First Advisor

Marvin H. Beeson

Term of Graduation

Spring 2021

Date of Publication

1986

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Basalt -- Oregon -- Salmon River Region, Geology -- Oregon -- Salmon River Region, Stratigraphic geology, Structural geology

DOI

10.15760/etd.5467

Physical Description

1 online resource (4, ix, 108 pages)

Abstract

Approximately 16 km of Columbia River basalt are exposed in the Salmon River area to the south and to the west of Mount Hood, Oregon. A maximum composite basalt section composed of 15 flows and totaling 461 m is exposed in discontinuous areas of outcrop.

The Columbia River basalt in the Salmon River area belongs to the Grande Ronde Basalt and the Frenchman Springs Member of the Wanapum Basalt. The Grande Ronde Basalt section (6 flows) was mapped as two chemically distinct units referred to as low-MgO (older) and high-MgO (younger) Grande Ronde Basalt. At least one interfingered flow of the Prineville chemical type occurs within the low-MgO section. The Prineville flow represents the oldest Columbia River basalt exposed in the Salmon River area.

The Frenchman Springs section (8 flows) is represented by four distinct chemical units referred to as the basalt of Ginkgo, the basalt of Silver Falls, the basalt of Sand Hollow, and the basalt of Sentinel Gap. Stratigraphic division of the Columbia River basalt units was based on geochemistry determined by INAA for trace elements and by x-ray fluorescence spectrometry for major oxides. The definition of geochemical units was aided by stratigraphic position, texture, and phenocryst/glomerocryst abundance.

The Grande Ronde Basalt and the Frenchman Springs basalt originated in the Columbia Plateau and flowed westward through the Cascade Range along a 72 km wide tectonic depression. The Prineville flow is chemically similar to flows that originated near the Bowman Dam (formerly the Prineville Dam) located 70-80 km to the southeast.

The distribution of Columbia River basalt in the Salmon River area was controlled by structures that developed gradually. The formation of these structures began during the incursion of the Grande Ronde Basalt and consisted of northeast-trending folds and a northwest-trending fault zone that displays components of strike-slip and vertical displacement. This approximately N 30° W-trending fault zone extends the entire length (30 km) of the Salmon River area.

The Ginkgo intracanyon flow may have passed through the Salmon River area within the projected continuation of the Mt. Hood-The Dalles syncline. Rapid infilling by subsequent flows restored the low relief nature of the basalt surface by the end of Frenchman Springs time.

Persistent north-south compressional stresses resulted in the development of large-scale folds that were imprinted by the existing, continually developing small scale-folds. Continual length-shortening resulted in northeast-trending thrusts and high angle-reverse faults along the weakened limbs of anticlinal structures. Thrust faults are associated with extensive breccias and may show up to 122 m of stratigraphic displacement. The northwest-trending (predominantly N 30° W) fault zone exists along the western edge of the Salmon River area.

Northeast-trending structures appear to terminate against it. The regional, right-lateral, strike-slip sense of motion along this zone is masked in the Salmon River area where terminated anticlines and synclines create a vertical sense of displacement. The N 30° W structural zone was active throughout the history of structural development in the Salmon River area and has a complimentary relationship with the northeast-trending structures. The prominent overall northwest-trend of the basalt outcrop pattern in the Salmon River area is related to the presence of the northwest-trending structures located along its western edge. Northeast-trending normal faults are the youngest structures recorded by the tectonic history of the Columbia River 3 basalt in the Salmon River area. This relationship was determined where normal faults cut previously formed thrust breccias. The late forming normal faults are not related to the predominant, northeast-trending structures and indicate a fluctuation in the regional, compressive stress pattern that may exist today. Normal faulting may also be related to the emplacement of semi-discordant sills of silicic volcanic material observed within the basalt section, and other intrusive features such as the Still Creek pluton located 1.5 km to the northeast.

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Comments

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Persistent Identifier

http://archives.pdx.edu/ds/psu/20365

Additional Files
b2.pdf (39637 kB)
Plate I
b1.pdf (23274 kB)
Plate II
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