QUATERNARY GEOLOGICAL FRAMEWORK OF NORTH-HOLLAND AND THE MARKERMEER (THE NETHERLANDS)
W.E. Westerhoff and E.F.J, de Mulder Geological Survey of The Netherlands P.O. Box 157 2000 AD HAARLEM
Abstract Quaternary geological data are presented as a basis for the extensive geohydrological and geotechnical studies performed to investigate possible harmful effects of the Markerwaard reclamation project.
1 .^Introduction A sound investigation on the impact of the proposed reclamation of the Markermeer on the adjacent land area of North-Holland must be based on a thorough knowledge of the composition of the subsurface of the entire region. For this reason, the Geological Survey of The Netherlands was asked to present a detailed geological framework of the study area. This framework has been translated into a geohydrological model by IWACO BV. Modifications of the ground water regime will have influence in the topmost 250 - 300 metres of the earth's crust, consisting of poorly consolidated clastic deposits saturated with brackish and saline water. Lithostratigraphy and chronostratigraphy of the Pleistocene deposits, as described in section 2, is based on a large number of deeper borings and is derived predominantly from the comprehensive study on the Pleistocene Geology of North-Holland by Breeuwer & Jelgersma (1979). The geological model of the Holocene (section 3) is the result of a detailed study of thousands of shallow borings and cone penetration tests. This model served as basis for geotechnical studies, done by the Delft Soil Mechanics Laboratory and the Heidemij Nederland BV. The composition of the Holocene coverbeds is expressed in a lithological succession legend (see section 4). On this basis, the land area surrounding the proposed polder was divided into subareas displaying certain soil mechanical and hydrological characteristics in relation to possible future changes in the ground water regime.
2. Pleistocene Geologically, The Netherlands forms part of the subsiding basin of the North Sea, which is filled with a poorly to unconsolidated sediment succession. The thickness of these sediments generally increases in NW direction (Van Staalduinen et al., 1979). The top of the consolidated hardrock consists of Upper Cretaceous limestones and is situated at a depth of 900 - 1000 metres below the surface in the study area. The Cenozoic deposits were not affected by substantial folding, and tectonic structures are confined to block faulting in deeper strata, which led to NW-SE oriented horsts and grabens. Coarse-grained clastic deposits such as sands and gravels, may serve as aquifers, whereas clay and peat beds are to be considered impermeable (aquicludes). The predominantly clayey Tertiary deposits may attain a 877 thickness of more than 500 metres. The top of the Tertiary is formed by a 20 - 50 metres thick marine-clay bed of Pliocene Age, belonging to the Oosterhout Formation (no. 1 in Fig. 1). The overlying Maassluis Formation (no. 2 in Fig. 1) is of Early Pleistocene Age and consists of shell-bearing sandy clays and fine grained sands. The topmost part of this shallow marine formation usually contains clay beds. During the Tiglian (see Fig. 2) the sea retreated from The Netherlands and the study area became part of the continent for a long period of geological time. In this area greyish-white, medium- to coarse-grained sands were supplied by North German rivers (Harderwijk Formation, no. 3 in Fig. 1). The lower part of this formation consists
Mean temp. Years Chronostratigraphy in July x10& 0° 10° 20'
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poorly permeable beds | I moderately permeable beds
0 poorly to moderately [ j well permeable beds J permeable beds
(5) numbers refer to formations, mentioned in the text Fig. 1 Schematic cross-section through the study area
Fig. 2 Chronostratigraphy and paleoclimatic curve of the Quaternary (modified after Zagwijn, 1975)
of medium-grained sands with gravel. The Harderwijk Formation is overlain by coarse-grained river sands and gravels of the Enschede Formation (no. 4 in Fig. 1), likewise of northeastern provenance. In the basal part of this formation of Menapian Age a thin clay bed that closes the underlying aquifer is often present. At the start of the 878 Middle Pleistocene, during the Cromerian, the supply of clastic material from the northeast diminished in favour of a supply by the southeastern rivers (Rhine and Meuse), which resulted in the deposition of multicoloured river sands. In this area these fine-grained to coarse-grained sands are assigned to the Urk/Sterksel Formation (no. 5 in Fig. 1) of Cromerian, Elsterian, and Holsteinian Age. In the upper part of this succession fine-grained sands and silty clays may occur. These sands are part of a higher-lying aquifer. During the Saalian, this part of The Netherlands was glaciated and was covered by large masses of inland ice. This glaciation resulted in the creation of glacial basins more than 100 metres deep, and ice-pushed ridges. Till was laid down by glaciers along the flanks and in the glacial basins, which after the retreat of the inland ice were filled with thick successions of varved clays (lake deposits). Together with medium- to coarse-grained fluvioglacial sands, these deposits belong to the Drente Formation (no. 6 in Fig. 1). During and after the withdrawal of the inland ice masses, which contributed considerably to the consolidation of the underlying unconsolidated deposits, the river Rhine found a westward path again. Multicoloured coarse-grained river sands (Kreftenheye Formation, of Saalian, Eemian, and Weichselian Age) with gravel of southeastern origin mixed with gravel of Scandinavian provenance were deposited in the study area. In the next interglacial period, the Eemian, the sea-level rose and marine shell-bearing sands and clay beds were deposited (Eem Formation). Thick clay successions occur in places in the former glacial basins. At the end of the Eemian, the sea-level dropped and marine sedimentation was again replaced by fluvial deposition, now of coarse-grained gravelly sands of southern provenance (Kreftenheye Formation, together with the Eem Formation indicated as no. 7 in Fig. 1). Fluvial sedimentation originating from the Rhine continued in this area well into the next and last glacial period, the Weichselian. During the Middle and Late Weichselian, fine-grained sands were transported by wind action and laid down as coversands (Twente Formation, no. 8 in Fig. 1). This resulted in a further smoothening of this area. The Kreftenheye Formation, the sandy parts of the Eem Formation, and the Twente Formation together constitute the uppermost aquifer in the study area.
3.Holocene During the Holocene, sedimentation in the study area was strongly influenced by the post-glacial rise of the sea-level. Initially, this resulted in the development of a peat bed, Basal Peat or Lower Peat (De Mulder & Bosch, 1982), in the topographycally lower places. Because of its impermeability, this Basal Peat bed is of great hydrological significance. Due to the continuing rise of the sea-level the sea invaded this peat landscape and a brackish lagoon was formed in which clays were deposited. The coast line migrated further to the east and the brackish lagoon was replaced by a tidal-flat area composed of an irregular complex of mud flats and sandy flats dissected by numerous narrow channels and some broad main tidal channels. Deep scouring of the underlying clay beds, the Basal Peat, and the topmost Pleistocene sands, took place in the main channels. These tidal channels were filled up predominantly with sandy deposits, which permitted hydrologically unobstructed contact between the uppermost Pleistocene aquifer and the Holocene sands (see Fig. 3). 879 v c a u (D I? -P to c m o » •H •P
as
880 was partofanon-marineenvironment,inwhichthermosaic predominant paroftheclasticmarinsedimentationtheshiftedfrom which issituatedonlyafewkilometrewestofthestudarea.Th peat islandsandfreshwaterlakewithorganicmudeposits. place inthentirarea.Fromthattimon,formerZuiderzearea This reclamationconsistedoftheintroductio aprimitivedrainag top ofthepeatthinclaybedslimiteddistribution. continued untilabout3000yearsago.Subsequently,peagrowthtook clastic sedimentation,interruptedbythedevelopmentofthinpeabeds followed bythedevelopmentofextensivpeaaccumulationsin the southerntowardsnortherpartofstudyarea,andthiwa Reclamation oftheselakes,wherthclasticsediment s stilllay Friesland (Borger,1975).Inthesoutherpartof thestudyarea,wher For thepurposesofthi study,thecompositionofHolocenbeds not occuronalargescaluntilthseventeenthcentury . Formore below thepeatabottom,startedinsixteent h centurybutdid created bythepeatextractionanstormeffects. the underlyingclasticdepositsarexposedatsurfac e inWest- disappearance ofenormousvolumethoriginal peat,andatpresen and oxidationofthepeat.Overcenturiesthi s causedthe system, whichloweredthegrounwatertablanthe n ledtosettling triggered byreclamationanpeatextractioith e MiddlAges. In spiteofthconstructioprimitivdikes,peatareas south (HollandPeat).Inthenortherpart(Westfriesland),marin which areupto20metres thick,hadtobeexpressessuchthath 4. Classificationofthelithologica l succession of thisarea,referencei s madetoDMulder&Bosch,1982. detailed informationoth e sedimentaryhistorandlithostratigraph the originalpeataccumulationswerconsiderably thicker,lakeswere increasing sizewerlostbymarinerosion,whic h wasstrongly invading fromtheNorth,whichinitiallyresultedindepositioo Fig. 4ClassificationoftheHolocenlithologicalsuccessio succession ofthegeohydrologicall y andsoil-mechanicallrelevant A B During Medievaltimesthepeatlandscapwaaffectedbysea About 5000yearsagothecoaslinreachediteasternmosposition, code Basal Peat present Basal Peat absent z Z 0 0 code Holocene absent sand sand Holocene absent sand Holocene sand Holocene "51 •S 2 0 1 0 0 2 2 1 2 1 1 peat>1m peat absent peat>1m claycaver>1m peat absent peat>1m clay cover>1m peat<1m, with peat>1m peat absent claycover>1 m peat<1m, with peat<1m, with peat absent clay cover>1m peat<1m, with m I o o o o v v o v o k v k o o k k without thinclaycover without thinclaycover without intercalated,peatbeds without thinclaycover thin claycover>0,25m without intercalatedpeatbeds thin claycover>0.25m intercalated peatbeds without thinclaycover thin claycover>Q,25m intercalated peatbeds without intercalatedpeatbeds intercalated peatbeds without intercalatedpeatbeds thin claycover>ft25m intercalated peatbeds 881 Type Bzlk Bz2o Ao1k Ao2o Azlk Az2o Az2v BzO Bz1o AoO Ao1o Ao2v AzO Az1o Bo1o Bo1k Bz2v BoO Bo2o Bo2v s S s s s s s s intercalated claybeds intercalated claybeds intercalated claybeds intercalated claybeds intercalated claybeds intercalated claybeds intercalated claybeds intercalated claybeds Y Bzlos Aolos Aolks Azlos Azlks Bolos Bzlks Type Bolks JAzlo jfazlQsj JBolos M Bolk || AolksjlAzlksl JBzIk IJBzIO |j AzO || AzO I I Aolo I j Bz2o j[~Bo2o 11 Az2v ! I BzO II Az2vJ I Bz2v
HHU peat [j|;j|ijl;i| day and lor sandy clay Hill day [ | sand l<10% day thickness >2ml \ j sand (Pleistocene) Fig. 5 Schematic cross-section with all classification types distinguished in the study area
AzIU
A/, •32 133 | \peat >1m [ j peut •;•// Pleistocene eroded with thin clay cover Fig. 6 Fragment of classification-type map lithological units would be optimally indicated. This led to the use of the classification shown in Fig. 4. The composition of the Holocene cover beds is expressed in maximally five levels, which are related to geotechnically significant beds. The highly impermeable Basal Peat bed is assigned to the first level of this classification, and the relatively permeable Holocene sand accumulations (thicker than 2 metres) to the second level. The Holland Peat and overlying clay beds occur on the third and fourth levels, at 882 both of which several subdivisions are distinguished on the basis of relative thickness and occurrence of intercalated peat beds. Finally on the fifth level, there is the geotechnically important intercalated clay bed in a Holland Peat succession, which occurs in the southern part of the study area. Fig. 5 gives a schematic cross-section including all classification types distinguished in the study area. The distribution of the classification types is shown on maps on a scale of 1:25,000 (Westerhoff and De Mulder, 1981). Figure 6 shows a fragment of this map. Additional maps on the same scale show the depth of the top of the Pleistocene, the depth of the top of the Holocene sands, the depth of the base of the Holland Peat, and the thickness of the clay bed on top of the Holland Peat.
Acknowledgements We wish to thank Dr. S. Jelgersma and Mr. J.B. Breeuwer for their comments; Mr. H. Bruinenberg for drafting the figures; Mrs. I. Seeger for reading the English text and Mr. I. Kraakman and Mrs. E.Y. Lamboo for typing the manuscript.
References Borger, G.J., 1975, De Veenhoop. Een historisch geografisch onderzoek naar het verdwijnen van het veendek in een deel van Westfriesland. Ph.D. Thesis, Amsterdam Breeuwer, J.B. & S. Jelgersma, 1979. De géologie van Noord-Holland. Rijks Geologische Dienst (rapp. no. OP 7106) Mulder, E.F.J, de & J.H.A. Bosch, 1982. Holocene stratigraphy Radio carbon datings and Paleogeography of central and northern North- Holland (The Netherlands). Meded. Rijks Geologische Dienst (in prep.) Staalduinen, C.J. van et al, 1979. The geology of The Netherlands. Meded. Rijks Geologische Dienst, vol. 31-2. Westerhoff, W.E. & E.F.J, de Mulder, 1981. Geologisch onderzoek i.v.m. studie schadeverwachting Markerwaard. Rijks Geologische Dienst (rapp. no. BP 10374). Zagwijn, W.H., 1975. Indeling van het Kwartair op grond van veranderin- gen in vegetatie en klimaat. In: Zagwijn, W.H. & C.J. van Staaldui nen, red.: Toelichting bij Geologische overzichtskaarten van Neder- land. Rijks Geologische Dienst, Haarlem
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