Abstract

In an attempt to understand the origin of the enigmatic Chessel-Noville hills in the Rhone River plain of western Switzerland, we have recorded two and three-dimensional ground-penetrating radar (georadar) data and drilled six shallow boreholes. Cross-sections and maps of horizons extracted from the georadar data reveal normal fault zones under the hill flanks and minor grabens and domes below the hill crests. Comparable graben-like structures with normal step faults converging with depth are observed at a nearby outcrop. Fine sand and silt within some of the exposed faults may have been dragged into the fault planes or injected as clastic dikes under high pore pressures. A silt unit encountered in two of the boreholes correlates with a strong georadar reflection. Although angular calcareous boulders are scattered across the surface, the early suggestion that the hills are remnants of a large historic rockfall is not compatible with the subsurface lithologies and structures. Neither is the frequently referenced hypothesis that they are moraines. Moreover, the absence of significant thrust faulting and lack of strong structural trends in the georadar data and at outcrops are inconsistent with the more recent interpretation of the hills as glaciotectonic ridges. Instead, our results indicate that the hills and their internal structures represent vertically uplifted and deformed fluvio-deltaic sediments. Diapirism offers a plausible explanation for the observed uplift and deformation. Saturated fine-grained sediments underlying the former Rhone Delta may have been compacted and overpressured during burial by pro-grading deltaic sands and thickening sequences of fluvial sands and gravels. Under these conditions, mud diapirs may develop. Actual diapiric uplift and deformation may have been triggered by a large historic earthquake (e. g., Tauredunum event of 563 A.D.) that would have induced additional over-pressurization, liquefaction and upward mobilization of the fine-grained sediments. Such an earthquake may also have triggered the rockfall responsible for the scattered calcareous boulders.

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