ABSTRACT

Within its type area, the Clarno Formation (Eocene, north-central Oregon) includes Oregon's two oldest Tertiary terrestrial mammal localities, numerous fossil plant localities, and many radiometrically dated rock units. Detailed lithostratigraphic mapping has now clarified the complex geologic history and stratigraphic framework which relate these localities and dates to each other and to the geologic record of adjacent areas. Of the five allostratigraphic units recognized within the local Clarno section, the second oldest includes the Nut Beds locality with vertebrates characteristic of the Bridgerian land mammal “age,” and the youngest includes the Hancock Quarry locality, the source of a large assemblage of Duchesnean vertebrates. Fossil plants are also preserved in both of these localities and in the middle unit. Those dates which are consistent with the local superpositional sequence also accord with dates on biostratigraphically correlated faunal assemblages elsewhere in North America

The Nut Beds local fauna, preserved within a lacustrine delta complex, includes species of Patriofelis, Orohippus, Hyrachyus, and Telmatherium which indicate a late Bridgerian North American Land Mammal “Age” (NALMA). This accords with an 40Ar/39Ar date of 48.32 Ma, but not with other dates near 44 Ma. A remarkably diverse floral record documents a paratropical to tropical rain forest environment, but, at about 14 million years older than the presently recognized Eocene/Oligocene boundary, it provides little evidence bearing on the rate of climatic cooling near the end of the Eocene.