Sedimentary products of avulsions in the Holocene Rhine–Meuse Delta, The Netherlands

Abstract

Abstract The main objective of this study was to describe the characteristics of deposits related to avulsion, to determine if avulsion locations can be recognized by characteristic sediment architecture. In the Rhine–Meuse Delta, the majority of avulsion sites is accompanied by crevasse–splay deposits with a relatively large areal extent. These crevasse–splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to ‘avulsion belt deposits’ described by Smith et al. [Sedimentology, 36 (1989) 1; Can. J. Earth Sci., 35 (1998) 453]. The lithology of all investigated crevasse–splay deposits ranges from coarse sand to silty clay. Sand is concentrated in crevasse splay channels, and silty and sandy clay occurs as distal deposits and as levee deposits between the channels. The total volume of sand is small, relative to clay and silt. Of a total of 91 avulsion sites, three sites were studied in detail, one each representing the sedimentary architecture of a full nodal avulsion, a failed avulsion, and a crevasse–splay complex that did not lead to avulsion. It is concluded that dating evidence of the abandoned and newly formed paleo-channel is always required to verify if avulsion really occurred, although the presence of large-scale sandy crevasse–splay deposits in combination with two main meandering channels often indicates avulsion. Based on a 2-D outcrop or just one cross section, it is impossible to determine whether a splay complex was associated with an avulsion or not, because the architecture and lithology in both cases can be identical. Not all avulsion locations are characterized by crevasse–splay deposits. There are two possible explanations for this: (1) splay deposits were eroded by the newly formed channel, or (2) splay deposits were never formed. This may be a result of reoccupation of an old channel. In this case, avulsive flow is immediately appropriated by the pre-existing channel and no large-scale splay formation occurs. Alternatively, splay deposits were not formed because avulsion resulted from headward erosion.