From river valley to estuary : the early-mid Holocene transgression of the Rhine-Meuse valley, The Netherlands

Abstract

Most present day estuaries formed within incised fluvial valleys, created during the last glacial, that drowned during post-glacial sea-level rise. The sedimentary archive of the associated river-mouth areas contains important information on estuarine evolution under different rates of sea-level rise. This thesis presents a study on the development of the mouth of the Rhine-Meuse system in the Rotterdam area, western Netherlands, between 12000-6000 BP. During the study tens of thousands of core descriptions and cone penetration test results, as well as seismic data, pollen and diatom analyses, tens of OSL-dates and hundreds of radiocarbon dates were used. The objectives were to explain: 1) the early-mid Holocene sedimentary succession of the Rhine-Meuse river-mouth area; 2) the development of the river-mouth area in the early-mid Holocene in response to rapid sea-level rise (SLR) and 3) the interaction of the fluvial and coastal systems during the early-mid Holocene transgression. Between 10.5-8 ka BP, the effects of sea-level rise started to influence the study area: groundwater rise resulted in the formation of extensive wetlands, fluvial flood basins became more frequently flooded, sediment-aggradation rates increased and finally the river valley changed into an estuary with adjacent tidal basins. Sea level reached rates of 1 m/100 yr before 8 ka BP with a period of 2 m/100 yr between 8.5-8.3 ka BP as a result of sea-level jumping. During the latter period sea level rose 4.06 0.5 m: 1.95 0.74 m background sea-level rise and 2.11 0.89 m sea-level jump. This jump is linked to the drainage of Lakes Agassiz and Ojibway in the Hudson Bay area and linked to the 8.2 event. After 8 ka BP the rate of SLR slowed down to 0.6 m/100 yr. North and south of the estuary, retrogradation of the coastline occurred at a faster rate than near the mouth at Hoek van Holland, leading to the formation of a promontory. In the upper estuary a bay-head delta was formed. Around 7.3 ka BP the main branch of the Rhine connected to a tidal inlet 20 km north of the promontory that was strongly eroded. The eroded sediment was partly used to fill in the abandoned estuary, but also to fill in tidal basins north of the promontory. The closure of the several tidal inlets resulted in poor drainage conditions and the wetland area increased rapidly after 6.5 ka BP and around 6 ka BP the coastline reached its most eastward position. The sedimentary succession was also described using sequence-stratigraphic concepts. The transition from lowstand to transgressive systems tract (TST) occurred when widespread basal-peat formation halted and gyttja accumulation and clastic deposition started. In large parts of the study area this occurred near-instantaneously during the sea-level jump. The highstand systems tract in the study area started after 6.5 ka BP with widespread peat formation in the closing tidal basins. The relatively late start of the TST is most likely typical for river-mouth areas along wide, low-gradient continental shelves where base-level changes are dominant for a relatively short period