Tidal Crevasse Splays As the Cause of Rapid Changes in the Rate of Aggradation in the Holocene Tidal Deposits of the Belgian Coastal Plain

Quaternary International 56 (1999) 3—13

Tidal crevasse splays as the cause of rapid changes in the rate of aggradation in the Holocene tidal deposits of the Belgian Coastal Plain

Cecile Baeteman! *, Dirk J. Beets", Mark Van Strydonck#

! Belgian Geological Survey, Jennerstraat 13, 1000 Brussels, Belgium " Geological Survey of the Netherlands, P.O. Box 157, 2000 AD Haarlem, The Netherlands # Royal Institute for Cultural Heritage, Jubelpark 1, 1000 Brussels, Belgium

Abstract

Detailed stratigraphic and sedimentologic work combined with high-resolution radiocarbon dating in an exposure of an upto 12 m thick Holocene succession near the town of Veurne in West Belgium has shown that filling of the coastal plain at continuous, but decelerating sea-level rise, occurs less smoothly than is usually assumed. Three tidal sequences are distinguished, separated by peat layers, the second peat and the surface peat, respectively. The second peat developed at about 6800 cal BP, after the rate of sea-level rise dropped from 7 m/ka to about 2.5 m/ka, indicating that sediment supply outran relative sea-level rise. In the excavation, the second peat drowns about 100 yr after it started and is covered by a thin veneer of inter- and supratidal beds, after which low intertidal to subtidal conditions prevailed for more than 1000 yr with no deposition. These 1000 yr stand for a gap of about 2 m sea-level rise. This gap is filled almost instantaneously by an ebb-dominated crevasse splay deposit, in which the daily rhythm of ebb and flood can be recognised in the sediments of the lower part and the fortnightly spring and neap tide alternation in the upper part. Crevassing is also considered important by the eventual flooding of the surface peat which is covered by about 1 m of inter- and supratidal deposits. Chaotic and catastrophic crevasse-splay channels in the surface peat are the precursors of the eventual flooding of the peat. Based on the observations in the excavation it is concluded that the current models of Holocene infill of coastal plains based on cores, often promote a gradualness which in reality does not exist. ( 1999 INQUA/Elsevier Science Ltd. All rights reserved.

1. Introduction a swamp and the gradual flooding of this swamp to change again into a tide-controlled back-barrier basin. This paper discusses rapid changes in the rate of ag- Based on detailed stratigraphical and sedimentological gradation and sedimentary environment of the Belgian work and high-resolution radiocarbon dating of the coastal plain near the city of Veurne (Fig. 1). The area Holocene succession exposed in an excavation pit near falls in what once was the estuary of the (now canalized) the town of Veurne (Fig. 1), we will show that in the river IJzer, which still drains the uplands South and estuarine environment long periods of non-deposition Southwest of the coastal plain. can alternate with short episodes of rapid sediment The knowledge of the development of Holocene accumulation. coastal plains of the Southern North Sea is mainly based The Holocene coastal plain of Belgium belongs to the on a large number of boreholes, with radiocarbon dating marine-dominated North Sea Lowlands which run from of peat intercalations and palynological investigations the Cretaceous marl outcrop at Blanc Nez near Calais to give a time-stratigraphic framework. This approach in northern France to northern Denmark. This long and almost always suggests a gradual filling of the tidal or in parts, broad coastal plain shows a wide variation in back-barrier basins more or less in pace with the rising coastal morphologies due to differences in tidal wave sea level. It moreover suggests a smooth transition to energy, the orientation of the shoreline with regard to the peat accumulation when the plain has been changed into predominant wind directions, and differences in sea-level rise and supply histories. Because of the circulation of the tidal wave in the southern North Sea, the tidal range * Corresponding author. varies from almost 5 m along the coast of Belgium to less

1991; Oost and De Boer, 1994; Beets et al., 1992; Baeteman, 1985) has shown that the plain was formed by the amalgamation of a large number of relatively small and shallow estuaries, as the rising sea invaded the valleys of the existing drainage pattern and the divides developed into headlands. These estuaries diﬀer in size from those on which the estuarine facies models of Dalrymple et al. (1992) and Zaitlin et al. (1994) are based, but are similar in the main processes of aggradation. They are smaller because many, although not all, of these estuaries formed in the valleys of local streams draining the direct surroundings. They are shallow for the same reason, but also because of the large distance from the lowlands to the shelf break, so that headward erosion during lowstands never reached the present coastal plain, in contrast to such estuaries as that of the Gironde in France (Allen, 1991) and Cobequid Bay in Canada (Dalrymple et al., 1990). Despite these diﬀerences, we will use the concepts and terminology developed by Dalrymple et al. (1992).

2. Holocene sea-level rise and sedimentary succession in the Belgian Coastal Plain

The Holocene coastal plain of western Belgium developed in the drainage basin of the Late Weichselian predecessor of the river IJzer, which at present debouches at the town of Nieuwpoort less than 10 km NE from Veurne (Fig. 1). The rising sea invaded the area by way of Fig. 1. Map of the Belgian coastal plain with location of the excavation the palaeovalley of the river which developed into an pit at Veurne. estuary with tidal channels and flats. As groundwater level rose with sea level, the flooded area was fringed by than 2 m along the Holland coast to increase again along freshwater marshes in which peat accumulated. This the Frisian Islands to the German Bight, where it peat, the so-called basal peat, is a time-transgressive unit measures over 4 m. Along the Danish coast the tidal shifting landward with the rising sea level. It forms the range drops rapidly to less than 1 m. Winds and waves base of the Holocene succession, where it is not eroded by are predominantly from the west. As shown by Van later channels. As the drainage basin of the river IJzer Straaten (1961) there is a close correlation between wind is relatively small, the fluvial sediment input can be and wave climate. The mean significant wave height is neglected (Baeteman and Denys, 1997). The estuary is a almost 2 m along the Frisian Islands and about 1 m tide-dominated system. All sand and mud was brought along the coast of Belgium (Sha, 1989). According to the in from sea by tidal currents. classification of Hayes (1979) and Davies and Hayes The general trend of the sea-level curve (Fig. 2) shows (1984), the coast of Belgium is tide-dominated (macrotidal that initially the sea-level rise was very rapid, rising at an to mesotidal), that of the Netherlands is a mixed-energy average rate of 7 m/ka in the period before ca. 7500 cal coast (mesotidal to microtidal), the German Bight is BP (Denys and Baeteman, 1995). This resulted in a very again tide-dominated (macrotidal to mesotidal), whereas rapid shift of the facies belts across the continental shelf the Denmark coast is wave-dominated (microtidal). The towards a position close to the present-day boundary of morphologies which go with this classification are only the coastal plain (Baeteman and Denys, 1997). At ca. partly fulfilled, due to sea-level rise and supply histories. 7500—7000 cal BP the relative sea-level curve shows During the Pleniglacial the Baltic ice sheet reached a distinct retardation to an average of 2.5 m/ka. Conse- Denmark and northern Germany. Consequently, sea- quently, the rapid landward shift of the facies belts level history of the lowlands varies strongly from Denmark stopped. Moreover, sediment supply now outran the to Belgium due to glacioisostatic and hydroisostatic creation of accommodation space by sea-level rise so that effects (Pirazzolli and Puet, 1991; Lambeck, 1993, 1995). the estuary was rapidly infilled. Instead, peat developed, Recent work on the Holocene development of the at first short-lived and locally, showing no regular pat- North Sea coastal plain (Streif, 1989, 1990; Flemming, tern in the distribution. However, by ca. 6800 cal BP peat C. Baeteman et al. / Quaternary International 56 (1999) 3—13 5

Fig. 2. Relative sea-level curve for the study area with indication of the average rate of sea-level rise (after Denys and Baeteman, 1995). accumulated on a more regional scale. Because of the peat is covered by a 1—2 m thick tidal flat deposit, form- lower rate of sea-level rise freshwater conditions were ing the final infill of the plain. maintained for about 200—300 yr. This ca. 10 cm thick peat layer is called the ‘second peat’ layer (Baeteman and Denys, 1997) and occurs at about 2.5 m below TAW 3. The Holocene sedimentary succession in outcrop: (Tweede Algemene Waterpassing: the Belgian ordinance the Veurne excavation pit datum referring to mean low water spring which is about 2 m below mean sea level). As a result of the lower rate of The excavation pit was upto 12 m deep with exposure the sea-level rise, the sequence deposited in the period walls of almost 300 m long and 170 m wide. As the entire between ca. 7800 and 5500 cal BP consists of successive Holocene sequence was exposed, the pit provided a peat beds alternating with tidal flat deposits. unique opportunity for detailed stratigraphical and sedi- The rate of relative sea-level rise continued to decele- mentological investigation and high-resolution radio- rate, and after ca. 5500—5000 cal BP it falls to an average carbon dating of shells and peat. All radiocarbon dates of only 0.70 m/ka (Fig. 2). This period corresponds well were carried out at the KIK-IRPA laboratory using the with the main development of the thickest intercalated conventional and AMS method (Van Strydonck and peat layer of the Flemish coastal plain, although the van der Borg, 1991; Forest and Van Strydonck, 1993) and beginning of the accumulation of this peat dates back to are indicated in calibrated years BP with the highest age as early as 6200 cal BP in certain areas (Baeteman, 1991). probability maxima, rounded to the nearest 0 or 5. This 1—2 m thick peat layer is known as ‘surface peat’ Altitudes are indicated in TAW. and generally occurs between 1 m below and 1 m above The Holocene sediments consist of clastic tidal depo- TAW. Between ca. 5600 and 4450 cal BP almost the sits interbedded with peat layers. The tidal deposits are entire plain was dominated by peat accumulation. Flood- mostly point-bar deposits of migrating channels and ing of this surface peat starts as early as ca. 4450 cal BP, associated muddy and sandy tidal flat deposits. Besides but in the distal parts of the coastal plain peat accumula- the basal peat, two distinct intercalated peat layers were tion proceeded until about 1500 cal BP (Baeteman, 1991). present: the second peat and the surface peat. The Holo- A more definite time limit of the end of this peat cannot cene succession has been subdivided into three main be given because of the wide range of dates. The surface units, named tidal sequences (Fig. 3). The lower tidal 6 C. Baeteman et al. / Quaternary International 56 (1999) 3—13 sequence is separated from the middle tidal sequence by Eocene clay, and an about 2.5 m thick sequence of tidal the second peat; the middle tidal sequence from the upper flat deposits interbedded between the basal peat at its tidal sequence by the surface peat. The Holocene se- base and the second peat at its top (Fig. 3). quence is underlain by few metres of Pleistocene deposits The tidal flat sequence consists of the alternation of covering Eocene clay. burrowed, fine sandy mud-rich intertidal deposits, small Throughout the vast extension of the excavation pit channels and gullies with fine sandy point-bar deposits and the different exposure walls, the relation between the indicating some lateral migration, sandy overbank channels, tidal flats and peat could be observed and this deposits of these gullies which grade into the intertidal within each of the three tidal sequences. muds, and two thin humic vegetation horizons with small roots at their base (Fig. 4). Dates of the base and top of 3.1. The lower tidal sequence the basal peat range between ca. 8110 and 7625 cal BP. The overlying second peat has an age of about 6835 cal The lower tidal sequence in the Veurne pit consists of BP. These ages are in perfect agreement with the an upto 5 m deep channel eroding into the top of the radiocarbon ages of shells sampled from the intertidal

Fig. 3. Composite stratigraphic scheme of the Holocene sequence in the excavation pit with location of the dated samples. Ages are in calibrated years BP. The thickness of the Holocene sequence ranges from 8 to 12 m.

Fig. 4. Stratigraphic section of the lower tidal sequence with indication of dated samples. C. Baeteman et al. / Quaternary International 56 (1999) 3—13 7 deposits and that of one of the vegetation horizons. From BP, and a large reworked oyster an age of ca. 7190 cal base to top these are (Fig. 4): BP. This coarse grained base is overlain by a fine sandy and mud-rich channel fill, which in itself consists of E Scrobicularia plana in living position at the base of the a stacked system of small channels. Fine parallel lamina- sequence with the burrows in the top of the basal peat tion and cross lamination of the climbing ripple type are having an age of ca. 7755 cal BP. the dominant sedimentary structures, in addition to some E Hydrobia sp. concentrated in shallow depressions on slumping at the base of the stacked channels. The sedi- top of the point-bar deposits of a gully just below the ments lack any infauna. Difference in grain size between second vegetation horizon having an age of ca. 7510 the coarse basal layer and the fine channel fill points to cal BP. an important change in hydraulic conditions. The sedi- E The vegetation horizon at the SE side of the exposure mentary structures of the fine channel fill and the com- with an age of ca. 7360 cal BP (Fig. 4). plete absence of infauna suggest rapid aggradation. As E Cerastoderma edule in living position in a strongly the cross-section of a channel is in equilibrium with the bioturbated and crumbly mud directly overlying the ebb- and flood-volume passing each tide (the tidal prism vegetation horizon having an age of ca. 7240 cal BP. at this locality) (O’Brien, 1969; Eysink, 1990), the change E Cerastoderma edule in living position just below the in hydraulic conditions must be due to an important second peat with an age of ca. 7090 cal BP. decrease in tidal prism. The system reacts by rapid ag- During deposition of the lower tidal sequence rate of gradation in the channel to re-establish equilibrium be- sea-level rise decreased from a mean of 7 m/ka to a mean tween cross-section and tidal prism. As the channel fill is of 2.5 m/ka (Fig. 2). Despite this overall high rate of overlain by the second peat layer, we think that it was sea-level rise, sediment supply was sufficient to outrun deposited shortly before the tidal basin, drained by this the demand, as the two vegetation levels and the second channel, silted up to supratidal level and changed into peat indicate silting up to supratidal levels. Although a freshwater swamp with peat accumulation. freshwater lenses must have been formed in the salt marshes for a vegetation horizon to develop, its duration 3.2. The middle tidal sequence was probably very short, as appears from the radiocarbon dates. However, the freshwater lens under- The second peat is covered by sediments of the middle lying the second peat lasted for about 100 yr (see tidal sequence which starts with mud and burrowed below). sandy mud, varying in thickness from about 1 m to The )5 m deep channel of the lower tidal sequence almost zero (Fig. 5). Cerastoderma edule in living position consists of a basal sand-rich layer with numerous shells near the base of the sandy mud have an age of ca. 6780 cal and clay pebbles, the latter derived from the eroded BP, which implies that peat accumulation of the second Eocene deposits. Two shell samples from the lag de- peat lasted for about a 100 yr. The sandy mudflat silts up posit were radiocarbon dated. A sample of reworked until above high water level resulting in a renewed forma- pelycepod Cerastoderma edule gave an age of ca. 7435 cal tion of a vegetation horizon. Just above this vegetation

Fig. 5. Details of the lower tidal sequence and lower part of the middle tidal sequence. 8 C. Baeteman et al. / Quaternary International 56 (1999) 3—13 horizon the fine-grained sediment has a tubular aspect lacks the dipping lateral accretion bedding of normal due to the activity of Barnea candida, a pelycepod which tidal channels, and it is completely filled by aggradation. bores itself into slightly indurated sediments. As the ani- There is no break in lithology in the unit, neither vertical mal cannot change its position once it has settled, it can from base to top, nor lateral between channel and lobes. only live in an environment with none or little sediment Three samples of reworked shells from the base of the accumulation. It, moreover, is restricted to subtidal to channel gave ages of ca. 5770, 5825 and 5495 cal BP. low intertidal conditions. Towards the east in the expo- However, stratigraphically they occur above the Barnea sure (Fig. 5), at a distance of only 30 m, the mud and samples (ca. 5550 cal BP), which implies that at least sandy mud peter out, and the bored vegetation horizon two of the samples contain shells reworked from older merges with the second peat. At both sites, the Barnea deposits. Another sample of reworked shells from a higher candida give similar ages, respectively ca. 5560 and level in the channel gave an age of ca. 5030 cal BP, which 5550 cal BP. Assuming that the vegetation horizon is seems to be too young in view of the ca. 5315 cal BP age slightly younger than the 6780 cal BP age of the of the base of the overlying surface peat (Fig. 3). Cerastoderma edule, there is a major gap between the The entire unit consists predominantly of fine sand and supratidal vegetation horizon and the borers of the low very fine sand. Mud occurs as reworked pebbles and intertidal to subtidal realm. As there is no indication for grains throughout most of the unit. Mud drapes depo- any erosion at this level, we can only assume that this gap sited during high water slack are found in the upper 2.5 m represents a long period of non-deposition. The borers of the unit; mud is only an important constituent in the are marine animals, but that does not imply that tidal upper metre, where it can form laminae of a few centi- conditions apply for the entire gap. In fact, the similar age metres thickness. In addition to the mud pebbles, shells of the two samples suggests that Barnea populated the and shell fragments form the coarser fraction of the sand. substrate at the end of this time gap. Considering the fact Shells are always transported, single valves. The entire that the surface peat starts to develop in the area from ca. sequence lacks burrowing, except for the upper metre. 6000 cal BP onward (Baeteman and Denys, 1997), it A thinning upward of the bedforms, due to the regular might well be that for most of the time the area formed decrease in the amount of aggradation during a tidal a lake within peat swamps. cycle is one of the main aspects of this tidal crevasse As sea-level rise was still in the order of 2.5 m/ka, this splay. The base of the channel sequence consists of upto time gap represents roughly 2 m of non-filled accommo- 1 m deep pockets, eroded into the underlying muds of the dation space. After ca. 5550 cal BP, this gap was rapidly lower tidal sequence, and filled by a conglomerate of mud filled by a tongue-shaped, fine sandy tidal crevasse splay pebbles and shells (Fig. 6). This conglomerate is covered (Figs. 3 and 6) consisting of an up to 30 m wide channel by about 1 m of even-bedded sand with scattered mud which laterally passes into lobes. The channel eroded pebbles, which is overlain by about 3 m thick alternation several metres into the underlying deposits, whereas the of climbing-ripple cross-laminated layers and even bed- lobes conformably cover the bored level discussed above. ded layers upto a depth of about 2 m below TAW (Figs. 6 Consequently, it varies in thickness from 6 to 7 m in the and 7). Most of the climbing ripples belong to the stoss- channel to ca. 2 m in the lobes. The master bedding in the depositional type of Ashley et al. (1982); angle of climb channel as well as in the lobes is horizontal; the channel varies but can become as steep as 30°. Even bedding

Fig. 6. Schematic columns of the middle tidal sequence. The upper part of the column on the right-hand side is given in detail in Fig. 7. C. Baeteman et al. / Quaternary International 56 (1999) 3—13 9

Fig. 7. Sedimentary succession of the upper part of the middle tidal sequence.

occurs as sharp, parallel lamination of the upper stage climbing ripple lamination. This suggests that flood is the plane bed or as sheets of massive, homogeneous sand subordinate current reworking the top part of the sedi- formed by a combination of fallout and traction from ment deposited during ebb. As will be discussed below, fast-moving, suspension-rich currents. The thickness of we infer that mean low water during deposition of the sequences of even-bedded and cross-laminated layers va- crevasse splay was at about 2.25 m below TAW (Fig. 7). ries between a few centimetres and half a metre. All major From there upwards, the character of the deposits cross-laminated layers point to a northward directed changes, in that far less sediment is deposited during current. As the North Sea and the most probable inlet for each tide. Tabular ebb-directed and trough-shaped flood- this tidal system was located north of the studied pit, we directed cross-lamination alternate rapidly between 2.25 assume this to be the ebb tidal current direction. Un- and 1.50 m below TAW, but more importantly, between interrupted climbing ripple sequences of almost 0.5 m in 1.50 and 0.30 m below TAW, the sequence shows a rhyth- thickness indicate rapid aggradation during the north- mic alternation of packages of thin even-bedded layers directed ebb current. and flaser-bedding highly suggestive of the fortnightly Indications for a southward directed flood current do neap-spring cycle (Tessier, 1993; Dalrymple et al., 1991). occur, but are usually restricted to isolated cross In the upper metre of the sequence the first burrowing laminated troughs in the top of sets of ebb-directed is seen. Non-burrowed sequences of parallel laminated 10 C. Baeteman et al. / Quaternary International 56 (1999) 3—13 mud alternate with strongly burrowed muddy sequences, One of the latter channels incised into the peat and which suggests a seasonal periodicity. truncated the underlying sediments of the middle tidal The sedimentary structures and the absence of infauna sequence until a depth of about 2 m below TAW over in the greater part of the sand body suggest that the a length of about 25 m. The channel form is symmetrical accumulation of this sand unit took little time. At and rounded with a very sharp erosional boundary a rough guess one might say that deposition of the (Fig. 8). Its infill is chaotic and without a channel lag. The succession below !2.25 m, where we can distinguish lower part is filled with fine sand, large irregular peat upto 0.5 m thick sequences formed during one tidal cycle, blocks, and sand and mud from the underlying middle was a question of days to weeks; the deposition of the tidal sequence. The latter material still displays the orig- succession between !2.25 and !0.30 m a question of inal lamination indicating that it slumped into the chan- months, and the upper metre a matter of a few years. On nel from the sides showing sharply truncated, concave a Holocene time scale the succession, one might say, was slide scarp boundaries. At the channel margins sand deposited almost instantaneously. occurs under the peat layer over a distance of several Although there is no major break in sedimentation, metres (Fig. 8), indicating that the peat was undermined by there is a small angular unconformity at about 0.50 m the currents scouring the deposits of the underlying middle below TAW. At the base of this level a number of small tidal sequence. The upper part of the infill consists of gullies are found. Using the data of Van der Spek and structureless sand with fewer and smaller peat blocks. Just Gerritsen (pers. comm., 1997) to reconstruct palaeotidal outside the channel, sand was also deposited on top of the range in the Southern North Sea, a mean tidal range of peat layer over a distance of about 5 m (Fig. 8). Beyond 3 m is assumed for the Veurne area around 5000 cal BP. there, the sand layer wedges out and is replaced by mud. This implies that mean sea level is situated near to this The chaotic filling, the absence of channel lag and small angular unconformity. As the crevasse splay was point-bar deposits, the undermined peat, and the pre- ebb-current-controlled, it could no longer aggrade above sence of fragments of the muds of the middle tidal mean sea level. The surface of the mudflats above MSL sequence, all indicate that the formation and filling of this were controlled by the main channel and have therefore channel occurred during one ‘catastrophic’ event. There- a slightly different attitude than those below MSL. The fore, it represents a scour-and-fill, formed by high-energy, gullies represent the drainage of the flats to the main catastrophic flows invading the freshwater marsh. Such channel. flows have the power to erode and produce both the After silting up, peat starts on the supratidal envi- channel form and its filling in a single event (Lucchi, ronment at about 5000 cal BP. This time, peat could 1995). Basically, the channel is a crevasse channel, accumulate continuously without any deposition for breaching the levee of a tidal channel which is not about 2500 yr. exposed in this pit, but which must be situated nearby. The main difference with the crevasse splay of the middle 3.3. The upper tidal sequence tidal sequence is probably a matter of accommodation space, which is much larger in the latter case. The upper tidal sequence consists of muddy and sandy The channel fill is covered by a 5—10 cm thick peat intertidal and supratidal deposits overlying the surface horizon, which could be followed laterally for several peat, and two different types of channels, the normal metres outside the channel where it merges with the laterally migrating channels characterised by point-bar surface peat (Fig. 8). This indicates that shortly after deposits, and more catastrophic, short-living channels the scour-and-fill of the crevasse channel, freshwater filled by fine sand and huge peat blocks breaching the conditions were re-established and peat accumulation surface peat (Fig. 3). resumed for a short period. Similar single event crevasse

Fig. 8. Details of the upper tidal sequence showing the scour-and-fill type of channel and the thin peaty horizon merging with the surface peat outside the channel fill. C. Baeteman et al. / Quaternary International 56 (1999) 3—13 11 channels in the surface peat and overlain by a thin peat a channel (e.g. Westerhoff et al., 1987). This is absolutely horizon were found at more localities in the excavation. not the case in the Veurne pit, and another explanation is This breaching of the peat swamp must have happened needed. Van der Spek and Beets (1992) show that at a shortly before ca. 2375 cal BP, which is the age of the peat high rate of sea-level rise the sediment supply in the horizon. At its turn, the thin peat horizon is overlain by coastal plain of Holland is insufficient to maintain inter- intertidal sandy muds with a fauna of Scrobicularia plana tidal flats, except for the direct surroundings of the in living position which gave an age of ca. 2085 cal BP channels. In their model tidal flats flank the channels and (Fig. 8). This implies that about 300 yr after the crevas- separate them from subtidal interchannel basins with sing of the peat swamp the area was again changed into predominantly mud deposition. However, even then and a tidal basin, which suggests that these crevasse channels far remote of the channels clay-rich mud is deposited, must be seen as precursors, heralding the eventual which would make living for Barnea difficult, if not change of the peat swamp into intertidal flats. As men- impossible. For that reason we assume that non-depo- tioned above, the intertidal and supratidal flats of the sition for such a long time at the site of the Veurne pit is upper tidal sequence were maintained by a lateral migra- not only due to a major shift in the position of the main ting channel eroding several metres into the channel channel, but perhaps also to isolation of the area due to deposits of the middle tidal sequence (Fig. 3). peat growth and consequently a change to a freshwater lake with minimal deposition. Around 5500 cal BP the lake is invaded by marine water so that it can be popu- 4. Discussion: processes of coastal behaviour lated by Barnea. Shortly thereafter the gap is filled by a tidal crevasse splay. Filling of the Belgian coastal plain at the site of Veurne Crevasse splays are common sediment bodies in fluvial excavation pit starts at about 7800 cal BP at a level of and deltaic settings (Collinson, 1996; Reading and almost 6 m below TAW which is in accordance to the Collinson, 1996). In tidal settings crevasse splays are Holocene MSL curve of Belgium as given by Denys and hardly mentioned (for an exception see Cloyd et al., Baeteman (1995). The basal peat on top of the Pleisto- 1990), although crevassing must be quite common where cene deposits is directly overlain by fine sandy and the channels show a high sinuosity in the mixed energy muddy tidal flat deposits with Scrobicularia plana bur- zone of the estuaries (Dalrymple et al., 1992). For crevass- rowing the top of the peat. Despite a high rate of sea-level ing to occur one needs levees and a higher than normal rise, sediment supply surpassed the creation of accommo- discharge of the channel. At high rates of sea-level rise dation space at this locality so that the intertidal flat and a limited supply of sediment, a situation as described deposits alternate with supratidal vegetation levels. This for the Holland tidal basin by Van der Spek and Beets evolution of one environment into another is not to be (1992) the intertidal flats separating channels and inter- generalised for the entire area. It is related to the position channel areas act as levees, and the sand lamina and beds of the channels, the sediment suppliers, with regard to the interbedded with the mud in the interchannel areas form studied outcrop. Consequently, the vegetation horizons the toes of crevasse splay deposits. Tidal wedges are have no stratigraphical implication, as their occurrence common along the channel in the estuary of the Wester- only depends on whether sufficient sediment was sup- schelde in the south-western Netherlands and in the plied to allow silting up of a particular part of the flats. Frisian Wadden Sea (Van Straaten, 1954; Van Veen, That the vegetation levels did not evolve to proper peat 1950) and they have the morphology of crevasse splay in accumulation is also due to the high rate of sea-level rise, alluvial terrains. Some of these wedges have a relatively as there was insufficient time to maintain a freshwater long life-time as they develop into short cuts for the ebb or lens below the supratidal flats. flood currents, others, on the contrary have a short exist- The situation changes as soon as the rate of sea-level ence and silt up rapidly after they were formed (Oost and rise decreased to an average of 2.5 m/ka. At about De Boer, 1994). In particular, the latter are comparable 6900 cal BP, the supratidal flat evolved into a freshwater to crevasse channels and fans in alluvial settings. The marsh with peat accumulation yielding the first inter- conditions for a higher than normal discharge is easily ful- calated peat layer for this particular area. Peat accu- filled when a storm from the west coincides with the flood mulation did not last for long, as by ca. 6780 cal BP phase. During such a storm the water stored in the sedimentation on intertidal flats recommenced. Sediment estuary reaches far above normal high-water level. When input still must have been sufficient as also here silting up the storm falls before ebb is reached, this huge amount of of the flat resulted in a thin vegetation horizon. As water is discharged, causing major shifts in the position described, this vegetation horizon has been bored by of the channel and the breaching of levees. According to a fauna of much younger Barnea candida, indicating that Flemming and Davies (1994) these storm events cause the site was deprived of sediment for a long time. These ebb-flow accelerations of upto 65% in the inlets. borers are more often found in tidal deposits, but usually The crevasse splay deposit of the middle tidal sequence in indurated sediments, preferentially peats, at the base of differs in many respects from those of the upper tidal 12 C. Baeteman et al. / Quaternary International 56 (1999) 3—13 sequence, in that the former fills up in a number of years lateral shift of the main channel. Similar to the situation a void of several metres thickness formed over a period of in the Holland tidal basin, these west—east-running chan- upto 1000 yr sea-level rise, whereas the latter forces its nels show a very restricted meandering belt during way into and over a peat deposit by scouring it during sea-level rise (Beets et al., 1992). probably one major ebb-surge storm event. The upper tidal sequence overlying the surface peat Acknowledgements represents a re-development of the tidal flat in the area and occurred in successive steps. The first step, probably We wish to thank Ad van der Spek and Bert van der before ca. 2400 cal BP was restricted to small crevasse Valk for assistance in the field. This paper is a contri- channels only. The renewed peat growth on the channel bution to IGCP Project 367, ‘‘Late Quaternary Coastal fill demonstrates that the environmental conditions of Records of Rapid Change: Application to Present and the freshwater marsh did not change drastically. How- Future Conditions’’. ever, freshwater conditions were not maintained for a long time, as less than 300 yr later, the area changed again in a tidal landscape with channels and intertidal References flats. Allen, G.P., 1991. Sedimentary process and facies in the Gironde estuary: a recent model of macrotidal estuarine systems. In: Smith, D.G., Reinson, G.E., Zaitlin, B.A., Rahmani, R.A. (Eds), Clastic 5. Conclusions Tidal Sedimentology. Canadian Society of Petroleum Geologists Memoir, Vol. 16, pp. 29—39. 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