LONGITUDINAL FACIES ARCHITECTURAL CHANGES OF A MIDDLE HOLOCENE ANASTOMOSING DISTRIBUTARY SYSTEM (RHINE-MEUSE DELTA, CENTRAL NETHERLANDS)

Longitudinal changes in channel planform and facies architecture occur over a 22-km long reach of an anastomosing Middle Holocene distributary system in the Rhine-Meuse delta (central Netherlands). A geological map and a sequence of cross sections show that the channel belt is characterized by sand body width/thickness ratios which decrease within the study area from approximately 25 upstream to well below 10 downstream, accompanied by a change in channel planform as indicated by a downstream increase of the number of crevasse splays. The lithological composition of the sand body is relatively homogeneous, but there is always a well-developed fining-upward sequence from sandy channel deposits to clayey natural levee deposits. Within-channel vertical aggradation appears to have been restricted during most of the period of activity of the distributary system, and since lateral accretion was also limited (especially in the downstream part) rapid accumulation of channel deposits seems to have occurred predominantly during the avulsion stage, leaving a narrow and shallow residual channel as the last remnant. The longitudinal change in channel pattern is mainly attributed to a downstream increase in the rate of (ground)water-level rise that resulted from Holocene sea-level rise. In the Rhine-Meuse delta a rate Of (ground)water-level rise of > 1.5 mm yr-1 is required for the creation of fully developed anastomosing fluvial systems (ribbons with a width/thickness ratio of < 15 accompanied by numerous crevasse splays). Values of 1.0-1.5 mm yr-1 enable the formation of fluvial systems with fewer crevasse splays and width/thickness ratios of about 25, which are interpreted as transitional to meandering.