Description
Poway rhyolite clasts are the dominant gravel type of the Eocene section in San Diego County. The clasts range from pebbles to boulders in size and are composed of slightly metamorphosed rhyolitic to rhyo-dacitic welded tuffs which have great resistance to mechanical abrasion. The shape evolution of the Eocene Poway rhyolite clasts is primarily due to transportation. Four sites were chosen to measure the clast dimensions for long, intermediate, and short axes from randomly selected cobbles. The different depositional environments studied were an Eocene river channel, alluvial fan apex, submarine canyon fill, and reworked distal fan facies. It was determined that depositional environments have had little effect on clast morphology; this is attributed to a short transport period between sites. The Poway rhyolite clast morphologies plot as disc-spheroidal shape on a variation of the Zingg (1935) model. The Eocene submarine canyon deposit and distal fan facies showed no distinct shape changes as a result of modern beach action.
