A Comparison of Bedrock Properties between Knickpoints and Non-Knickpoint Reaches: Implications for the Roles of Bedrock Properties in Determining Fluvial Erodibility
Abstract
Bedrock erodibility exerts a fundamental control on landscape evolution, especially in the context of fluvial incision, the mechanism which redistributes material across most of Earth's terrestrial surface. Yet the roles of specific, measurable bedrock properties in determining erodibility in fluvial settings remain elusive. Therefore, erodibility is often underrepresented in studies seeking to model landscape evolution or infer past events from landscape form.
Knickpoints (KPs) often result from channel adjustment to resistant material, and act as markers for changes in bedrock erodibility. Thus, lithologic KPs provide a practical focal point for studies seeking to demystify erodibility. Because KP form is a product of erosive mechanism and channel hydraulics, which are adjusted to maintain erosion rate in resistant material, KP form can be compared to local bedrock properties (tensile strength, spacing and orientation of discontinuities) to infer how these properties influence erodibility. To the same end, bedrock properties can be compared between KP and non-KP reaches, representing end-members of the local erodibility spectrum. Here, we present results of KP analysis in bedrock-alluvial channels within the Valley and Ridge of Virginia. Channels intersect the Devonian Brallier Fm., comprised of dominant mudstones and occasional turbidic sandstones. At one site (North Mt.), we examine 5 KPs formed in a repeated stratigraphic interval of thick-bedded sandstones. Because rock strength, discontinuity spacing, and climate are equal between KPs, but dip varies from 30-90° due to small folds, this is an ideal setting to test the effect of discontinuity orientation on KP form. At a second location 20km west (Allen Hollow), we examine 8 KPs which also coincide with thick sand beds, in contrast with the thinly bedded mudstones underlying reaches in-between KPs, providing a setting to compare the effects of discontinuity spacing and rock strength on erodibility. Preliminary results reveal 1) a positive correlation between dip angle and KP slope, and 2) clear distinctions in discontinuity spacing and rock strength between KP and non-KP bedrock, the largest difference being that of discontinuity spacing. These results highlight discontinuity networks as dominant influencers on fluvial bedrock erodibility.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP33D2438C
- Keywords:
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- 1824 Geomorphology: general;
- HYDROLOGYDE: 1825 Geomorphology: fluvial;
- HYDROLOGYDE: 1856 River channels;
- HYDROLOGYDE: 5419 Hydrology and fluvial processes;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS