JOHN DAY FOSSIL BEDS Geology and Paleoenvironments of the Clarno Unit John Day Fossil Beds National Monument, Oregon

The Clarno Unit of the John Day Fossil Beds National Monument includes a complex sequence of Eocene volcaniclastic sediments, volcanic flows and intrusions. Through section description and mapping in this type area of the Clarno Formation we have generated a geologic framework for numerous fossil sites, including several newly discovered sites.

Two widespread conglomeratic units of andesitic composition in the middle Eocene (Bridgerian-Uintan) part of the Clarno Formation are separated by red claystones including several paleosols. The lower unit, conglomerates of the Palisades, consists of channel and floodplain debris-flow conglomerates and hyperconcentrated flood flow or lahar runout deposits. The overlying conglomerates of Hancock Canyon also contain channel and floodplain debris-flow conglomerates, but have in addition, fluvially reworked conglomerates and pebbly sandstones, reworked tuff beds, a distinctive amygdaloidal basalt flow and the "Nut Beds" fossil site. The Palisades unit is interpreted as a debris flow apron on which there was little fluvial reworking. The Hancock Canyon unit is interpreted as a debris flow apron to a braidplain in an area of complex topography, including hills of a pre-existing dacite intrusion. Both accumulated on footslopes of a large andesitic stratovolcano.

Above the conglomerates are thick but discontinuous red claystones, which record a long period of local volcanic quiescence, slow floodplain aggradation and soil formation. An abrupt climatic change is inferred during accumulation of the red beds because the lower sequence of paleosols is mainly Ultisols, whereas the upper sequence is mainly Alfisols. The fossil Ultisols, like paleosols and fossil plants from the "Nut Beds" can be taken as evidence of a climate that was subtropical (mean annual temperature or MAT 23-25°C) and humid (mean annual precipitation of MAP of 1500-2000 mm). Both fossil plants and soils are more like those of modern southern Mexico, than northern Mexico or Central America. Most fossil plants were transported, but aguacatilla (Meliosma) dominated paleosols of both swamps and lowland colonizing forest, and fresh ash was colonized by ferns (Saccoloma) and fresh alluvium by horsetails (Equisetum). Permineralized forests with sycamore (Macginitea) and katsura (Joffrea) of temperate climatic affinities in the lahars are similar to montane forests of southern Mexico. Thus they reflect an ecotone boundary rather than paleoclimatic change. The abrupt transition to Alfisols in the upper Clarno red beds may represent a decline in both temperature (to MAT 19-23°C) and rainfall (MAP 1000-1500 mm), with dry seasons.

Disconformably overlying the red beds are gray-brown siltstones and conglomerates of the "Mammal Quarry" which has yielded a titanothere-dominated fossil fauna. This is the most ancient known fauna of the late Eocene Duchesnean NALMA and of the White River Chronofauna. Paleosols in the beds of the "Mammal Quarry" show better preservation of primary volcanic grains, and may represent a climatic drying (MAP 550-1000 mm) or increased sedimentation due to volcanic influences.

The Clarno Formation is overlain abruptly by an ash-flow tuff of the basal John Day Formation, here newly dated by single-crystal ⁴⁰Ar/³⁹Ar techniques at 39.22±0.03 Ma. Additional new radiometric dates include 38.4±0.07 Ma for a tuff and 33.62±0.19 Ma for the "Slanting Leaf Beds", both in the lower John Day Formation. The fossil leaf beds are thus very earliest Oligocene in age because the Eocene-Oligocene boundary is currently recognized at 34 Ma. Thus the Clarno Formation is entirely Eocene and the John Day Formation ranges from late Eocene to early Miocene in age.