Interpreting the Concept of the Sedimentation Unit

Abstract

ABSTRACT Consideration is given here to the interpretation of Otto's concept of the sedimentation unit and to Apfel's definition of a phase. Otto's definition tacitly assumes that the stratified internal structure of the sedimentation unit is causally related to small erratic fluctuations or perturbations within the transport system. These are referred to as nonassignable causes. Both Otto and Apfel selected velocity as the critical and diagnostic parameter, and they related the bedded structure of the unit to chance variations in velocity, or to fluctuations in the competency of the transporting current. Laboratory experiments involving the deposition of artificial sedimentation units demonstrate that: (a) a well-defined bedded structure can develop under steady-state con itions of flow (velocity) and sediment transport, (b) velocity fluctuations represent but one of a number of important processes responsible for the sorting and segregation of grain sizes into a bedded structure. Furthermore, the experimental evidence suggests that a microchange in regimen may trigger the deposition of a group of laminae rather than a single lamina. A genetic interpretation of the laminated internal structure is not particularly meaningful until such time as further information becomes available on the mechanics of depositional processes. Therefore the connotation essentially constant used by Otto in his definition of the sedimentation unit must be interpreted rather liberally in the light of present knowledge, because well-defined lamination can be formed under an a parently wide range of flow conditions, including the special case of uniform flow conditions. It is claimed, therefore, that these considerations tarnish the conceptive framework of the sedimentation unit.