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

摘要: Bedded airfall tuffs, mainly rhyolitic in composrtion and locally very thick, occur throughout the Browns Park Formation (upper Oligocene to upper Miocene) northwestern Colorado northeasternmost Utah Frrst mentioned m geologic literature by Bradley (1935), they have recelved only cursory attention other than for purpose of radiometric dating. The present writer began study tuffs 1980, hoping use them as time-stratigraphic marker beds wrthin formatron. Several tuff-rich stratrgraphw sections were measured numerous samples collected. results petrographic petrochemical studies these are presented. Refractive mdices vitric shards from 52 increase slightly irregularly with apparently decreasing age samples. Seemingly, trendpoints toward less felsic compositions time. Quartz, feldspar, especially heavy-mineral suites studied petrographically. Because components (phenocrystic minerals) not generally readily distrnguishable contaminating detrital grains, time-composition trends assemblages problematical drfficult establish. Clinopyroxene likely origin may Increase slrghtly abundance wrth decreasrng age. ratio apatite (mamly arrfall origin) zrrcon (mixed origins) appears decrease Recalculated analyses major oxides 24 whole-rock that appear be least contaminated detrrtus show followrng variations age: increased silica, potash, total alkalis, titanra, decreased alumma, iron oxides, magnesra, lime, soda, soda: potash ratio. In general, indicate became rncreasingly more felsrc time, albeit irregularly. This contradicts conclusions drawn refractrve-mdex determinations on same Preliminary correlations airfall-tuff petrochemistry, refractive indices glass, mineralogy population heavy minerals, been partly successful Mrnor-element distribution provides useful informatron, but data incomplete. Prevarling westerly winds mid- late-Tertrary time brought ash into correlative units elsewhere Rocky Mountain region distant sources volcanically active areas western United States. Bulk major-oxide selected eruptrve centers Basrn Range province commonly resemble those Brown tuff. Source silicic eruptive material In, necessarily limited to, Utah, Nevada, eastern Calrfomia, southwestern Idaho, southeastern Oregon.