RELATIVE ERUPTION CHRONOLOGY AND TEPHRA-VENT CORRELATIONS OF THE SAND MOUNTAIN VOLCANIC CHAIN, CENTRAL OREGON CASCADES

The Sand Mountain Volcanic Chain (SMVC) in the central Oregon Cascades consists of approximately twenty volcanic cones with associated explosive tephra and lava flows. The cones define a north-trending bifurcated lineament that parallels the Horse Creek fault zone on its west side and exists in a down dropped graben that hosts several basaltic cones of Pleistocene and Holocene age. The SMVC cones are all about 3 ka in age and their conspicuously fine grain size and blocky texture has been interpreted to indicate a phreatomagmatic origin. Cone lineaments influenced by local bedrock structure can erupt in sequences with definable orders in specific directions or in a seemingly random, non-sequential orders along the lineaments. Understanding the sequence of the tephra producing eruptions at the SMVC is important considering the chain is intersected by a major transportation route and recent magma recharge at nearby South Sister Volcano indicates regional magmatism is still active.

To determine the eruption sequence for the SMVC, tephra samples were collected in 100 to 200 cm deep trenches up to 15 km east of the chain, and scoria and basaltic lava samples were collected at or adjacent to each cone. Major elements and select trace elements were determined on the tephra, scoria, and lava samples by x-ray fluorescence (XRF) to assist geochemical correlation of individual tephra layers in the tephra stratigraphy with specific cones or cone groups in the chain. Based on variation diagrams involving SiO₂, MgO, Al₂O₃, FeO and total alkalis, many of the tephra layers originated from central Sand Mountain cones, whereas two samples were sourced from Nash Crater. Scatter on variation diagrams involving major elements such as K₂O, P₂O₅, and TiO₂ suggests either complex magmatic processes or tephra that were not sampled on a fine enough scale and therefore represent a mixture of source cones. Advanced statistical analysis such as principal component analysis should effectively connect individual tephra layers to individual cones or cone groupings. Additionally, mineral chemistry will be collected by electron microprobe to determine if variations in solid solution series minerals, such as plagioclase and olivine, between eruptions can also be used to correlate tephra layers and cones.