MIOCENE COASTAL VEGETATION PRESERVED BY VOLCANIC ERUPTION AT CAPE BLANCO, OREGON

The Cape Blanco flora of coastal Oregon is preserved in a shale lens 0.8 m below the top of a 6-m-thick redeposited tuff within shallow marine “sandstone of Floras Lake” (informal name). Previous dating of the flora and formation was based on marine mollusk biostratigraphy as Newportian (14-20 Ma) Here we report a single crystal plagioclase 40Ar-39Ar plateau age for the plant-bearing tuff of 18.24 ± 0.86 Ma. The pumiceous-vitric tuff is andesitic in bulk chemical composition. The tuff bed has well to poorly sorted cm to m beds with silt to pebble sized grains. Larger clasts are predominantly pumice. Deposition of the tuff in shallow water is suggested by common scour-and-fill and graded beds on scales of about a meter, some showing reverse grading of pumice and coarse organic matter. The fossil plants were curled, skeletonized and fragmented before deposition, but not associated with a fossil soil. Redeposited upslope leaf litter is the likely source of fossil material. The tuff bed is distinct from the adjacent well sorted marine sandstone which contains such indicators of nearby rocky coasts as mussels, barnacles, and sparse conglomerate lenses.. We suggest that the tuff and its plant fossils are evidence of an aggrading delta caused by an eruption in the source area that dramatically increased sediment available for transport and deposition. The recent eruptions of the Soufrière Hills Volcano, Montserrat provide a modern analogue for this type of coastline extension.