Geoffrey Houbrechts

Residual doses in recent alluvial sediments from the ardenne (S Belgium)

Residual Doses in Recent Alluvial Sediments From the Ardenne (S Belgium) We report on our first investigations into the potential of optical dating for determining the rate of river flood sedimentation in the Ardenne region (S Belgium). Two samples collected from a recent alluvial deposit were used to investigate the extent of resetting in different particle size fractions of quartz (4-11 μm, 63-90 μm, 90-125 μm, 125-180 μm, 180-212 μm and 212-250 μm) as well as in polymineral fine (4-11 μm) grains. Both samples show satisfactory OSL and IRSL characteristics. The IRSL signals from the polymineral fine grains yield an equivalent dose (De) of 3-4 Gy, while a De of 0.3-0.6 Gy was measured using large aliquots of quartz. Small aliquot analyses of 63-90 μm and 212-250 μm quartz grains confirm that the coarser fraction contains more grains with lower Des. Furthermore, for a modern sample (< 3 years old), ~60% of the aliquots yields a De consistent with zero, indicating that these contain only well-bleached grains. These findings suggest that it might be possible to extract the true burial dose from dose distributions measured using small aliquots of coarse-grained (e.g. 212-250 μm) quartz.

Fluvial architecture of Belgian river systems in contrasting environments:implications for reconstructing the sedimentation history

Accurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Ambleve catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Ambleve catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking.

Holocene floodplain deposition and scale effects in a typical European upland catchment: A case study from the Amblève catchment, Ardennes (Belgium)

This study quantifies Holocene alluvial sediment deposition in the Belgian Amblève catchment (1080 km2), situated in the Ardennes uplands. An extended coring data set is used for a quantitative description and a quantification of the alluvial depositions. The floodplains fall into three main types: the upper and lower floodplains and the steep reaches. Total Holocene alluvial sediment deposition amounts to 32 Tg, or 0.029 Tg/km2. Dating of sediments using iron slag as tracer shows that on average 42% of contemporary storage was accumulated during the last 600 years. Radiocarbon dating of fluvial deposits at eight sites in the catchment confirms that the majority of the sediment is relatively recent. The increased sedimentation rates for this period are related to anthropogenic land use, possibly enhanced by climatic variations. A fluvial sediment budget was constructed for this 600 yr time period and shows that export from the catchment has about the same importance as storage in the floodplains, while lateral reworking of existing floodplain deposits only affects half the quantity of sediments. Overall, floodplain sediment storage in the Amblève catchment, comparable with other upland catchments, is of lesser importance compared with catchments dominated by loess. This can mainly be explained by lower sediment supply resulting from less intense anthropogenic land use, despite the higher sediment transport efficiency through the fluvial system. Floodplain sediment storage is scale-dependent, with initially a sharp increase in catchment area-specific sediment deposition, followed by a steady decrease with increasing catchment area.

The Semois Valley in Southern Ardenne: Short-Wavelength, Large-Amplitude Meanders Incised into a Slaty Basement

The incised meanders of the Semois Valley, in Southern Ardenne, are remarkably elongated, forming a nearly 100-km-long meander belt. Their elongation is conditioned by the relative orientation of the meander loops and the regional slaty cleavage, as theorized by Strahler. These meanders already existed in the Early Quaternary, some cut-off meanders lying more than 100 m above the present floodplain. Although the Quaternary incision rates of the Semois were low (≤0.05 mm/year), several meanders look more entrenched than ingrown, probably because lateral erosion was fairly slow (~0.3–0.5 mm/year), owing to the large quantity of material to evacuate by a rather small river. The combined aesthetic and scientific interests of this particularly well-developed meander belt speak in favour of the preservation of the nicest meanders as geomorphosites.

A Unique Boulder-Bed Reach of the Amblève River, Ardenne, at Fonds de Quarreux: Modes of Boulder Transport

The Ambleve valley at Fonds de Quarreux displays one of the most spectacular incised landscapes of the Ardenne massif. Cut into highly resistant Cambrian quartzites and surrounded by steep wooded hillslopes, a 3-km-long gorge is cluttered with hundreds of huge quartzite boulders up to several metres in size. These boulders originate from the locally outcropping Cambrian Venne Formation and reached the riverbed through periglacial slope processes. Up to 2-m-large boulders have been transported by the river over distances of several kilometres, some of them being observed 90 km downstream in the Meuse terraces of the Campine Plateau. We show that such transport of very large boulders over long distances cannot occur through purely fluvial processes and conclude that ice rafting was the most likely transportation mode, active during Pleistocene cold periods and probably supplemented by other ice breakup-related processes. We also briefly present nearby sites of geomorphological value within the Ambleve catchment.

The Periglacial Ramparted Depressions of the Hautes Fagnes Plateau: Traces of Late Weichselian Lithalsas

Once thought to be of human origin, ramparted circular or elongate depressions that densely cover many, but not all, summit surfaces of the Hautes Fagnes area, NE Ardenne, have been recognized since the 1950s to represent relics of periglacial mounds. We examine first the regional distribution of these periglacial ramparted depressions (PRDs), whose location seems controlled mainly by elevation and lithologic factors. Geomorphological characteristics of the PRDs and trenching data obtained from several ramparts consistently show that these landforms initially developed as lithalsas through accumulation of segregation ice in the mineral soil, whose later thaw produced the presently observed ramparted depressions. 14C dating of, and pollen data from, peat layers pre- and postdating the formation of the periglacial mounds, further supported by the presence of the 12.9-ka-old Laacher See tephra in the underlying peat, point to a period of rapid formation during the second half of the Younger Dryas. Mean annual air temperature and mean temperature of July derived for the Hautes Fagnes area (~600 m asl) during the Younger Dryas from a compilation of NW European pollen data are in excellent agreement with the temperature values observed in the western Labrador Peninsula where lithalsas are currently active, showing that PRDs are valuable paleoclimate indicators.