Sediment grain size vs. parent rocks lithology: insights from the Avisio River and its drainage area (Dolomiti, Italy)

Grain size of clastic sediment is generally regarded as the product of physical processes active during transport and deposition. Here we investigate the influence of the parent rock lithology on the original grain-size of daughter sediments by combining in situ and laboratory analyses of outcrop, detritus and sediments.

As study site we chose a relatively small catchment area (147,7 km² planar, 185,4 km² accounting for elevation) located in Valle di Fassa (Dolomites, IT). This is done to consider the analysed samples representative of the sediment produced at the source, disregarding the effect of sediment transport. In this area three main lithologies outcrop, all in similar proportions: (a) dolostones, (b) mafic to intermediate volcanics and (c) limestones and sandy limestones. We: i) quantified the source rock distribution with a GIS-based geospatial analysis, ii) analysed the outcrops of these lithologies with 3D drone photogrammetry and in-situ Schmidt hammer rebound test to estimate fractures, bedding and rock strength, iii) performed image analysis and sieving to obtain grain-size of detritus collected at the base of outcrops, and iv) finally, performed both grain-size and compositional analyses of each grain-size fraction between 16 cm and 0.075 mm on the sediment samples collected from a sandy-gravelly fluvial bar of the Avisio River reaching sediments from the studied outcrops.

The results obtained show a significant relationship between outcrops fracture spacing, grain size of detritus, and grain size and composition of river sediment: dolostones tend to be over-represented in the gravelly sediments, while volcanic grains dominate the sandy grain-size. Still, none of the sediment samples analysed has a similar proportion between the three lithologies, as the one of the GIS-derived catchment area‘s.

These findings suggest that sediment grain size at the origin is strongly controlled by lithology-dependent weathering processes, active on parent rocks. This is expected to have a significant effect on facies development along the routing systems of clastic sediments. In addition, different grain-sizes preserve very different images of the same source area. Therefore the control of parent-rocks lithology on daughter sediment grain-size must be carefully considered  when approaching provenance studies aimed at paleo-geological reconstructions as well as for facies tract predictions.