A petrologically constrained timeframe for early dolomitization in the mixed carbonate-siliciclastic Miocene Dam Formation, Qatar

The timing of dolomitization is difficult to constrain. Many studies suggest that it is early, but the rates at which dolomitization occurs in nature are rarely defined. This study aims to pinpoint the timing of dolomitization in sedimentary environments using detailed petrography and sedimentological observations from two near-surface research cores in the lower part of the Dam Formation (lower Dam) in southwestern Qatar. The lower Dam, deposited in a shallow-marine-estuarine setting with frequent sea-level fluctuations, presents strong sedimentological and stratigraphic constraints for the timing of dolomitization. The highly variable environmental conditions resulted in deposition of marine to brackish-water carbonates, land-derived siliciclastics, and intermittent subaerial exposure, creating a high-frequency cyclicity that allows for cycle-by-cycle diagenetic analysis in stratigraphically distinct depositional units. Observations from core and thin-section petrography permit the identification of five distinct lithofacies, which are classified as either carbonate-dominated or siliciclastic-dominated lithotypes. Dolomite, the most abundant mineral in the carbonate-dominated intervals, occurs as a fabric-retentive very fine crystalline (VFxn) or isopachous dolomite. Crosscutting relationships in each stratigraphically distinct depositional unit indicate that replacive dolomite predates all other diagenetic mineral phases, including palygorskite, illite, quartz cement, pyrite, and exposure-related blocky and poikilotopic calcite cements. Diagenetic calcite cements are most abundant immediately below exposure surfaces, gradually decrease in abundance down section, and are generally absent directly above exposure surfaces. Collectively, these observations imply that blocky and poikilotopic calcite cements postdate dolomitization, supporting a model where dolomitization of a given depositional cycle occurred syndepositionally, before the deposition of the overlying cycle. With an estimated deposition rate of the Dam Formation of between approximately 5.6 + or - 1.4 and 3.3 + or - 0.9 cm kyr (super -1) , dolomitization in some cycles likely occurred in approximately 5.4 + or - 1.4 to 9.1 + or - 2.3 kyr, consistent with findings from Holocene dolomite studies, and considerably shorter (< 100X) than those extrapolated from high-temperature laboratory experiments. The findings from this study imply that sedimentary carbonates can be dolomitized relatively rapidly on a cycle-by-cycle basis.