Date of Graduation

8-2024

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Marshall, Jill A.

Committee Member

Dumond, Gregory

Second Committee Member

Covington, Matthew D.

Keywords

Talus; Persistence; Erodibility; Buffalo River

Abstract

Quantifying bedrock properties that control the persistence of fallen rock material (talus) along bedrock walls bounding river valleys may be a key factor in understanding what controls the rate at which these river valleys widen. My work is a component of a collaborative research project focused on the overarching hypothesis that the persistence vs. erodibility of talus that buffers river valley walls can be a first-order control on valley widening rates. I hypothesize that the primary control for creating erodible or persistent talus is the fracture spacing of bedrock valley walls as it sets the maximum block size of collapsed wall material. My field area, the free?flowing Buffalo National River cuts through the flat lying strata of the climatically uniform and tectonically dormant Ozarks of Northwest Arkansas. Here the upper river valley is wide when the river flows through the cherty limestone of the Boone Formation and narrows downstream when the river flows through the massively bedded sandstone of the Everton Formation. I have captured 16 Gigapan gigapixel panoramas at the cm-scale of both narrow and wide bedrock valley walls above talus piles in the Buffalo River. Using the image analysis software Image J and Adobe Illustrator, I mapped the fracture spacing and density of these valley walls. My findings suggest that fracture patterns differ between wide and narrow river valleys. In the wide valleys of the Boone Formation, average fracture spacings range from 0.04 m to 0.29 m and average fracture densities range from ~17 �� ��2 to ~40 �� ��2 . In the narrow valleys of the Everton Formation, average fracture spacings range from 0.99 m to 2.9 m and average fracture densities range from ~1.3 �� ��2 to ~3.6 �� ��2 . Additionally, I found that fracture spacing variations within a single thinly bedded rock wall were controlled by its bedding layer thickness, indicating that both a vertical and horizontal scanline is needed to acquire fracture data that accurately represents the entire rock wall. These findings indicate that fracturing may be a control on bedrock valley width by setting the vulnerability of a talus-buffered rock wall to a river’s eroding flows and transport capacity.

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