First Advisor

Martin J. Streck

Term of Graduation

Summer 2025

Date of Publication

9-26-2025

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Basalt petrogenesis, Columbia River Basalt Group, Compositional Data Analysis, Machine learning, Picture Gorge Basalt, Rock classification

Physical Description

1 online resource (x, 121 pages)

Abstract

For more than a century, scientists have studied the geologic stratigraphy of eastern Oregon, which chronicles a rich history of volcanic activity spanning millions of years. Recent and ongoing geologic mapping of the northeastern Harney Basin -- along a corridor east of Highway 395 between the towns of Burns and John Day -- provides a more detailed understanding of an area originally mapped only in reconnaissance during the 1960s. At that time, geologic units were poorly delineated, often with entire quadrangles mapped as a single unit. For example, over 90% of the Calamity Butte quadrangle was previously mapped as 'Tba' (Tertiary basalt and andesite), and similarly substantial portions of the adjacent Jump-off Joe Mountain quadrangle were mapped as 'Ts' (Strawberry Volcanics). This area lies at the intersection of two of the oldest formations of the Columbia River Basalt Group (CRBG) – the Picture Gorge Basalt (PGB) and Steens Basalt – as well as just south of the younger Strawberry Volcanics (SVB). Accurately assigning newly mapped basalt units is therefore critical to refining the regional volcanic framework.

Geochemical, petrographic, and age data from 48 unassigned basalt samples, mapped and collected during this study as well as compiled from recent quadrangle maps, were compared with regional basalt lava members and classified using machine learning algorithms. These models predicted that the samples corresponded to one of the following three units: (1) Picture Gorge Basalt (PGB), as suggested by Cahoon et al. ; (2) Steens Basalt, as suggested by Camp et al. ; or (3) Strawberry Volcanics Basalt (SVB). While these units are compositionally similar, distinct differences appear in bivariate plots of key major oxides (TiO₂, Al₂O₃, P₂O₅), trace elements (V, Zn, La, Ba, Th, Nb, Y, Hf, Ta, Sr, Sc, Zr), and elemental ratios (TiO₂/P₂O₅, Cr/Ni, La/Nb, La/Yb, Ba/Nb, Zr/Y, Tb/Yb).

Using these diagnostic geochemical attributes and machine learning results, 31 unassigned basalts were classified as PGB, 6 as Steens Basalt, and 11 as SVB. This new dataset highlights unique characteristics among regional basalt lava members, extends the known distributions of PGB, Steens Basalt, and SVB, and enhances our understanding of the diverse volcanic history of eastern Oregon.

Rights

© 2025 Angela Rae Stetson

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

https://archives.pdx.edu/ds/psu/44223

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