Vol.:(0123456789) 1 3 Carbonates and Evaporites (2020) 35:101 https://doi.org/10.1007/s13146-020-00634-0 ORIGINAL ARTICLE Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan Hamad ur Rahim 1,2,3  · Mumtaz Muhammad Shah 1  · Mercè Corbella 3  · Dídac Navarro‑Ciurana 3 Accepted: 7 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic lime- stone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO 3 ) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non- stoichiometric (33.39–55.08 mol% of CaCO 3 ) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO 3 ) in origin. Stable isotope studies reveal non-depleted δ 18 O and δ 13 C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ 18 O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ 13 C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydro- thermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes. Keywords Samana Suk formation · Diagenetic evolution · Dolomitization · Stoichiometry · Stable isotope geochemistry Introduction Carbonate rocks are more prone to diagenetic alteration such as dolomitization (Machel 2004; Moore 2001; Tucker and Wright 1990). The study of dolomitized limestone succes- sion is the topic of intensive research from the past decades due to presence of major hydrocarbon reservoirs worldwide (Zenger et al. 1980; Land 1985; Warren 2000; Cantrell et al. 2004) . Besides this, various economic minerals associated * Mumtaz Muhammad Shah mshah@qau.edu.pk 1 Department of Earth Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan 2 Earth Sciences Division, Pakistan Museum of Natural History, Garden Avenue, Shakaraparian, Islamabad 44000, Pakistan 3 Department de Geologia, Facultat de Ciencies, Universitat Autonoma de Barcelona, Edifici Cs s/n, Bellaterra, 08193 Cerdanyola del Valles, Spain