FLUVIAL ENVIRONMENTS | Sediments

Abstract

Fluvial sediments consist of channel (channel bar, channel fill) and overbank (natural levee, crevasse splay, flood basin) deposits. Channel deposits are comprised predominantly of sand and gravel and typically occur as lenticular bodies or thick tabular sheets with complex bedding geometries and erosive bases. Overbank deposits consist primarily of silt and clay and lesser amounts of sand, and often encase sand bodies. Organic and chemical sediments such as peat and carbonate can be important constituents of overbank sediments in humid and arid settings, respectively. Fluvial sediments are present in a diverse array of environments that range from tectonically quiescent continental interiors to actively subsiding continental margins and foreland basins. The geologic setting controls the abundance of fluvial sediments, the completeness of the record of fluvial activity, and the architecture of the deposits. In continental interiors where erosion predominates, fluvial sediments are often represented by down-stepping flights of terraces underlain by sand and/or gravel (see Terrace Sequences). Fine-grained overbank deposits are often rare in these settings. Periods of fluvial aggradation and sediment reworking are punctuated by river incision and soil formation. Records of fluvial activity in these settings are relatively incomplete. Along continental margins, fluvial sediments are represented by relative thick successions of muddy overbank and sandy channel deposits. Stacked deposits of Quaternary fluvial sediments in these settings provide relatively complete and long-term records of fluvial activity.