Mechanisms of near-normal sea water dolomitisation: Mesohaline-reflux or syn-depositional?

The Eocene Epoch is characterised by multiple hyperthermals, which promoted extensive carbonate sediment deposition and dolomitisation, particularly across the Arabian Plate. The Dammam Formation, a key Eocene aquifer and hydrocarbon-bearing unit in the Arabian platform, is extensively dolomitised. Several models have been proposed to explain its dolomitisation, many of which invoke sea water or its derivatives as the dolomitising fluid. However, the specific sea water-driven mechanisms remain poorly understood. More broadly, the effectiveness of commonly used diagenetic proxies (e.g. stable isotopes, stoichiometry and trace elements) in distinguishing between closely related sea water-driven dolomitisation processes has not been systematically assessed. This study addresses these issues by integrating petrographic, mineralogical, chemical and stable isotopic (δ¹³C and δ¹⁸O) analyses of a shallow subsurface core from the Dammam Formation. Two distinct dolomite types were identified: Type I (TI), predominantly subhedral and Type II (TII), exhibiting euhedral and subhedral textures. Both types are pervasive, fabric non-selective and non-mimetic. The dolomite pre-dates all other diagenetic processes, such as the formation of palygorskite, calcite, pyrite and stylolite, indicating early, shallow emplacement. Mineralogical data reveal that TI dolomite is near-stoichiometric (51.4 mol%CaCO₃), and well-ordered, while TII is non-stoichiometric (56.3 mol%CaCO₃) and poorly ordered. Trace element concentrations (Fe, Mn, Sr) align with some previously reported sea water dolomites. The δ¹⁸O values (−2.59‰ to 0.95‰) and temperatures (24–40°C) suggest formation within Eocene marine conditions, with potential evidence of warmer fluids or recrystallisation. The δ¹³C values (0.89–1.83‰) indicate buffering during dolomitisation, consistent with Eocene sea water. Contrary to previous interpretations suggesting sabkha mixing or evaporated sea water, this study proposes that the Saudi Dammam dolomite formed primarily through normal-to-mesohaline sea water dolomitisation, driven most probably by reflux of mesohaline fluids. This study adds to the understanding of sea water-driven dolomitisation, emphasising that recrystallisation can make distinction between mesohaline reflux and syn-depositional sea water-driven mechanism, in shallow marine dolomitised carbonates, challenging.