Evolution of lithic clasts in a subaquatic debris flow: Conclusions from the Rzucewo site, Poland

Research on clast evolution in subaquatic debris flows is rare and experiments on this issue are insufficient due to technical limitations. To fill this knowledge gap, we identify the determinants of morphological and lithological diversity of pebbles in Late Pleistocene deposits of subaquatic debris flows. The advantage of the study site is the ability to trace these aspects along the entire cross-section of the subaquatic fan, which is unique. Lithology, size, form (using Sneed and Folk's method), and roundness (using Powers' method) of pebbles ranging in size from 20 to 60 mm in two debrites were analysed. Additionally, the grain size composition of debrites was determined through wet sieving. As a result of inheritance from debris transported in the ice, the majority of pebbles in all samples show lithology similar to the till of the same age and a form close to sub-equant. A downslope increase in the share of more oblate and prolate clasts is recognized, which is explained by the transfer of pebbles more susceptible to buoyancy to the debris-flow front. This may mimic clast form changes caused by abrasion. Moreover, a tendency to transfer the largest pebbles can be observed. The roundness of pebbles increases along with the debris-flow route, despite its relatively short length, mainly due to friction against matrix and collisions with other coarse particles. This trend is observed in different petrographic groups of rocks, regardless of their abrasion resistance. It is most likely that the rotation of pebbles has helped in smoothing out their edges and faces. Our results show that a moderately dynamic environment, with a slope angle relatively smaller compared to experiments using flume channel, but a debris-flow distance similar to those studies, can alter the morphology of pebbles. While experiments focus on subaerial environments, the findings appear to be useful in studies of subaquatic debris flows.