Can the Size Distributions of Talus Particles be Predicted from Fracture Spacing Distributions on Adjacent Bedrock Cliffs?
Abstract
What controls the size of sediments produced on hillslopes and supplied to river channels? This is an important but unanswered question in geomorphology and sedimentology. One hypothesis is that the initial size distribution of rock fragments eroded from bedrock is related to the distribution of spacing between pre-existing fractures in the bedrock. Slopes of talus that accumulate below eroding cliffs provide a simple natural experiment to test this hypothesis. We studied talus slopes and cliff faces at more than 20 locations in California, USA, where cliff retreat rates were previously measured by Moore et al., 2009. Rock types included andesite, basalt, granodiorite and meta-sediment. To quantify fracture spacing we measured fracture frequency and orientation along scan lines at the base of the cliff. We also used scaled photographs of the cliff face to characterize the shape, size and surface area of discrete blocks. We measured talus particle size distributions using surface point counts along transects oriented downslope from the cliff face, and mapped facies of distinct size distributions. To explore the effect of chemical weathering on talus size we sampled cliff faces and talus particles for x-ray fluorescence analysis to test for depletion of labile cations relative to source rock. Preliminary results suggest that talus size distributions are strongly correlated with bedrock fracture spacing, although systematic differences do occur. In some cases, talus sizes are larger than the spacing between fractures because the detached particles still retain truncated fractures. In other cases, talus is smaller than cliff fracture spacing, presumably because particle size is reduced by fragmentation on impact and weathering during transport down the talus slope. Further analysis will explore whether cliff retreat rate and extent of chemical weathering, as well as rock type and local climate, can explain between-site differences in the size of particles produced.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP31A0929V
- Keywords:
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- 1625 Geomorphology and weathering;
- GLOBAL CHANGEDE: 1815 Erosion;
- HYDROLOGYDE: 1865 Soils;
- HYDROLOGYDE: 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS