Dolomitization and dolomite cementation of upper Triassic limestones, Wadi Sha'am, Ras Al Khaimah

Abstract

The outcropped Upper Triassic carbonates in Wadi Sha'am, Ras Al Khaimah, have been subjected to pervasive dolomitization and dolomite cementation, which occur along and in the vicinity of faults and associated fractures. Field observations, petrographic, stable and strontium isotope analyses have been combined to constrain the timing of dolomitization episode(s), as well as the origin and evolution of dolomitizing fluids. Associated with fracturing and brecciation, saddle dolomite and host dolostones have similar δ 13CVPDB (+0.52‰ to +1.78‰ and +0.21‰ to +1.78‰, respectively) and δ18OVPDB (-9.98‰ to -8.03‰ and -9.40‰ to -5.30‰, respectively) stable isotope data and high strontium isotope ratios (0.708858 to 0.708864 and 0.709256, respectively). The precipitation of saddle dolomite was followed by the precipitation of coarse-crystalline equant calcite (δ 13CVPDB = -1.03‰ to +0.38‰, δ 18OVPDB = -7.10‰ to -4.68‰) and minor amounts of prismatic quartz. Two regional orogenic events (i.e. the obduction of Oman-UAE ophiolites and the Cenozoic thrusting phase) make it possible for hot basinal brines (referred to as hydrothermal fluids in literature) to flow through active faults and fractures. Assuming that saddle dolomite and equant calcite were precipitated from evolved basinal brines with calculated δ 18OSMOW more than +2‰, the estimated temperatures (91°C to 107°C for saddle dolomite and 49°C to 64°C for calcite) indicate a cooling phase, which was probably related to obduction of Oman ophiolites. Calcite cement filling fractures that cut across the dolostones and saddle dolomite have low δ13CVPDB (-10.81‰ to -8.43‰) and δ18OVPDB (-11.93‰ to -11.80‰) isotope values and low strontium isotope ratio (0.708393), indicating the involvement of meteoric water during the uplift and erosion phase. This study thus elucidates the influence of regional tectonic history and related basinal fluids on the diagenetic evolution of carbonate succession. The formation of large vugs might have been caused by the undersaturation of hot basinal brines in terms of calcite. Low porosity in the dolostones should be attributed to the extensive carbonate cementation.

Date of Award	2016
Original language	American English
Supervisor	Sadoon Morad (Supervisor)