A Study of Mid-Miocene Yellowstone Hotspot Volcanics and the Search for the Steens Mountain Reversal

It has been suggested that SE Oregon, NW Nevada and SW Idaho were quickly covered by Steens-type basalts at the beginning of Yellowstone hotspot volcanism. Only Steens Mountain has been studied in detail, but exploratory paleomagnetic sampling at six other sites suggests that the R-N polarity reversal recorded at Steens is common throughout the area (Mankinen et al., 1987). By sampling a large number of lava flow sequences and using current paleomagnetic and Ar/Ar dating techniques, we hope to compile a more detailed composite record of the magnetic field during the Steens Mountain reversal and locate it exactly in the geomagnetic polarity timescale of Cande and Kent (1995). Current dates for the reversal between 16-17 Ma need to be refined. After finding and dating reversals throughout the area, we can then use the stratigraphic position of the reversal within the lava flow sequence to place the local volcanic events into the larger geographical framework with a greater time precision than radiometric dating alone could give us. With enough volcanic sequences, the propagation of initial Yellowstone hot spot magmatism should be better understood. We have sampled two flow-on-flow sequences in the area. Initial core analysis and portable fluxgate magnetometer readings on hand samples from the Pueblo Mountains suggest a flow sequence that is normal at the top and reversed at the bottom. Data from the Summit Springs section suggest a thick normally magnetized top with a few transitional flows near the bottom. Next we plan to drill several other sections suspected to be Steens equivalents, including a recently scouted section in the Owyhee Mountains that shows a normal top and reversed bottom.