ANALYSIS AND INTERPRETATION OF CONGLOMERATE ALONG MAJOR RING FAULTS OF THE SILVERTON CALDERA, SAN JUAN MOUNTAINS, COLORADO

The San Juan volcanic field is the site of a major mid-Cenozoic ‘ignimbrite flare-up’ and contains multiple calderas. The Silverton Caldera is the least studied of the well-preserved calderas in the field. The Crystal Lake Tuff, that is most likely the unit that erupted on its formation has been located and analyzed outside of the caldera rim, however a correlative unit has not been described within the caldera, and this has been explained by either erosion of infill after eruption or deposition only as outflow due to complete evacuation of the magma chamber.

Small localized conglomerates in calderas can have multiple possible origins such as collapse breccias, autobreccias, agglomerates, hot avalanches, and/or lag breccias marking eruption channelways, all of which give information on caldera eruption and collapse history. Collapse breccias are defined by angular fragments of broken rock in a fine grained matrix. A conglomerate interbedded with tuffs in close proximity to the outer ring fault on the western rim of the caldera has been classified as a caldera collapse breccia. A unit in a similar position near the upper edge of the NW caldera rim has been interpreted in previous mapping as a lahar.

The pebble to boulder sized matrix supported polymict conglomerate has rounded clasts of a narrow range of volcanic and volcaniclastic composition. The unit while approximately 400ft thick in areas shows no grading within and weak to no internal flow banding. Petrography of samples from multiple locations within the unit show pervasive propylitic alteration throughout. Clast composition, while all intermediate volcaniclastic with generally abundant plagioclase crystals varies, but common clast types include those with flow banding as well as perlitic fracturing within the clast matrix. The matrix shows texture corresponding to those of welded ignimbrites. It is comprised mostly of fragments of glass and ash and was at least locally welded, showing small amounts of divitrification, including some vug infilling of hydrothermal quartz. The petrographic nature of the clasts and matrix will be discussed. Comparison between the conglomerate matrix and Crystal Lake Tuff may contribute pertinent information about whether eruption of this tuff was related to the formation of the caldera.