THE OLIGOCENE WEST ELK BRECCIA: EVIDENCE FOR MASSIVE VOLCANIC DEBRIS AVALANCHES IN THE EASTERN GUNNISON RIVER VALLEY, WEST-CENTRAL COLORADO, U.S.A.

• Main Author: Whalen, Patrick J.
• Format: Others
• Published: UKnowledge 2017
• Subjects: volcanic debris avalanches; debris flows; volcaniclastics; megaclasts; jigsaw fractures; volcanic breccia; Geology; Sedimentology; Volcanology
• Online Access: http://uknowledge.uky.edu/ees_etds/47; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1055&context=ees_etds

The West Elk Breccia has been studied since the late 1800’s with many interpretations regarding its origin. One unrecognized possibility is that parts of it are debris-avalanche deposits. This study has recognized evidence for this interpretation at three scales: volcano scale, outcrop scale, and intra-outcrop scale. At the volcano scale, a scarp in the old volcano reveals underlying Mesozoic bedrock, suggesting sector collapse. At the outcrop scale, megablocks of the original edifice, up to hundreds of meters in length, have atypical orientations and are surrounded by a gravel matrix. At the intra-outcrop scale, jigsaw-fit fracturing and rip-up clasts are common in distal deposits, which are documented in analogous debris-avalanche deposits. Similar to the debris-avalanche deposit at Mt. Shasta, medial-to-distal-matrix volcaniclast content decreases by 23%; Paleozoic and Mesozoic clasts increase by 5%; and the size of megablocks decreases. The geochemical and petrographic signatures reveal breccia blocks composed of pyroxene-andesite, a more silicic matrix facies, and the andesitic-to-dacitic East Elk Creek Tuff, all compositions that corroborate previous work on this northern extension of the San Juan volcanic field. Measured sections in the 100-km² study area allow for an estimation of total formation volume of approximately 8.5 km3.