Field characteristics of deposits from spatter-rich pyroclastic density currents at Summer Coon volcano, Colorado

The Oligocene, deeply eroded Summer Goon composite volcano contains mafic a ndesite deposits that are massive to poorly bedded, have abundant flattened and deformed spatter clasts, have Varying proportions of dense lithic clas ts, and are supported mostly by a coarse-ash matrix. Although superficially these deposits resemble typical facies from Strombolian eruptions (emplace d ballistically, by fallout, and by rolling and local grain-avalanches down steep cone slopes), there are several Lines of evidence that lead to an in terpretation that the deposits were emplaced by pyroclastic density current s. These include local coarse-tail grading, deformation of spatter clasts i n a down-flow direction, incorporation of matrix ash and lapilli into flatt ened spatter clasts, imbrication of large clasts, plastering of spatter on stoss sides of large lithic blocks and lenses of lithic-rich material on le e sides, deposition on angles less than the angle of repose, and a paucity of clast shapes associated with Strombolian mechanisms. The deposit charact eristics are consistent with rapid sedimentation from a low-particle-concen tration, turbulent flow onto an aggrading bed. We infer two potential mecha nisms for generating these density currents: (1) explosive magma-water inte raction involving lithic debris and relatively unfragmented melt, and (2) c ollapse of oversteepened upper cone slopes due to rapid accumulation of spa tter from voluminous Strombolian eruptions. (C) 2000 Elsevier Science B.V. All rights reserved.