Recognition of Fluvial Environments by Particle-Size Characteristics

Abstract

ABSTRACT Although fluvial deposits represent a depositional continuum from channel to back swamp environments, various fluvial facies can be defined from CM patterns. The dispersal of sediment particles within a fluvial system is closely related to their mode of transport. Traction and graded-suspension loads are confined to stream channels, whereas uniformly suspended material in the upper part of the water column escapes the channel during overbank flow; the effects of these relationships are evident on CM diagrams. As a first approximation, graded, uniform, and pelagic CM patterns can be interpreted as representative of channel-proximal, flood plain, and flood basin environments. Ternary plots of sand, silt, and clay contents of fluvial sediments also produce characteristic patterns. These patterns are compared with those of sediments deposited from transport modes other than dilute suspension. Although ternary diagrams are less useful for environmental interpretation than are CM plots, their common use in reporting sedimentologic data merits consideration of their basic patterns. Recognition of the presence and relative abundance of different fluvial facies should be valuable in general considerations of paleoflow regime, paleoslope stability, and factors relating to local and regional events which affected equilibrium conditions in large drainage basins.