Evaluating reflux dolomitization using a novel high-resolution record of dolomite stoichiometry: A case study from the Cretaceous of central Texas, USA

Refluxing brines are routinely invoked to explain dolomitization in peritidal settings. One such example is the Cretaceous Upper Glen Rose Formation (UGR) of central Texas, United States. The UGR is characterized by high-frequency cycles of variably dolomitized limestones interpreted to reflect periodic deepening and shallowing in a peritidal setting. Previous work suggests that reflux commenced near the top of each depositional cycle when evaporative brines seeped downward, causing dolomitization of the underlying sediments. To test this model, a novel high-resolution lithological, mineralogical, and geochemical data set is presented. These data show that transgressive facies successions exhibit a pattern of decreasing dolomite abundance, dolomite stoichiometry, and delta O-18 (relative to Vienna Peedee belemnite [VPDB]), and increasing dolomite crystal size. In contrast, regressive facies successions are characterized by a pattern of increasing dolomite abundance, dolomite stoichiometry, and delta O-18 (VPDB), and decreasing dolomite crystal size. No such variability is observed laterally. The patterns suggest that fluids did not migrate and evolve downward or basinward during reflux, which would have produced a systematic decrease in dolomite abundance and dolomite stoichiometry within each cycle. The data instead support a model of syndepositional dolomitization whereby each facies was dolomitized before deposition of the overlying sediments. The mineralogical and geochemical trends thus reflect temporal changes in water depth as well as a combination of the Mg/Ca, temperature, and salinity of the formative solutions. These results suggest that syndepositional dolomitization in peritidal environments may be a more common and widespread occurrence than previously recognized, particularly during greenhouse conditions.