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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esurf-2020-54
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esurf-2020-54
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  06 Jul 2020

06 Jul 2020

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This preprint is currently under review for the journal ESurf.

Sediment size on talus slopes correlates with fracture spacing on bedrock cliffs: Implications for predicting initial sediment size distributions on hillslopes

Joseph P. Verdian1, Leonard S. Sklar1,2, Clifford S. Riebe3, and Jeffrey R. Moore4 Joseph P. Verdian et al.
  • 1Department of Earth and Climate Sciences, San Francisco State University, San Francisco, 94302, USA
  • 2Department of Geography, Planning and Environment, Concordia University, Montreal, H3G1M8, Canada
  • 3Department of Geology and Geophysics, University of Wyoming, Laramie, 82071, USA
  • 4Department of Geology and Geophysics, University of Utah, Salt Lake City, 84112, USA

Abstract. The detachment of rock fragments from fractured bedrock on hillslopes creates sediment with an initial size distribution that sets the upper limits on particle size for all subsequent stages in the life of sediment in landscapes. We hypothesize that the initial size distribution should depend on the size distribution of latent sediment (i.e., blocks defined by through-going fractures) and weathering of sediment before or during detachment (e.g., disintegration along crystal grain boundaries). However, the initial size distribution is difficult to measure, because the interface across which sediment is produced is often shielded from view by overlying soil. Here we overcome this limitation by comparing fracture spacings measured from exposed bedrock on cliff faces with particle size distributions in adjacent talus deposits at 15 talus-cliff pairs spanning a wide range of climates and lithologies in California. Median fracture spacing and particle size vary by more than tenfold and correlate strongly with lithology. Fracture spacing and talus size distributions are also closely correlated in central tendency, spread, and shape, with b-axis diameters showing the closest correspondence with fracture spacing at most sites. This suggests that weathering has not modified latent sediment either before or during detachment from the cliff face. In addition, talus has not undergone much weathering after deposition and is slightly coarser than the latent sizes, suggesting that it contains some fractures inherited from bedrock. We introduce a new conceptual framework for understanding the relative importance of latent size and weathering in setting initial sediment size distributions in mountain landscapes. In this framework, hillslopes exist on a spectrum defined by the ratio of two characteristic timescales: the residence time in saprolite and weathered bedrock, and the time required to detach a particle of a characteristic size. At one end of the spectrum, where weathering residence times are negligible, the latent size distribution can be used to predict the initial size distribution. At the other end of the spectrum, where weathering residence times are long, the latent size distribution can be erased by weathering in the critical zone.

Joseph P. Verdian et al.

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Joseph P. Verdian et al.

Joseph P. Verdian et al.

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Short summary
River behavior depends on the size of rocks they carry. Rocks are born on hillslopes where erosion removes fragments from solid bedrock. To understand what controls the size of rock fragments, we measured the spacing between cracks exposed in 15 bare-rock cliffs, and the size of rocks on the ground below. We found that, for each site, the average rock size could be predicted from the average distance between cracks, which varied with rock type. This shows how rock type can influence rivers.
River behavior depends on the size of rocks they carry. Rocks are born on hillslopes where...
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