ORIGINS OF CONE-FORMING BRECCIAS OF THE SUMMER COON VOLCANO

The Summer Coon Volcano (SCV), located in the San Juan Volcanic Field about 10 km north of Del Norte in southwest Colorado, is a heavily eroded composite volcano. The cone unit of SCV is dominated by breccia deposits, making it depositionally unique among composite volcanoes. Hypotheses regarding the origin of breccia in the SCV cone unit have been the subject of controversy and are the focus of this study. Several sites within the cone unit were carefully examined to determine the nature and succession of the breccia deposits. Measurement and description of sections, photography, sampling and field checks were employed at SCV to determine the lithologic character of the cone unit. Most of the breccia in the SCV cone unit formed as the result of mass wasting, either as a direct or indirect result of eruptive activity. Both hot and cold lahar deposits account for the greatest volume of breccia within the SCV cone. The bulk of SCV lavas are flow-brecciated, which has led to its miss-identification among past workers. Pyroclastic flow deposits are rare within the cone unit, with only one being identified. Conclusions from this study of SCV include: 1) Most of the breccia in the SCV cone unit is laharic in origin. However, there is a higher proportion of lava represented in the cone unit than previous workers realized. SCV cone unit deposits appear deceptively monotonous because the majority of them are breccia. 2) Nearly all of the deposits preserved at SCV fall into a series of proximal volcaniclastic facies (Cas and Wright, 1987). 3) Although breccia dominated, the origin of deposits of the SCV cone unit are not out of place within the succession of deposits in a normal composite volcano. Rather, they represent a unique combination of formative processes, that give the lithology of the cone a deceptively monotonous appearance.