Orientation of gravels and soft-sediment clasts in subaqueous debrites – implications for palaeodirection reconstruction: case study from Puck Bay, northern Poland

Abstract

Subaqueous debrites on a fan were recognized at Rzucewo (N Poland). Analysis of debrite fabric enabled debris-flow palaeodirections to be traced on the fan. The long axis azimuth and dip direction of lithic clasts (gravels) as well as soft-sediment clasts (SSC) were measured in the debrites. The results obtained indicate a palaeotransport direction to NNW, similar to the palaeocurrents interpreted from the asymmetrical ripples. However, detailed measurements of both gravels and SSC orientation show variability of palaeoflow directions between NW and NE sectors. In the proximal part of the fan, gravels indicate variable dip directions and dip angles, and a mostly scattered distribution of a-axis orientation. In the medial and distal parts of the fan, clasts are better ordered and usually their a-axes are oriented upslope or transverse to the palaeoslope inclination direction. Probably during the decelerating and halting of debris flows, the compression processes induced clast rotation and a change of inclination. In the distal part of the fan, debrites reveal a distinct variability of palaeoflow directions caused by flattened topography in which debris-flow lobes split and spread freely in different directions. Finally, we conclude that the individual sets of clast fabric usually indicate only local debris-flow directions.