Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

Abstract

The upper Palaeocene–lower Eocene Umm er Radhuma Formation in the subsurface of Qatar is dominated by subtidal carbonate depositional packages overlain by bedded evaporites. In Saudi Arabia and Kuwait, peritidal carbonate depositional sequences with intercalated evaporites and carbonates in Umm er Radhuma have been previously interpreted to have been dolomitized via downward reflux of hypersaline brines. Here, textural, mineralogical and geochemical data from three research cores in Qatar are presented which, in contrast, are more consistent with dolomitization by near‐normal marine fluids. Petrographic relationships support a paragenetic sequence whereby dolomitization occurred prior to the formation of all other diagenetic mineral phases, including chert, pyrite, palygorskite, gypsum, calcite and chalcedony, which suggests that dolomitization occurred very early. The dolomites occur as finely crystalline mimetic dolomites, relatively coarse planar‐e dolomites, and coarser nonplanar dolomites, all of which are near‐stoichiometric (50.3 mol% MgCO3) and well‐ordered (0.73). The dolomite stable isotope values (range −2.5‰ to +1‰; mean δ18O = −0.52‰) and trace element concentrations (Sr = 40 to 150 ppm and Na = 100 to 600 ppm) are compatible with dolomitization by near‐normal seawater or mesohaline fluids. Comparisons between δ18O values from Umm er Radhuma dolomite and the overlying Rus Formation gypsum further suggest that dolomitization did not occur in fluids related to Rus evaporites. This study provides an example of early dolomitization of evaporite‐related carbonates by near‐normal seawater rather than by refluxing hypersaline brines from overlying bedded evaporites. Further, it adds to recent work suggesting that dolomitization by near‐normal marine fluids in evaporite‐associated settings may be more widespread than previously recognized.

Preservation of organic carbon in dolomitized Cambrian stromatolites and implications for microbial biosignatures in diagenetically replaced carbonate rock

“…An average crystal size of µm was estimated using confocal Raman microscopy. Microcrystalline textures in dolomite (<10 microns) are thought to be from fluids that are highly saturated with respect to dolomite (Sibley, 1991) and are common in early nearsurface dolomitization (Moore, 1989;Sibley, 1991;Lukoczki et al, 2020;Ryan et al, 2020).…”

Biostratigraphy and depositional environments of thePalaeocene–Eocenesuccession at Um El Huwaitat area, Eastern Desert, Egypt

“…The cloudy cores as being cloudy because they have calcite inclusions (Sibley, 1982). The clear rims as a product of a second dolomitization event (Jones, 2007) interprets the cloudy (partially dissolved) cores as being primary dolomite that gets recrystallized into the clear pristine rims (Ryan, Kaczmarek, & Rivers, 2020). The lime‐mudstone was likely deposited in a low energy, inhabitable environments.…”

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

“…Metre‐thick gypsum deposits are interbedded with the dolomitized rocks of the Rus Formation in southern Qatar (Al‐Saad, 2003) and decimetre‐thick gypsum beds are locally present in the Dam Formation (Dill et al ., 2005). The presence of dolomite in these rocks, however, is ubiquitous, and so does not correlate well with the presence of evaporites, or even with evidence of shallow‐restricted environments, where concentrated seawater might be expected (Rivers et al ., 2019c; Ryan et al ., 2020). In contrast, bulk isotope and cross‐cutting relationships in the partially‐dolomitized Eocene Dammam Formation point to dolomite replacement after stabilization to low Mg‐calcite in meteoric waters.…”