Depositional and diagenetic studies of the middle Jurassic Samana Suk Formation in the Trans Indus Ranges and western extension of Hill Ranges, Pakistan: an integrated sedimentological and geochemical approach

Abstract

The middle Jurassic Samana Suk Formation is well exposed in Himalayan foreland fold and thrust belt forming a good hydrocarbon reservoir of the Indus Basin; however, the combined sedimentological and geochemical studies are not conducted so far. An integrated approach using field, petrographic, geochemical, and isotopic studies was used to better understand the depositional and diagenetic processes within the formation. The formation is predominantly composed of thin to medium-bedded limestone with intercalation of shale. Field observations reveal sedimentary and diagenetic features such as cross bedding, sole marks, ripple marks, convolute bedding, stylolites, dissolution marks and patchy dolomitization. Microfacies associations include mudflat microfacies associations (mudstone MF-1, dolo-mudstone MF-2), lagoonal microfacies associations (siliciclastic bio-packstone MF-3, peloidal bioclastic packstone MF-4, bioclastic wackestone MF-5, and peloidal wackestone MF-6), barrier/shoal microfacies association (peloidal grainstone MF-7, ooidal–peloidal bioclastic grainstone MF-8, ooidal grainstone MF-9, and bioclastic peloidal grainstone MF-10). The above-mentioned microfacies associations suggest the deposition in the ramp settings (mudflats, lagoonal and shoal). The diagenetic features include: micritization, mechanical/chemical compaction, dissolution, neomorphism, cementation, dolomitization and fracturing. Selective replacement of grain dominated facies represents fabric retentive replacive dolomite RD-I formed at the early phase, followed by matrix replacive dolomite RD-II. Late-stage diagenetic alteration is marked by fabric-destructive dolomite RD-III. Geochemical data show a consistent decrease in salinity from the early to late diagenetic phases characterized by elevated Na and K concentration and reduced Fe and Mn concentration. Furthermore, stable isotopic data of limestone and dolomite phases show non-depleted δ¹³ C values ranging from + 0.26 to + 1.86‰ VPDB suggesting no external supply of carbon after the deposition of the carbonate units. The non-depleted δ¹⁸ O values ranging from − 1.96 to − 0.45‰ VPDB of dolomite phases represents seawater signatures, and hence may have formed in surface processes of marine water in mudflat settings/evaporitic conditions. Paleogeographically, Samana Suk Formation exhibits similar depositional conditions with the western coastline of the Tethys.