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 northeasternmost Utah. 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 stratigraphic sections were measured and numerous samples were collected. The results of petrographic and petrochemical studies of these samples are presented. -from Author