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Dutch tidal flats
Sedimentary Geology, 18 (1977) 13--23 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
13
DUTCH TIDAL FLATS
J.D. DE JONG
Department of Soil Science and Geology, Agricultural University Wageningen, Wageningen (The Netherlands) (Received November 24, 1976)
ABSTRACT De Jong, J.D., 1977. Dutch tidal flats. Sediment. Geol., 18: 13--23. Physiographic elements and their sediments of the tidal-flat environment are given both from the classical Wadden Sea in the northern part of The Netherlands and from the estuarial areas in the southwest. Sedimentary processes are briefly mentioned. Sediments of both areas are supplied by incoming currents from the North Sea, which in their turn receive sediment from the rivers Rhine and Meuse. Sediments show fining and coarsening-upward sequences with about similar probabilities of occurrence; these types of sequence have been observed next to each other both in lateral in vertical sections. The Dutch coast shows two different types of coastal elements, i.e. coastal barriers in the western part and cheniers in the northern area, bordering the Wadden Sea. The present Wadden Sea shows a relation between the frequency of channels in the tidal-flat areas and the tidal inlets between the barrier islands; channels are less distinctly developed along coasts where barrier islands are lacking. This finding has consequences in the sedimentary geology of fossil tidal-flat environments.
INTRODUCTION
Interpretation of tidal-flat deposits in the geological record requires a knowledge of sedimentation patterns and sedimentary sequences in different recent tidal-flat environments. Salient aspects of the results of detailed investigations in the tidal flats of the Netherlands coastal area by Van Straaten in the 1950's and by Terwindt and several others during the last ten years are synthesized in the present paper. The main emphasis of Van Straaten's (1954a) investigations was on the topstratum tidal-flat and salt-marsh deposits in the Wadden Sea, although channel deposits have not been neglected. Terwindt (1971, 1975) studied the tidal channels and subtidal parts of shoal deposits in the estuarine environment of the southwestern Netherlands, and concentrated on analysis of vertical sequences. Van Straaten's observations covered the Wadden Sea and the southwestern area (Fig. 1). To state that Terwindt's findings also apply to the Wadden Sea would go t o o far at this stage.
14 The object o f the present review of part of the literature on the Dutch tidal sediments is t o p u t t oge t he r some data of an e n v i r o n m e n t of which the tidal character is indisputable. The data presented may serve as one of the starting points in the discussions on the presumed tidal character of ancient sediments. SEDIMENTARY ENVIRONMENT The Dutch coast forms part of an area with storm-wave and meso-tidal environments, with semi-diurnal tides (Davies, 1972). West winds are dominant. Tidal ranges vary along the Dutch coast; from 170 to 240 cm at one of the western Wadden islands, and from 250 to 320 cm at the m o u t h of an estuary in the northeast; from 120 to 180 cm only along the central part of the Dutch coast; f r om a p p r o x i m a t e l y 300 cm at neap tide to 450 cm at spring tide at the m o u t h of one of the estuaries in the southwest, the West Scheldt, whereas in the most inward parts the tidal ranges are 550 cm. Three different subenvironments may be distinguished in the tide-dominated environments: (a) the supratidal subenvironment, above mean high tide: salt marshes with a dense h al ophyt i c vegetation; (b) the intertidal subenvironment, between mean low and high tides: tidal flats proper; (c) the subtidal subenvironment, below mean low tide: channels, sandbars and shoals below the water surface, and tidal deltas on b o t h sides of the tidal inlets. Alternation of silt--sand and clay--silt--sand layers with a wavy lamination is very c o m m o n in the salt marshes. The tidal flats, channels and subtidal deposits vary f rom pure sand and alternations of sand and mud to pure mud. The sandy layers show laminations and small- and large-scale ripple bedding. Flaser and lenticular bedding (Reineck, 1975) is very c o m m o n ; climbing ripples are seldom present. Du tch tidal flats occur in two separate areas: the Wadden Sea in the n o r t h e r n part and the estuaries in the southwestern part of the Netherlands (Fig. 1). The Wadden Sea forms part of the shallow part of the North Sea along the Dutch, German, and Danish coasts, extending over a distance of a p p r o x i m a t e l y 450 km. The Wadden Sea is separated from the North Sea and sheltered by a chain of islands occurring 25 km from the mainland in the west to 8 km in the east. The Wadden Sea is c o m p o s e d of a series of tidal basins separated by watersheds. The surface areas of the tidal basins range up to 820 km 2. The ebb currents in the Wadden Sea converge into the relatively narrow tidal inlets, 3--5 km in the western and 5--8 km in the eastern part, and reaching depths up to 30 m. The channels in the tidal inlets reach deeper levels than the adjacent sea b o t t o m . The tidal inlets of the Wadden Sea branch into channels away f r om the inlets, and end finally against the watersheds b etween the different tidal basins. The channels do n o t show a regular
15
tida~ fiats s.s. coastal dunes c0
0
20 km
/
AFnst@FaamL~
J
~
t
~
(3
\
Fig. 1. Tidal flats proper in the Netherlands. In the Wadden Sea in the north, and the estuaries, in the south west, the tidal flats proper have been indicated in black; salt marshes have been omitted. The coastal dunes only show another young (after 1200 A.D.) coastal feature.
shallowing because bifurcations in relatively deep tidal wedges are very common. The estuaries in the southwestern Netherlands show a shape and orientation, with respect to prevailing westerly winds, different from the Wadden Sea. Moreover, there is an important supply of fresh water from the rivers Rhine and Meuse and a subordinate a m o u n t from the river Scheldt. The axes o f the estuaries are transverse to the coast. Estuaries are up to 22 km in width at their m o u t h s and depths o f 40 m are c o m m o n . The map of the tidal areas in the north and southwest (Fig. 1) reveals that supratidal and intertidal areas cover a larger part in the northern area: water-
16
~2 ~5
d r~
•
J
--x
Fig. 2. Coastal d e p o s i t i o n a l e n v i r o n m e n t s a r o u n d 7000 B.P., simplified a f t e r Zagwijn (1974). 1 = P l e i s t o c e n e a n d o l d e r f o r m a t i o n s , a n d H o l o c e n e fluvial d e p o s i t s at t h e surface; 2 = p e a t ; 3 = coastal barriers, p r e s u m e d ; 5 = intertidal and lagoonal deposits.
shed areas, on the landward side of the western Wadden islands, and the entire Wadden Sea farther east belong to this realm, whereas similar areas are more or less limited to the sheltered and most landward parts in the southwestern estuaries. Apparently, the estuarine environment with its fluvial outflow and the relatively large tidal ranges does n o t have the quiet conditions required for the development of tidal flats. Sedimentological properties of tidal deposits in both areas show t h a t great lateral variations in lithofacies occur; a particular layer cannot be traced over any significant distance. Changes in the channel patterns may explain this; shifting of channels is strongest where sandy material is found in the tidal basins which may in its turn be caused by displacements of channels near the inlets. Channels show a more or less fixed position in the inner parts of the tidal basins where incision occurs in less erodible layers of the salt marshes or
17
of older peat and clay layers. Displacement of channels implies strong reworking of the adjacent sections. Therefore, thicknesses of tidal deposits are mainly determined by the depth of channels and this, in turn, results in seaward increase in the thickness of the deposits. The tidal-flat sediments are predominantly fine-grained sands with less than 5% of silt and clay, and less than 10% medium-grained sand. Sorting is very good because of the endless repetition of selective processes in tidal currents and waves. Sorting is poorer when there are larger percentages of clay and silt. Less perfect sorting may also be brought about by the supply of comparatively poorly sorted material, e.g. by erosion of channel bottoms of older, glacial or fluvioglacial formations in the Wadden Sea, and by the
]1 3~~5~42 ~-~
[
~'~:~iiiiiii:~i:~i!iii~ :...........................~::i~:~i:~:~T:~:~:=~i
---~-~
~~::~:~:~:~~::-:: ~.
J
2 r-./
i
Fig. 3. Coastal d e p o s i t i o n a l e n v i r o n m e n t s a r o u n d 4;~O0 B.P., simplified a f t e r Zagwijn (1974). 1 = P l e i s t o c e n e and o l d e r f o r m a t i o n s , and H o l o c e n e fluvial d e p o s i t s at t h e surface; 2 = p e a t ; 3 = coastal barriers, p r e s u m e d ; 4 = coastal barriers; 5 = i n t e r t i d a l and lagoonal deposits.
18
alternating deposition of coarser and finer sediment during successive stages of tides. The study of Holocene sediments in the coastal area of the western Netherlands has revealed that under transgressive conditions tidal sedimentation has taken place in lagoons landward of coastal barrier systems roughly between 7 0 0 0 and 4 0 0 0 years B.P. (Figs. 2, 3). Regression with the subsequent development of a peat cover terminated these conditions. In the northern coastal area a similar sequence of events took place but tidal sedimentation continued until about two thousand years later (Roeleveld, 1974; Figs. 4, 5). The present Wadden Sea is, geologically, a very young feature. Less than
,ql/~, H c o A S T A L
BARRIERS ~
..............
~2 ~4 ~5
_
, I~'
:t_
]
/
Fig. 4. Coastal depositional e n v i r o n m e n t s around 3 4 0 0 B.P., after Pons et al. ( 1 9 6 3 ) and Roeleveld ( 1 9 7 4 ) . 1 = Pleistocene and older f o r m a t i o n s , and H o l o c e n e fluvial deposits at the surface; 2 = peat; 3 = coastal barriers, presumed; 4 = coastal barriers; 5 = intertidal and lagoonal deposits.
19
ARRIERS
~2 ~s ~4 ~s
?
2
Fig. 5. Coastal d e p o s i t i o n a l e n v i r o n m e n t s around 2 0 0 0 B.P., after Pons et al. ( 1 9 6 3 ) and R o e l e v e l d ( 1 9 7 4 ) . 1 = P l e i s t o c e n e and older f o r m a t i o n s and H o l o c e n e fluvial d e p o s i t s at the surface; 2 =peat; 3 = coastal barriers, p r e s u m e d ; 4 = coastal barriers; 5 = intertidal and lagoonal deposits.
one thousand years ago large parts of the present Wadden Sea still formed part of an inhabited peat country. Storm floods during the 14th century inundated and eroded large parts of these areas and several of the present embayments originated. Possibly the construction of dikes has decreased the possibilities of flooding of parts of the hinterland; consequently floods and currents were stronger in the area seaward of the dikes. Comparison of old and new maps of the Wadden Sea shows that progradational sedimentation has been the result. Studies on the evolution of the Netherlands coastal area (Pons et al., 1963) show that the pattern of the southwestern estuarine area is the result of storm floods during short transgressive periods in the last three thousand years {Hageman, 1969).
20 SEDIMENTARY PROCESSES The main processes of sedimentation are (Van Straaten, 1954a): (1) Vertical filling of channels, gullies and creeks. The deposits are sandy, with a pronounced m u d d y composition in many cases. (2) Lateral deposition. Channels are displaced by lateral or downstream migration of meanders. Cohesiveness of sediment in mud flats makes channels meander; sand flats show braided gully courses. (3) Vertical deposition. The upward growth of horizontal parts of tidal flats is of the same order as the relative rise of sea level. (4) Trapping of sediment by plants. This process is evident on salt marshes, natural levees have been developed along creeks. (5) Concentration of sediments by suspension feeders. Concentration of fine-grained suspended sediment increases landwards in the Wadden Sea; consequently finer grained sediments are concentrated in this part of the environment. Postma's theory (1954), further elaborated b y Van Straaten and Kuenen (1957) and Postma (1961) explains the landward transport of suspended matter as being due to the inward decrease of tidalcurrent velocities and depths combined with settling and scour-lag effects. PROVENANCE OF SEDIMENTS Wadden Sea sediments have n o t been supplied b y the few small rivers debouching into this sea. Mineralogical investigations (Crommelin, 1940, 1945; Favejee, 1951) show that sand-, silt- and clay-sized material has been supplied from the North Sea b o t t o m . The same holds true for the southwestern estuaries. Little sand brought down by the rivers Rhine and Meuse has reached the North Sea in recent times (Terwindt et al., 1963). Rather, the sand of the Wadden Sea is being derived from the shallow zones of the North Sea adjacent to the Wadden islands, and the beaches and dunes of these islands. Samples from the Wadden Sea and an estuary contain open marine forams and shell debris derived from offshore (Van Straaten, 1956). Certain fossils in the sediments are indigenous to the tidal-fiat environments, namely, shells and silica skeletons (diatoms). Analyses of manganese and mercury contents of sediments have revealed that only minor quantities of the m u d in the West Scheldt (one of the southwestern estuaries) was supplied by the river Scheldt. The major portion comes from the North Sea. The mud brought into the Wadden Sea is transported directly by longshore currents in a narrow zone along the coast of the western Netherlands (De Groot, 1964; De Groot et al., 1971). THE TIDAL-FLAT SEDIMENTARY SEQUENCE AND DEPOSITIONAL HISTORY Fining-upward sequences have been found for a migrating estuarine channel in one of the southwestern estuaries (Oomkens and Terwindt, 1960).
21
However, more work on this subject has revealed 'that there is little difference in the frequency distribution of fining- and coarsening-upward sequences of lithofacies in the different areas.' So from the present data it is impossible to conclude that inshore and tidal-inlet deposits are characterized b y finingupward sequences of lithofacies. Coarsening-upward sequences have almost the same probability of occurrence. It was c o m m o n l y observed that 'finingand coarsening-upward sequences alternate above each other.' (Terwindt, 1971, p. 521) The same author subsequently concluded that 'the vertical succession of l i t h o f a c i e s . . , may be nonsequential (with sharp upper and lower boundaries of the lithofacies) or sequential (with gradual transitions). Most of them are fining-upward. However, coarsening-upward and other types of sequences are also observed.' (Terwindt, 1975, p. 89). During an earlier phase of research Van Straaten (1963) established that in tidal-flat topstratum deposits the lateral migration of channels or silting up of whole areas may result in changes of the environment during the course of time. This process may lead to vertical sections consisting from b o t t o m to t o p of sandy channel-floor deposits, overlain by more clayey low tidal-flat sediments and more sandy high tidal-flat material followed b y clayey marsh deposits. Reconstruction of coastal landscapes with the help of detailed studies of subrecent Holocene deposits in parts of the Netherlands coastal area has shown that tidal-flat sequences, mudflats, and marshes with decreasing marine influence, have evolved into slightly brackish swamps. Reeds eventually start growing, indicating a final rapid freshening of the environment (Van Straaten, 1957; Roeleveld, 1974). For additional references on this subject see De Jong (1971). Relative rise of sea level in the Netherlands coastal areas has played an important role in the development of coastal barriers in the western part of the country. Because of continuous reworking by the transgressing sea, a new barrier was formed when the rate of relative sea-level rise was retarded (De Jong, 1971). Along the northeastern part of the coast conditions were somewhat different. The 'lateral extension of the salt marsh was interrupted several times during transgressive phases. During these transgressive phases the salt marsh was temporarily pushed back and covered by its own erosional p r o d u c t s . . . On the basis of their origin and their situation on top of the original marsh the ridges must be defined as cheniers' (Roeleveld, 1974, p. 140). A landscape with ridges and swales parallel to the present coast of the Wadden Sea, with the mainland representing cheniers, is the result of this development. A schematic section through the Wadden Sea deposits shows two phases of deposition: an older one and the present one. The former originated when sea level was still lower. After the silting up of inlets at the end of the first phase salt-marsh deposits and peat layers were formed. The present Wadden Sea may be considered as the somewhat deeper part of a coastal area in which the supply of fresh water hardly influences the
22
nature of the sedimentary environment. Clastic material is supplied through tidal creeks as the minor inner branches of large tidal-channel systems. These latter channels entered the area inside the barrier belt through the tidal inlets. The Wadden Sea has been an environment of continuous deposition of tidal-flat sediments interrupted only by regressive phases which are manifested in the form of peat layers. About one thousand years ago man began to build dikes and the landward part of the tidal-flat environment did not undergo any further accretion. SEDIMENTARY SYNTHESIS
Sequential analyses of the tidal-flat deposits give some clue as to the sedim e n t a r y environment. After taking into account some degree of compaction of the primary thickness, the thickness of individual sequences roughly agrees with the depth of the tidal channels. This depth is, in turn, a function of the tidal ranges which are larger in marine funnel-shaped embayments where extreme values may be reached whereas tidal ranges along open oceanic coasts are less. The mineralogical composition of the tidal sediments reflects the composition of the unconsolidated sediments on the adjacent sea floor. It is evident from the map (Fig. 1) that channels in the Wadden sea are more or less extensions of the tidal inlets in much the same way as Shinn et al. (1969) have described from the Bahamas. Location of inlets therefore controls channel location. Extending this observation it is proposed that channel frequency on a tidal flat may be a function of a protective barrierisland chain with tidal inlets whereas, along open coasts channels should be developed in a much less distinctive manner. REFERENCES C r o m m e l i n , R.D., 1 9 4 0 . De h e r k o m s t van h e t zand van de W a d d e n z e e . Tijdschr. K. Ned. Aardrijksk. G e n o o t . , 57: 3 4 7 - - 3 6 1 . C r o m m e l i n , R.D., 1 9 4 3 . De h e r k o m s t van h e t W a d d e n s l i b m e t k o r r e l g r o o t t e b o v e n 10 m i c r o n . V e r h . Geol. M i j n b o u w k . G e n o o t . , Geol. Ser., 13: 2 9 9 - - 3 3 3 . Davies, J.L., 1972. G e o g r a p h i c a l v a r i a t i o n in coastal d e v e l o p m e n t . In: K.M. C l a y t o n ( E d i t o r ) , G e o m o r p h o l o g y Texts, 4. Oliver a n d B o y d , E d i n b u r g h , 204 pp. De G r o o t , A.J., 1 9 6 4 . Origin a n d t r a n s p o r t of m u d ( f r a c t i o n < 1 6 m i c r o n ) in coastal w a t e r s f r o m t h e Western S c h e l d t t o t h e D a n i s h f r o n t i e r . In: L.M.J.U. van S t r a a t e n ( E d i t o r ) , Deltaic a n d Shallow Marine Deposits. D e v e l o p m e n t s in S e d i m e n t o l o g y , 1. Elsevier, A m s t e r d a m , pp. 9 3 - - 1 0 1 . De G r o o t , A.J., de G o e y , J.J.M. a n d Zegers, C., 1971. C o n t e n t s a n d b e h a v i o u r of merc u r y as c o m p a r e d w i t h o t h e r h e a v y m e t a l s in s e d i m e n t s f r o m t h e rivers R h i n e a n d Ems. In: J.D. de J o n g ( E d i t o r ) , S e d i m e n t o l o g y ; R e s e a r c h o n S e d i m e n t o l o g y a n d Sedim e n t a r y G e o l o g y in t h e N e t h e r l a n d s . Geol. M i j n b o u w , 50: 3 9 3 - - 3 9 8 . De J o n g , ,I.D., 1 9 7 1 . T h e s c e n e r y of t h e N e t h e r l a n d s against the b a c k g r o u n d of H o l o c e n e geology; a review of t h e r e c e n t l i t e r a t u r e . Rev. G6ogr. Phys. G6ol. Dyn., 2 ( 1 2 ) : 1 4 3 - 162.
23 Favejee, J.Ch.L., 1951. The origin of the "Wadden"-mud. Meded. Landbouwhogesch., Wageningen, 51: 113--141. Hageman, B.P., 1969. Development of the western part of the Netherlands during the Holocene, Geol. Mijnbouw, 48: 373--388. Oomkens, E. and Terwindt, J.H.J., 1960. Inshore estuarine sediments in the Haringvliet (Netherlands). In: J.D. de Jong (Editor), Bijdragen tot de geologie en hydrologie van het Deltagebeid. Geol. Mijnbouw, 39: 701--710. Pons, L.J., Jelgersma, S., Wiggers, A.J. and de Jong, J.D., 1963. Evolution of the Netherlands coastal area during the Holocene. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (2): 197--208. Postma, H., 1954. Hydrography of the Dutch Wadden Sea. Arch. N~erl. Zool., 10 (4): 106 pp. (Thesis, Univ. Groningen). Postma, H., 1961. Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res., 1: 148--190. Reineck, H.-E., 1975. German North Sea tidal flats. In: R.N. Ginsburg (Editor), Tidal Deposits; A Casebook of Recent Examples and Fossil Counterparts. Springer, New York, N.Y., pp. 5--12. Roeleveld, W., 1964. The Groningen Coastal Area. Thesis, Vrije Universiteit, Amsterdam, 252 pp. Shinn, E.A., Lloyd, M.R. and Ginsburg, R.N., 1969. Anatomy of a modern carbonate tidal-flat, Andros Island, Bahamas. J. Sediment. Petrol., 39: 1202--1228. Terwindt, J.H.J., 1970. Observation on submerged sand ripples with heights ranging from 30 to 200 cm occurring in tidal channels of S.W. Netherlands. Geol. Mijnbouw, 49: 489--501. Terwindt, J.H.J., 1971. Litho-facies of inshore estuarine and tidal-inlet deposits. In: J.D. de Jong (Editor), Sedimentology; Research on Sedimentology and Sedimentary Geology in the Netherlands. Geol. Mijnbouw, 50: 515--526. Terwindt, J.H.J., 1975. Sequences in inshore subtidal deposits. In: R.N. Ginsburg (Editor), Tidal Deposits; A Casebook of Recent Examples and Fossil Counterparts. Springer, New York, N.Y., pp. 85--99. Terwindt, J.H.J., de Jong, J.D. and van der Wilk, E., 1963. Sediment movement and sediment properties in the tidal area of the Lower Rhine (Rotterdam Waterway). Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (2): 243--258. Van Straaten, L.M.J.U., 1954a. Composition and structure of recent marine sediments in the Netherlands. Leidse Geol. Meded., 19: 1--110. Van Straaten, L.M.J.U., 1954b. Sedimentology of Recent tidal flat deposits and the psammites du Condroz (Devonian). Geol. Mijnbouw, 16: 25--47. Van Straaten, L.M.J.U., 1956. Composition of shell beds formed in tidal flat environment in the Netherlands and in the bay of Arcachon (France). Geol. Mijnbouw, 18: 209-226. Van Straaten, L.M.J.U., 1957. The Holocene deposits. In: L.M.J.U. van Straaten and J.D. de Jong (Editors), The excavation at Velsen. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Set., 17: 158--183. Van Straaten, L.M.J.U., 1963. Aspects of Holocene sedimentation in the Netherlands. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (1): 149--172. Van Straaten, L.M.J.U. and Kuenen, Ph.H., 1957. Accumulation of fine grained sediments in the Dutch Wadden Sea. Geol. Mijnbouw, 19: 329--354. Zagwijn, W.H., 1974. Palaeogeographic evolution of The Netherlands during the Quaternary. Geol. Mijnbouw, 53: 369--385.
13
DUTCH TIDAL FLATS
J.D. DE JONG
Department of Soil Science and Geology, Agricultural University Wageningen, Wageningen (The Netherlands) (Received November 24, 1976)
ABSTRACT De Jong, J.D., 1977. Dutch tidal flats. Sediment. Geol., 18: 13--23. Physiographic elements and their sediments of the tidal-flat environment are given both from the classical Wadden Sea in the northern part of The Netherlands and from the estuarial areas in the southwest. Sedimentary processes are briefly mentioned. Sediments of both areas are supplied by incoming currents from the North Sea, which in their turn receive sediment from the rivers Rhine and Meuse. Sediments show fining and coarsening-upward sequences with about similar probabilities of occurrence; these types of sequence have been observed next to each other both in lateral in vertical sections. The Dutch coast shows two different types of coastal elements, i.e. coastal barriers in the western part and cheniers in the northern area, bordering the Wadden Sea. The present Wadden Sea shows a relation between the frequency of channels in the tidal-flat areas and the tidal inlets between the barrier islands; channels are less distinctly developed along coasts where barrier islands are lacking. This finding has consequences in the sedimentary geology of fossil tidal-flat environments.
INTRODUCTION
Interpretation of tidal-flat deposits in the geological record requires a knowledge of sedimentation patterns and sedimentary sequences in different recent tidal-flat environments. Salient aspects of the results of detailed investigations in the tidal flats of the Netherlands coastal area by Van Straaten in the 1950's and by Terwindt and several others during the last ten years are synthesized in the present paper. The main emphasis of Van Straaten's (1954a) investigations was on the topstratum tidal-flat and salt-marsh deposits in the Wadden Sea, although channel deposits have not been neglected. Terwindt (1971, 1975) studied the tidal channels and subtidal parts of shoal deposits in the estuarine environment of the southwestern Netherlands, and concentrated on analysis of vertical sequences. Van Straaten's observations covered the Wadden Sea and the southwestern area (Fig. 1). To state that Terwindt's findings also apply to the Wadden Sea would go t o o far at this stage.
14 The object o f the present review of part of the literature on the Dutch tidal sediments is t o p u t t oge t he r some data of an e n v i r o n m e n t of which the tidal character is indisputable. The data presented may serve as one of the starting points in the discussions on the presumed tidal character of ancient sediments. SEDIMENTARY ENVIRONMENT The Dutch coast forms part of an area with storm-wave and meso-tidal environments, with semi-diurnal tides (Davies, 1972). West winds are dominant. Tidal ranges vary along the Dutch coast; from 170 to 240 cm at one of the western Wadden islands, and from 250 to 320 cm at the m o u t h of an estuary in the northeast; from 120 to 180 cm only along the central part of the Dutch coast; f r om a p p r o x i m a t e l y 300 cm at neap tide to 450 cm at spring tide at the m o u t h of one of the estuaries in the southwest, the West Scheldt, whereas in the most inward parts the tidal ranges are 550 cm. Three different subenvironments may be distinguished in the tide-dominated environments: (a) the supratidal subenvironment, above mean high tide: salt marshes with a dense h al ophyt i c vegetation; (b) the intertidal subenvironment, between mean low and high tides: tidal flats proper; (c) the subtidal subenvironment, below mean low tide: channels, sandbars and shoals below the water surface, and tidal deltas on b o t h sides of the tidal inlets. Alternation of silt--sand and clay--silt--sand layers with a wavy lamination is very c o m m o n in the salt marshes. The tidal flats, channels and subtidal deposits vary f rom pure sand and alternations of sand and mud to pure mud. The sandy layers show laminations and small- and large-scale ripple bedding. Flaser and lenticular bedding (Reineck, 1975) is very c o m m o n ; climbing ripples are seldom present. Du tch tidal flats occur in two separate areas: the Wadden Sea in the n o r t h e r n part and the estuaries in the southwestern part of the Netherlands (Fig. 1). The Wadden Sea forms part of the shallow part of the North Sea along the Dutch, German, and Danish coasts, extending over a distance of a p p r o x i m a t e l y 450 km. The Wadden Sea is separated from the North Sea and sheltered by a chain of islands occurring 25 km from the mainland in the west to 8 km in the east. The Wadden Sea is c o m p o s e d of a series of tidal basins separated by watersheds. The surface areas of the tidal basins range up to 820 km 2. The ebb currents in the Wadden Sea converge into the relatively narrow tidal inlets, 3--5 km in the western and 5--8 km in the eastern part, and reaching depths up to 30 m. The channels in the tidal inlets reach deeper levels than the adjacent sea b o t t o m . The tidal inlets of the Wadden Sea branch into channels away f r om the inlets, and end finally against the watersheds b etween the different tidal basins. The channels do n o t show a regular
15
tida~ fiats s.s. coastal dunes c0
0
20 km
/
AFnst@FaamL~
J
~
t
~
(3
\
Fig. 1. Tidal flats proper in the Netherlands. In the Wadden Sea in the north, and the estuaries, in the south west, the tidal flats proper have been indicated in black; salt marshes have been omitted. The coastal dunes only show another young (after 1200 A.D.) coastal feature.
shallowing because bifurcations in relatively deep tidal wedges are very common. The estuaries in the southwestern Netherlands show a shape and orientation, with respect to prevailing westerly winds, different from the Wadden Sea. Moreover, there is an important supply of fresh water from the rivers Rhine and Meuse and a subordinate a m o u n t from the river Scheldt. The axes o f the estuaries are transverse to the coast. Estuaries are up to 22 km in width at their m o u t h s and depths o f 40 m are c o m m o n . The map of the tidal areas in the north and southwest (Fig. 1) reveals that supratidal and intertidal areas cover a larger part in the northern area: water-
16
~2 ~5
d r~
•
J
--x
Fig. 2. Coastal d e p o s i t i o n a l e n v i r o n m e n t s a r o u n d 7000 B.P., simplified a f t e r Zagwijn (1974). 1 = P l e i s t o c e n e a n d o l d e r f o r m a t i o n s , a n d H o l o c e n e fluvial d e p o s i t s at t h e surface; 2 = p e a t ; 3 = coastal barriers, p r e s u m e d ; 5 = intertidal and lagoonal deposits.
shed areas, on the landward side of the western Wadden islands, and the entire Wadden Sea farther east belong to this realm, whereas similar areas are more or less limited to the sheltered and most landward parts in the southwestern estuaries. Apparently, the estuarine environment with its fluvial outflow and the relatively large tidal ranges does n o t have the quiet conditions required for the development of tidal flats. Sedimentological properties of tidal deposits in both areas show t h a t great lateral variations in lithofacies occur; a particular layer cannot be traced over any significant distance. Changes in the channel patterns may explain this; shifting of channels is strongest where sandy material is found in the tidal basins which may in its turn be caused by displacements of channels near the inlets. Channels show a more or less fixed position in the inner parts of the tidal basins where incision occurs in less erodible layers of the salt marshes or
17
of older peat and clay layers. Displacement of channels implies strong reworking of the adjacent sections. Therefore, thicknesses of tidal deposits are mainly determined by the depth of channels and this, in turn, results in seaward increase in the thickness of the deposits. The tidal-flat sediments are predominantly fine-grained sands with less than 5% of silt and clay, and less than 10% medium-grained sand. Sorting is very good because of the endless repetition of selective processes in tidal currents and waves. Sorting is poorer when there are larger percentages of clay and silt. Less perfect sorting may also be brought about by the supply of comparatively poorly sorted material, e.g. by erosion of channel bottoms of older, glacial or fluvioglacial formations in the Wadden Sea, and by the
]1 3~~5~42 ~-~
[
~'~:~iiiiiii:~i:~i!iii~ :...........................~::i~:~i:~:~T:~:~:=~i
---~-~
~~::~:~:~:~~::-:: ~.
J
2 r-./
i
Fig. 3. Coastal d e p o s i t i o n a l e n v i r o n m e n t s a r o u n d 4;~O0 B.P., simplified a f t e r Zagwijn (1974). 1 = P l e i s t o c e n e and o l d e r f o r m a t i o n s , and H o l o c e n e fluvial d e p o s i t s at t h e surface; 2 = p e a t ; 3 = coastal barriers, p r e s u m e d ; 4 = coastal barriers; 5 = i n t e r t i d a l and lagoonal deposits.
18
alternating deposition of coarser and finer sediment during successive stages of tides. The study of Holocene sediments in the coastal area of the western Netherlands has revealed that under transgressive conditions tidal sedimentation has taken place in lagoons landward of coastal barrier systems roughly between 7 0 0 0 and 4 0 0 0 years B.P. (Figs. 2, 3). Regression with the subsequent development of a peat cover terminated these conditions. In the northern coastal area a similar sequence of events took place but tidal sedimentation continued until about two thousand years later (Roeleveld, 1974; Figs. 4, 5). The present Wadden Sea is, geologically, a very young feature. Less than
,ql/~, H c o A S T A L
BARRIERS ~
..............
~2 ~4 ~5
_
, I~'
:t_
]
/
Fig. 4. Coastal depositional e n v i r o n m e n t s around 3 4 0 0 B.P., after Pons et al. ( 1 9 6 3 ) and Roeleveld ( 1 9 7 4 ) . 1 = Pleistocene and older f o r m a t i o n s , and H o l o c e n e fluvial deposits at the surface; 2 = peat; 3 = coastal barriers, presumed; 4 = coastal barriers; 5 = intertidal and lagoonal deposits.
19
ARRIERS
~2 ~s ~4 ~s
?
2
Fig. 5. Coastal d e p o s i t i o n a l e n v i r o n m e n t s around 2 0 0 0 B.P., after Pons et al. ( 1 9 6 3 ) and R o e l e v e l d ( 1 9 7 4 ) . 1 = P l e i s t o c e n e and older f o r m a t i o n s and H o l o c e n e fluvial d e p o s i t s at the surface; 2 =peat; 3 = coastal barriers, p r e s u m e d ; 4 = coastal barriers; 5 = intertidal and lagoonal deposits.
one thousand years ago large parts of the present Wadden Sea still formed part of an inhabited peat country. Storm floods during the 14th century inundated and eroded large parts of these areas and several of the present embayments originated. Possibly the construction of dikes has decreased the possibilities of flooding of parts of the hinterland; consequently floods and currents were stronger in the area seaward of the dikes. Comparison of old and new maps of the Wadden Sea shows that progradational sedimentation has been the result. Studies on the evolution of the Netherlands coastal area (Pons et al., 1963) show that the pattern of the southwestern estuarine area is the result of storm floods during short transgressive periods in the last three thousand years {Hageman, 1969).
20 SEDIMENTARY PROCESSES The main processes of sedimentation are (Van Straaten, 1954a): (1) Vertical filling of channels, gullies and creeks. The deposits are sandy, with a pronounced m u d d y composition in many cases. (2) Lateral deposition. Channels are displaced by lateral or downstream migration of meanders. Cohesiveness of sediment in mud flats makes channels meander; sand flats show braided gully courses. (3) Vertical deposition. The upward growth of horizontal parts of tidal flats is of the same order as the relative rise of sea level. (4) Trapping of sediment by plants. This process is evident on salt marshes, natural levees have been developed along creeks. (5) Concentration of sediments by suspension feeders. Concentration of fine-grained suspended sediment increases landwards in the Wadden Sea; consequently finer grained sediments are concentrated in this part of the environment. Postma's theory (1954), further elaborated b y Van Straaten and Kuenen (1957) and Postma (1961) explains the landward transport of suspended matter as being due to the inward decrease of tidalcurrent velocities and depths combined with settling and scour-lag effects. PROVENANCE OF SEDIMENTS Wadden Sea sediments have n o t been supplied b y the few small rivers debouching into this sea. Mineralogical investigations (Crommelin, 1940, 1945; Favejee, 1951) show that sand-, silt- and clay-sized material has been supplied from the North Sea b o t t o m . The same holds true for the southwestern estuaries. Little sand brought down by the rivers Rhine and Meuse has reached the North Sea in recent times (Terwindt et al., 1963). Rather, the sand of the Wadden Sea is being derived from the shallow zones of the North Sea adjacent to the Wadden islands, and the beaches and dunes of these islands. Samples from the Wadden Sea and an estuary contain open marine forams and shell debris derived from offshore (Van Straaten, 1956). Certain fossils in the sediments are indigenous to the tidal-fiat environments, namely, shells and silica skeletons (diatoms). Analyses of manganese and mercury contents of sediments have revealed that only minor quantities of the m u d in the West Scheldt (one of the southwestern estuaries) was supplied by the river Scheldt. The major portion comes from the North Sea. The mud brought into the Wadden Sea is transported directly by longshore currents in a narrow zone along the coast of the western Netherlands (De Groot, 1964; De Groot et al., 1971). THE TIDAL-FLAT SEDIMENTARY SEQUENCE AND DEPOSITIONAL HISTORY Fining-upward sequences have been found for a migrating estuarine channel in one of the southwestern estuaries (Oomkens and Terwindt, 1960).
21
However, more work on this subject has revealed 'that there is little difference in the frequency distribution of fining- and coarsening-upward sequences of lithofacies in the different areas.' So from the present data it is impossible to conclude that inshore and tidal-inlet deposits are characterized b y finingupward sequences of lithofacies. Coarsening-upward sequences have almost the same probability of occurrence. It was c o m m o n l y observed that 'finingand coarsening-upward sequences alternate above each other.' (Terwindt, 1971, p. 521) The same author subsequently concluded that 'the vertical succession of l i t h o f a c i e s . . , may be nonsequential (with sharp upper and lower boundaries of the lithofacies) or sequential (with gradual transitions). Most of them are fining-upward. However, coarsening-upward and other types of sequences are also observed.' (Terwindt, 1975, p. 89). During an earlier phase of research Van Straaten (1963) established that in tidal-flat topstratum deposits the lateral migration of channels or silting up of whole areas may result in changes of the environment during the course of time. This process may lead to vertical sections consisting from b o t t o m to t o p of sandy channel-floor deposits, overlain by more clayey low tidal-flat sediments and more sandy high tidal-flat material followed b y clayey marsh deposits. Reconstruction of coastal landscapes with the help of detailed studies of subrecent Holocene deposits in parts of the Netherlands coastal area has shown that tidal-flat sequences, mudflats, and marshes with decreasing marine influence, have evolved into slightly brackish swamps. Reeds eventually start growing, indicating a final rapid freshening of the environment (Van Straaten, 1957; Roeleveld, 1974). For additional references on this subject see De Jong (1971). Relative rise of sea level in the Netherlands coastal areas has played an important role in the development of coastal barriers in the western part of the country. Because of continuous reworking by the transgressing sea, a new barrier was formed when the rate of relative sea-level rise was retarded (De Jong, 1971). Along the northeastern part of the coast conditions were somewhat different. The 'lateral extension of the salt marsh was interrupted several times during transgressive phases. During these transgressive phases the salt marsh was temporarily pushed back and covered by its own erosional p r o d u c t s . . . On the basis of their origin and their situation on top of the original marsh the ridges must be defined as cheniers' (Roeleveld, 1974, p. 140). A landscape with ridges and swales parallel to the present coast of the Wadden Sea, with the mainland representing cheniers, is the result of this development. A schematic section through the Wadden Sea deposits shows two phases of deposition: an older one and the present one. The former originated when sea level was still lower. After the silting up of inlets at the end of the first phase salt-marsh deposits and peat layers were formed. The present Wadden Sea may be considered as the somewhat deeper part of a coastal area in which the supply of fresh water hardly influences the
22
nature of the sedimentary environment. Clastic material is supplied through tidal creeks as the minor inner branches of large tidal-channel systems. These latter channels entered the area inside the barrier belt through the tidal inlets. The Wadden Sea has been an environment of continuous deposition of tidal-flat sediments interrupted only by regressive phases which are manifested in the form of peat layers. About one thousand years ago man began to build dikes and the landward part of the tidal-flat environment did not undergo any further accretion. SEDIMENTARY SYNTHESIS
Sequential analyses of the tidal-flat deposits give some clue as to the sedim e n t a r y environment. After taking into account some degree of compaction of the primary thickness, the thickness of individual sequences roughly agrees with the depth of the tidal channels. This depth is, in turn, a function of the tidal ranges which are larger in marine funnel-shaped embayments where extreme values may be reached whereas tidal ranges along open oceanic coasts are less. The mineralogical composition of the tidal sediments reflects the composition of the unconsolidated sediments on the adjacent sea floor. It is evident from the map (Fig. 1) that channels in the Wadden sea are more or less extensions of the tidal inlets in much the same way as Shinn et al. (1969) have described from the Bahamas. Location of inlets therefore controls channel location. Extending this observation it is proposed that channel frequency on a tidal flat may be a function of a protective barrierisland chain with tidal inlets whereas, along open coasts channels should be developed in a much less distinctive manner. REFERENCES C r o m m e l i n , R.D., 1 9 4 0 . De h e r k o m s t van h e t zand van de W a d d e n z e e . Tijdschr. K. Ned. Aardrijksk. G e n o o t . , 57: 3 4 7 - - 3 6 1 . C r o m m e l i n , R.D., 1 9 4 3 . De h e r k o m s t van h e t W a d d e n s l i b m e t k o r r e l g r o o t t e b o v e n 10 m i c r o n . V e r h . Geol. M i j n b o u w k . G e n o o t . , Geol. Ser., 13: 2 9 9 - - 3 3 3 . Davies, J.L., 1972. G e o g r a p h i c a l v a r i a t i o n in coastal d e v e l o p m e n t . In: K.M. C l a y t o n ( E d i t o r ) , G e o m o r p h o l o g y Texts, 4. Oliver a n d B o y d , E d i n b u r g h , 204 pp. De G r o o t , A.J., 1 9 6 4 . Origin a n d t r a n s p o r t of m u d ( f r a c t i o n < 1 6 m i c r o n ) in coastal w a t e r s f r o m t h e Western S c h e l d t t o t h e D a n i s h f r o n t i e r . In: L.M.J.U. van S t r a a t e n ( E d i t o r ) , Deltaic a n d Shallow Marine Deposits. D e v e l o p m e n t s in S e d i m e n t o l o g y , 1. Elsevier, A m s t e r d a m , pp. 9 3 - - 1 0 1 . De G r o o t , A.J., de G o e y , J.J.M. a n d Zegers, C., 1971. C o n t e n t s a n d b e h a v i o u r of merc u r y as c o m p a r e d w i t h o t h e r h e a v y m e t a l s in s e d i m e n t s f r o m t h e rivers R h i n e a n d Ems. In: J.D. de J o n g ( E d i t o r ) , S e d i m e n t o l o g y ; R e s e a r c h o n S e d i m e n t o l o g y a n d Sedim e n t a r y G e o l o g y in t h e N e t h e r l a n d s . Geol. M i j n b o u w , 50: 3 9 3 - - 3 9 8 . De J o n g , ,I.D., 1 9 7 1 . T h e s c e n e r y of t h e N e t h e r l a n d s against the b a c k g r o u n d of H o l o c e n e geology; a review of t h e r e c e n t l i t e r a t u r e . Rev. G6ogr. Phys. G6ol. Dyn., 2 ( 1 2 ) : 1 4 3 - 162.
23 Favejee, J.Ch.L., 1951. The origin of the "Wadden"-mud. Meded. Landbouwhogesch., Wageningen, 51: 113--141. Hageman, B.P., 1969. Development of the western part of the Netherlands during the Holocene, Geol. Mijnbouw, 48: 373--388. Oomkens, E. and Terwindt, J.H.J., 1960. Inshore estuarine sediments in the Haringvliet (Netherlands). In: J.D. de Jong (Editor), Bijdragen tot de geologie en hydrologie van het Deltagebeid. Geol. Mijnbouw, 39: 701--710. Pons, L.J., Jelgersma, S., Wiggers, A.J. and de Jong, J.D., 1963. Evolution of the Netherlands coastal area during the Holocene. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (2): 197--208. Postma, H., 1954. Hydrography of the Dutch Wadden Sea. Arch. N~erl. Zool., 10 (4): 106 pp. (Thesis, Univ. Groningen). Postma, H., 1961. Transport and accumulation of suspended matter in the Dutch Wadden Sea. Neth. J. Sea Res., 1: 148--190. Reineck, H.-E., 1975. German North Sea tidal flats. In: R.N. Ginsburg (Editor), Tidal Deposits; A Casebook of Recent Examples and Fossil Counterparts. Springer, New York, N.Y., pp. 5--12. Roeleveld, W., 1964. The Groningen Coastal Area. Thesis, Vrije Universiteit, Amsterdam, 252 pp. Shinn, E.A., Lloyd, M.R. and Ginsburg, R.N., 1969. Anatomy of a modern carbonate tidal-flat, Andros Island, Bahamas. J. Sediment. Petrol., 39: 1202--1228. Terwindt, J.H.J., 1970. Observation on submerged sand ripples with heights ranging from 30 to 200 cm occurring in tidal channels of S.W. Netherlands. Geol. Mijnbouw, 49: 489--501. Terwindt, J.H.J., 1971. Litho-facies of inshore estuarine and tidal-inlet deposits. In: J.D. de Jong (Editor), Sedimentology; Research on Sedimentology and Sedimentary Geology in the Netherlands. Geol. Mijnbouw, 50: 515--526. Terwindt, J.H.J., 1975. Sequences in inshore subtidal deposits. In: R.N. Ginsburg (Editor), Tidal Deposits; A Casebook of Recent Examples and Fossil Counterparts. Springer, New York, N.Y., pp. 85--99. Terwindt, J.H.J., de Jong, J.D. and van der Wilk, E., 1963. Sediment movement and sediment properties in the tidal area of the Lower Rhine (Rotterdam Waterway). Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (2): 243--258. Van Straaten, L.M.J.U., 1954a. Composition and structure of recent marine sediments in the Netherlands. Leidse Geol. Meded., 19: 1--110. Van Straaten, L.M.J.U., 1954b. Sedimentology of Recent tidal flat deposits and the psammites du Condroz (Devonian). Geol. Mijnbouw, 16: 25--47. Van Straaten, L.M.J.U., 1956. Composition of shell beds formed in tidal flat environment in the Netherlands and in the bay of Arcachon (France). Geol. Mijnbouw, 18: 209-226. Van Straaten, L.M.J.U., 1957. The Holocene deposits. In: L.M.J.U. van Straaten and J.D. de Jong (Editors), The excavation at Velsen. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Set., 17: 158--183. Van Straaten, L.M.J.U., 1963. Aspects of Holocene sedimentation in the Netherlands. Verh. K. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 21 (1): 149--172. Van Straaten, L.M.J.U. and Kuenen, Ph.H., 1957. Accumulation of fine grained sediments in the Dutch Wadden Sea. Geol. Mijnbouw, 19: 329--354. Zagwijn, W.H., 1974. Palaeogeographic evolution of The Netherlands during the Quaternary. Geol. Mijnbouw, 53: 369--385.