Relative Eruption Chronology and Tephra-Vent Correlations of the Sand Mountain Volcanic Field, Oregon High Cascades

Abstract

Tephra stratigraphy east of the SMVF was sampled to characterize the major and trace element geochemistry, along with olivine and plagioclase mineral chemistry, in attempts to correlate the tephra with specific cones and cone groups in the SMVF chain. Three geochemical groups are represented in the tephra stratigraphy, defined by relative proportions of SiO2, TiO2, Sr, Cr, and Ni, and determined by SAHN Clustering analysis. These three groups are defined in the basal, middle, and upper portions of the stratigraphy and generally correlate with the three major geochemical groups defined in the SMVF lava flows. Correlation between the tephra and lava flow geochemical subgroups are complicated by magma mixing, magma recharge, heterogeneous crystal growth, sample mixing, tephra reworking, density stratification during fallout, and error during sample preparation for XRF analysis. Regardless, possible subgroup correlations generally agree with the lava flow stratigraphy and interpretations by Deligne et al. (2016) that the Lost Lake Group is the oldest, the Sand Group is second oldest, and the Nash Group is the youngest, while some cones in the Sand Nash Groups were intermittently active around the same time. This refutes radiometric ages produced in the 1960s and 1970’s that interpreted the lost Lake Group of cones as the youngest in the chain.