Stratigraphic architecture of the Rigi megafan. (a) Photo showing Rigi Mountain. The fan apex deposits crop out on the SE margin of the photo. Accordingly, the deposits at the top of the section (part δ) chronicle a successively more proximal situation. This could potentially blur the interpretation of a progradational trend recorded by the stratigraphic trend of the section. Nevertheless, because shifts towards a more proximal sedimentary environment occurs also on the steep ridge, displayed on the NW margin of the photo (between parts α and γ), the coarsening- and thickening-upward record does include a progradational trend of the Rigi megafan. (b) Stratigraphic section22 that is based on a compilation from field-based investigations24. The ages are based on magneto-polarity stratigraphies and have been used to correlate the Rigi conglomerates with other sections in the foreland basin. The figure also shows the results of the sedimentologic investigations, including trends in grain size, channel depths, and palaeo-slopes. Palaeoflow data is based on orientations of gutter casts and cross-beds that we have measured in the field. The number in the circle of the rose diagrams indicates the number of observations. These measures have been combined with literature data22,24 into representative rose diagrams. We note that post-depositional compaction could bias the measured channel depths. However, related effects are expected to be larger where the channels are eroded into mudstone (unit A), because mudstones are likely to be compacted to a higher degree than the conglomerates. Therefore, it is likely that underestimates of channel depths in unit A would be larger than in unit B, which contains conglomerates only. The effect is an amplification of the decreasing channel-depth trend illustrated in Fig. 2b. (c) Summary of results (largest grain size D
, channel depth d, slope S) for the base (α) and the top (β) of unit A, and also for the base (γ) and the top (δ) of unit B. The subdivisions into α, β, γ and δ of the Rigi section are also displayed on the photo (Fig. 2a) and for the section (Fig. 2b).