Upper and lower plane bed definitions revised

Abstract Sedimentary structures in ancient deposits are clues to reconstruct past geohazards. While parallel lamination formed by plane beds is one of the most common sedimentary event such as turbidites, formative conditions for remain unclear. In literature, two types (upper and lower beds) exist supposed develop under different shear stresses, particle sizes, flow regimes. Here, we present new phase diagrams based on compilation existing data regarding hydraulic clarify formation processes associated with beds. The indicated that form separate populations gap between them corresponds threshold condition entrainment into suspension. Lower when sediment particles move only bed load. This space can be discerned from fine sand gravel differs conventional view which limited grain sizes above 0.7 mm. addition, our suggest upper appear active suspended Our analyses demonstrate load contributes beds, whereas other mechanisms also produce fine-grained flows low stress. Thus, results this study interpretations Bouma’s T d division need reconsidered. bedform newly established will useful estimating geologic records