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The earliest evolution of the North Alpine Foreland Basin in Switzerland was characterized by deposition in small, structurally partitioned sub-basins during the Late Cretaceous and Early Tertiary, rather than in a single, large foredeep. These sub-basins, which were probably located between old rift margin fault-blocks reactivated during Alpine compression, were incorporated into the thrust wedge during thin-skinned deformation. In eastern Switzerland, the most external sub-basins with respect to the orogenic wedge (North Helvetic Flysch and Blattengrat units) have at their base an unconformity attributed to flexural forebulge erosion. More internal sub-basins (Sardona and Prattigau units) contain a conformable succession from the underlying passive margin stage and are dominated by deep-water sedimentation. In western Switzerland, both external sub-basins, now found in the Helvetic Diablerets and Wildhorn nappes, and deep-water internal sub-basins (Hochst-Meilleret Flysch, Neisen Flysch, Tarentaise Flysch) preserve a well-developed basal unconformity. Comparison of the eastern and western Swiss transects shows important intrabasinal lateral variations to be present. The western Swiss area was a topographic high for much of the Late Cretaceous and Early Tertiary; this is demonstrated by the increased chronostratigraphic gap at the karstified basal unconformity surface in western Switzerland. The strata onlapping this unconformity young to the west, suggesting that drowning of the emergent area was delayed compared with the east. In addition, reactivation and uplift of the rift margin structures occurred earlier in western Switzerland compared with eastern Switzerland. There is therefore strong evidence for lateral topographic gradients in the early foreland basin caused by differential amounts of tectonic reactivation of rift margin structures. In the early foreland basin-fill, these lateral variations are as important in determining depositional patterns as strike-normal changes across the basin. |
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