SCREEN WASHING ZEOLITIZED VOLCANICLASTIC SEDIMENTS FOR MICROFOSSILS AT JOHN DAY FOSSIL BEDS NATIONAL MONUMENT IN OREGON

The Turtle Cove Member of the John Day Formation is known for having one of the richest and most diverse Arikareean (Oligocene) faunas in the world. The John Day Formation is composed of fossil- bearing volcaniclastic sediments and tuff layers, many of which have been radiometrically dated. The well-defined stratigraphy exposed in the John Day facies, along with the geologically short spans of time between tuff layers, allows us to assign ages to the fossils found both in-situ and on the surface. Microfossils, the fossilized remains of tiny organisms, represent populations of abundant small species, which are often underrepresented in museum collections due to collection and preservational biases. In the case of the John Day Formation, existing museum collections, including tens of thousands of specimens, show a strong bias towards larger taxa. Screen washing sediments in fossiliferous units helps to reduce these collection biases, providing a more complete picture of past ecosystems. A series of screen washing experiments, involving different lithological units, wash methods, and chemical treatments were undertaken to test the efficacy of screen washing John Day Formation strata. Qualitative and quantitative analyses of these experiments has allowed us to develop a reliable screen washing procedure, which has yielded a diverse assemblage of underrepresented specimens from the region, and has proven to be an effective microfossil recovery method that can be utilized by the park. These finds will aid current and future studies in biostratigraphy, paleoecology, and evolution. (This is a Geocorps submission).