Airfall Tuff in the Browns Park Formation, Northwestern Colorado and Northeastern Utah

Bedded airfall tuffs, mainly rhyolitic in composition and locally very thick, occur throughout the Browns Park Formation (upper Oligocene to upper Miocene) in northwestern Colorado and northeastern most Utah. First mentioned in the geologic literature by Bradley (1935), they have received only cursory attention other than for the purpose of radiometric dating. The present writer began study of the tuffs in 1980, hoping to use them as time-stratigraphic marker beds within the formation. Several tuff-rich stratigraphy sections were measured and numerous samples were collected. The results of petrographic and petrochemical studies of these samples are presented. Refractive indices of vitric shards from 52 samples increase slightly and irregularly with apparently decreasing age of the samples. Seemingly, the trend points toward less felsic compositions with time. Quartz, feldspar, and especially heavy-mineral suites from these from these 52 samples were studied petrographically. Because airfall components (phenocrystic minerals) are not generally readily distinguishable from contaminating detrital grains, time-composition trends in airfall assemblages are problematical and difficult to establish. Clinopyroxene of likely airfall origin may increase slightly in abundance with decreasing age. The ratio of apatite (mainly of airfall origin) to zircon (mixed origins) appears to decrease with time. Recalculated analyses of major oxides for the 24 whole-rock samples that appear to be least contaminated by detritus show the following variations with decreasing age: increased silica, potash, total alkalis, and titania, and decreased alumina, total iron oxides, magnesia, lime, and soda, and soda: potash ratio. In general, petrochemical results indicate that Browns Park tuffs became increasingly more felsic with time, albeit irregularly. This contradicts the conclusions drawn from refractive-index determinations on the same samples. Preliminary correlations of airfall-tuff beds by petrochemistry, refractive indices of glass, and mineralogy and population of heavy minerals, have been partly successful Minor-element distribution provides useful information, but data are incomplete. Prevailing westerly winds in mid- to late-Tertiary time brought ash into the Browns Park Formation and correlative units elsewhere in the Rocky Mountain region from distant sources m numerous volcanically active areas in the western United States. Bulk mineralogy and major-oxide compositions and variations from selected eruptive centers in the Basin and Range province commonly resemble those of Brown Park tuff. Source areas of silicic eruptive material were in, but not necessarily limited to, Utah, Nevada, eastern California, southwestern Idaho, and southeastern Oregon.