Pleistocene Volcanic Ash Deposits in Utah

Abstract

The major western Pleistocene volcanic ashes are reviewed with emphasis on their occurrence in Utah. The Pleistocene Bishop ash, Pearlette type 0 ash, Mazama ash, and several local ashes have been identified in Utah. The value of these extensive ash layers as stratigraphic markers depends on the ability to distinguish between them. Techniques employed to distinguish the ashes are discussed with emphasis on using the chemical composition of the volcanic glass shards. The major element chemistry of the glass portion of these ashes is presented together with that of other Cascade ashes and the Green Mountain Reservoir ash. It is noted that in the concentrations of major elements these ashes are all chemically similar to rhyolite, and only titanium, calcium, and iron are useful in separating the ash layers. The trace element con ten t (titanium, barium, manganese, iron, chlorine, rubidium, yttrium, zirconium, strontium, and niobium) of these ashes is more conclusive in identifying the ash layers. Some heretofore undescribed ashes from the east flank of the Oquirrh Mountains are not chemically similar to any other well characterized Pleistocene ash and are chemically distinct from Tertiary ashes occurring in the Salt Lake Group of the Bonneville Basin as well.