DATING THE COLUMBIA RIVER FLOOD BASALTS BY CORRELATION OF INTERBEDDED SILICIC ASH LAYERS

Possible links between the eruptions of the Columbia River flood basalts (CRB) and mid-Miocene climate change can only be confirmed if the ages of the eruptions can be precisely and accurately compared with those of climate proxies. This endeavor is currently hampered because there is no consensus on the eruption ages of the basalts, with estimates varying by hundreds of thousands of years [1,2]. This is largely because most ages quoted for the CRB lavas are derived from direct ⁴⁰Ar/³⁹Ar dating of basaltic material and have large associated errors. In contrast, the extremely high precision now obtainable by ⁴⁰Ar/³⁹Ar dating of sanidine, and the growth of an extensive database of North American Neogene silicic ashes of known ages (many determined on sanidine) and chemical compositions [3] suggests an alternative approach based on geologic relations between basalt flows and contemporaneous, widely-dispersed ash layers. We have focused on two stratigraphic intervals in the CRB succession both located at the eastern edge of the Columbia Plateau. Interbedded silicic ashes have two source regions, the ancestral Cascade arc and early rhyolites of the Yellowstone hotspot erupted in the Oregon-Nevada border region. Both source areas are ~500 km distant and most ashes lack dateable phenocrysts but glass shards are amenable to analysis for major and trace elements and hence ash layers can be chemically correlated with dated units. Using this approach, we have identified a fused tuff in the R1 magnetostratigraphic interval of the Grande Ronde Basalt near Orofino, ID as a distal equivalent of the Tuff of Trout Creek Mountains, NV-OR, dated at 16.47 ± 0.04 Ma [4]. This is consistent with the basalt chronology developed by Jarboe et al. [1]. We have also correlated an ash layer in the Miocene Lake Clarkia beds with the Cold Springs Tuff of the Santa Rosa-Calico volcanic field, NV dated at 15.50 ± 0.09 and 15.56 ± 0.04 Ma [5]. Miocene Lake Clarkia formed when the course of the ancestral St. Maries River, ID was temporarily dammed by a flow of the Priest Rapids Member of the Wanapum Basalt. This result suggests that the Priest Rapids lavas may be be older than previously thought. References: 1. Jarboe et al. 2010, Chem Geol; 2. Barry et al. 2013, GSA SP 497; 3. Nash & Perkins 2010, PloS One; 4. Henry et al. 2012, AGU abs; 5. Brueseke et al. 2008, Bull Volcan