INPUTS OF TEPHRA AND SULFURIC ACID TO AN ANCIENT GREAT PLAINS PLAYA (OLIGOCENE OF NEBRASKA)

Historic, sulfur-rich volcanic eruptions have altered global climate for as much as five years, and much larger events known from the geologic record are widely considered as contributory factors to several of the planet’s greatest mass extinctions. At Scotts Bluff, Nebraska, early Oligocene strata of the lower Arikaree Group contain several tephra beds. One of these contains abundant calcite pseudomorphs after gypsum. Previous work has shown that sulfate from this tephra bears a strong 17O anomaly indicative of oxidation of reduced sulfur species by atmospheric ozone or hydrogen peroxide. The most likely source of the tephra and sulfate was a caldera eruption in the eastern Great Basin at about 28 ma. Our sedimentologic study shows that tephra and volcanogenic sulfate were deposited and preserved within a small, surface-discharging playa that developed on the irregular upper surface of eolian siltstones of the subjacent White River Group. Sulfuric acid percolated downward within the vadose zone, dissolving smectite cement within underlying sandstones, reddening these rocks along an irregular alteration front. Preserved fine-scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis. Displacive growth of gypsum at the playa center folded surficial tephra beds and forced tephra into underlying sandstones, forming elongate cones. The large mass fraction of gypsum (now mostly replaced by calcite) in the playa sediments suggests a huge, long-distance delivery of sulfate aerosols.