Upper and lower plane bed definitions revised

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