POLISHED COBBLES DERIVED FROM PHYSICAL ABRASION IN HYPERCONCENTRATED FLOOD FLOWS OF THE OLIGOCENE COLESTIN FORMATION, JACKSON COUNTY, OREGON

The Oligocene Colestin Formation is approximately 1200 m thick and consists of epiclastic volcanic sandstones, vitric and crystal tuffs, matrix- and grain-supported conglomerates, planar stratified sandstones and conglomerates, and mudstones. Deposition of these sediments is consistent with fluid, pyroclastic, hyperconcentrated flood, and debris flows in continental volcanic environments associated with the development of the Western Cascades. Polished cobbles of quartzite, chert, gneiss, vein quartz, and metavolcanic rock fragments derived from the ancestral Klamath Mountains occur within matrix-supported conglomerates deposited by hyperconcentrated flood and debris flows. Volcanic rock fragments of andesite, dacite, and rhyolite within these same matrix-supported conglomerates are not polished, suggesting that the durability of the cobbles is significant to polishing. The formation of the polished cobbles in the matrix-supported conglomerates is hypothesized to have developed following a two-step process. First, the cobbles were rounded by fluid flow in rivers and streams sourced in the ancestral Klamath Mountains. Second, the rounded cobbles were polished by physical abrasion in hyperconcentrated flood flows. Polished cobbles in the Oligocene Colestin Formation were produced by physical abrasion and may provide an alternate explanation for the occurrence of numerous polished cobbles in Mesozoic rocks previously interpreted as gastroliths.