East Elk Creek

East Elk Creek

 

Volcanic Stratigraphy of East Elk Creek

 

East Elk Creek is located about 15 km west of Gunnison above the Curecanti National Recreation Area Group Campground. We have used the site for years to look at the local volcanic stratigraphy and to study zonations in ash flow tuffs. A basic description of the geology of the area and units is found below.

At East Elk Creek the base of the cliff is West Elk Breccia, a unit consisting of debris flows, and fluvial deposits derived from the flanks of West Elk Volcano. In the photo below the West Elk breccia consists of a large debris flow that form steep cliffs and pinnacles. The debris flow is overlain by tuffaceous conglomerate that makes up the top of the West Elk Breccia. The Tuff of East Elk Creek is a local unit, found for about 20km as discontinuous outcrops in the mesas north of Blue Mesa Reservoir. This unit is a complex series of ash flow, ash fall, tuffaceous sandstones, and lake deposits that presumably accumulated in fluvially carved valleys formed in the drainages between West Elk Volcano to the north and the San Juan volcanoes to the south. The top two units are ash flow tuffs derived from the large calderas of the western San Juan Mountains. In this location, the Dillon Mesa Tuff is an unwelded ash flow about 5m thick. It is overlain the the much larger and more extensive Sapinero Mesa Tuff. In this location, the Sapinero Mesa Tuff is much thicker and densely welded.

 

This view shows the same area from the top of the mesa looking toward the center of West Elk Volcano. The most prominent unit is the cliff of densely welded Sapinero Tuff that forms the edge of the mesa. The Tuff of East Elk Creek and the Dillon Mesa Tuff ,that can be seen in the foreground, pinch out just around the corner as the West Elk Breccia thickens. On top of the mesa Fish Canyon Tuff makes low easily eroded hills and the high point on the photo is capped by Carpenter Ridge Tuff.

 

The next set of photos concentrate on the contact between the Dillon Mesa Tuff and Sapinero Mesa Tuff. The Dillon Mesa Tuff. The Dillon Mesa Tuff is unwelded and locally incised by small stream channels that are filled with tuffaceous conglomerate. In addition the upper Dillon Mesa Tuff is oxidized red.

The base of the Sapinero Mesa Tuff is a white layer that has parallel lamination and contains clasts of the underlying conglomerate. It is interpreted as a base surge associated with the ash flow. Above that is a partially welded zone that grades up into the vitrophyre.

 

The twophotos to the right and below show the same area from different angles.