Abstract

The Beaver River canyon area in the southern Tushar Mountains is situated on the southwestern flank of the Tertiary Marysvale volcanic field. Today the lower part of this deep gorge offers a spectacular geologic cross-section that reveals many of the details of the events that led to the build-up of this field.

The Delano Peak Tuff Member and intermediate lava of the Miocene and Oligocene Bullion Canyon Volcanics, exposed at the bottom of lower Beaver canyon, record early episodes in the build-up of the Marysvale volcanic field. Following the emplacement of these units, two stratovolcanoes were active simultaneously in the immediate vicinity of present-day lower Beaver canyon. These volcanoes were sources for material incorporated in the Mount Dutton Formation, the largest volcanic accumulation on the southern flank of the Marysvale field. The topography created by these two vents controlled distribution of younger lavas and ash-flow tuffs as well as the present course of Beaver River.

Two cooling units of the Osiris Tuff, an early Miocene ash-flow tuff, erupted from a distant source to the northeast, spread into the area after the close of Mount Dutton activity. They lapped around the volcanoes and ponded in the lowlands adjacent to them. The tuff of Lion Flat, a Miocene ash-flow tuff derived from a more local source, did likewise. This was followed shortly by emplacement of the formation of Lousy Jim, a thick sequence of rhyodacitic lava flows.

Basin-range normal faulting began in the Miocene and has continued into the Recent. The faulting was accompanied by eruption of alkaline mafic lava flows. The main phase of this faulting elevated the Tushar Mountains to their present relief and led to the downcutting of the Beaver River to form Beaver Canyon.