Clast imbrication in coarse-grained mountain streams and stratigraphic archives as indicator of deposition in upper flow regime conditions

Abstract

Abstract. Clast imbrication is one of the most conspicuous sedimentary structures in coarse-grained clastic deposits of modern rivers but also in the stratigraphic record. In this paper, we test whether the formation of this fabric can be related to the occurrence of upper flow regime conditions in streams. To this end, we calculated the Froude number at the incipient motion of coarse-grained bedload for various values of relative bed roughness and stream gradient as these are the first-order variables that can practically be extracted from preserved deposits. We found that a steeper energy gradient, or slope, and a larger bed roughness tend to favor the occurrence of supercritical flows. We also found that, at the onset of grain motion, the ratio ϕ between the critical shear stress for the entrainment of a sediment particle and its inertial force critically controls whether flows tend to be super- or subcritical during entrainment. We then mapped the occurrence of clast imbrication in Swiss streams and compared these data with the hydrologic calculations. Results indicate that imbrication may record supercritical flows provided that (i) ϕ values are larger than ca. 0.05, which is appropriate for streams in the Swiss Alps; (ii) average stream gradients exceed ca. 0.5 ± 0.1°; and (iii) relative bed roughness values, i.e., the ratio between water depth d and bed sediment D84, are larger than ∼ 0.06 ± 0.01. We cannot rule out that imbrication may be formed during subcritical flows with ϕ values as low as 0.03, as demonstrated in a large number of flume experiments. However, our results from Alpine streams suggest that clast imbrication likely reflects upper flow regime conditions where clasts form well-sorted and densely packed clusters. We consider that these differences may be rooted in a misfit between the observational and experimental scales.