Pebble clusters as a directional structure in fluvial gravels: modern and ancient examples

Abstract

Pebble clusters abound in both the Recent alluvial gravels and the Late Paleozoic fluvial conglomerates of the Intrasudetic Basin. In fact, pebble clusters are small-scale pebble jams forming characteristically in front of obstacles. Some may also contain sand shadows behind them. Generally, any object protruding above the surface of accumulation may act as such an obstacle. As a rule, the structures parallel mean (paleo) flows. C-fabric measurements of flat clasts grouped to form pebble clusters revealed rather a high dispersion in dip directions attributable to perturbations of flow passing round the obstacles. Field evidence indicates that the best paleocurrent data may be obtained measuring the orientation of flat obstacles only. In alluvial environment pebble clusters (as well as imbrication in general) require turbulent, rapid or nearly so flows and may be considered to be indicative of such (paleo) hydraulic conditions. The presence of pebble clusters in debris-flow deposits is also briefly discussed.