GEOLOGIC MAPPING OF THE CLARNO UNIT OF JOHN DAY FOSSIL BEDS NATIONAL MONUMENT, EASTERN OREGON, WITH IMPLICATIONS FOR THE CENOZOIC PLANT FOSSIL RECORD

Detailed mapping of federal lands provides opportunities to better characterize sites of geologic, paleontologic, and archeologic interest, but most current geologic maps are obsolete. To address this problem, the authors mapped the Clarno Unit of John Day Fossil Beds National Monument in eastern Oregon as part of a GSA GeoCorps America project.

The early to mid Eocene Clarno Formation is exposed across the majority of the Clarno Unit and contains lava flows and fossiliferous volcaniclastic strata interbedded with paleosols. Lower units consist of highly altered lahars that form a gentle to open anticline, a dacite dome (~54 Ma), and an andesite flow (~51 Ma). Onlapping strata include thick lahars with interbedded basalt and andesite flows, as well as paleosols. These units are capped by Member A ignimbrite of the John Day Formation (~40 Ma). The Clarno Unit contains abundant plant and several mammal fossil localities, including the well-known Nut Beds and Mammal Quarry. Vertical east-west trending faults with dominant dextral separation and possible north-south trending faults are present. Stratigraphic relationships are obscured by lateral discontinuity of volcaniclastic units, large Quaternary landslides, and the dacite dome.

Geologic mapping resulted in reinterpretation of Clarno geologic history and the Oregon fossil record. Previous authors concluded that the dacite dome intruded the basal lahars, forming a topographic high onto which younger lahars were deposited. We found no evidence of intrusive relationships and hypothesize that the dome was exhumed after solidifying. The oldest well-dated terrestrial Cenozoic fossils within Oregon overlie the lower Clarno andesite (~51 Ma). New leaf and wood fossils discovered in lahars below the andesite extend this record and provide evidence of even older terrestrial Cenozoic fossils. Unfortunately, the base of the lahars is not exposed, and age relations with the ~54 Ma dacite could not be determined due to faulted contacts. Detrital zircon geochronology of the lahars could provide a maximum age constraint for the fossils.

The recent finds extend the maximum age of fossils recorded from John Day Fossil Beds and underscores the importance of updated geologic mapping and continued exploration of federal lands.