River incision and migration deduced from 36Cl cosmic-ray exposure durations: The Clue de la Cerise gorge in southern French Alps
Introduction
Vertical superposition of layers with different resistances to fluvial incision result in complex incision rate patterns in space and time (Shtober-zisu et al., 2018). For instance, rivers flowing through rock sequences bearing large lithological contrasts and a strong tectonic structuration often form so-called rectangular or transverse drainage patterns (Oberlander, 1965; Stokes et al., 2008; Twidale, 2004): some branches of the streams run parallel to tectonic structures, while others cross-cut them at high angles to follow the average slope of the topography. Rectangular drainage patterns are particularly well developed in folded or tilted sedimentary formations with contrasted lithologies (Howard, 1967). They can also occur on rocks of relatively uniform lithology, in which case they are controlled by the presence of faults and joints (Anton et al., 2015). Incision mechanisms and corresponding eroded volumes ought to be different depending on the stream direction with respect to the tectonic and stratigraphic structure. For instance, on a dipping stratigraphic pile with a soft rock layer (marls) overlying a more resistant one (limestones), the erodibility contrast will cause a dipward migration of bedding-parallel streams and progressive denudation of the strong layer surface. Meanwhile, bedding-perpendicular streams will incise vertically both layers, resulting in the upstream (retrogressive) migration of a knickpoint controlled by the lithological contrast (Fig. 1). Quantifying the rate at which rivers incise and migrate on their substrate has a potential interest for landscape modelling and mass budget estimates, as the presence or absence of lateral channel migration may lead to large differences in the amount of eroded material (Fig. 1). Moreover, recent studies evidenced that gorge formation can be extremely rapid even with moderate discharge (Anton et al., 2015), which points out the need for a better quantification of bedrock incision rates.
In this study, the recent incision and migration rates were estimated on a spectacular (1000 m long, 400 m high) ~N-S striking gorge cross-cutting the N-dipping Jurassic and Cretaceous sediments in the southernmost part of the external Alps fold-and-thrust belt. In this area, the Esteron River, a large tributary of the Var River, displays a typical rectangular drainage pattern with ENE and NNW striking branches (Fig. 2A and B). The objective of this study was to examine at which rate bedding-perpendicular and bedding-parallel river branches have incised and migrated through time, in order to infer how such transverse drainage patterns develop.
Section snippets
Geological, tectonic and geomorphological setting
The study area encompasses the southernmost units of the external Castellane fold-and-thrust belt, which consists of a Lower Jurassic to Miocene sedimentary cover partly decoupled from its underlying Paleozoic to Permian basement by Upper Triassic evaporites (Fig. 2A). West of the Var River, Miocene shortening was accommodated by both thin- and thick-skin thrusts and ~E-W trending folds (e.g., Jourdon et al., 2014 and references therein), while some right-lateral faulting occurred along the Var
Recent river incision rates in the var catchment
Recent studies based on Cosmic Ray Exposure (CRE) dating of river polished surfaces in the Var catchment have highlighted periods of large incision rates (several mm/yr) since the last 15–20 ka (Rolland et al., 2017; Saillard et al., 2014). In the Vésubie River, which is the next large tributary of the Var River upstream the Esteron, CRE ages range from 2 to 18 ka, leading to an average incision rate of 2.2 mm/yr (Saillard et al., 2014). In the Tinée River, another tributary draining the
Methods
We sampled the rectangular drainage pattern in the upper part of the ‘Clue de la Cerise’ gorge, where Jurassic limestones are deeply incised by the Esteron River. There, the top surface of the Jurassic limestones is well preserved, as well as polished incised surfaces on the gorge cliffs (Fig. 3B and C). 18 samples were gathered along two vertical river polished profiles and three others on the denuded north-dipping surface layer of the Jurassic limestones, on the upstream segment of the gorge.
Results
Along the highest vertical profile, CRE ages range from ~21 ka at the top of it to ~11 ka at the bottom located higher than 5 m above the surface of the river (Table 1 and Fig. 4). Along the lower vertical profile, CRE ages range from 7 to 3 ka, while surface samples have CRE ages between 2 and 8 ka. All samples but 3 align on two age/height linear trends which slopes correspond to mean incision rates of 1 and 0.3 mm/yr, respectively, although the latter is only constrained by 4 points (Fig. 4
Climatic significance of CRE 36Cl ages and incision rates
As detailed in Section 5, the obtained CRE 36Cl ages in this study show age/height linear trends implying incision rates of 0.3 and 1 mm/yr before and after the Younger Dryas glacial phase at ~12 ka, respectively (Fig. 4). These new data acquired in the SW part of the Alpine range are consistent with data collected in other parts of the Alpine belt (Brocard et al., 2003; Montgomery and Korup, 2010; Valla et al., 2010), which all large incision rates the Late Quaternary. These results obtained
Conclusion
We sampled and measured in situ-produced 36Cl concentration in 12 samples along a ~15 m high river polished cliff in a thick, homogeneous limestone layer and on the bedding surface at the top of it. Samples from the bedding surface and from the cliff provide consistent age/height relationships, allowing us to precise a geometrical and chronological model for the formation of a rectangular fluvial drainage pattern. According to this model (Fig. 1) the bedding-parallel part of the channel
Acknowledgments
This study has been funded by the French CNRS INSU Syster and Aleas Research programs and by the UCA-IDEX RiTMICA project. We are grateful to Martin Stokes, and anonymous reviewer and to Editor Markus Stoffel for their insightful comments.
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