Early Carbonate Diagenesis of the Miocene Dam Formation in Southwestern Qatar

Date of Award

12-2023

Degree Name

Doctor of Philosophy

Department

Geological and Environmental Sciences

First Advisor

Stephen E. Kaczmarek, Ph.D.

Second Advisor

Peter J. Voice, Ph.D.

Third Advisor

John M. Rivers, Ph.D.

Fourth Advisor

Paul J. Moore, Ph.D.

Keywords

Carbonates, dolomite, dolomitization models, early diagenesis, miocene, shallow marine

Abstract

The timing and mechanisms of platform-scale dolomitization have been the subject of a long and contentious scientific debate known as the “dolomite problem.” The current study provides new insights through an investigation of the Miocene mixed carbonate-siliciclastics-evaporites Dam Fm in southwestern Qatar. Petrographical, mineralogical, and geochemical data are integrated from two research cores, the Dam-01 and Dam-02. The Dam Fm provides an ideal site to investigate dolomitization because these rocks are: (i) relatively young, (ii) have not been deeply buried, (iii) exhibit no evidence of diagenetic resetting, and (iv) characterized by alternating meter-scale depositional cycles that help constrain the relative timing of bed-scale diagenetic events.

Three principal lithotype intervals are identified in the Dam Fm, including carbonate-dominated, siliciclastic-dominated, and evaporite-dominated. Mineralogical characterization of the Dam Fm shows that dolomite is the most abundant mineral within the carbonate-dominated intervals. Calcite within the Dam Fm occurs as a cement at the tops of depositional cycles, and is associated with evidence of exposure. Based on petrographic cross-cutting relationships, dolomite predates all other diagenetic mineral phases, including palygorskite, illite, quartz cement, gypsum cement, pyrite, and exposure-related blocky calcite cement, which suggests that dolomitization occurred early, during deposition, and prior to subaerial exposure. This suggests that the dolomite formed syndepositionally on a cycle-by-cycle basis before the deposition of the overlying sediments.

The mineralogical and geochemical data of the lower Dam Fm show considerable spatial and stratigraphic variability. Stoichiometric dolomites in both Dam-01 and Dam-02 cores are more abundant within open lagoon facies, while more Ca-rich dolomites are more abundant in restricted water facies. The δ18Ocarb values also show a relationship with depositional facies, whereby dolomites from open lagoon facies in Dam-01 are characterized by more negative δ18Ocarb (average = -2.67‰) compared to those associated with restricted water facies (average = -1.35‰). In Dam-02, however, dolomites associated with both lagoon facies and restricted water facies are characterized by more similar δ18Ocarb values (-1.1‰ to -1.24‰). In contrast, carbonate intervals in both cores with more abundant calcite have δ18Ocarb values that are up to 6 ‰ more negative.

Petrographic, geochemical, mineralogical, and paleontological data suggest that the dolomites of the lower Dam Fm formed syndepositionally in a dynamic, estuarine setting where surface fluids were periodically mixed between marine-derived and meteoric sources. The patterns and trends further suggest that conventional models of dolomitization cannot explain dolomitization of the Dam Fm. Instead, a new model is presented whereby early, rapid dolomitization of water bottom carbonate sediments occurred in an estuary setting characterized by regular fluctuations in fluid chemistry.

Access Setting

Dissertation-Abstract Only

Restricted to Campus until

12-1-2025

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