DECIPHERING THE GENETIC EVOLUTION OF EPISODIC DIAGENETIC MODIFICATIONS IN CRETACEOUS KAWAGARH FORMATION, HAZARA BASIN, PAKISTAN; INSIGHTS FROM GEOCHEMICAL INVESTIGATIONS

Abstract

The Cretaceous Kawagarh Formation is exposed in various parts of Hazara Basin. The Formation consist of micritic limestone and dolomite. Field investigations, petrographic studies, Mineralogical studies, geochemical analysis and isotopic analysis are used to decode the formation and mechanism of dolomites in the Kawagarh Formation. Two sections were sampled (Bagnothar and Kala Pani) to find out the mechanism of dolomitization. Three types of dolomites were recognized according to grain size and grain textures i.e. medium to coarse grain matrix replacive planar-p dolomite (RD1), medium grain matrix replacive subhedral dolomite (RD2) and medium to coarse grain matrix replacive nonplanar dolomite (RD-3). Beside replacive dolomites, two type of dolomite cements i.e. fracture filling fine grained dolomite cements (CD1) and coarse grain dolomite cements (CD2) were also reported. Voids and fracture filling cementing calcites (CC) i.e. Twin and syntaxial were also reported. XRD study shows that all the phases’ dolomites are stochiometric and of high order ranging 0.925945 to 0.952155 (with an average value of 0.921275). The analytical results demonstrate that dolomites have high Fe and Mn contents but low in Na and K content. Isotopic analysis indicates that dolomite phases show a varied range δ¹⁸O signature. Dolomites are highly depleted in δ¹⁸O ranging -3.89‰ to -4.94‰ V-PDB (with an average value of -4.66‰ V-PDB). Dolomites and calcites support the diagenetic alteration of the Kawagarh Formation by dolomitization temperatures. The stable carbon isotope δC¹³ values of all dolomite phases slightly depleted from the original ranging 0.77‰ to 1.32‰ V-PDB (with an average value of 1.08‰ V-PDB). The δ¹³C values of carbonates rocks is less affected by temperature. Compactional features in field observation, grain to grain contacts and medium to coarse grained dolomites having irregular boundaries between dolomites crystals in petrographic studies are the clear indication of deep burial. Geochemistry, mineralogical studies and isotopic results also support the deep burial dolomitization.