Date of Award
Doctor of Philosophy
Geological and Environmental Sciences
Dr. Stephen E. Kaczmarek
Dr. John M. Rivers
Dr. R.V. Krishnamurthy
Dolomitization, carbonate diagenesis, carbonate sedimentology, recrystallization, carbonate geochemistry
Eocene carbonates comprising the Umm er Radhuma (UER), Rus, and Dammam Formations, cover 80% of the surface and extend >300 m into the subsurface of Qatar. These rocks record marine sedimentation in shallow sub-tidal to restricted settings. Despite undergoing only shallow burial (<1000 m), extensive diagenetic alteration has occurred. Given that little work on this topic has been published previously, the objectives of this work are to document diagenesis in the Eocene, and to integrate petrography, mineralogy, and geochemistry to delineate the timing and environments of diagenesis of these Eocene carbonates. Detailed petrographic data show that dolomitization occurred early in the UER, before the formation of diagenetic chert, palygorskite, pyrite, calcite, and gypsum cements. Bulk dolomite δ18O values, coupled with clumped isotope-derived (Δ47) temperatures and dolomitizing fluid δ18O values, suggest dolomitization took place in near-normal marine fluids, perhaps during shallow burial. These data challenge the current paradigm of large-scale, top-down hypersaline reflux dolomitization in the UER. Associated depth trends of increasing crystal size and with increasing stoichiometry and cation ordering but variable δ18O further suggest that UER dolomites were extensively recrystallized prior to the formation of all other mineral phases. This finding challenges an analysis of the literature which suggests that extensive dolomite recrystallization only happens late in the diagenetic history and/or with deep burial. Similarly, the overlying Rus Fm. consists of dolomitized peritidal facies interbedded with gypsum, as well as more open marine deposits higher in the formation. However, petrographic data shows dolomite formed prior to gypsum cementation, and bulk δ18O and δ13C data suggest a non-evaporative origin. The intimate association between dolomites and meteoric calcites elsewhere in the section indicate that Rus dolomites have likely been recrystallized multiple times, possibly in meteoric-related fluids. The key findings of this study are that early diagenesis can be extremely complex, and that current paradigms related to reflux dolomitization and dolomite recrystallization may need to be reevaluated.
Ryan, Brooks H., "Integrating Detailed Petrography, Geochemistry, and Mineralogy to Elucidate Extensive Early Diagenesis in the Eocene Carbonates of Qatar" (2020). Dissertations. 3562.