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Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan
Carbonates and Evaporites ( IF 1.4 ) Pub Date : 2020-09-20 , DOI: 10.1007/s13146-020-00634-0
Hamad ur Rahim , Mumtaz Muhammad Shah , Mercè Corbella , Dídac Navarro-Ciurana

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 limestone 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 CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C 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 δ18O 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 δ13C 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 hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

中文翻译:

巴基斯坦西北部小喜马拉雅山脉中侏罗纪 Samana Suk 组的成岩演化和相关的白云石化事件

Samana Suk 组侏罗系碳酸盐岩广泛出露于喜马拉雅山前陆地区,是印度河流域上游的主要储集层。这些碳酸盐由鲕粒、泥质含化石和泥晶灰岩单元组成,这些单元已经通过成岩作用,特别是白云石化作用进行了广泛的改造。现场观测显示了两种不同类型的白云岩地质体(i)层状平行层状,和(ii)斑片状白云岩单元。地层基部为层状平行层状白云岩,Samana Suk 组中上部为片状白云岩。在研究区,两个地质体的白云石化强度从 NW 到 SE 增加。岩相学研究根据质地、晶体大小和形态揭示了六相白云岩和三相方解石。这些相是:基质置换白云岩(MD-I 到 MD-III);胶结白云岩包括置换胶结白云岩(RD)、鞍形胶结白云岩(SD)和晚期胶结白云岩(CD);方解石相包括CC-I和CC-II。XRD 分析表明层状基质白云岩(MD-I、MD-II)是化学计量的(CaCO3 的 51.08-51.86 mol%)并且含有高达 95% 的矿物白云石。片状白云岩胶结物是非化学计量的(CaCO3 的 33.39-55.08 mol%),含有约 65% 的矿物白云岩,而鞍状白云岩的起源也是非化学计量的(51.57 至 53.50 mol% 的 CaCO3)。稳定同位素研究揭示了基质白云岩的未耗尽 δ18O 和 δ13C 值(MD-I,MD-II) 代表侏罗纪碳酸盐岩的同期海水特征,因此可能是由蒸发过程形成的。白云岩胶结物 (RD, SD) 显示出耗尽的 δ18O 值,这表示温度升高,与其形成的热液源有关。裂缝充填方解石 (CC-II) 显示出较少的枯竭值,表明受浅至中度埋藏影响的大气流体。云岩显示耗尽的 δ13C 值表明它们是由大气水形成的。现场、岩石学和地球化学研究表明,Samana Suk 组的成岩演化是一个多阶段的过程。在第一阶段,由于海水在潮间带环境中的表面过程,可能形成了包括海相胶结、层状白云石化在内的海洋成岩过程,而第二阶段的成岩作用是由于埋藏相关过程,包括由于富镁流体沿裂缝、断层、层理面和柱状岩等弱平面运动而发生的热液白云石化作用。最后阶段包括由于抬升相关过程形成热液裂缝充填方解石、置换黄铁矿和白云岩。
更新日期:2020-09-20
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