Abstract

The Eocene Uteland Butte Member of the Green River Formation in the Uinta Basin is characterized by lacustrine carbonate depositional cycles consisting of calcareous shales, limestones, and dolomites that have been interpreted to reflect climatically driven lake level fluctuations. Previous work suggests that dolomitization of three distinct stratigraphic intervals in the Uteland Butte Member—the PZ-1, PZ-1′, and PZ-2—occurred during low lake levels and was driven by a combination of density-driven downward reflux and evaporative pumping of concentrated brines. The current study uses a novel high-resolution mineralogical dataset to evaluate these proposed dolomitization mechanisms. Mineralogical data from three drill cores show that the dolomitized intervals are characterized by variations in dolomite abundance (relative to calcite), dolomite stoichiometry, and cation ordering (015:110), all of which covary with depositional facies. In the near-basin margin core and near-basin center cores, the PZ-1 interval is characterized by a shallowing-upward facies trend that corresponds to an increase in stoichiometry, dolomite abundance, and cation ordering. In the PZ-1′ interval both the near-basin margin and near-basin center cores exhibit shallowing to deepening facies patterns up core that correspond to an increase and a subsequent decrease in dolomite stoichiometry. Similarly, dolomite abundance in this interval exhibits an increase then a decrease. The PZ-2 interval is also characterized by a shallowing to deepening facies pattern, which corresponds with an increase then a decrease in stoichiometry. Lateral trends between cores indicate that basinward facies have less dolomite, and that the dolomite is less stoichiometric compared to their more landward counterparts. Collectively, these observations argue against a simple model of top-down reflux dolomitization driven by evaporative pumping. Instead, the vertical and lateral relationships between depositional facies and mineralogical properties in the PZ-1′ and PZ-2 intervals suggest that dolomitization may have occurred syndepositionally, and that the observed mineralogical patterns were driven by differences in fluid chemistry associated with lake level fluctuations through time. These findings are broadly consistent with previous studies on peritidal marine carbonates showing that dolomite mineralogy can record temporal and spatial paleoenvironmental changes that can be utilized to evaluate dolomitization mechanisms.