Sedimentary architecture of Late Cretaceous to Early Eocene flysch sequences in the Swiss Alps

The Campanian/Maastrichian to Lutetian Alpine flysch sequences of the Schlieren- and Gurnigel nappes record deposition in an ocean-continent subduction setting related to Alpine orogenesis. Despite extensive studies of these flysch deposits, the existence of a source-to-sink relationship between the two units remains debated. Here, we logged 50-70 m-thick successions of the Gurnigel and Schlieren deposits at two sites, respectively, at a scale of 1:20. The ages of the analysed sediments range from the Thanetian to the Lutetian. We measured the paleoflow directions using sole marks and cross-bedding, and conducted drone surveys to document the large-scale depositional architecture. Our aim was to reconstruct a potential proximal-to-distal relationship between the two sequences.

In the Schlieren nappe, the analysed sediments are dominated by coarse-grained (grain size up to 2 mm) sandstone beds <5 m thick, characterised by a matrix-supported fabric and sole marks at their bases. The finer-grained sandstone beds (grain size up to c. 0.6 mm) are <50 cm thick. They display a massive, grain-supported fabric with normal grading at the base, followed by parallel lamination and occasionally ripple marks at the top. Mudstone beds (clay and silt fraction) are up to 30 cm thick. They are massive to parallel-laminated and locally show bioturbation. Mudstone beds contribute to <10% to the entire suite. Paleoflow directions scatter between the NE and SE. Drone surveys disclose the presence of troughs up to 7 m deep and ten meters wide. They are cut into sandstone beds and backfilled with coarse-grained, massive to laminated sandstones.

By contrast, the Gurnigel sequences are dominated by a succession of sandstone beds with mudstone interbeds. Sandstone beds are <1.5 m thick. They have a planar base, are medium- to fine-grained (grain size ranging from c. 0.1 to 0.6 mm) and show a fining-up trend. Individual beds display a succession of sedimentary structures occasionally starting with a massive fabric. It is followed by mm-scale plane lamination, ripple marks with convolute bedding and sub-mm laminations towards the top. Mudstone interbeds, up to 30 cm thick, are massive to parallel laminated and strongly bioturbated, comprising up to 40% of the surveyed outcrop. Drone imagery shows that laterally continuous, horizontally layered beds dominate the overall architecture. However, lenticular sandstone beds with scours up to 50 cm deep occur locally. Sole marks and cross bedding indicate paleoflow toward the S and W.

The sedimentary structures indicate that the Schlieren sediments were deposited predominantly by (hyper)concentrated, friction-controlled flows and concentrated currents where grain-grain interactions dominate. In contrast, the Gurnigel sediments most likely accumulated from surge-like turbidity flows driven by dynamic pressure. Although the inferred surges recorded in the Gurnigel sediments could, in principle, have resulted from flow separation – with coarse-grained material accumulating in the more proximal Schlieren area and finer-grained fractions being deposited in a more distal setting such as the Gurnigel realm – we discard this interpretation. This conclusion is supported by the opposite paleoflow directions, which indicate that no source-to-sink relationship existed between the two depositional systems.