Detailed investigation of dolomites in the Middle Jurassic Samana Suk Formation, Kahi section, Nizampur Basin, NW Himalayas, Pakistan

Diagenetically modified carbonate rocks are more common in the rock record. Among these modifications, multiphase dolomitization is the most common process. The Middle Jurassic carbonates of the Samana Suk Formation are extensively altered by the dolomitization process in the Kahi section, Nizampur Basin. The primary objective of this study is to investigate this multiphase dolomitization and to elucidate its possible mechanism. Field investigation shows both host limestone (oolitic, fossiliferous, and massive) and dolomites. Dolomite bodies are of both beddings: parallel to and crosscutting the bedding planes. Different types of dolomites were recognized on the basis of color contrast as dark gray replacive dolomite, light gray dolomite, brownish dolomite, and yellowish dolomite. Along with the replacive phase, void- and fracture-filling cementing saddle dolomite and cementing calcites are recognized in the field. Petrographic studies show the complex diagenetic history of the Samana Suk Formation from near-surface diagenesis, including micritization, neomorphism, and several varieties of dolomites. These varieties are as follows: RD1 is very fine- to fine-grained dolomite; RD2 is medium- to coarse-grained and anhedral to subhedral dolomite; RD3 is coarse- to very coarse-grained and planar euhedral zoned dolomite; and RD4 is coarse-grained euhedral to subhedral ferroan dolomite. In addition, cementing saddle dolomite SD consists of large crystals with curved faces showing sweeping extinction. Cementing calcite phases are as follows: CC1 is granular mosaic; CC2 is twin; CC3 is fracture-filling; and CC4 is ferroan calcite. The stable isotope values of limestone (delta (super 18) O is -7.13 to -0.73 per mille V-PDB, and delta (super 13) C is -0.05 to 1.32 per mille V-PDB) show depletion with respect to the Jurassic marine signature. The values of multiphase dolomites RD1-RD4 and SD (delta (super 18) O is -8.65 to -3.16 per mille, and delta (super 13) C is -3.56 to 2.09 per mille) indicate multiphase dolomitization. The CC1-CC3 values (delta (super 18) O is -11.07 to -8.97 per mille and delta (super 13) C is -2.14 to 0.76 per mille) indicate highly depleted values of delta (super 18) O, showing hydrothermal origin. From field, petrography, and geochemistry data, it is deduced that a possible source of Mg for hydrothermal dolomites is activation of faults and fractures during active tectonic regime in the area and might be related to activation and reactivation of the Kahi Thrust system.