Search

The >800-m-thick, Oligocene Ohanapecosh Formation records voluminous sedimentation of volcanic clasts in the Ancestral Cascades arc (Washington State, USA). Most volcaniclastic beds are dominated by angular pumice clasts and fi amme of andesitic composition, now entirely devitrifi ed and altered. All beds are laterally continuous and have uniform thickness; fi ne sandstone and mudstone beds have features typical of deposits from low-density turbidity currents and suspension settling. Erosion surfaces, crossbeds, and evidence of bi-directional oscillatory currents (i.e., wave ripples and swaley and hummocky cross-stratifi cation) are almost entirely absent. We infer that the setting was subaqueous and below wave base. The abundance of angular pumice clasts, crystals and dense volcanic clasts, and the extreme thickness of several facies, suggest they were derived from magmatic volatiledriven explosive eruptions. The extremely thick beds are ungraded or weakly graded, and lack evidence of hot emplacement, suggesting deposition from subaqueous, watersupported, high-concentration volcaniclastic density currents. Some of the thickest beds contain coarse, rounded, dense clasts at their base and are interbedded with accretionary lapilli-bearing mudstone; these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic fl ows that crossed the shoreline. Shallow basaltic intrusions and mafi c volcanic breccia composed of scoria lapilli indicate the presence of intra-basinal scoria cones that may have been partly subaerial. The range in facies in the Ohanapecosh Formation is typical of below-wave-base, continental (lacustrine) basins that form in proximity to active volcanic arcs, and includes eruption-fed and resedimented facies. Extreme instantaneous aggradation rates are related directly to explosive eruptions, and sediment pathways refl ect the locations of active volcanoes, in contrast to conventional sedimentation processes acting in non-volcanic environments. [ABSTRACT FROM AUTHOR]