WAS COLORADO MINERAL BELT VOLCANISM RESPONSIBLE FOR SODIUM-CARBONATE EVAPORITE DEPOSITS IN THE EOCENE GREEN RIVER FORMATION?

Sodium carbonate evaporite minerals deposited in lakes are important both as economic deposits of soda ash, and as indicators of past atmospheric CO₂ concentration. Modern deposits generally appear to be associated with magmatic sources of CO₂, which provide the excess alkalinity required to form Na-carbonate instead of gypsum and halite. Paradoxically, the world’s largest known ancient soda ash deposit, consisting of trona in the Green River Formation in Wyoming, is not spatially associated with evidence for coeval magmatism. Magmatism was active ~200 km southeast within parts the Colorado Mineral Belt however, suggesting that mantle-derived magmatic CO₂ could have been transported northwestward by rivers flowing into Eocene Lake Gosiute. CO₂-enriched waters draining from the Colorado Mineral Belt could likewise help account for nahcolite deposits in the Piceance basin in northwest Colorado. Field studies near the Colorado-Wyoming border confirm that rivers did in fact enter Lake Gosiute from the southeast, depositing multi-story fluvial and deltaic sandstone bodies with generally northwest-directed paleocurrents. These deposits are consistent with a proposed “Aspen River” that flowed from central Colorado toward the lake (Smith et al., 2014, EPSL). Detrital zircon age analyses will help to further assess a possible provenance connection between the sandstone deposits and Eocene volcanic rocks. Other regional volcanic centers appear less likely to have influenced evaporite deposition in Green River Formation lakes. The Absaroka Volcanic Province was active ~300 km to the north, but sedimentary provenance studies suggest that it did not drain into Lake Gosiute. The Challis Volcanic Province ~500 km to the northwest did drain into the lake, eventually filling it with volcaniclastic detritus, but major eruptions did not commence until almost 1 my after peak trona deposition.