Geochemistry of volcanic ash in the Salt Lake group, Bonneville Basin, Utah, Idaho, and Nevada

Diverse outcrops within the Bonneville Basin of non-marine sedimentary rocks older than the Pleistocene and younger than the Norwood Tuff (37 m.y.) have been questionably included in the Salt Lake Group. This work attempts to determine whether these rocks represent a single contemporaneous unit or a series of distinct episodes, and if one unit is represented, to establish possible correlations between the sampled outcrops. The approach undertaken utilized the idea that a specific eruptive source produced volcanic glass which was unique to its trace element chemistry. Thus the determination of the chemistry of the glass portion of volcanic ashes, through statistical deter­mination of chemical types present, identified the chemical group that the sample was affiliated with. It was hoped that at least one ash group or layer was both substantially chemically unique and wide­ spread, and thus traceable between outcrops. Volcanic ash layers were sampled at twelve distinct localities within the Bonneville Basin from outcrops previously mapped as the Salt Lake Group. From these twelve localities, 64 X-ray samples and 40 micro­ probe samples of the glass separates were prepared. The extensive sample preparation involved separation of the glass shards from all other associated grains such as quartz, feldspar, and mafics, and extensive cleaning to remove any calcite cement or fine adhering particles. The clean shards were then chemically analyzed by electron microprobe. X-ray fluorescence, and classical wet chemical techniques. CIPW norm calculations, differentiation indices, and location on the ternary diagram for the quartz-albite-orthoclase-water system sug­gested a chemical equivalence with a rhyolitic or granitic rock. Also implied was the similarity to initial liquids produced by partial melt- ting of crustal quartzo-feldspathic material. Similarity matrices based on correlation and distance established the presence of six chemical groups. After a statistical test of the validity of the six chemical groups by a discriminant function analysis, a stepwise discriminant analysis was performed on most samples to deter­ mine their probability of belonging to one of the six chemical groups. The analysis produced a set of Mahalanobis D2 values for each sample of each chemical group; these established the level of significance (for most samples 0.05) for classification within one of the six chemical groups. Noteworthy is an ash layer, here referred to as the Goose Creek Ash, which correlates from NE Utah (Mink Creek section) to NW Utah (Goose Creek section) to N central Utah (Rush Valley section). The Goose Creek Ash is also present at Dove Creek and Cotton Thomas sections. Also of interest is the apparent regional restrictions of an ash group at both the northwestern and eastern sampled sections. A general trend from west to east of younger ash layers was observed, suggesting an eastward shift of volcanic activity in time. All localities may be correlated with one another, and thus repre­sent a distinct event in geologic time. Therefore the term Salt Lake Group is a valid unit name representing the mid-to-late Tertiary non-marine sedimentary rocks at these localities. Volcanic ash from Promontory Point, known to be Pearlette, was chemically compared to twelve Pleistocene glass separates of volcanic ash on the east flank of the Oquirrh Range. No positive correlation was noted.