IMPACT OF LOCAL TECTONICS ON THE DIAGENETIC ALTERATIONS OF PRECAMBRIAN UTCH KHATTAK FORMATION EXPOSED IN GADWALIAN SECTION, HAZARA RANGES, PAKISTAN

Abstract

Late Precambrian Utch Khattak Formation has been studied in detail at Gadwalian Section, Gandghar Range Haripur, Khyber Pakhtunkhwa Pakistan. Extensive fieldwork was conducted to mark and measure igneous intrusions in the form of dykes and to investigate diagenetic events associated with thermal effect of igneous intrusions on outcrop level. 47 representative rock samples were collected from affected zone of the 246-meter-thick carbonate sequence out of which 27 thin sections were prepared to delineate diagenetic alterations for the interpretation of effect of igneous intrusions on the host limestone of Utch Khattak Formation. Detail Petrographic study reveals that the studied formation is composed of Five main lithological units i.e., Argillaceous limestone which remained unaffected during this thermal effect, host limestone, dolomitic limestone which shows slight diagenetic effect, dolomite which is an indication of complete replacement and Marble. Marble was present at the periphery of igneous intrusions along the contact aureole zone followed by dolomite zone and then by dolomitic limestone. This study reveals that high energy is required for the diagenetic alteration of impure limestone i.e., Argillaceous limestone in our case. Petrographic study claims that the dolostone of Precambrian Utch Khattak Formation is of secondary nature for which the precursor limestone was deposited on intertidal to subtidal and inner shelf marine environment. Stoichiometric study explains that dolomite of the studied formation displays significant variation in crystal ordering. Diagenetic processes involved in reservoir modification were dissolution, dolomitization, micritization, compaction, neomorphism and cementation. Diagenetic events like dissolution, dolomitization and mechanical compaction show positive affect on the reservoir character of the target formation, while the chemical compaction, micritization, neomorphism and cementation plays a negative role by decreasing the reservoir potential of the carbonate successions of the Utch Khattak Formation. Field features, petrographic interpretation and geochemical investigations suggests that dolomitization model for Utch Khattak Formation is Burial Compaction Model and Hydrothermal Dolomitization.