Parent rocks control on grain-size of daughter sediments and implications for provenance studies: insights from the Avisio River (Dolomites, Italy)

Grain-size of clastic sediment is generally considered to be the result mainly of physical processes active during transport and deposition (e.g. grain-size sorting by dimension/density, sediment by-pass); less importance is generally given to other factors, such as the parent rock lithology. In this work, we investigate the control exerted by the parent rock lithology on the grain-size of daughter sediments. Our approach combines fieldwork-based sediment characterization (in-situ grain-size measurements and petrographic analysis of pebbles > 2 cm), laboratory analyses (sieving, measurement and petrographic point counting at the microscope on grains < 2 cm) and geospatial statistics of the source area.

To do so, we selected as study site a sandy-gravelly bar of the Avisio River, located in Valle di Fassa (Dolomites, Italy) which is sourced by a relatively small catchment area made by very different parent rocks. This allows us to disregard the effect of sediment transport from the source to the sampled depositional site, i.e. to consider the analysed samples representative of the sediment produced at the source by the studied catchment. The lithologies exposed in the catchment area are mainly represented by (i) dolostones, (ii) mafic to intermediate volcanics and (iii) limestones and sandstones, which all outcrop in similar proportions. From the fluvial bar, we collected sand and gravel samples analysing their dimensional (pebble measurements and grain-sizes sieving) and compositional properties (rock identification and sedimentary petrography). We analysed both the overall grain-size and composition of the collected samples and the composition of each grain-size fraction between 16 cm and 0.075 mm contained in each sample. Moreover, we performed a GIS-based geospatial analysis of the sediments source area to quantify the rock type distribution and have the true geology of the source region to be compared with its image provided by its daughter sediments.

Our results show a significant relationship between grain-size and sediment composition: gravels are mainly made by dolostone pebbles, while sands are mainly composed of volcanic grains. This trend persists across the separated grain-size portions: sediment fractions > 1 mm are richer in dolostone grains, while sediments fractions < 1 mm are richer in volcanic grains, and proves that dolostone and volcanic rocks feed at the source daughter sediments with dramatically different grain-size curves. Moreover, none of the samples shows the same proportion of the compositional distribution derived from the GIS-based geospatial analysis (i.e., similar proportion between the three lithologies considered).

These findings indicate that since their origin, sediment grain-size is strongly controlled by different weathering effects on the diverse parent rocks and therefore distinct grain-sizes provide very different geologic scenarios for the same source rock geology. This, while often overlooked, significantly impact provenance studies aimed to paleo-geologic reconstructions and must be also carefully considered in facies tract models, challenging the assumption that grain-size variations along depositional systems are solely due to physical processes acting on sediments having at the origin a simple grain-size distribution.