WEATHERING, SPRINGS, AND THE ORIGIN OF NA-EVAPORITES IN THE EARLY EOCENE GREEN RIVER FORMATION, WYOMING

Na-evaporite minerals, carbonate-rich mudstone, and alluvial detritus of the Wilkins Peak Member of the Green River Formation and laterally equivalent Cathedral Bluffs Member of the Wasatch Formation all record deposition in a closed, intermontane sedimentary basin. The Early Eocene watershed boundaries of this basin can be semi-accurately reconstructed, and the duration of the Wilkins Peak member has been well established via ⁴⁰Ar/³⁹Ar dating of interbedded tuff horizons. A mass balance based on these constraints indicates average annual fluxes of 3.7 ± 0.9 t, 16 ± 11 t, and 420 ± 79 t of dissolved Na and Ca and of alluvial detritus, respectively. If all of these products were derived from weathering of the surrounding landscape, this implies average chemical erosion of 63 ± 22 t/km² yr, average physical erosion of 420 ± 79 t/km² yr, and average denudation of 175 ± 30 m/Myr. These rates rival the highest documented non-glacial Quaternary rates for crystalline bedrock, and may reflect the presence of elevated atmospheric pCO₂ concentrations. This interpretation is consistent with a previously documented warm climate during the Early Eocene Climatic Optimum.

Alternatively, some solutes and CO₂ could have entered the basin via hydrothermal springs. For example, springs provide the bulk of water and solutes to modern Lake Magadi in the Kenya Rift. Spring deposits and silicified strata are common within Green River Formation and equivalent strata. Silicified intervals up to 30 m in vertical relief occur within the lower to middle part of Wilkins Peak member, roughly equivalent to the interval that contains the majority of its bedded trona. The volumetric significance of spring systems relative to surface weathering processes has not yet been assessed however, requiring further petrographic, hydrologic, and geochemical studies.