Pebble roundness as a tool to reconstruct fluvial transport distances

Abstract

Pebbles become rounded during fluvial transport. Roundness, along with other shape factors, size and lithology, are commonly used in sedimentology to link the fluvial transport distance and consequent abrasion prior to deposition. Although roundness is being used as a fundamental property, the method of assessment of roundness is mainly qualitative and there is no model to define the roundness as a proxy of transport distance. Here, we propose a new model that quantifies the relation between roundness and fluvial transport distance, and apply it to pebbles/clasts of multiple lithologies from both modern sediment as well as conglomerates deposited by ancient river system. We collect field data from two Himalayan rivers in Nepal over a length of around 50 km. We use the Isoperimetric Ratio (IR = 4*π*A/P2, where A and P are the cross-sectional area and perimeter of the pebble, respectively) that we correct for the effect of elongation (“Normalized Isoperimetric Ratio - IRn”) to quantify roundness. In the field data, we find a linear relation between IRn and transport distance, with the slope of the line varying with the rock type and percentile considered. Based on these results, we propose a non-linear roundness model, as theory predicts the ratio should asymptotically converge towards a value of 1 (perfectly rounded). This asymptotic behaviour occurs over distances greater than 50 km. Using this new model, we find that the Pliocene conglomerates of the paleo-Bagmati River preserved at the base of the Kathmandu intermontane basin were deposited by a river that was once part of a more expansive river network but has since been a victim of drainage capture.