The Green River salt mystery: What was the source of the hyperalkaline lake waters?

Bicarbonate-rich source waters were needed to form the largest sodium carbonate evaporite deposits in the geologic record, the early and middle Eocene Green River trona (NaHCO₃·Na₂CO₃·2H₂O) in the Bridger basin, Wyoming, and nahcolite (NaHCO₃) in the Piceance Creek basin, Colorado. Large modern and Pleistocene trona deposits are associated with magmatic activity and Na⁺-HCO₃ ⁻-rich hydrothermal inflow waters, either within the depositional basin (Lake Magadi, Kenya) or at great distances (Searles Lake, California). No evidence exists for magmatic sources of CO₂ near the Green River Formation. Several regional volcanic centers were active 300 km or more to the north, but drainage reconstructions show that waters from these areas did not discharge into the Green River Formation lakes during evaporite deposition. Alternatively, Na⁺-HCO₃ ⁻-rich waters could have drained northwestward from the Colorado Mineral belt to the Bridger basin via the proposed Aspen River. A river originating in the Colorado Mineral belt (Sawatch uplift) could also have provided source waters to the Piceance Creek basin. Field evidence, however, has not yet documented these flow paths, and specific Eocene volcanic centers and hydrothermal source areas have yet to be identified. Other explanations for the elevated alkalinities needed to form thick sodium carbonate evaporites include accelerated silicate mineral weathering rates during a period of high atmospheric pCO₂ and Eocene warmth. Amplified chemical weathering may explain the cluster of sodium carbonate evaporites in the USA and China that are Eocene in age. Another possible source of alkalinity to the Green River lakes is fault-controlled upward migration of a deep sedimentary source of CO₂. The large amount of organic matter preserved in the evaporitic Wilkins Peak and Parachute Creek Members raises the possibility that decay of organic matter in Green River lakes could have added sufficient CO₂ and alkalinity to produce waters capable of precipitating trona and nahcolite, although such degradation of organic matter has not created hyperalkalinity in modern lakes.