Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar. Issue 6 (8th April 2020)

• Record Type: Journal Article
• Authors: Ryan, Brooks H.; Kaczmarek, Stephen E.; Rivers, John M.
• Editors: Hollis, Cathy
• Is Part Of: Sedimentology. Volume 67:Issue 6(2020)
• Page Start: 2917
• Page End: 2944
• Publication Date: 2020-04-08
• Subjects: Diagenesis -- dolomite -- evaporite -- geochemistry -- petrography; Sedimentology -- Periodicals; 552.5
• DOI: 10.1111/sed.12726
• Languages: English
• ISSNs: 0037-0746

Abstract

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 δ 18 O = −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 δ 18 O 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.