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Erosional features in the lagoonal Almere Member (“sloef”) of the Groningen Formation (Holocene, central Netherlands)
Sedimentary Geology, 13 (1975) 253--265 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
E R O S I O N A L F E A T U R E S IN THE L A G O O N A L A L M E R E MEMBER ( " S L O E F " ) O F THE G R O N I N G E N F O R M A T I O N (HOLOCENE, CENTRAL N E T H E R L A N D S )
A.J. VAN LOON and A.J. WIGGERS
Instituut voor Aardwetenschappen der Vrije Universiteit, Amsterdam (The Netherlands) (Received February 2, 1975; revised and accepted March 20, 1975)
ABSTRACT Van Loon, A.J. and Wiggers, A.J., 1975. Erosional features in the lagoonal Almere Member ("sloef") of the Groningen Formation (Holocene, central Netherlands). Sediment. Geol., 13: 253--265. Various erosional features in the lagoonal Almere Member stress the importance of wave action and bottom currents for the depositional conditions in the lagoonal environment. Two erosive structures, named "peat hole" and "rocking hole" are briefly described. These structures have not yet been mentioned in the international literature.
INTRODUCTION
The so-called "sloef", which forms the Almere Member of the Groningen Formation (Van Loon and Wiggers, 1975b), is a young Holocene (0--1600 A.D.) lagoonal sediment. It is exposed in ditches in the reclaimed polders of the IJsselmeer (the former Zuiderzee) (Fig. 1). The average grain size is small: most of the grains are encountered in the fraction 16--50 pm (Wiggers, 1955; Van Loon and Wiggers, 1975a,c). Nevertheless, the depositional environment cannot be consfdered to have been continuously quiet, since various expressions of erosional forces can be observed. A period with strong erosion existed at a b o u t 1200 A.D. In many locations the previously deposited sediments of this member have been removed entirely. This fact gave rise to the -- n o w abandoned -- idea that sedimentation started as late as 1200 A.D. instead of a b o u t 0 A.D. The sediment that was formed during this erosive period consists frequently of a relatively coarse layer (lag deposit) with shells, pebbles and w o o d fragments, and it is called the Valvata layer. It belongs to the bed with code S1 IIIa. Other indications for strong erosion have been found in erosion holes in the underlying peat (peat holes). Another indication is given by the erosion
254
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(due to wave action) of peat areas, surrounding the lake. In this paper this p h e n o m e n o n is called "coastal erosion". The latter two types o f erosion are rarely found. Usually the erosive forces resulted in minor effects, such as scour-and-fill structures, which tend to be of a small size. Although the distinction between erosional and abrasional p h e n o m e n a in a lagoonal environment is a rather arbitrary one, some abrasional features will be described in a f o r t h c o m i n g paper. It will deal with the abrasion of the undulating Pleistocene sandy subsoil, o f out croppi ng glacial till, and of Late Weichselian river dunes. COASTAL EROSION Detailed mapping o f the Almere Member (Bodemkundige Code- en Profielenkaart van de Noordoostpolder, 1947--1956; Bodemkundige Code- en Profielenkaart van Oostelijk Flevoland, started in 1968) showed that the various beds o f this memb e r are n o t present everywhere. In some cases this may be due to non-deposition, e.g. caused by t o o high a position of older sediments
255
Fig. 2. Fossilized peat "cliff" (dark). The contemporaneous lagoonal sediments consist of peat detritus (lower right) and finely laminated humic material (middle right). Due to compaction, the surface of the peat now lies at a lower level than that of the corresponding laminated siliciclastic deposits. Both are overlapped by a younger bed of the Almere Member (bent dark layer).
(Zuur, 1948); in other cases this may be the result of erosion. The latter is the case, for instance, with large parts o f the beds S1 III b and S1 III a, which may only be represented by the Valvata layer. A third reason for the local absence of some beds is related to the extension of the lagoon in which these sediments were deposited. The lagoon was located within a peat area, the borders of which were continuously eroded by wave action. By this process the extent of the lagoon increased, and y o u n g beds overlapped the remnants of the underlying peat layer. At some localities, a fossilized peat " c l i f f " has been observed in the sides of ditches. In such cases peat blocks have been found, t h a t were obviously eroded from the cliff. At present these blocks are embedded in the siliciclastic lagoonal sediment (Fig. 2). EROSIONAL SURFACES
During the sedimentation of the Almere Member the erosion was n o t restricted to lateral directions, but it also t o o k place vertically. Usually the b o t t o m erosion was of only local importance. Nevertheless, a few periods existed in which considerable erosion affected relatively large parts of the lagoonal area. This resulted in erosional surfaces t h a t are characterized by a
256
lag deposit, since the finer particles were carried off in suspension. The lag deposit represents a rather thick sediment, since the originally deposited material must have consisted (just like the other parts of this member) mainly of clay and silt (Wiggers, 1955; Van Loon and Wiggers, 1975a,c). The occurrence of such erosional surfaces is assumed to be the result of changes in the hydrographic conditions. An analogous situation existed after the construction of the Afsluitdijk (Enclosing Dam) in 1932, when these conditions changed very rapidly. In some parts of the IJsselmeer this resulted in fairly severe erosion of the marine Zuiderzee Member (1600--1932 A.D.), while in other parts the eroded material was redeposited as the fresh to slightly brackish IJsselmeer Member. The position of shells of Mya arenaria, visible at the surface (Fig. 3), gives an indication for the rate of erosion between 1932 and the m o m e n t at which the polder Oostelijk Flevoland fell dry (1957). These molluscs lived at a depth of a b o u t 10 cm below the surface. One of the lag deposits within the Almere Member can be traced over a vast area in the Noordoostpolder. Since this coarse layer contains abundant specimens of the fresh-water snail Valvata piscinalis, it is called the Valvata layer. A photograph showing the typical facies of this band has been pub-
Fig. 3. Erosional surface in Oostelijk Flevotand, as visible just after reclamation in 1957. Many specimens of Mya arenaria are present, still in living position. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
257
Fig. 4. Depression in the Valvata layer with abundant specimens of Valvata piscinalis {small) and Unio turnidus (large).
lished earlier (Van Loon and Wiggers, 1975c). Apart from the Valvata shells, the Valvata layer contains many other larger particles, especially w o o d fragments that were derived from the underlying wood-containing peat, large shells of the fresh-water mussel Unio tumidus, and pebbles. The strong erosional forces that affected the lagoonal b o t t o m left their traces in the form of frequent depressions, in which the Valvata layer (usually 1--3 cm thick) may reach a thickness o f several decimeters (Fig. 4). In cross-section these depressions resemble channels, b u t thorough investigations revealed that they do n o t have an elongated character. Dating by means of archaeological finds indicates that this layer was formed at a b o u t 1200 A.D. (Van Loon and Wiggers, 1975b). Ente (1973) correlated the formation of the Valvata layer with a breakdown in 1170 A.D. of a peat barrier between parts of the lagoon. According to this author, this would have led to a rapid burial of the living gastropods b y a layer of peat detritus. We are of the opinion, however, that changing hydrographic conditions were the main factor in the genesis o f the Valvata layer, as is indicated by the lag-deposit character. All these developments were possibly the result of a change in climate that started at that time, leading towards the "Minor Ice Age" (Lamb, 1961). Simultaneously an eastward shift of the western coast of The Netherlands t o o k place, leading to a deepening of the coastal profile. The sand that became available by this process was used for the formation of the Younger Dunes (Jelgersma et al., 1970).
258
Fig. 5. Decapitated contorted structures. This indicates abrasion by bottom currents. Similar lag deposits, although thinner and more locally developed, are also present. Especially on the boundary between the beds S1 II a and S1 I b (Wiggers, 1955; Van Loon and Wiggers, 1975b), these features have frequently been observed during our recent investigations. Erosional and abrasional phenomena on a smaller scale are abundant, as is shown by innumerable "decapitated" structures (Fig. 5). PEAT HOLES In places where the Almere Member directly overlies peat, it sometimes can be observed that parts of the peat have been removed as a kind of blocks. This resulted in steep pits in the b o t t o m , which have been described in Dutch reports as "veenkuilen". We propose to name such phenomena "peat holes" (Fig. 6). The same erosional process occurred during the time of deposition of the overlying Zuiderzee Member, as is shown by the sediments within the holes. The dating of these pits as more or less contemporaneous with the filling up is confirmed by recent observations in these polders: shortly before the polders were reclaimed, various canals were excavated, which appeared to have been filled up for the greater part at the m o m e n t the polders came dry. This indicates a very rapid sedimentation in depressions, caused by b o t t o m currents that continuously transport material, until it is trapped in depressions. The occurrence of peat holes is restricted to a few areas, and they are best known from the northern part of the Noordoostpolder, from the areas around the former islands of Urk and Schokland, and from parts of Oostelijk Flevoland, south of Ketelhaven.
259
Fig. 6. Peat hole with finely laminated sediments inside. Bent contacts may be caused by subsequent differential compaction. Note that material was supplied from the right, as shown by the uppermost laminae. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
Fig. 7. Various shapes of peat holes• Compare with Fig. 8 for reconstruction of their genesis. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
260
The sediment in the holes usually is thinly laminated. The laminae end rather abruptly against the sides of the holes, although differential compaction afterwards may have caused a slightly bent contact (Fig. 6). The nearly horizontal position of the laminae indicates that these depressions are n o t the result of a postsedimentary disturbance, e.g. load casting. The erosional origin is confirmed by the shape of the holes, which shows many variations. Some are more or less rectangular, while others are more or less rounded in diameter (Fig. 7). Based on observations like those in Fig. 7, Wiggers (1955) published a figure, in which he showed the genesis of some steep, rectangular holes near Urk (Fig. 8). It should be noted, that only a small part of the holes can be explained in his way. The peat area near Urk was covered by a thin clay layer, on which sand lenses were locally deposited. This sand, called Urkzand, was derived from abraded glacial till. According to Wiggers' theory, the peat subsided (due to
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Fig. 8. Genesis of rectangular peat holes near Urk. After Wiggers (1955).
261
compaction) at the depositions] sites of the sand lenses. This led to a synsedimentary subsidence of the depositional surface, resulting in a continuation of the sand deposition. This phenomenon will be dealt with in more detail in a forthcoming paper. Due to the synsedimentary subsidence the peat and clay layers around the sand lenses were pushed up, and the resulting higher parts were decapitated by abrasion afterwards. As a result of the rigidity of the peat during this folding, vertical cracks were formed, which facilitated subsequent erosion of the peat. The holes that were formed in this way were rapidly filled up. Another argument for the erosion of large peat blocks is the presence of such blocks, embedded elsewhere in the Almere Member in the siliciclastic beds. In many cases, however, the blocks would have been totally destroyed during transport, yielding peat detritus for the many humic laminae in this sediment.
Fig. 9. Irregular lamination, showing small current ripples and scour-and-fill structures.
262
The peat holes were, as indicated above, rapidly filled with bed-load material. B o t t o m currents, however, sometimes may have been too erosive, and possibly eddies may have been formed in the holes. The rounded character of some holes is thought to be the result of the erosional action by eddies. SCOUR-AND-FILL STRUCTURES
One of the characteristics of the Almere Member is the fine parallel lamination (Koopstra, 1962; Van Loon and Wiggers, 1975c). In many places, however, this rather quiet picture is disturbed by the frequent occurrence of small wash-outs and current ripples (Fig. 9). Although such structures are most typical of channel bottoms, they may be formed under certain conditions when water flows over an unconsolidated sediment surface (Reineck and Singh, 1973). This latter origin must be assumed for the struct~Lres found in these deposits. They resemble the structures described by Reineck and Singh in several respects, e.g. the inclined position of the filling-in laminae and their size (a few cm to several m). Reconstruction of paleocurrent directions is, however, in most cases impossible, since an elongated character is rare. The few cases that might be impor-
Fig. 10. Superposition o f various scour-and-fill structures. These are usually filled in laterally, as is shown by the lamination (dark parts have a high c o n t e n t o f h u m i c material).
263
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Fig. 11. Diagram showing relationship between depth and width of scour-and-fill structures in the Almere Member.
tant for the determination of these directions will be dealt with in a future paper. The scour-and-fill structures occur quite frequently in all sections t h a t were investigated (Van Loon and Wiggers, 1975a), either in an isolated position, or in groups. It seems t h a t the presence of such structures has favoured the conditions for the formation of new ones, since t h e y are often f o u n d eroding each other (Fig. 10). Even in these cases, where the various structures may have fairly different sizes, the d e p t h / w i d t h ratio is rather constant (Fig. 11). Since observations were mainly made in the sides of ditches, it was in most cases impossible to check whether some larger structures should be considered as scour-and-fill structures or as channels. It is probable, however, t h a t real channels are a very rare p h e n o m e n o n in the Almere Member. ROCKING HOLES
Erosive depressions in the m u d d y surface sediment of the lagoon were n o t only formed by b o t t o m currents, but also by objects lying u p o n the b o t t o m . These tool marks should be considered as intermediate between stationary and moving tool marks, for the resulting structures were caused by a rocking movement of the objects, due to wave action. There is no argument for a considerable transport of the objects.
264
Fig. 12. Rocking hole with large peat fragment at the bottom. This block may have been formed during coastal erosion, or during the formation of a peat hole. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
Various kinds of objects may be involved. In most cases it concerns peat blocks (Fig. 12), that were formed by either coastal erosion, or during the formation of peat holes. Other objects that have been encountered are shipwrecks (e.g. Van Loon and Wiggers, 1975c, fig. 5), w o o d fragments (branches and trunks), clay balls, and even large shells. All these objects seem to have been light enough to be rocked b y wave action, b u t t o o heavy to be dragged over the b o t t o m . The occurrence of really heavy objects like pebble~ (Van Loon and Wiggers, 1975c) never led to such structures, b u t did cause load casting. The structure that is shown in Fig. 12 must have been formed in the following way. During the sedimentation of the Almere Member an eroded peat block (1) was deposited at the lagoonal b o t t o m . This happened at level (2). After deposition, the peat block was exposed to wave action, which resulted in a rocking movement of the block. Due to this process the base of the peat block became rounded, which in its turn facilitated the rocking movement during the next phases of wave action. This movement resulted in the formation of an erosive hole (3), that was somewhat larger than the peat block itself. Bumping of the block against the walls of the hole led to steep, sometimes even overhanging sides (4). The eroded sediments, as well as the eroded peat of the base of the block, were partly removed and partly deposited in the hole itself (5). When the hole had become so deep that wave action no longer had an important influence, the rocking action stopped, and
265
the hole gradually became filled up with new lagoonal sediments (6).' After all these processes compaction t o o k place. The youngest sediments in the hole (6) that consist of nearly pure clay and silt were especially compacted. This process led to a downward bending of the sediments that cover the rocking hole (7). The presence of large objects on a b o t t o m consisting mainly of silt and clay is rare in the geological record. This may be the reason w h y this structure has (as far as we know) never been described before. We propose to name this kind of tool mark "rocking hole". In our opinion, a lagoon is the most favourable environment for this phenomenon, although it may occasionaily be found in other environments (fluvial, shallow marine, lacustrine) as well. ACKNOWLEDGEMENTS
The authors wish to express their thanks to the "Rijksdienst voor de IJsselmeerpolders" for permission to reproduce some of their photographs.
REFERENCES Bodemkundige Code- en Profielenkaart van de Noordoostpolder, 1947--1956. Directie van de Wieringermeer (Noordoostpolderwerken), Kampen. Bodemkundige Code- en Profielenkaart van Oostelijk Flevoland, started in 1968. Rijksdienst voor de IJsselmeerpolders, Lelystad. Ente, P.J., 1973. De IJsseldelta. Kamper Almanak, 1973: 137--164. Jelgersma, S., De Jong, J., Zagwijn, W.H. and Van Regteren Altena, J.F., 1970. The coastal dunes of the western Netherlands; geology, vegetational history and archeology. Meded. Rijks Geol. Dienst N.S., 21: 93--167. Koopstra, R., 1962. Een vergelijking van de geologische indelingen van Noordoostpolder en Oostelijk Flevoland. Rapp. Kartering Oostelijk Flevoland na Droogvalling, 1 0 : 2 1 PP. Lamb, H.H., 1961. Atmospheric circulation and climatic changes in Europe since 800 A.D. INQUA (Warsaw, 1961), 2: 291--318. Reineck, H.-E. and Singh, I.B., 1973. Depositional Sedimentary Environments. Springer, New York, N.Y., 439 pp. Van Loon, A.J. and Wiggers, A.J., 1975a. Holocene lagoonal silts (formerly called "sloef") from the Zuiderzee. Sediment. Geol., 13: 47--55. Van Loon, A.J. and Wiggers, A.J., 1975b. Litho-, bio-, and chronostratigraphy of the Holocene Dutch "sloef" (Almers Member of the Groningen Formation). Meded. Werkgr. Tert. Kwart. Geol., 12: 3--24. Van Loon, A.J. and Wiggers, A.J., 1975c. Composition and grain-size distribution of the Holocene Dutch "sloef" (Almere Member of the Groningen Formation). Sediment. Geol., 13: 237--251. Wiggers, A.J., 1955. De wording van het Noordoostpoldergebied. Thesis, Vrije Universiteit van Amsterdam, 214 pp. Zuur, A.J., 1948. Over de bodemkundige gesteldheid van de zuidelijke kom van het IJsselmeer. Intern Rapp. Directie Wieringermeer (Noordoostpolderwerken), Kampen, 8 PP.
E R O S I O N A L F E A T U R E S IN THE L A G O O N A L A L M E R E MEMBER ( " S L O E F " ) O F THE G R O N I N G E N F O R M A T I O N (HOLOCENE, CENTRAL N E T H E R L A N D S )
A.J. VAN LOON and A.J. WIGGERS
Instituut voor Aardwetenschappen der Vrije Universiteit, Amsterdam (The Netherlands) (Received February 2, 1975; revised and accepted March 20, 1975)
ABSTRACT Van Loon, A.J. and Wiggers, A.J., 1975. Erosional features in the lagoonal Almere Member ("sloef") of the Groningen Formation (Holocene, central Netherlands). Sediment. Geol., 13: 253--265. Various erosional features in the lagoonal Almere Member stress the importance of wave action and bottom currents for the depositional conditions in the lagoonal environment. Two erosive structures, named "peat hole" and "rocking hole" are briefly described. These structures have not yet been mentioned in the international literature.
INTRODUCTION
The so-called "sloef", which forms the Almere Member of the Groningen Formation (Van Loon and Wiggers, 1975b), is a young Holocene (0--1600 A.D.) lagoonal sediment. It is exposed in ditches in the reclaimed polders of the IJsselmeer (the former Zuiderzee) (Fig. 1). The average grain size is small: most of the grains are encountered in the fraction 16--50 pm (Wiggers, 1955; Van Loon and Wiggers, 1975a,c). Nevertheless, the depositional environment cannot be consfdered to have been continuously quiet, since various expressions of erosional forces can be observed. A period with strong erosion existed at a b o u t 1200 A.D. In many locations the previously deposited sediments of this member have been removed entirely. This fact gave rise to the -- n o w abandoned -- idea that sedimentation started as late as 1200 A.D. instead of a b o u t 0 A.D. The sediment that was formed during this erosive period consists frequently of a relatively coarse layer (lag deposit) with shells, pebbles and w o o d fragments, and it is called the Valvata layer. It belongs to the bed with code S1 IIIa. Other indications for strong erosion have been found in erosion holes in the underlying peat (peat holes). Another indication is given by the erosion
254
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BELGIUM 0 i
30 I
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/~r~ " / ~ ". "OOSTELIJK'
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,t~') - f
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10
20
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Fig. 1. Location of the reclaimed polders in the IJsselmeer area.
(due to wave action) of peat areas, surrounding the lake. In this paper this p h e n o m e n o n is called "coastal erosion". The latter two types o f erosion are rarely found. Usually the erosive forces resulted in minor effects, such as scour-and-fill structures, which tend to be of a small size. Although the distinction between erosional and abrasional p h e n o m e n a in a lagoonal environment is a rather arbitrary one, some abrasional features will be described in a f o r t h c o m i n g paper. It will deal with the abrasion of the undulating Pleistocene sandy subsoil, o f out croppi ng glacial till, and of Late Weichselian river dunes. COASTAL EROSION Detailed mapping o f the Almere Member (Bodemkundige Code- en Profielenkaart van de Noordoostpolder, 1947--1956; Bodemkundige Code- en Profielenkaart van Oostelijk Flevoland, started in 1968) showed that the various beds o f this memb e r are n o t present everywhere. In some cases this may be due to non-deposition, e.g. caused by t o o high a position of older sediments
255
Fig. 2. Fossilized peat "cliff" (dark). The contemporaneous lagoonal sediments consist of peat detritus (lower right) and finely laminated humic material (middle right). Due to compaction, the surface of the peat now lies at a lower level than that of the corresponding laminated siliciclastic deposits. Both are overlapped by a younger bed of the Almere Member (bent dark layer).
(Zuur, 1948); in other cases this may be the result of erosion. The latter is the case, for instance, with large parts o f the beds S1 III b and S1 III a, which may only be represented by the Valvata layer. A third reason for the local absence of some beds is related to the extension of the lagoon in which these sediments were deposited. The lagoon was located within a peat area, the borders of which were continuously eroded by wave action. By this process the extent of the lagoon increased, and y o u n g beds overlapped the remnants of the underlying peat layer. At some localities, a fossilized peat " c l i f f " has been observed in the sides of ditches. In such cases peat blocks have been found, t h a t were obviously eroded from the cliff. At present these blocks are embedded in the siliciclastic lagoonal sediment (Fig. 2). EROSIONAL SURFACES
During the sedimentation of the Almere Member the erosion was n o t restricted to lateral directions, but it also t o o k place vertically. Usually the b o t t o m erosion was of only local importance. Nevertheless, a few periods existed in which considerable erosion affected relatively large parts of the lagoonal area. This resulted in erosional surfaces t h a t are characterized by a
256
lag deposit, since the finer particles were carried off in suspension. The lag deposit represents a rather thick sediment, since the originally deposited material must have consisted (just like the other parts of this member) mainly of clay and silt (Wiggers, 1955; Van Loon and Wiggers, 1975a,c). The occurrence of such erosional surfaces is assumed to be the result of changes in the hydrographic conditions. An analogous situation existed after the construction of the Afsluitdijk (Enclosing Dam) in 1932, when these conditions changed very rapidly. In some parts of the IJsselmeer this resulted in fairly severe erosion of the marine Zuiderzee Member (1600--1932 A.D.), while in other parts the eroded material was redeposited as the fresh to slightly brackish IJsselmeer Member. The position of shells of Mya arenaria, visible at the surface (Fig. 3), gives an indication for the rate of erosion between 1932 and the m o m e n t at which the polder Oostelijk Flevoland fell dry (1957). These molluscs lived at a depth of a b o u t 10 cm below the surface. One of the lag deposits within the Almere Member can be traced over a vast area in the Noordoostpolder. Since this coarse layer contains abundant specimens of the fresh-water snail Valvata piscinalis, it is called the Valvata layer. A photograph showing the typical facies of this band has been pub-
Fig. 3. Erosional surface in Oostelijk Flevotand, as visible just after reclamation in 1957. Many specimens of Mya arenaria are present, still in living position. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
257
Fig. 4. Depression in the Valvata layer with abundant specimens of Valvata piscinalis {small) and Unio turnidus (large).
lished earlier (Van Loon and Wiggers, 1975c). Apart from the Valvata shells, the Valvata layer contains many other larger particles, especially w o o d fragments that were derived from the underlying wood-containing peat, large shells of the fresh-water mussel Unio tumidus, and pebbles. The strong erosional forces that affected the lagoonal b o t t o m left their traces in the form of frequent depressions, in which the Valvata layer (usually 1--3 cm thick) may reach a thickness o f several decimeters (Fig. 4). In cross-section these depressions resemble channels, b u t thorough investigations revealed that they do n o t have an elongated character. Dating by means of archaeological finds indicates that this layer was formed at a b o u t 1200 A.D. (Van Loon and Wiggers, 1975b). Ente (1973) correlated the formation of the Valvata layer with a breakdown in 1170 A.D. of a peat barrier between parts of the lagoon. According to this author, this would have led to a rapid burial of the living gastropods b y a layer of peat detritus. We are of the opinion, however, that changing hydrographic conditions were the main factor in the genesis o f the Valvata layer, as is indicated by the lag-deposit character. All these developments were possibly the result of a change in climate that started at that time, leading towards the "Minor Ice Age" (Lamb, 1961). Simultaneously an eastward shift of the western coast of The Netherlands t o o k place, leading to a deepening of the coastal profile. The sand that became available by this process was used for the formation of the Younger Dunes (Jelgersma et al., 1970).
258
Fig. 5. Decapitated contorted structures. This indicates abrasion by bottom currents. Similar lag deposits, although thinner and more locally developed, are also present. Especially on the boundary between the beds S1 II a and S1 I b (Wiggers, 1955; Van Loon and Wiggers, 1975b), these features have frequently been observed during our recent investigations. Erosional and abrasional phenomena on a smaller scale are abundant, as is shown by innumerable "decapitated" structures (Fig. 5). PEAT HOLES In places where the Almere Member directly overlies peat, it sometimes can be observed that parts of the peat have been removed as a kind of blocks. This resulted in steep pits in the b o t t o m , which have been described in Dutch reports as "veenkuilen". We propose to name such phenomena "peat holes" (Fig. 6). The same erosional process occurred during the time of deposition of the overlying Zuiderzee Member, as is shown by the sediments within the holes. The dating of these pits as more or less contemporaneous with the filling up is confirmed by recent observations in these polders: shortly before the polders were reclaimed, various canals were excavated, which appeared to have been filled up for the greater part at the m o m e n t the polders came dry. This indicates a very rapid sedimentation in depressions, caused by b o t t o m currents that continuously transport material, until it is trapped in depressions. The occurrence of peat holes is restricted to a few areas, and they are best known from the northern part of the Noordoostpolder, from the areas around the former islands of Urk and Schokland, and from parts of Oostelijk Flevoland, south of Ketelhaven.
259
Fig. 6. Peat hole with finely laminated sediments inside. Bent contacts may be caused by subsequent differential compaction. Note that material was supplied from the right, as shown by the uppermost laminae. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
Fig. 7. Various shapes of peat holes• Compare with Fig. 8 for reconstruction of their genesis. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
260
The sediment in the holes usually is thinly laminated. The laminae end rather abruptly against the sides of the holes, although differential compaction afterwards may have caused a slightly bent contact (Fig. 6). The nearly horizontal position of the laminae indicates that these depressions are n o t the result of a postsedimentary disturbance, e.g. load casting. The erosional origin is confirmed by the shape of the holes, which shows many variations. Some are more or less rectangular, while others are more or less rounded in diameter (Fig. 7). Based on observations like those in Fig. 7, Wiggers (1955) published a figure, in which he showed the genesis of some steep, rectangular holes near Urk (Fig. 8). It should be noted, that only a small part of the holes can be explained in his way. The peat area near Urk was covered by a thin clay layer, on which sand lenses were locally deposited. This sand, called Urkzand, was derived from abraded glacial till. According to Wiggers' theory, the peat subsided (due to
4i
metres b e l o w surface
0
5
I
I
lOm f
Fig. 8. Genesis of rectangular peat holes near Urk. After Wiggers (1955).
261
compaction) at the depositions] sites of the sand lenses. This led to a synsedimentary subsidence of the depositional surface, resulting in a continuation of the sand deposition. This phenomenon will be dealt with in more detail in a forthcoming paper. Due to the synsedimentary subsidence the peat and clay layers around the sand lenses were pushed up, and the resulting higher parts were decapitated by abrasion afterwards. As a result of the rigidity of the peat during this folding, vertical cracks were formed, which facilitated subsequent erosion of the peat. The holes that were formed in this way were rapidly filled up. Another argument for the erosion of large peat blocks is the presence of such blocks, embedded elsewhere in the Almere Member in the siliciclastic beds. In many cases, however, the blocks would have been totally destroyed during transport, yielding peat detritus for the many humic laminae in this sediment.
Fig. 9. Irregular lamination, showing small current ripples and scour-and-fill structures.
262
The peat holes were, as indicated above, rapidly filled with bed-load material. B o t t o m currents, however, sometimes may have been too erosive, and possibly eddies may have been formed in the holes. The rounded character of some holes is thought to be the result of the erosional action by eddies. SCOUR-AND-FILL STRUCTURES
One of the characteristics of the Almere Member is the fine parallel lamination (Koopstra, 1962; Van Loon and Wiggers, 1975c). In many places, however, this rather quiet picture is disturbed by the frequent occurrence of small wash-outs and current ripples (Fig. 9). Although such structures are most typical of channel bottoms, they may be formed under certain conditions when water flows over an unconsolidated sediment surface (Reineck and Singh, 1973). This latter origin must be assumed for the struct~Lres found in these deposits. They resemble the structures described by Reineck and Singh in several respects, e.g. the inclined position of the filling-in laminae and their size (a few cm to several m). Reconstruction of paleocurrent directions is, however, in most cases impossible, since an elongated character is rare. The few cases that might be impor-
Fig. 10. Superposition o f various scour-and-fill structures. These are usually filled in laterally, as is shown by the lamination (dark parts have a high c o n t e n t o f h u m i c material).
263
30
20
'i
.:'" ":~
o
~
0
'''~"°f'~t~l~t'~'''"~m' 50
,
, 100
. . . .
I 150
,
,
,
,
I 200
,
,
,
,
i 250
Fig. 11. Diagram showing relationship between depth and width of scour-and-fill structures in the Almere Member.
tant for the determination of these directions will be dealt with in a future paper. The scour-and-fill structures occur quite frequently in all sections t h a t were investigated (Van Loon and Wiggers, 1975a), either in an isolated position, or in groups. It seems t h a t the presence of such structures has favoured the conditions for the formation of new ones, since t h e y are often f o u n d eroding each other (Fig. 10). Even in these cases, where the various structures may have fairly different sizes, the d e p t h / w i d t h ratio is rather constant (Fig. 11). Since observations were mainly made in the sides of ditches, it was in most cases impossible to check whether some larger structures should be considered as scour-and-fill structures or as channels. It is probable, however, t h a t real channels are a very rare p h e n o m e n o n in the Almere Member. ROCKING HOLES
Erosive depressions in the m u d d y surface sediment of the lagoon were n o t only formed by b o t t o m currents, but also by objects lying u p o n the b o t t o m . These tool marks should be considered as intermediate between stationary and moving tool marks, for the resulting structures were caused by a rocking movement of the objects, due to wave action. There is no argument for a considerable transport of the objects.
264
Fig. 12. Rocking hole with large peat fragment at the bottom. This block may have been formed during coastal erosion, or during the formation of a peat hole. Photograph by the "Rijksdienst voor de IJsselmeerpolders".
Various kinds of objects may be involved. In most cases it concerns peat blocks (Fig. 12), that were formed by either coastal erosion, or during the formation of peat holes. Other objects that have been encountered are shipwrecks (e.g. Van Loon and Wiggers, 1975c, fig. 5), w o o d fragments (branches and trunks), clay balls, and even large shells. All these objects seem to have been light enough to be rocked b y wave action, b u t t o o heavy to be dragged over the b o t t o m . The occurrence of really heavy objects like pebble~ (Van Loon and Wiggers, 1975c) never led to such structures, b u t did cause load casting. The structure that is shown in Fig. 12 must have been formed in the following way. During the sedimentation of the Almere Member an eroded peat block (1) was deposited at the lagoonal b o t t o m . This happened at level (2). After deposition, the peat block was exposed to wave action, which resulted in a rocking movement of the block. Due to this process the base of the peat block became rounded, which in its turn facilitated the rocking movement during the next phases of wave action. This movement resulted in the formation of an erosive hole (3), that was somewhat larger than the peat block itself. Bumping of the block against the walls of the hole led to steep, sometimes even overhanging sides (4). The eroded sediments, as well as the eroded peat of the base of the block, were partly removed and partly deposited in the hole itself (5). When the hole had become so deep that wave action no longer had an important influence, the rocking action stopped, and
265
the hole gradually became filled up with new lagoonal sediments (6).' After all these processes compaction t o o k place. The youngest sediments in the hole (6) that consist of nearly pure clay and silt were especially compacted. This process led to a downward bending of the sediments that cover the rocking hole (7). The presence of large objects on a b o t t o m consisting mainly of silt and clay is rare in the geological record. This may be the reason w h y this structure has (as far as we know) never been described before. We propose to name this kind of tool mark "rocking hole". In our opinion, a lagoon is the most favourable environment for this phenomenon, although it may occasionaily be found in other environments (fluvial, shallow marine, lacustrine) as well. ACKNOWLEDGEMENTS
The authors wish to express their thanks to the "Rijksdienst voor de IJsselmeerpolders" for permission to reproduce some of their photographs.
REFERENCES Bodemkundige Code- en Profielenkaart van de Noordoostpolder, 1947--1956. Directie van de Wieringermeer (Noordoostpolderwerken), Kampen. Bodemkundige Code- en Profielenkaart van Oostelijk Flevoland, started in 1968. Rijksdienst voor de IJsselmeerpolders, Lelystad. Ente, P.J., 1973. De IJsseldelta. Kamper Almanak, 1973: 137--164. Jelgersma, S., De Jong, J., Zagwijn, W.H. and Van Regteren Altena, J.F., 1970. The coastal dunes of the western Netherlands; geology, vegetational history and archeology. Meded. Rijks Geol. Dienst N.S., 21: 93--167. Koopstra, R., 1962. Een vergelijking van de geologische indelingen van Noordoostpolder en Oostelijk Flevoland. Rapp. Kartering Oostelijk Flevoland na Droogvalling, 1 0 : 2 1 PP. Lamb, H.H., 1961. Atmospheric circulation and climatic changes in Europe since 800 A.D. INQUA (Warsaw, 1961), 2: 291--318. Reineck, H.-E. and Singh, I.B., 1973. Depositional Sedimentary Environments. Springer, New York, N.Y., 439 pp. Van Loon, A.J. and Wiggers, A.J., 1975a. Holocene lagoonal silts (formerly called "sloef") from the Zuiderzee. Sediment. Geol., 13: 47--55. Van Loon, A.J. and Wiggers, A.J., 1975b. Litho-, bio-, and chronostratigraphy of the Holocene Dutch "sloef" (Almers Member of the Groningen Formation). Meded. Werkgr. Tert. Kwart. Geol., 12: 3--24. Van Loon, A.J. and Wiggers, A.J., 1975c. Composition and grain-size distribution of the Holocene Dutch "sloef" (Almere Member of the Groningen Formation). Sediment. Geol., 13: 237--251. Wiggers, A.J., 1955. De wording van het Noordoostpoldergebied. Thesis, Vrije Universiteit van Amsterdam, 214 pp. Zuur, A.J., 1948. Over de bodemkundige gesteldheid van de zuidelijke kom van het IJsselmeer. Intern Rapp. Directie Wieringermeer (Noordoostpolderwerken), Kampen, 8 PP.