Introduction
Study area and location of the cross sections and the offshore seismic lines.During the last glacial, the Weichselian, incised-valleys systems formed as a result of low sea levels (up to 120 m lower than today) or due to rapid climate changes. After the Last Glacial Maximum (~21,000 years ago), ice sheets started to melt and sea levels began to rise. As a consequence, most of the incised-valley system drowned and transformed into estuaries or deltas. In the western Netherlands, Rotterdam area, this change occurred 11,000-8,000 years ago during a period of rapid sea-level rise (1 m/100 yr). Our understanding of the developments in this crucial time, as of the contemporary rate of sea-level rise, is poor due to lack of data.
Objectives
The aim of this project is to reconstruct the transgression of the fluvial valley, with emphasis on the interlinked response of the coastal and fluvial to rapid sea-level rise. To do so, beside collecting new data, existing subsurface data from many different sources are integrated in a single database. With this data, past sea levels and the development of the study area are reconstructed. Using the results, sequence stratigraphic models will be tested.
Methods
Gathered existing subsurface data consist of >50,000 core descriptions, > 50,000 cone penetration tests and seismic data. New data are mostly collected by hand drilling or from occasional available construction pits. Additional palaeoenvironmental information is coming from diatom and pollen analyses. For the study area, hundreds of radiocarbon dates and tens of OSL-dates were already available. During this project, ~50 radiocarbon dates and ~10 OSL dates from key locations are obtained. With all these data, delta-wide cross sections and palaeogeographic maps are constructed.
Results
For the period 9,000-7,500 years ago a new sea-level curve was constructed, showing sea level to have risen from 24 to 10 m below the present level, an average of 90 cm/100 yr. This induced groundwater-level rise and subsequent peat formation and fluvial aggradation. Shortly after 8,500 years ago, large parts of the Rhine-Meuse valley in the study area were transgressed. Tidal influence increased and at several locations the environments turned brackish: the fluvial valley had changed into an estuary that was located at roughly the same spot as today. On adjacent sides of the estuary, back-barrier basins with tidal channel systems came into existence. Constant coastal retrogradation occurred, although slower near the estuary mouth than in front of the back-barrier basins. This is due to thick stacks of erosion-prone clay and peat layers within and offshore the estuary mouth. As a result a promontory formed. In the estuary, several distributaries debouched. Around ~7.2 ka BP, the main Rhine distributary started to debouch near Leiden, forming a new main estuary. The old main estuary was quickly transgressed and the promontory eroded. The available sediments were transported by offshore currents into the northern back-barrier basins. As a consequence, sediment delivery started to keep with sea-level rise and coastal retrogradation near these basin halted ~6,300 yr ago. In other parts along the Dutch coast, farther away from sediment sources, retrogradation continued for many more centuries.
Pictures
Visiting a construction pit of 20 m deep in Rotterdam, Here deposits of 9,000-8,000 years old are exposed.Links
http://www.geo.uu.nl/fg/palaeogeography/
http://uu.academia.edu/MarcHijma
http://igitur-archive.library.uu.nl/fysgeo/2009-0306-203511/UUindex.html
Marc Hijma's PhD thesis: From river valley to estuary : the early-mid Holocene transgression of the Rhine-Meuse valley, The Netherlands
Please contact Marc Hijma for more info on this project