Palaeogeographic development of the Rhine-Meuse delta.

Assen: Van Gorcum, 250 pp

Summary

  1. Introduction
    1. General background and objectives
    2. History of the research
    3. The present database
    4. Organization of this book
  2. Geological development of the Rhine-Meuse delta
    1. Tertiary and Pleistocene
    2. Holocene
      1. Fluvial area and back-barrier coastal plain
      2. Estuarine and back-barrier tidally influenced area
      3. Barrier beach and coastal dune area
    3. Human influence and modern developments
  3. River morphology, sedimentation and fluvial style
    1. Braided rivers
    2. Straight rivers
    3. Meandering rivers
    4. Anastomosing rivers
    5. Controlling factors
  4. Research methods
    1. Mapping
    2. Lithological description of corings
    3. Lithostratigraphic units
    4. Dating and correlating channel belt fragments
      1. Geological and geomorphological mapping
      2. Calcium carbonate content
      3. Soil formation
      4. Relative depth of overbank deposits
      5. Gradient of the top of the (sandy) channel deposits
      6. Gradient of the top of the natural levees
      7. Pollen analysis
      8. Archaeological artifacts
      9. Historical evidence
      10. Dendrochronology
      11. 14C analysis
      12. Palaeogeographic reconstruction
  5. Maps and cross-sections
    1. The geological-geomorphological map and palaeogeographic maps of the Rhine-Meuse delta by H.J.A. Berendsen, K. Cohen & E. Stouthamer
    2. Longitudinal (W-E) geological cross-section of the Rhine-Meuse delta by H.J.A. Berendsen, W. Boasson, K. Cohen, R. Isarin, B. Makaske & E. Stouthamer
  6. Aggrading Holocene river systems
    1. The Benschop river system
    2. The Utrecht river system
    3. The Krimpen river system
    4. The Maas river system
    5. The Est river system
    6. The Graaf river system
    7. The Linschoten river system
  7. The palaeogeographic evolution during the Late Pleistocene
    1. 7.1 Pre-Weichselian terraces
    2. 7.2 Pleniglacial terrace (Lower terrace)
    3. 7.3 Younger Dryas terrace (Terrace X)
  8. The palaeogeographic evolution during the Holocene
    1. Early Holocene incised river systems: 10000 - 8000 yr BP
    2. Evolution from 8000 - 5000 yr BP
    3. Evolution from 5000 - 3000 yr BP
    4. Evolution from 3000 - present
  9. Factors influencing river evolution during the Holocene
    1. Shape of the Pleistocene valley
    2. Lithological composition of the substrate
    3. Sealevel rise
    4. Neotectonic movements
    5. Coastal evolution
    6. Increased discharge and sedimentation rate
    7. Human interference
    8. Changes in fluvial style
  10. Avulsion
    1. Avulsion and palaeogeography
    2. Factors controlling the avulsion history
    3. Avulsion parameters
  11. Summary

Summary:

Over the years, a large amount of data has become available regarding the (Holocene) evolution of the Rhine-Meuse delta, the Netherlands. This dataset, that is a result of a long-standing effort of data collection by Utrecht University, most likely is without a counterpart anywhere else in the world. Its unique potential as a data source for numerous aspects of fluvial sedimentology, fluvial geomorphology, Quaternary geology, environmental science, hydrology, archaeology, engineering geology and even oil reservoir modelling, has become increasingly recognized during the last decennium.

It is the aim of this book:

The palaeogeographic development of the Rhine-Meuse delta is closely related to the avulsion history, that was studied simultaneously by STOUTHAMER (in prep.). Avulsion is defined as the abandonment of a part or the whole of a channel belt by a stream in favor of a new course. The palaeogeographic development is largely determined by the avulsion history. The Holocene Rhine-Meuse delta offers a unique palaeo-environment to study both palaeogeography and avulsions on a timescale of millenia, for several reasons: (1) the relatively complete geological record, as a result of rapid aggradation during the Holocene, governed by relative sealevel rise and land subsidence, and (2) the availability of the extremely detailed database of Utrecht University.

The palaeogeographic reconstruction presented in this book leads to many new insights in the avulsion history, and many other characteristics of the fluvial system, like factors influencing avulsion, interavulsion period (= period of existence of individual channel belts), avulsion frequency (= number of avulsions per time interval in a given area), and avulsion duration (= time between initiation of a new channel and complete abandonment of the previous channel), see STOUTHAMER (in prep.).

The history of the research is briefly described in the introduction. Over the past 40 years, more than 1400 undergraduate students have participated in the field course in the Rhine-Meuse delta. This field course involved detailed geological and geomorphological mapping, on a scale of 1 : 10,000, using core descriptions as a main tool. Coring density varies from about 30 per km2, to 350 corings per km2. The database contains over 200,000 lithological borehole-descriptions, of which 80,000 are now available in a digital format. In addition, more than 1200 14C dates were available for this study, and 36,000 archaeological artifacts from the ARCHIS database.

In Chapter 2 the geological development of the Rhine-Meuse delta is described since the Miocene, when the Rhine was a small stream, draining the Graben of the Lower Rhine Embayment. Neogene uplift of the Rhenish Massif (Germany) and the Ardennes (Belgium) led to increasing drainage areas both for the Rhine and Meuse (=Maas). Marine deposition predominated in the Netherlands up till about the early Pleistocene. As a result of glaciation during the Pleistocene, supply of debris increased and led to rapid regression. The area between Bonn (Germany) and the Dutch border was uplifted during the Quaternary. Here, alternating glacial and interglacial conditions produced a series of terraces. The hinge line between net erosion and net sedimentation practically coincides with the Dutch-German border. In the subsiding North Sea basin the Quaternary sequence reaches a thickness of up to 1000 m. The Pleistocene lowstand deltas of the Rhine-Meuse system occur far seaward of the highstand deltas, parts of which were only preserved in subsiding basins. A characteristic of the Rhine-Meuse delta is, that it was strongly influenced by the successive glaciations of NW Europe. The Saalian ice cap reached the Netherlands and significantly altered the landscape, forming 100 m high glaciotectonic ridges that are still important elements in the landscape. At present these ice-pushed ridges still partly control the width of the fluvial plain, thereby determining the areal extent of the Holocene delta. As an introduction to the palaeogeographic development, described in Chapters 7 and 8, the Holocene evolution of the delta is briefly described for the fluvial area, the back-barrier coastal plain and the estuarine and back-barrier tidally influenced area.

In Chapter 3 the morphology and sediment characteristics associated with different fluvial styles are briefly explained, for the readers not familiar with the used terminology. This Chapter is mainly devoted to the differences between braided, meandering, straight and anastomosing rivers, and their associated deposits.

In Chapter 4 the research methods, used in mapping, dating and describing the lithological successions in the Holocene delta, are dealt with extensively. Emphasis is on dating methods, especially radiocarbon dating and the construction of gradient lines (GTS-lines) of the top of Holocene channel belts. These methods proved to be most valuable in making palaeogeographic reconstructions.

Chapter 5 gives some technical information about the geological-geomorphological map (Addendum 1). The map shows the ages of the Holocene channel belts, organized in 500 yr time intervals. All radiocarbon dates that are used to construct the map, and that are of geological interest are incorporated in Appendix 1 and 2. They are also on the enclosed CD-ROM. In Appendix 3 details of the various channel belts indicated on the map are given, including dating evidence and references. Appendix 4 summarizes dating results, both as 14C ages and as calendar year ages (calibrated 14C ages). Addendum 2 shows the palaeogeographic development of the delta, based on the age determinations of the channel belts. The palaeogeographic maps of Addendum 2 are believed to be accurate to within ± 200 yr.

In section 5.2 the longitidunal (W-E) cross-section of the delta is briefly explained, as well as the way in which it was constructed.

In Chapter 6 the Holocene channel belts are grouped in several 'river systems', based on age, source area, discharge volume, or direction of (palaeo-)flow. The following river systems are distinguished: Benschop river system, Utrecht river system, Krimpen river system, Maas river system, Est river system, Graaf river system, Linschoten river system.

The palaeogeographic evolution during the Late Weichselian is described in Chapter 7. Alternating climatic conditions led to the formation of two partly buried terraces: the Lower terrace of Pleniglacial (pre-Allerød-interstadial) age, and terrace X of Younger Dryas age. The difference in elevation between the terraces is about 2 m in the eastern part of the delta. Gradient lines converge upon each other in a western direction; the terrace intersection of the Pleniglacial and Younger Dryas terraces is located near Rotterdam. Both terraces are covered by a tough clayey layer, that was deposited by incipient meandering rivers during the Allerød-interstadial and the Early Holocene respectively. Both layers are described as the Wijchen Member of the Kreftenheye Formation. The occurrence of a vegetation horizon in both layers is caused by a period of non-deposition. In the western part of the delta, where no elevation differences between the terraces exist, the occurrence in single cores of two vegetation layers in the Wijchen Member enables to distinguish the Lower terrace, even near the terrace intersection. A way of distinguishing terrace X is to look for pumice granules, that were derived from the Laacher See volcanic eruption around 11,230 yr BP. Other indications are the ages of infilled residual channels: these are of Allerød-interstadial and Younger Dryas Stadial age on the Lower terrace, and of Early Holocene age on terrace X. Eolian dunes of Younger Dryas age virtually always were found overlying the Wijchen Member of Allerød-interstadial age at the top of the Lower terrace. The only exception found so far are dunes south of the river Maas in the Maaskant area. Here the dunes directly overlie the Lower terrace, and the Wijchen Member is lacking. This is explained as a result of NW tilting of the upthrown Peel Horst. Because of the higher position of this part of the Peel Horst, the Wijchen Member was not deposited here. The position of the dunes, relative to the braidplain surfaces that provided the sand for their formation, indicates southwestern and northern winds during the Younger Dryas. No dunes were found on the Younger Dryas terrace X, although they may have been formed.

Chapter 8 describes the palaeogeographic evolution during the Holocene, based on Addendum 2. During the Early Holocene, rivers were still incised, leading to a period of non-deposition, that lasted until the terrace intersection of the Late Weichselian terraces and Holocene deposits had shifted upstream of a given location. Sealevel rise is held responsible for the inland shift of the terrace intersection. Aggradation started in the near-coastal area of the present Holocene delta between 9000 and 8000 yr BP, and reached the western fault of the Peel Horst around 6000 yr BP. After 6000 yr BP the terrace intersection shifted rapidly inland, although the rate of sealevel rise decreased. This indicates, that the Peel Horst was a topographic high at that time.

The oldest Holocene channel belts on the map (Addendum 1) belong to the Benschop river system. Between approximately 8000 and 4000 yr BP a remarkable river pattern developed in the central western part of the delta, roughly in the area between Gorkum-Vianen and Rotterdam-Gouda, consisting of an anastomosing complex of straight, and narrow channels with a low width/thickness ratio of the sandbody. Most of these channels had a NE-SW direction, and debouched into the Meuse estuary near Rotterdam. They may be regarded as large crevasse channels that belong to 'failed avulsions'.

Around 5500 yr BP a major avulsion occurred near the Peel Boundary Fault at Wijk bij Duurstede, which led to complete abandonment of the Benschop river system around 5350 yr BP, and the formation of the Utrecht river system. The Old Rhine channel (Oude Rijn) became the main Rhine branch, and remained active until 1122 AD, when its trunk course (the Kromme Rijn) was dammed at Wijk bij Duurstede, and its drainage was diverted to the River Lek, that gradually had become more important.

Although many avulsions occurred within the upstream part of the Utrecht river system, discharge remained concentrated towards the Leiden estuary. Only after 2000 yr BP, when discharge and sediment load probably increased, and coastal erosion occurred in the Maas estuary near Rotterdam, main discharge shifted to the southwest. A gradient advantage now developed in this direction and rivers debouched through the Maas tidal inlet. As a result, the Oude Rijn degraded, as well as the Leiden estuary. Coastal development therefore also seems to have played a crucial role in river evolution.

After 1000 yr BP human influence became increasingly important. Main man-induced changes include the embankment of the rivers, digging of canals, and the construction of groynes and weirs.

In Chapter 9 the palaeogeographic evolution is explained as a complex interaction of various factors influencing the river dynamics. These factors are: the shape of the Pleistocene valley, lithological composition of the substrate, sealevel rise, neotectonic movements, coastal evolution, discharge and sediment load variations, and human interference. Examples are given of the influence of each factor. Especially the influence of neotectonics on Holocene fluvial evolution in the Rhine-Meuse delta downstream of the hinge line is a new phenomenon for the Netherlands, and now seems to be supported by convincing evidence. Neotectonics probably also influenced avulsion locations, as nodal avulsions were concentrated in the Peel Horst fault zones, and many other avulsions occurred also on faults. The final section of Chapter 9 is devoted to changes in fluvial style. It is shown, that a spatial succession of fluvial styles exists, that was also present in the past, and that is related to gradient and substrate conditions. This succession shifted inland as a result of sealevel rise. Late Weichselian and Holocene rivers in the Rhine-Meuse delta all were of the anastomosing type, as defined by MAKASKE (1998). Straight anastomosing rivers continued to exist up to the present, but a unique lithofacies is present in the west-central part of the delta. This facies is probably caused by 'failed avulsion' and large-scale crevassing. A new time-space model of river channel pattern is presented in Fig. 9.15.

During the Holocene, avulsion (Chapter 10) was an important process in delta building, and the location of avulsion sites is influenced by the same factors that influenced the palaeogeographic evolution. Many avulsions may be related to neotectonic movements of the Peel Horst and the Roer Valley Graben. Avulsion frequency initially seems to have been determined essentially by rapid sealevel rise (during the Atlantic). A second maximum of the avulsion frequency was reached between 3000 and 1700 BP, which may be related to increased discharge and/or by within-channel sedimentation or both. Avulsion frequency seems to have a periodicity of 500 years. Although this periodicity is not statistically significant, it may be real. The period of existence of channel belts varied widely throughout the Holocene, but shows no significant trend over time and space. It remained on average constant at around 1000 14C yr, with a standard deviation of 700 14C yr. When only the best dated channel belts are taken into account, the average period of existence of channel belts is 1280 ± 820 cal yr. The average avulsion duration is ~325 yr. It seems to be constant over time, although it varied widely, from instantaneous avulsion (14C yr. This implies that the average interavulsion period can be estimated to be ~600-700 14C yr, or ~800-900 cal yr, but variation is considerable. If avulsion duration and interavulsion period are constant, then the number of coeval channels in the Rhine-Meuse delta was essentially determined by avulsion frequency.