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Discussion papers | Copyright
https://doi.org/10.5194/esurf-2018-35
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 May 2018

Research article | 03 May 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

Clast imbrications in coarse-grained sediments suggest changes from upper to lower flow regime conditions

Fritz Schlunegger and Philippos Garefalakis Fritz Schlunegger and Philippos Garefalakis
  • Institute of Geological Science, University of Bern, Switzerland

Abstract. Clast imbrications are presumably the most conspicuous sedimentary structures in coarse-grained clastic deposits. In this paper, we test whether the formation of such a fabric is related to changes from lower to upper flow regime conditions in streams. To this extent, we calculate the Froude number at the incipient motion of coarse-grained bedload for various values of relative bed roughness and stream gradient. We then compare the results with data from modern streams and stratigraphic records. The calculations show that upper flow regime conditions most likely establish where average stream gradients exceed c. 0.5±0.1°, and where relative bed roughness values are larger than ∼0.06±0.01. Similarly, data from modern streams reveal that imbricated clasts are found where channels are steeper than c. 0.5±0.2°, and where relative bed roughness values exceed ∼0.07. Likewise, imbricated conglomerates are encountered in late Oligocene foreland basin sequences where paleo-slopes were greater than 0.4°. We use these relationships to propose that clast imbrications occur where channel gradients exceed a threshold, which appears large enough for upper flow regime conditions to establish. We finally relate the formation of an imbricated arrangement of clasts to a mechanism where material transport occurs through rolling, or pivoting. This process requires a large shear force and thus a large flow velocity upon transport, which is likely to be associated with shifts from the lower to the upper flow regime. Our results thus suggest that clast imbrications are suitable recorders of upper flow regime conditions upon sediment transport.

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Fritz Schlunegger and Philippos Garefalakis
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Fritz Schlunegger and Philippos Garefalakis
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Latest update: 21 Jul 2018
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Clast imbrications, which is a depositional fabric where clasts overlap each other similar to a run of toppled dominoes, are presumably the most conspicuous sedimentary structures in coarse-grained fluvial deposits. However, the conditions leading to this fabric has been contested. Here, we calculate the hydrological conditions for various stream gradients. We find that clast imbrications most likely form where channel gradients exceed a threshold and where upper flow regime conditions prevail.
Clast imbrications, which is a depositional fabric where clasts overlap each other similar to a...
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