ENRICHING THE TEPHROSTRATIGRAPHIC RECORD OF THE PACIFIC NORTHWEST WITH U-Pb ZIRCON GEOCHRONOLOGY AND PRINCIPLE COMPONENT ANALYSIS

Tephrostratigraphy (volcanic glass correlation) is a chronologic cornerstone in paleontology and paleoclimatology studies in the Pacific Northwest. This is highlighted in studies regarding the Mid-Miocene Climatic Optimum (MMCO), which marks the last time that atmospheric CO₂ concentrations were as high as modern levels, correlating to a peak of significant warming during an otherwise cooling trend through the Cenozoic. The utility of tephrostratigraphy is constrained by the ability to make quantitative correlations between tephras and by the density, accuracy, and precision of geochronologic anchor horizons. While bivariate diagrams can be useful in correlating/distinguishing between samples, they often fail to capture the full chemical signature of a sample, and as such faulty correlations can be made. Principle component analysis by singular value decomposition (SVD-PCA) is a dimensionality reduction technique which has shown to be useful in testing correlation hypotheses based on chemical compositions. Additionally, direct radioisotopic dates testing of the correlation distal tephras are sparse.

We present new U-Pb zircon geochronology for several key Pacific NW locales across the MMCO, including the Sucker Creek Formation along the Oregon-Idaho border (15.77 to 14.81 ± 0.01 Ma) the Mascall Formation of central Oregon (16.13 Ma to 13.56 ± 0.02 Ma), and the Bully Creek Formation of eastern Oregon (15.69 to 15.45 ). This new geochronology extends the stratigraphic ranges of these basin sequences, opening up new possibilities for correlation. Additionally, we reexamine previously correlated tephras in the light of this new geochronology and with SVD-PCA using a wider suite of trace, minor, and major elements, such as those correlated to the regional Bully Creek Tuff, the Dinner Creek Tuff, the Mascall Tuff, and others. SVD-PCA analyses of several tephras exhibit a wider range of chemical compositions than previously interpreted, suggesting that chemical zoning within eruptions can complicate tephrostratigraphic correlations. We will document how the integration of high-precision U-Pb zircon geochronology and SVD-PCA geochemical correlation of tephras enriches the Mid-Miocene tephrostratigraphy of the Pacific Northwest.