GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS OF BLUE BASIN AT JOHN DAY FOSSIL BEDS NATIONAL MONUMENT, OREGON

John Day Fossil Beds National Monument in east-central Oregon contains one of the most complete records of Cenozoic fossils extending from the early Eocene to the late Miocene.

Through an internship with the GeoCorps America program, a partnership between the Geological Society of America and the National Park Service, a geologic map was created of Blue Basin, a kilometer-squared area within the Sheep Rock Unit of the monument. Strata exposed here include the lower units (A-G) of the Turtle Cove Member of the Oligocene John Day Formation. These rocks are composed primarily of volcaniclastic sediment, air fall tuff, and welded tuff units, many of which are zeolitized. These are also highly productive fossil horizons, including one of the richest and most diverse Oligocene faunas in the world, which have been studied for nearly 150 years. Detailed mapping of the structure and stratigraphy of Blue Basin is an essential tool for continued paleontological and geological research at the park, and because the area includes one of the park’s most popular trails it also helps further visitor services.

The geologic map was created using high resolution aerial photos and includes some cross sectional views of highly prospected outcrops from photo points, which will help familiarize workers and visitors with the geology of Blue Basin.

A large number of previously unmapped and unrecognized faults have been identified in the map area. These include numerous northwest trending near-vertical normal faults with ~50m stratigraphic separation as well as north-northeast trending strain hardened fault sets that may have formed at a somewhat different time. The NNE-trending faults appear as conjugate sets of normal faults dipping both to the west and the east with <10m stratigraphic separation; some strike slip slickenlines are also observed on these fault surfaces. The faulting of Blue Basin may have formed as a result of Basin and Range extension as far north as the mapping region and may reflect the transition from Mid to Late Miocene NE-SW directed extension to Pliocene NW-SE directed extension. The monument is also caught between large NW trending right lateral fault zones, complicating the structure. Finally, late stage Tertiary landslides form the topography of the north side of Blue Basin, displacing large blocks by hundreds of meters.