DIACHRONEITY IN THE NORTH AMERICAN EOCENE-OLIGOCENE FLORAL RECORD AND CLIMATIC DETERIORATION

The Eocene and Oligocene floras of the western United States show a climatic deterioration that has been variously called the “terminal Eocene event” or “Oligocene deterioration.” Late Eocene floras in Oregon, Washington, California, Alaska, and Colorado document a final episode of warm subtropical conditions before a decline to much cooler and drier conditions in all of these places. However, the dating of these floras has long been problematic. Recent argon/argon dates and magnetic stratigraphy have greatly improved their correlation. In Colorado, the climatic change seems to have been extremely rapid and occurred between the Florissant and Antero floras, and dated between 33.89 and 34.07 Ma, or latest Eocene in age, although the 32.9 Ma Pitch-Pinnacle flora suggests that the deterioration took place in the early Oligocene. In northeast California, the dating and floral sequences are not as precise, so the climatic change could have occurred between 31.5 and 34.0 Ma (probably early Oligocene). In western Oregon (Eugene-Fisher Formations), the change is gradual and occurs between the early Oligocene Goshen flora (33.4 Ma) and the early Oligocene Rujada flora (31.3 Ma). In the John Day region of Oregon, it occurs before the oldest Bridge Creek flora, dated at 33.6 Ma (right at the Eocene-Oligocene boundary). Thus, only two of these four floral sequences (Eugene, Oregon, and Cedarville, California) show the early Oligocene climatic change consistent with that documented in the global marine record, while the climatic change was abrupt in the late Eocene in Colorado between 33.89 and 34.07 Ma, and also sometime during the late Eocene (before 33.6 Ma) in central Oregon. This discrepancy is not yet resolved, so further dating and magnetic stratigraphy is required to understand it.