Diagenesis of the Middle Eocene Upper Dammam Subformation, Qatar: Petrographic and isotopic evidence

The subsurface Middle Eocene Upper Dammam Subformation in Qatar displays a complex diagenetic history related to sea level variations. The studied rocks are made up of dolomitic limestones and pervasive dolomites. Two main types of dolomite are identified based on an integrated petrographic/isotopic/mineralogic study: dolomites type (I) and (II), which affect the lower (the Umm Bab member) and the upper (the Abaruq member) parts of the Upper Dammam Subformation respectively. Dolomite type (I) is pervasive and composed of relatively coarse crystals that exhibit polymodal size distribution and different shapes of crystal boundaries (non-planar, planar-s and planar-e). Meanwhile, dolomite type (II) is selective, fine-textured and made up of unimodal equant euhedral rhombs. The marine delta O-18 (+0.6 to +2.8 parts per thousand) values of dolomite type (I) are consistent with formation during an early transgressive phase where the rising sea level possibly caused marine pore-water to push mixing and meteoric waters ahead of it. The active circulation in the marine pore-water zone led to pervasive dolomitization. The depleted delta C-13 (-5.9 to -8.9 parts per thousand PDB) values of dolomite type (I) indicate that dolomitization possibly occurred within the sulphate reduction zone beneath the sediment/water interface when an oxygen-minimum zone impinged the bottom surface of the carbonate platform. The presence of mimetic and non-mimetic dolomite textures is related to kinetic factors including the rate of dissolution of the CaCO3 precursor, reactant surface area and precipitation kinetics. Dolomite type (II) was formed during a subsequent highstand period during shelf progradation. The delta O-18 (+0.4 to +2.6 parts per thousand PDB) and delta C-13 (-0.9 to -4.6 parts per thousand PDB) values, the fine crystalline fabric and the planar intercrystalline boundaries argue for its early diagenetic origin within the subtidal zone above the oxygen-minimum zone. During further shelf progradation the studied rocks have been emplaced within a mixing marine-meteoric zone. During this period the dolomitization process was interrupted by a silicification phase. The precipitation of quartz and the resultant silica fabrics are controlled by variations in the P-CO2 and the pH of the porewater. At the later stages of the highstand system tract, when sea level started to fall, a meteoric lens possibly developed at the shelf margin. The section was subjected to meteoric diagenesis. A late meteoric origin is suggested for calcite mosaics, that fill some secondary pores, based on their extremely depleted delta O-18 values (-5.7 to -11.8 parts per thousand), variable delta C-13 values (-2.6 to -7.8 parts per thousand PDB), low-Mg composition and their equant form.