ORPHAN BASALTS OF THE GREATER JOHN DAY TO BURNS CORRIDOR, EASTERN OREGON: UNDERSTANDING THE ORIGINS OF UNASSIGNED BASALTS

Our previous and ongoing mapping efforts and new maps by others in the NE part of the Harney Basin, along a corridor east of highway 395 and between the towns of Burns in the south and John Day in the north, provide a more detailed geological account of an area only mapped originally in reconnaissance in the 1960s, where geologic units were poorly delineated, often blanketing whole quadrangles as essentially one unit. For example, more than 90% of the Calamity Butte quadrangle was mapped previously as ‘Tba’ – Tertiary basalt and andesite –, and similar amounts mapped as ‘Ts’ – Strawberry Volcanics – in the adjacent Jump-off Joe Mountain quadrangle. This new mapping has also revealed a series of mid Miocene basalt units scattered across the quadrangles. As this area lies at the intersection of the two oldest formations of the Columbia River Basalt Group (CRBG), the Picture Gorge and Steens Basalt, as well as just south of the slightly younger Strawberry Volcanics, assigning newly mapped basalt units an identity is important for a better understanding of the regional volcanic framework. With XRF and ICP-MS data, we examined trace element, major oxide and element ratio relationships with PGB, Steens Basalt, and basalts of the Strawberry Volcanics through compositional diagrams. We correlated compositions of 47 unassigned basalt and basaltic andesite samples from the John Day – Burns corridor and validated predictions using machine learning modules. PGB, Steens Basalt, and basalts of the Strawberry Volcanics (SVB) exhibit comparable compositions, but differences emerge in key trace elements (Ba, Th, Nb, La, Zr, Hf, Tb, Y, Yb), trace element ratios, and variations in major oxides (TiO₂, Al₂O₃, and P₂O₅). Using these key attributes to correlate unassigned basalts, we classified 36 as PGB, 10 as SVB, and 1 as Steens Basalt. Forest-based machine learning models mirrored these classifications. These predictions emphasize unique characteristics among lava members, expand volume and distribution estimates for the PGB, Steens Basalt and the SVB, and enhances our understanding of the diverse volcanic framework of eastern Oregon.