SULFUR AND OXYGEN ISOTOPE CHARACTERIZATION OF AN EOCENE PLAYA, NORTHERN HIGH PLAINS, USA

An Oligocene volcaniclastic ash bed exposed at Scotts Bluff National Monument, NE, contains sulfate that exhibits a highly positive ¹⁷O anomaly, a measure of the deviation from the terrestrial fractionation line. The anomalous sulfate is believed to have originated from volcanic eruptions to the west where emitted volcanic sulfur gases lingered in the troposphere and became oxidized by ozone, creating dry fogs. Upon settling, the fogs saturated playas with sulfate and abundant gypsum precipitated. This is the only known case in the region where sulfate in playa deposits has an ¹⁷O anomaly. Volcanic activity persisted through much of the Cenozoic in western North America. A question arises, then, if dry fogs settling are a common origin of sulfate in other Cenozoic playa deposits in northern High Plains, or is the Scotts Bluff playa truly an exception. Nearby, at Wolf Butte, NE, we located a gypsum-rich sequence in the Big Cottonwood Creek Member of the Chadron Formation (Eocene) deposited in an apparent playa setting. Here the gypsum occurs as massive layers rather than discrete lensoid or rosette crystals as in Scotts Bluff. A complete stable isotope characterization was performed on the gypsum sulfate collected from an ~ 15 m section to determine if the sulfate has a similar or contrasting origin to that in the Scotts Bluff playa.

We found that none of the sulfate at Wolf Butte has an ¹⁷O anomaly and the d³⁴S values range from +12.2 to +14.8‰. The source of sulfate at this location, therefore, greatly differs from that at Scotts Bluff (¹⁷O anomaly reaching 5.8‰ and d³⁴S ranging from ~0 to +7.5‰). Potential sources of sulfate at Wolf Butte include weathering of Cretaceous shale and selenite-bearing paleosols. Enrichment in ³⁴S is likely due to subsequent bacterial sulfate reduction in groundwater or surface water. Using d¹⁸O, d¹⁷O, and d³⁴S measurements and sedimentological observations, we conclude that Scotts Bluff and Wolf Butte playas represent two distinctly different systems that existed during the Cenozoic in the northern High Plains.