Chapters

1. Introduction

• Geologic Setting and Previous Work

2. Basalt Flow Stratigraphy

• Grande Ronde Flows
• Post-Grande Ronde Flows
• Feeder Dikes

3. Structure

• Faults Coincident with the Olympic-Wallowa Lineament
• The Hite Fault System
• Blue Mountains Uplift
• Graben Related Faults
• Shallowly Dipping Faults
• Wanapum Basalt Feeder Dikes
• Interaction Between the Major Fault Systems
• Discussion

4. Conclusions

References

Appendices

A. Descriptions of the five principal stratigraphic sections

B. Chemical analyses by XRF of basalt flows and dikes

C. Slickenside and fault plane data

D. Procedure for field use of the portable fluxgate magnetometer

E. Road log for Tiger Creek Road with fault outcrop descriptions

F. Fault outcrop descriptions for Maps 3 and 4

Abstract of poster presentation derived from this thesis

Complete contents of poster presentation derived from this thesis

Sample location maps

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THE OLYMPIC-WALLOWA LINEAMENT, HITE FAULT SYSTEM, AND COLUMBIA RIVER BASALT GROUP STRATIGRAPHY IN NORTHEAST UMATILLA COUNTY, OREGON

The significance of strike-slip faulting on the Columbia Plateau, including displacement along the structural zone coincident with the Olympic-Wallowa Lineament (OWL-zone) and along the Hite fault System (HFS), has long been controversial, in part because of difficulty in determining strike-slip displacements in horizontal flows of the Columbia River Basalt Group (CRBG). Even the significance of apparent strike-slip displacement of vertical feeder dikes has been questioned because of the possibility of en echelon dike emplacement. This investigation was undertaken to clarify the type and timing of displacements on faults of the OWL-zone and HFS. The area immediately southeast of Walla Walla, Washington, and Milton-Freewater in northeast Oregon was chosen for study because it is located at the intersection of the OWL and HFS and contains many important exposures.

Numerous faults in this area, both those mapped by earlier workers and those observed for the first time in this study, were examined to determine striae, associated minor structures, and displacements. To determine displacements, which occur entirely within horizontal flows of the CRBG, it was necessary to distinguish the numerous basalt flows. This has been done by building a local stratigraphic framework through the sampling of a series of sections on opposite sides of major faults and correlating the flows by their petrologic and paleomagnetic character and chemical composition. 347 samples of basalts were analyzed for 27 major and trace elements by XRF to accomplish this.

The main conclusions are (1) that there is significant, albeit still somewhat circumstantial, evidence that the structures acted as right-lateral (OWL-zone) and left-lateral (HFS) fault zones prior to the eruption of the Columbia River basalts, (2) that there has been 300 meters of exposed syn- and post-CRBG vertical displacement of flows on the Hite fault, (3) that virtually all of the fault zones studied are dominated by horizontal striae, and (4) that the OWL-zone and HFS probably represent conjugate fault systems with real, but limited, post-CRBG strike-slip displacements.