Microfacies, Diagenesis and Depositional Environments of Samana Suk Formation Middle Jurassic Carbonates Exposed in South East Hazara and Samana Range

This document presents the results of detailed work on the microfacies analysis, depositional environments and diagenetic history of the Middle Jurassic carbonates system, (The Samana Suk Formation) investigated in the three measured outcrop sections (total thickness 785m) in the Hazara and Kohat area. The three sections (Sanghar Gali, Bagh and Samana Range) lie along the northern rim of the Upper Indus Basin of Pakistan. Rocks exposed in the three above mentioned localities were investigated in the field for facies and sedimentary structures. 800 samples were collected and each sample was further investigated on polished slab, and 850 thin-sections were prepared for detailed petrographic analysis. All the data generated was interpreted for depositional environments and diagenetic imprints. This work is also supported by selected SEM photographs, forty two oxygen and carbon isotope analyses and fourteen trace element analyses. The data from the three studied sections is presented semi-quantitatively in Sedimentary Synthesis Logs and Diagenetic Feature Logs. The results are also supported by biostratigraphic analysis of selected shales and limestone samples. All the results of these studies were integrated to propose a likely depositional or sequence stratigraphic model of the Samana Suk Formation. Benthic foraminiferal data suggest that carbonate sedimentation started in Bajocian (Middle Jurassic) and might have continued until Callovian time. The data has also confirmed the Bathonian age from the middle part of the formation in the Samana Range Section. On the whole, the faunal/floral assemblage suggest sedimentation of the Samana Suk Formation occurred on the shallow to very shallow marine setting typical of gently dipping shelf/ramp., The assemblages suggest various episodes of sea level rise and fall. Thirty microfacies have been identified that were deposited in variety of environments. Generally the formation is dominated by oolitic grainstone, that have deposited in shallow marine subtidal to intertidal environments as an oolitic shoal complex on a gently inclined carbonate ramp/platform which might have attained slightly rimed shape in a later stage. Within the whole succession about 19, 20 and 31 shallowing upward cycles have been interpreted in the Sanghar Gali, Bagh and Samana Range Sections respectively. These cycles vary in numbers and thickness in the three studied sections and do not correlate individually across the study area. Such cycles normally start from mudstone (M2) and/or wackestone (W1 or W2) and terminate at oolitic grainstone while occasionally complete cycles go up to pelletal pack-grainstone (PG1) and/or unfossiliferous mudstone (M1), representing the tidal flat facies. These cycles are parasequences which started from subtidal environments and terminate in intertidal environments and rarely end in supratidal environment. Twenty six different diagenetic features have been identified in the Samana Suk Formation. Overall, diagenetic sequence ranges from marine phreatic to deep burial. However dominant influences of meteoric phreatic and mixed meteoric marine environments during shallow burial have also been observed. Dolomite, though minor component relative to limestone, is present in several forms in the Samana Suk Formation such as layers and thin beds, dolomitized burrows, grain selective and cement/matrix selective dolomite, molds and fracture filling dolomite, dolomite developed along sutured seams and stylolites, and pervasive dolomite. Dolomite in the Sanghar Gali and Bagh Sections is common, while in the Samana Range Section it is rare. Dolomitized burrows and pervasive dolomite in the upper 35m of the Bagh Section are volumetrically important. Dolomite layers, beds and pervasive dolomite were developed in meteoric marine mixing zone. Bio-pumping together with bacterial activities in the burrow network, related to the burrowing organisms, was probably responsible for the preferential dolomitization of burrow fillings. The grainstone horizons originally had good primary intergranular/ interparticle porosity, which was first reduced by early mechanical compaction and later on occluded during diagenesis by seven different types of calcite cements. Three phases of dissolution and four phases of fracturing have been identified which have generated secondary intergranular porosity (i. e., mouldic, vuggy and fracture) which has also been occluded. Two sets of fractures are filled with medium to coarse blocky calcite, 3rd set is filled with internal sediments whereas 4th set is filled with dolomite. The high resolution microfacies and cyclicity, biostratigraphic abundance data has been used to interpret the significant depositional surfaces such as flooding surfaces (transgressions) and sequence boundaries (regressions). Four flooding surfaces and four sequence boundaries have been interpreted which divides the whole Samana Suk Formation into four 3rd order sequences. These sequences have only highstand and transgressive system tracts. This interpretation is based on the most striking contrast in the microfacies. The proposed model shows proximal facies (mainly oolitic shoal and back shoal facies) towards the east-southeast while distal or open marine facies (mainly mudstone/marl) towards the west-northwest. This also implies that the lowstand system tract is probably preserved further west of the Samana Range section.