Geophysical investigation

At site Rotem-2 we conducted one combined 2-D resistivity/induced polarisation profile (Wenner-Schlumberger array with 2 m electrode spaing), three geo-radar profiles with 100 MHz antenna, and four shallow handborings (Fig. 7). The results are shown in Fig. 8.

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Fig. 8 – 2-D resistivity, 2-D induced polarisation, 100 MHz geo-radar and borehole profile at site Rotem-2.

The 2-D resistivity image shows a remarkable change of resistivity values below the foot of the scarp:

• SSW of the scarp, in footwall position, the Maas gravels are only thin (generally < 6 m) and characterised by high resistivities (> 750 W.m); the top of this terrace is at an elevation of about 34 m, and there is only a thin cover (± 1 m thick) of low-resistivity sediments;
• NNE of the scarp, in hanging wall position, the Maas gravels get abruptly thicker (> 10 m, unfortunately the penetration depth of the profile is not sufficient to observe the base of the terrace) and their resistivity values are relatively low (< 550 W.m); the top of the gravel is ± 1 m lower (at ± 33 m), and the low-resistivity cover is also thicker. The lower resistivity of the hanging wall gravel is thought to reflect a higher water content, which would suggest that the fault plane represents a hydrological barrier.

The 2-D induced polarisation profile shows circular patches of elevated IP values in a few places at the base of the gravel layer in the footwall, but the largest anomaly is situated exactly where the fault plane is suspected. This elongated, subvertical anomaly shows a slight dip towards the NNE, as anticipated. We have observed similar IP anomalies at several places in the Maas valley where a large hydrological contrast exists between dry footwall deposits and wet or water-saturated hanging wall deposits. We hypothesise that the elevated IP values are caused by infiltrated clays and/or concentrations of Fe- and Mn-oxides along the fault plane.

The geo-radar profile has a more limited depth range. The strong consistent reflector corresponds to the top of the Maas gravel terrace, as shown by ground-truth evidence from handborings. This surface shows a sharp step of about 1 m at the exact same position as the resistivity and IP anomalies: in the footwall the gravel top is slightly above 34 m, in the hanging wall it is at 33 m. The elevation of the gravel top is consistent with that of the late Weichselian Maasmechelen terrace. We show for the first time that the top of this terrace is faulted.

The handborings, finally, show that the gravel is overlain by a thin layer (± 20 cm) of fine-medium sands, interpreted as late Weichselian coversands (eolian), and by 1-1.5 m of sandy loam, possibly corresponding to Holocene alluvium. The coversands show the same fault offset as the gravel top. However, the total offset of the gravel top is slightly less than the topographic offset, which suggests that the latest faulting event post-dates most, but probably not all, of the sandy loam, and may thus be intra-Holocene. Trenching will help us establish if the sandy loam is affected by faulting as well, and hopefully yield a more precise age bracket for the most recent faulting event.