Roadcut 1

.TOURING

COLORADO GEOLOGY

WEST ELK BRECCIA

	This roadcut is located about 3 miles east of Gunnison along Hwy. 50. The rocks exposed at this locality are part of the West Elk Breccia. These volcanic rocks are considered lower Oligocene in age.
The source of these volcanics is from the West Elk volcanic center located a few miles to the north of this locality. This volcanic area is a deeply eroded, composite strato-volcano. The breccia is as thick as 1200 m in places and has been subdivided into three facies: the Basal cone, Chaotic, and Volcaniclastic. The oldest, the Basal cone is the most altered volcanic rocks in the area consisting of "porphyritic, massive and brecciated, dark-gray,brown, grayish-green, red and purple rocks of basaltic andesite, microgranular diorite and quartz dioritie." This facies can be 600 m thick in places. Some of these rocks may represent lava domes and explosion breccias.	The middle facies, the Chaotic, consists of a "melange of propylitized and silicified lava flows, flow breccias, and tuff breccias that were locally fed by distinctive, dense, greenish-gray and brown siliceous dikes and dikes of intrusive breccia." This facies can be 400 m thick.

The uppermost facies, the Volcaniclastic, consists of "gently dipping, crudely layered tuff breccias, local ash beds, laharic breccias, lava flows, minor tuffaceous conglomerate and epiclastic deposits." The breccia beds contain angular and poorly sorted volcanic fragments as large as 3 m across. The conglomerates appear to be air-fall tuff reworked by streams. This roadcut appears to be this type of material. Well-rounded cobbles and boulders of Precambrian rocks, quartzite, hypabyssal, prophyries, and volcanic rocks can be seen in the outcrop. This facies has a maximum thickness of 500m.
The quoted passages above are taken from Gaskill, Mutschler, and Bartleson, 1981, West Elk Volcanic Field, Gunnison and Delta Counties, Colorado: New Mexico Geological Society Guidebook, 32nd Field Conference, Western Slope, Colorado.