REEVALUATING EVAPORATIVE REFLUX DOLOMITIZATION IN PERI-TIDAL CARBONATES

Debating the timing, fluid composition, and mechanism of dolomitization has become somewhat of a cottage industry. Early seawater dolomitization, microbial mediation, evaporative reflux, mixing-zone dolomitization, hydrothermal convection, late burial dolomitization, are but a few of the models proposed in the literature. The most widely cited of these is perhaps the evaporative reflux model, which posits that dense, concentrated brines formed in evaporative settings reflux downward, thus delivering warm, Mg-rich fluids that promote dolomitization of the underlying limestone strata. This model is particularly attractive for dolomites in peri-tidal carbonate successions that are associated with interbedded or overlying evaporites. Although this model has been invoked over a wide range of temporal and spatial scales, recent work suggests that top-down reflux in evaporative settings may be less common than we think. Textural, mineralogical, and geochemical data from three examples of seemingly evaporative reflux dolomites - the Paleocene-Eocene Umm er Radhuma (UER) Formation in Qatar, the Cretaceous Upper Glen Rose (UGR) Formation in central Texas, and the Ordovician Saluda Formation in southern Indiana – are discussed. The UER is a large marine dolostone capped by a thick evaporite sequence, but patterns in dolomite stoichiometry, trace element concentrations, and δ¹⁸O_carb suggest dolomitization took place in marine fluids, and not by large-scale top-down reflux during evaporite deposition of the overlying Rus Fm. In the UGR, high-frequency depositional cycles closely track patterns in dolomite stoichiometry, dolomite crystal size, and δ¹⁸O_carb, suggesting that rather than top-down reflux, dolomitization occurred syndepositionally and was largely driven by sea level fluctuations. In the Saluda, a laminated dolo-mudstone interpreted to have formed in a shallow, restricted lagoon, dolomite stoichiometry and δ¹⁸O_carb are also more consistent with dolomitization in marine fluids. Collectively, these case studies suggest that, like the mixing-zone and microbial models of dolomitization, the evaporative reflux model may require further scrutiny.