonate sediments of the Amb Formation,gradually replaced by pure carbonates of the Wargal Formation.Similar carbonate platform facies are also located elsewhere in northern Pakistan i.e.Swat,Hazara,Peshawar Basin,and Kashmir;however,these deposits are intensely metamorphosed due to ongoing India-Eurasia collision.The carbonates are major hydrocarbons reservoirs and their reservoir potential is the outcome of diagenetic modification.Based on detailed petrography,Scanning Electron Microscopy(SEM),Energy Dispersive Spectroscopy(EDS),Cathodoluminescence(CL),and stable isotopic analyses,strong digenetic modification of the reservoir potential within the Zaluch Group is indicated.Initially the rocks were subjected to marine and meteoric diagenesis,characterized by dissolution and cementation.The burial diagenesis is marked by stylolites,neomorphism,and dislocation of grains.The burial phase is followed by uplifting and subsequent enhancement of fracturing through circulating meteoric fluids.The progressive diagenesis under various phases and subsequent evolution of porosity and permeability has greatly reshaped the reservoir potential of strata.The outcrop,petrographic,SEM,plug porosity/permeability analyses have shown moderate,low,and moderate reservoir potentials for the Amb,Wargal,and Chhidru formations.