CORRELATING THE STEENS (R0-N0) MAGNETIC POLARITY REVERSAL ACROSS THE OREGON PLATEAU

Recent work has extended the Columbia River Basalt Group to include the lavas of the Oregon Plateau and suggests that the oldest Columbia River flows (the Imnaha member) are coeval with the Steens Basalts of the Oregon Plateau. Half a dozen known volcanic sections on the Oregon Plateau preserve reverse to normal (R-N) geomagnetic polarity transitions and many others are likely to exist. Provided the Steens reversal (R-N) is typical (2-10 ky duration), it offers a means to precisely correlate these sections to construct the spatio-temporal history of early Yellowstone hotspot basaltic eruptions. This is particularly useful during the voluminous Steens eruptions because no ash layers exist as regional markers. We have determined the age of plagioclase separates from two transitional flows from the type section at Steens Mountain (SM) to be 16.69 ± 0.19 Ma and 16.48 ± 0.18 Ma. Because the Earth's magnetic field reversed rapidly during this time, with less than 200 ky occurring between some R-N transitions, precise ages are needed to be certain a particular R-N transition is indeed the Steens reversal. Similarities of transitional magnetic field behavior are also used to strengthen claims of equivalency, but some similarities may be misleading due to the presence of persistent dipole or nondipole transitional fields. For example, sections in the Santa Rosa Range (120 km SW of SM), and the Sheep Creek Range (250 km SE of SM) record reversals that resemble that from SM, but yield preliminary ages (unpublished data, and John et al., 2000) that suggest they record earlier and later reversals, respectively. On the other hand, a comparison our highly detailed paleomagnetic study of the reversal path found at Catlow Peak (70km SSE of SM) to the detailed Steens Mountain reversal record strongly suggests the two reversals are the same. Preliminary ages of plagioclase separates from normally magnetized rocks at Summit Springs 60km NNE of SM (17.0 ± 0.2 Ma) and the reversely magnetized rocks at Pueblo Mountains 60km S of SM (16.7 ± 0.4 Ma) suggest these sections are also coeval with the Steens reversal. However, work that is in progress (involving additional rock-age determinations and further analysis and data reduction that will increase precision) is needed to definitively correlate these sections.