Gilles Rixhon

Fluvial terraces of the Amblève: a marker of the Quaternary river incision in the NE Ardennes massif (Western Europe).

The geomorphological analysis of the terraces of the Ambleve, an Ardennian subtributary of the Meuse, allows us to reconstruct eleven distinct levels (T1 to T11). Like those of most rivers draining the Ardennes massif, the Ambleve terrace profiles also converge in the upstream direction. Moreover, the “Main Terrace” complex, widely recognized in the Rhine and Meuse systems (including their major tributaries draining the Ardennes-Eifel massif), has also been identified in the Ambleve valley. However, the lack of terrace remnants in the ~ 10km-long reach of the Quarreux gorge, combined with the knickpoint observed in the presentday long profile in this reach, leaves some uncertainty in the overall profile reconstruction. Despite the presence of a paleokarst filled by alluvial material in the lower Ambleve and diversely dated between ~ 0,5 and ~ 1 Ma, firm chronological data about the Quaternary evolution of the Ambleve are still lacking. Therefore, we base our discussion of the temporal evolution of the Ambleve incision on geometrical correlations with dated terraces of the Meuse downstream of Liege.

Valley downcutting in the Ardennes (W Europe): Interplay between tectonically triggered regressive erosion and climatic cyclicity

While climatic models of valley downcutting discuss the origin of terrace staircases in valleys of middle Europe within the frame of alternating cold and temperate periods of the Quaternary, other models, starting from a base level fall imposed by an initial tectonic signal, describe the response of the drainage network mainly as the propagation of an erosion wave from the place of base level fall (the margin of the uplifted region) toward the headwaters, the two types of model being rarely confronted. In the Ardennes (West Europe), cosmogenic 10 Be and 26 Al ages have recently been calculated for the abandonment of the Younger Main Terrace (YMT) level, a prominent feature at mid-height of the valleysides marking the starting point of the mid-Pleistocene phase of deep river incision in the massif. These ages show that the terrace has been abandoned diachronically as the result of a migrating erosion wave that started at 0.73 Ma in the Meuse catchment just north of the massif, soon entered the latter, and is still visible in the current long profiles of the Ardennian Ourthe tributaries as knickpoints disturbing their upper reaches. At first glance, these new findings are incompatible with the common belief that the terraces of the Ardennian rivers were generated by a climatically triggered stepwise general incision of the river profiles. However, several details of the terrace staircases (larger than average vertical spacing between the YMT and the next younger terrace, varying number of post-YMT terraces in trunk stream, tributaries and subtributaries) show that a combination of the climatic and tectonic models of river incision is able to satisfactorily account for all available data. The cosmogenic ages of the YMT also point out a particular behaviour of the migrating knickpoints, which apparently propagated on average more slowly in the main rivers than in the tributaries, in contradiction with the relation that makes knickpoint celerity depend directly on drainage area. We tentatively suggest a process accounting for such anomalies in migration rates.

Applying Pattern Oriented Sampling in current fieldwork practice to enable more effective model evaluation in fluvial landscape evolution research

Field geologists and geomorphologists are increasingly looking to numerical modelling to understand landscape change over time, particularly in river catchments. The application of landscape evolution models (LEMs) started with abstract research questions in synthetic landscapes. Now, however, studies using LEMs on real-world catchments are becoming increasingly common. This development has philosophical implications for model specification and evaluation using geological and geomorphological data, besides practical implications for fieldwork targets and strategy. The type of data produced to drive and constrain LEM simulations has very little in common with that used to calibrate and validate models operating over shorter timescales, making a new approach necessary. Here we argue that catchment fieldwork and LEM studies are best synchronized by complementing the Pattern Oriented Modelling (POM) approach of most fluvial LEMs with Pattern Oriented Sampling (POS) fieldwork approaches. POS can embrace a wide range of field data types, without overly increasing the burden of data collection. In our approach, both POM output and POS field data for a specific catchment are used to quantify key characteristics of a catchment. These are then compared to provide an evaluation of the performance of the model. Early identification of these key characteristics should be undertaken to drive focused POS data collection and POM model specification. Once models are evaluated using this POM/POS approach, conclusions drawn from LEM studies can be used with greater confidence to improve understanding of landscape change.

The picturesque Ardennian valleys: Plio-Quaternary incision of the drainage system in the uplifting Ardenne

As a response to Late Cenozoic tectonic uplift of the Ardenne massif, the Meuse River and its drainage system deeply incised the landscapes of Southern Belgium. Well-preserved terrace staircases flanking the main Ardennian valleys show a break at the height of the Main Terrace Complex, whose younger level (YMT) marks the transition towards the steep-sided lower part of valley transverse profiles. Numerical 10Be/26Al dates obtained for the YMT in various places along the lower Meuse—lower Ourthe—Ambleve line yield an age around 620 ka for the pulse of tectonic uplift responsible for increased incision rates and fast valley deepening. They also show diachronic abandonment of the YMT, indicating that post-YMT erosion invaded the drainage system through the migration of knickpoints originating in the ~20 m base level lowering initially created at the margin of the en-bloc uplifted region. A study of knickpoints in modern long profiles of Ardennian rivers fully confirms this view. Assimilation of the new data imposes a revised model be proposed for river incision in the Ardenne. Integrating the climatic control with the various modes of tectonic control over river incision, this conceptual model offers a sounder frame of the Quaternary river system evolution in the Ardenne, also explaining observed cases of stream piracy. Basin average denudation rates published for the Ardenne are also briefly compared with valley incision rates.