Abstract

Microprobe analyses of glass and radiocarbon dating of samples associated with three basaltic volcanic ashes in the deposits of Lake Bonneville make the ashes useful stratigraphic markers for correlation within the Bonneville basin. Two of the ashes were derived from tuff cones (Pavant Butte and Tabernacle Hill) that erupted into Lake Bonneville in what is now the Black Rock Desert of south-central Utah. Near Kanosh, Utah, the Pavant Butte ash is interbedded with barrier-beach lagoon marl that is slightly less than 16,000 yr old. Five radiocarbon dates on carbonate materials stratigraphically associated with the ash elsewhere in the Sevier Desert average 15,300 yr B.P. Lake Bonneville was in its transgressive phase, about 15 m below the Bonneville Shoreline, when the Pavant Butte eruption began. At the Tabernacle Hill tuff cone, a basalt flow that has pillows, wave-rounded cobbles, and tufa on its outer margins was erupted into Lake Bonneville shortly after the tuff cone at or near the Provo Shoreline. The Tabernacle Hill tuff cone and basalt flow are older than a radio-carbon date of about 14,300 yr B.P. on tufa collected from the outer margin of the basalt flow, but younger than the Bonneville Flood at about 14,500 yr B.P. The third ash, informally termed the "Thiokol" ash, has been found interbedded with Lake Bonneville deposits in two exposures in northern Utah and in sediment cores from the Great Salt Lake. It is about 25,000 yr old, and may have been erupted from a volcanic field on the northwest shore of Great Salt Lake or from the Snake River Plain.

Microprobe analyses of glass from samples of the Black Rock Desert ashes show them to have similar chemical compositions, but glass from Tabernacle Hill can be distinguished from Pavant Butte glass by its higher concentrations of CaO and P2O5. Systematic differences in chemical composition between samples of Pavant Butte glass can be explained by comagmatic processes. "Thiokol" glass has less SiO2, AI2O3, MgO, and Na2O but greater total iron and P2O5 than do the Black Rock Desert glasses.

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