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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 δ 18 O (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 δ 18 O (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.

Various geochemical proxies are used to constrain the diagenetic origin and evolution of ancient dolomites. Dolomite stoichiometry (mole % MgCO3) and cation ordering, two mineralogical attributes that define dolomite, have also been shown to demonstrate utility in this regard. Observations from laboratory experiments and field studies suggest that these attributes broadly reflect the fluid chemistry and temperature of the dolomitizing environment. The degree to which these parameters reflect global conditions during dolomitization (e.g., seawater chemistry, eustasy, atmospheric pCO2) and long-term geological processes is poorly understood, however. Here, a large dataset consisting of mineralogical data from over 1,690 Phanerozoic dolomites from various geographic locations, stratigraphic ages, platform types, and depositional environments are queried to investigate the broader geological controls on dolomite stoichiometry and cation ordering in dolomites formed by early, near-surface dolomitization. A suite of statistical analyses performed on the global dataset indicate: 1) despite wide ranges at the eon, period, and epoch level, dolomite stoichiometry and cation ordering broadly increase with geologic age; 2) significant variations in dolomite stoichiometry and cation ordering throughout the Phanerozoic do not correlate with global parameters, such as seawater chemistry, eustasy, orogenic events, and ocean crust production; 3) dolomites associated with restricted depositional settings, such as restricted lagoons, and the intertidal and supratidal zones, are more stoichiometric than dolomites associated with open marine settings, such as the deep-subtidal and shallow-subtidal zones; and 4) dolomites from shallow ramps and epeiric carbonate platforms are generally more stoichiometric than dolomites from open shelves and isolated carbonate platforms. These observations permit a number of inferences to be drawn. First, the principal signal observed in the data is that local environmental conditions associated with platform type and depositional setting are the strongest control on dolomite mineralogy. The observation that more stoichiometric dolomites correlate with shallow and restricted depositional environments is consistent with laboratory experiments that show environmental factors, such as higher Mg:Ca, temperature, and salinity of the dolomitizing fluids yield more stoichiometric dolomite. Second, a weaker secondary signal is also observed such that dolomite stoichiometry and cation ordering both increase with geologic age, suggesting that progressive recrystallization driven by mineralogical stabilization during burial is also occurring. Collectively, these data suggest that spatial and temporal variations in stoichiometry and cation ordering reflect the interplay between local dolomitizing conditions near the surface and long-term mineralogical stabilization during burial.

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