Ebb-tidal delta deposits along the west Frisian Islands (The Netherlands): process, facies, architecture, and preservation more

In: Clastic Tidal Sedimentology, D. G. Smrth, G. E Reinson, B. A. Zeitlin and R. A. Rahmani (ads.). Canadian Society of Petroleum Geologists. Memoir 16, p. 199-218. ! 7I r Ebb-tidal delta deposits along the west Frisian Islands (The Netherlands): processes, facies architecture and preservation L. P. SHA I AND P. L. DE BOER Comparative Sedimentology Division, Institute of Earth Sciences, University of Utrecht, p.D. Box 80.021, 3508 TA Utrecht, The Netherlands ABSTRACT The morphologies of the ebb-tidal deltas of Texel, Terschelling and Ameland inlets and the resulting sand transport patterns are greatly influenced by the interaction of shore-parallel tidal currents with tidal currents through the inlets. These three inlets and their ebb-tidal deltas are similar in many aspects, and together they differ from North American Atlantic coast examples and from those of the East Frisian Islands. Sedimentary facies and sequences, and sand body geometry of these three inlets/ebb-tidal deltas are strongly controlled by this interaction of offshore tidal currents, tidal currents through the inlets, and waves. The interaction and relative strengths of both tidal current systems determine the flow patterns and orientations of the main ebb channels in the ebb-tidal deltas. Different orientations of the (asymmetrical) ebb-tidal deltas as compared to those along the East Frisian Islands and the U.S. east coast, as well as a change of the general orientation of the main ebb channel of the ebb-tidal delta of Texel Inlet since the 16th century, are related to differences and changes in the relative importance of tidal currents and waves. Waves and longshore drift tend to force the ebb-tidal deltas into a downdrift asymmetry, whereas the shore-parallel tidal currents tend to force the ebb-tidal delta into an updrift asymmetry. Under the present regime of waves and longshore drift in this area, the critical boundary between updrift and downdrift asymmetry appears to be at a tidal prism on the order of 500 x 1()6 ml. The volume of ebb delta sand bodies is approximately 1 km' (Texel), a considerable part of which can be preserved in the fossil record. Holocene deposits off the West Frisian Islands indeed show that preservation of ebb-tidal delta deposits during a rise of relative sea level is possible under certain conditons. The strength of tidal influence is of great importance for determining both the basal boundary and the seaward extent of the ebb delta systems. Thus, it is inferred that preservation potential of ebb-tidal delta deposits can be positively correlated with the relative dominance of ebb-tides during their formation. RESUME Les morphologies des deltas de man:!e de type reflux des petits bras de mer de Texel, Terschelling et Ameland, ainsi que les modeles de transport de sable associes sont grandement influences par l'interaction des courants de maree paralleles a la cote et des courants de maree a travers les petits bras de mer. Ces trois petits bras de mer et leurs deltas de maree de type reflux sont en plusieurs points similaires entre eux, mais ils different toutefois des exemples provenant de la cote atlantique de l'Amerique du Nord et des lIes Frisonnes orientales. Pour les trois exemples etudies, les facies et sequences sedimentaires, et la geometrie du corps sableux sont fortement controles par I'interaction entre les courants de maree littoraux, les courants de maree a travers les petits bras de mer et les vagues. L'interaction et la force relative des deux systemes de courants de maree determinent Ie patron d'ecoulement et l'orientation des principaux chenaux de reflux des deltas. Les differentes orientations des deltas asymetriques de maree de type reflux, com parables a celles des lIes Frisonnes orientales et de la cote est des Btats-Unis, et Ie changement de I'orientation generale du principal chenal de reflux du delta du petit bras de mer Texel depuis Ie 16ieme siecIe, sont relies aux differences et modifications de l'importance relative des courants de maree et des vagues. L'asymetrie des deltas de maree de type reflux different selon les composantes; les vagues et la derive littorale ont tendance a produire une asymetrie dans Ie sens de la derive littorale tandis que les courants de maree paralleles a la cote conduisent a une asymetrie inverse au sens de la derive littorale. Sous Ie regime actuel des vagues et de la derive littorale de cette region, un prisme de maree de I'ordre de 500 x 10 m 3 semble etre la limite critique entre une asymetrie dans Ie sens de la derive littorale et une asymetrie inverse au sens de celle-ci. Le volume des corps sableux du delta de maree atteint jusqu'a 1 km 3 (Texel) dont une quantite importante peut etre preservee dans les archives geologiques. Dans les depots holocenes au large des tIes Frissonnes occidentales, les depots deltai'ques de type reflux, deposes durant une hausse relative du niveau de la mer, sont effectivement preserves sous certaines conditions. La force de l'influence des marees est importante pour determiner tant la limite basale que l'etendue vers la mer des systemes de delta de type reflux. Alors, il faut en deduire que Ie potentiel pour la preservation des depots de delta de maree de type reflux peut etre positivement correlee avec la dominance relative des marees descend antes durant leur formation. 'Present address: Geological Survey of the Netherlands, P.O. Box 157, 2000 AD Haarlem, The Netherlands 199 200 L. P. SHA AND P. L. DE BOER INTRODUCTION Ebb-tidal delta deposits are accumulations of sandy sediment at the seaward side of tidal inlets (Hayes, 1975). They depend on the presence of barrier islands, which largely are the result of wave action and tidal currents through the tidal inlets. Generally micro-and mesotidal regimes are considered to favour the evolution and maintenance of tidal inlets (Hayes, 1979, 1980), but it should be pointed out that the tidal prism rather than the tidal amplitude defines the strength of tidal currents within and adjacent to the tidal inlets (Sha, 1989c). Waves and tidal currents through the inlets have been demonstrated to be responsible for the formation and morphodynamic behaviour of ebb-tidal deltas (Bruun and Gerritsen, 1959; O'Brien, 1969; Oertel, 1972; Goldsmith et al., 1975; Bruun, 1978). Thus, regional differences in the geometry of ebb-tidal deltas are logically explained on the basis of differences in wave climate and tidal range (Oertel, 1975; Hayes, 1975, 1979, 1980; Hubbard et al., 1977; Nummedal et al., 1977; Nummedal and Fischer, 1978). Facies and sequence models have been proposed by Oertel (1973) and Imperato et al., (1988) in relation to the controlling dynamic processes (Hubbard et al., 1979). It is to be expected that sand body architecture and the preservation potential of ebb delta systems are closely related to the morphodynamics. Studies about this relationship are scarce, however, and comparisons between ancient examples and modern environments are not common. This paper focusses on processes, sedimentary facies and sequences, sand body architecture, and preservation potential of ebb-tidal delta deposits in the West Frisian Islands (Figs. I, 2). REGIONAL SETTING OF THE WEST FRISIAN ISLANDS The tidal wave in the North Sea rotates anticlockwise around three amphidromic points. Along the Dutch coast is progrades from the southwest to the northeast, and produces nearshore reversing tidal currents parallel to the NORTH SEA $CHIERMONNIKOOG • N Islands ------ Ameland Inlet LEGEND 10m below MSL ....., channel deeper than 10m Sm MSL FRIESLAND TEXEL o IJSSELMEER 10 20 30km .... NOORD-HOLLAND Figure 1. Location of the Texellnlet, Terschelling Inlet and Ameland Inlet ebb-tidal deltas. Ten-metre depth lines indicate the position of the ebbdeltas; black areas are channels within the ebb-tidal deltas. EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 201 MSL 10 MSL 10 20 30 40 50 m The Holocene-Pleistocene interface [3 Holocene ebb delta sand Holocene ~ Early Holocene peat WelchseDan deposits Eemian deposits ~ Pleistocene ~ t £ -~-:~-I Saalian deposits Figure 2. Fence diagram of the Texel ebb delta sand body. Five stratigraphic units are shown. The Saalian deposits at the base, found in the northern part of the ebb-tidal delta, are of glacial origin. They are poorly sorted, silty, clayey sands with gravel. The Eemian deposits above are coarse to fine grained. shell-rich sand covered by a grey, firm. clay layer. The overlying Weichselian deposits are mainly clayey silt and fine grained sand with interlayers of peat in the upper part and fine grained sand in the lower part. Peat layers at the base of the Holocene represent the early transgressive phases, and are found only onshore along the coast of Den Helder. The recent ebb delta sands cOnsist of fine to medium grained sand with abundant marine shell fragments. coast. The tidal range varies from 1.4 m near Texel Inlet to about 3 m in the German Bight (Dijkema el al., 1980). Related to this increase in tidal amplitude toward the northeast, tidal inlets become wider and barrier islands smaller. The predominant wave direction is from the west, producing a net northeastward longshore drift. The average wave height is about 1.2 m off Texel. A net northeastward, longshore, residual current results from predominantly southwesterly winds along the Dutch North Sea coast and from deformation of the tidal wave in the nearshore shallow waters (Van Straaten, 1961; Nummedal and Fischer, 1978; Postma, 1982; Sha, 1989b). The real residual current pattern is very complicated and varies with the actual wind conditions (ej Van der Giessen et al., 1990). Most of the ebb-tidal deltas along the Wadden Islands face the north and northwest, that is, they are asymmetrical to the right when seen from the land (Fig. 8, see below). However, the inlets of Texel, Terschelling, and Ameland face the southwest (Fig. 1). PROCESSES IN THE EBB-TIDAL DELTAS SEDIMENT TRANSPORT PATTERNS Sediment transport in the ebb-tidal delta of Texel Inlet was studied by means of current measurements, echosounding measurements of bedform asymmetry, measurements of foreset orientation and comparisons with historical morphological profiles. The results show mostly flood-oriented net sediment transport in the minor tidal channels and over the shoals (Fig. 3). Ebb-dominated sand transport occurs in the larger and deeper channels. Sand, which is transported to the seaward margin of the ebb-tidal delta by the ebb flow through the southwest-directed main ebb channels, is carried north by flood-dominated residual tidal currents along the ebb delta front. Landward sediment transport dominates the northern part of the ebb-tidal delta. Sand is deposited north of the ebb delta shoal 'Noorderhaaks' owing to the weak, rotational tidal currents (Fig. 3). In that area, waves transport sand shoreward, especially 202 L. P. SHA AND P. L. DE BOER 6 N j I 6r' ! / ----'t TEXEL N TEXEL NOORDHOLLAND ~ O.5m/sec current averaged over 0-3 m depth a 1 2 3 km =-==1 6. 6 hou rs before HW at Hoek of Holland Figure 3. (Left) Schematic current and sand transport pattern in the ebb-tidal delta of Texellnlet Arrows indicate the net sand transport directions_ Morphological situation as in 1985_ (Right) Tidal current roses for a tidal cycle in the ebb-tidal delta of Texellnlet integrated from the "Stroomatlas van Nederland" (MMAH. 1963)_ Figures indicate time in hours before or after high water at the Hook of Holland_ Arrow length represents current velocities, averaged from the water surface to 3 m below the surface. Morphological situation as in 1963. in the form of swash bars. Part of the sand returns to the inlet through the flood-dominated channel 'Molengat' in the north (Figs. 2, 3). The orientations of the larger bedforms in the channels (measured at different times during the tidal cycle) do not change in response to changing ebb and flood tides (Fig. 4). Orientations of foresets in the upper decimetres of the sediment generally are the same as those of the bedforms at the surface. Sand transport estimated on the basis of current velocity data (Sha, 1989b) is in accordance with the pattern shown in Figure 3. Despite the continuous rise of relative sea level, large areas of the Wadden Sea continue to be dominated by sand. Thus, there must exist a dominance of sand transport through the inlets by flood tidal currents. De Boer (1979) calculated a figure of 17 x 1{)6 ml per year for deposition of sediment (largely sand) in the western part of the Wadden Sea. Large sand waves indeed indicate such a transport through the inlet. The North Sea and the adjacent coast are the sources of the ebb-tidal delta sands as well as of much or all of the sand in the Wadden Sea. Sediment transport patterns similar to those in the Texel ebb-tidal delta are found in the ebb-tidal deltas of Terschelling Inlet and Ameland Inlet (Fig. 5). Historical morphological profiles indicate erosion and deepening of the channels, and seaward progradation of the ebb delta lobe in the southern part of the ebb-tidal delta of Texel Inlet (Fig. 6). In the northern part of the ebb delta, however, the ebb-tidal delta shoal has been subjected to partial erosion since at least 1896. Such progradation and ero- sion patterns are caused by large-scale changes in the dynamic regime, resulting from an increase in the tidal prism as discussed below. A5 0 s N E .c Q c. ., 20 404-----------------------r---------------~ o Distance (m) 1000 1750 Figure 4. Echo sounding profile recorded at mid-ebb, showing the floodoriented bedforms. Location: channel east of most southeasterly rose diagram in Figure 3b. EFFECT OF TIDAL CURRENT INTERACTION Ebb-dominated sediment transport in all these ebb-tidal deltas is directed to the southwest. This is a result of the interaction between nearshore tidal currents parallel to the coast with tidal currents through the inlets (Fig. 7). Thus strong, bidirectional tidal currents are produced on the left side of the ebb deltas, and a weak, rotational tidal current EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 203 l' I 10m / .. VUe/and o 3 km "" 5 km Figure 5. Schematic sand transport pattern in the ebb-tidal deltas of Terschelling Inlet (modified after Kool and de Boer; 1983) and of Ameland Inlet (modified after Lubberts and De Vos, 1987). P P Mid-flood Progradational eblKlelta lobe erosion Inlet ~:~\j Barrier Island -- ~--...... I --\ -Island - channel I 30 / 1.5 km l I\ '" ) Lagoon 40- Figure 6. Geomorphological section pop' showing the evolution of the Nieuwe Schulpengat channel since 1930. Inlet channel at the right and progradational ebb delta lobe to the left. Based on loading maps of the Ministry of Waterworks. For location see Figure 3. pattern on the right side (Fig. 3). This tidal current pattern favours shoal development in the part of the ebb-tidal delta northeast of the inlet, and channel development to the southwest. MORPHOLOGICAL PATTERNS TEXEL, TERSCHELLING, AMELAND EBB-TIDAL DELTAS - - Mid-ebb ...- - - .. -Barrier Island Barrier Island The most important features of the three larger inlets (Texel, Terschelling, Ameland) are very similar. The ebb-tidal deltas and the main ebb channels are asymmetrical to the left (Figs. 3, 5), the large, ebb-dominated channels have clear seaward-prograding distal lobes (Fig. 6), and, smaller flooddominated channels commonly develop on the landward side, adjacent to the main ebb channels. On the north-northeast side of the inlets, sandy tidal delta shoals occupy large subtidal to supratidal areas (up to 6.5 Lagoon Figure 7. Schematic current patterns at mid-flood and mid-ebb showing the effect of the interaction of tidal currents parallel to the coast with the tidal currents through the inlet. Thus, strong, bidirectional currents are produced on the southwest side of the ebb-tidal deltas, and a weak, rotational tidal current pattern occurs on the northeast side. See also Figure 3b. 204 km:). Flood-dominated channels, which occur northeast of the shoals, tend to silt up, and become abandoned through attachment of the shoals to the island to the northeast. SMALL ER EBB-TIDAL DELTAS L. P. SHA AND P. L. DE BOER Unlike the above three large ebb-tidal deltas, the smaller ebb deltas along the Wadden Sea coast and their main ebb channels are asymmetrically oriented to the east (Fig. 8). Explanations for such an orientation are based on the fact that wind-driven waves attack the coast predominantly from the west, producing an easterly longshore drift that forces the ebb-tidal deltas to be asymmetrical to the east (Luck, 1976; Nummedal and Penland, 1981). CAU SE OF UPDRIFT AND DOWNDRIFT ASYMMETRY tidal currents. Their example, however, concerned a much smaller system (width of the inlet about 500 m, and depth about 10 m). Lynch-Bloose and Kumar (1976) proposed th at a constant tidal regime tends to produce an updrift growth of the downdrift part of the inlet system in quiet periods, whereas strong waves and storms may cause sudden shifts in the downdrift direction. EVOLUTION OF THE CHA NN EL SYSTEl\1 I N THE EBB-TIDA L DELTA OF TEX EL I N LET The difference between the large and the small ebb-tidal deltas seems to be controlled by the relative importance of tidal currents (offshore and through the inlet) on the one hand , and wave-generated currents on the other. Of the three larger ebb-tidal deltas with an updrift asymmetry, the asymmetry is largest in the ebb delta of Texel Inlet (tidal prism about 1050 million mJ) and smallest in the ebb delta of Ameland Inlet, which has a tidal prism of only about 450 million m J. Terschelling Inlet has a tidal prism of about 850 million ml. Tidal prisms through the inlets with downdrift oriented ebb-tidal deltas are smaller. Thus, the tidedominated ebb-tidal deltas, controlled by interaction of offshore and inlet tidal currents are updrift asymmetrical, and the ones dominated by wave-induced longshore drift are more downdrift asymmetrical. Lynch-Blosse and Kumar (1976) interpreted changes in Brigantine Tidal Inlet, New Jersey and also attached value to the relative importance o f waves/ longshore drift versus The above mechanism also explains the morphological evolution of the ebb-tidal delta of Texel Inlet in relation to the evolution of the hydrodynamic regime. Reconstructions on the basis of historical navigation maps produced since the 16th century show that the ebb delta of Texel Inlet was asymmetrically oriented north-northwest before the middle 18th century (Fig. 9). It became symmetrical between the late 18th century and the early 19th century, and since the early 19th century it has been directed toward the southwest. The historical maps show that the maximum inlet depth has increased from 25 m in about 1583 to about 50 m at present. By combining data from modern examples in the western Wadden Sea with depth data read from old maps, a relationship between the maximum inlet depth and the tidal prism was established (Fig. 10; Sha, 1990a). The tidal prism through Texel Inlet is interpreted to have increased from about 240 x 10" m) in 1583 to 1050 x 10" m' in 1970. This increase is related primarily to sea level rise and the result ing increase of tidal amplitude. The increase in tidal volume is also due in large part to mismanagement by man when draining and exploiting extensive peat areas which previously existed in the Wadden Sea (Eisma and Wolff, 1980). Moreover, the construction of dikes since about the yea r 1000 <:} 5 km N 2 m below MSL channel deeper than 6 m Figure S. Morphology of the ebb-tidal deltas along the East Frisian Islands (after FitzGerald et al., 1984). Note that the ebb-tidal deltas and their main ebb channels are asymmetrical to the east. EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 205 Wadden S .... ..... .. " ea ........... Wadden Sea ' . .......... . ' - - - - - - " about 3 km . about 3 km Wadden Sea I J ) J ~/Il ) J ' } -----.! /) j '" ~') ) / ----------- , \ ~ AD prevented natural subsidence to be compensated for by accumulation of fine grained sediment, so that repeated bursts of dikes during recurring severe storms caused sudden increases in volume of the Wadden Sea. In addition, the closure of tidal inlets in other areas along the Wadden Sea coast has contributed to the increase of the tidal prism through Texel Inlet. The closure of the 'Ijsselmeer' (Fig. 1) in 1932, paradoxically led to a 20 per cent increase in tidal amplitude near Texe!. The increase of the tidal prism in Texel Inlet during the last few centuries may be the reason for the change of orientation (from downdrift to updrift) of the ebb-tidal delta in historical times. It appears that the change in orientation occurred when the tidal prism was about 700 x ICY' ml (Figs. 9, 10). At present the inlet of Ameland has a slight updrift asymmetry, and the tidal prism is only about 450 x ICY' ml. This dissimilarity must be due to differences between the deltas with respect to wave attack and tidal regime, and possibly also to changes in these factors in historical time. Morphological changes, as seen in Texel Inlet, also occurred in the inlet of Terschelling. The evolution of the historical tidal prism has not been reconstructed there, but available measurements (Ministry of Waterworks, unpub!. data) do indicate an increase in tidal prism during recent decades. TIME 1100 TEXEL INLET '\ \ ---------. -'--- D '\', \---\ \ '\ \ \ \ Figure 9. Schematic representation of the ebb·tidal delta of Texel Inlet, reconstructed on the basis of historical maps. A. 16th century; B. 17th to 18th century; C. 18th to 19th century; D. 19th to 20th century. TIDAL PRISM RELATION 1000 U1 fW 0 1970 cr w 900 1916 800 ' 51 L iii 0 0 :> u z 700 n :J 17 g774 600 :E ;; ::; U1 -' 1~15 it "-' 0 500 ;: " 400 In 1~08 300 J~ 1~8: 1500 1700 1800 1900 2000 200 1 '600 YEARS Figure 10. Estimated historical tidal prisms of Texel Inlet, based on the comparison of maximum depths of channels on historical maps with maximum depths of tidal inlets and tidal prisms of the present·day tidal inlets along the Wadden Sea. 206 L. P. SHA AND P. L. DE BOER 1608 / I 1 1623 1681 17 ,12 I f 1583 , I I I \ \ \ 1608 " 6 ",I I \ \ " , , \ ' '\ \ \ 1583 TEXEL N \ '\ '" " 'TLand Diep van Texel HELDER 1530/" HELDER Spaengers Gat a 3km b 1712 6 N Molengat HELDER De Slenck or Noorder Gat or Het Nieuwe Gat or Spangaerds Gat (1732) C d Figure 11. Rotation and migration of channels in the northern part of the ebb-tidal delta of Texel Inlet. Solid lines indicate pOSitions and periods in which the channels acted as the main ebb channel. EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 207 CYCLI CA L SHIFTI NG OF SH OAL S AND CHA NN ELS An important feature of the ebb-tidal delta of Texel Inlet is the clockwise rotation of the channels, together with the onshore movement of intertidal-supratidal shoals (Westenberg, 1974; Liefhebber and Berg, 1976). Such a phenomenon is cyclical and four cycles have been recognized since 1583 (Fig. 11). The cycle starts with the development of a new main (ebb) channel, which then gradually rotates to the north. The channel degenerates into a marginal flood channel, while a new main ebb channel develops farther to the southwest. Rotation of the channel occurs together with the onshore movement of the shoal (Fig. 12). The shoreward- / -t I -6 --1851 ---- - 1863 - - _ . 1874 - .. - .. - 1896 N DEN HELDER 1901 --1908 2 km o I 1 2 3 km I Figure 13. Channel development in the 'Noordwest Gronden' offshore Terscheliing (after Kool and de Boer, 1983). Figure 12. Migration of the inter-and supratidal shoal 'Onrust' between 1851 to 1908. The shoal attached to Texel shortly after 1908. migrating shoal buries the channel at the eastern side, and is transformed into a sand spit. The recent cycle started when the shoal 'Onrust' attached to Texel early during this century. At present the shoal 'Noorderhaaks' (Fig. I, 2) is migrating toward the island and the 'Molengat' marginal flood-tidal channel is decreasing in importance (pers. comm., Netherlands Institute of Sea Research). Attachment of the shoal 'Noorderhaaks' to the island of Texel may be expected in the not too distant future, either by a gradual merger or, more likely, during a severe storm, which will cause filling of the 'Molengat' channel. Similar cyclic changes also occur in the ebb-tidal deltas of Terschelling and Ameland inlets. This is evident from the clockwise rotation of the flood channel in 'Noordwest Gronden' in the ebb-tidal delta of Terschelling Inlet (Fig. 13). Also a shore-attached supratidal shoal and abandoned channel are presently seen in the ebb-tidal delta of Ameland Inlet (Figs. 5, 14). Fitzgerald (1984) described similar cyclical patterns of growth and decay of the ebb-tidal delta at Price Inlet, South Carolina. Fitzgerald attributed this to the onshore migration of large bar complexes every 4 to 7 years. The mechanism that causes the above-described cyclical migration is not fully clear. The most obvious explanation is that the system tends to be gradually pushed, by the action of waves and longshore currents, to a more westnorthwesterly orientation. Thus the length of the main (ebb) channel increases. As a result the length of the tidal-current pathway through the main inlet channel increases (Fig. 7), and the tidal currents through this channel become weaker. At some point the distance covered by the ebb (and flood) currents (of decreasing strength) through the main ebb channel reaches a critical limit. At this stage the channel becomes ineffective and becomes a marginal, flood-dominated channel, while a channel farther to the south takes over the role of main ebb channel. Other flood-dominated channels farther north diminish in importance accordingly, and waves and storms push the larger ebb delta shoal over the marginal flood channel toward the island. By this process the channels rotate clockwise, and produce typical lateral migration patterns in one direction only. Figure 14. The shore-attached shoal on the ebb-tidal delta of Ameland Inlet (Fig. 5) . The picture was taken in 1987, from the supratidal shoal which was almost attached to the island . Note the abandoned channel behind the supratidal shoal, and Ameland in the background. The now abandoned channel was 15 m deep in 1983, but only 3 m deep in 1987. 208 L. P. SHA A ND P. L. DE BOER TEXEL 3 km .. . DEN . HELDER I , , , , I / I I I .' , " . : ' -...../ Depth line of -10m MSL ,..-.... Contour of mean grain sizes in microns Figure 15. Facies distribution in the ebb-tidal delta of Texel Inlet. ACDL: active delta-lobe fac ies; DSDL: distal lobe faci es; ABDL : abandoned ebb-delta lobe facies; SW: swash-bar facies ; SS : supratidal shoal facies; TCH : tidal channel facies ; LPT: locally protected facies; ISTSS: innershelf tidal sand sheet facies. EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 209 ebb channels. It consists of prograding, lobe-like sand bodies influenced by both tidal currents and waves. Waves dominate the seaward side of the lobe, and tidal currents generally dominate the landward side. Wave-and tidal current-produced sedimentary structures are commonly interlayered (Fig. 16a). FACIES 2 (DSDL) SEDIMENTARY FACIES The distribution of surface sediments is closely related to sediment transport patterns and morphodynamics (Sha, I 990b), and is similar for the ebb-tidal deltas of the three major inlets. This is in accordance with the many morphodynamic characteristics that these systems have in common. The ebb-tidal delta deposits in the three inlets can be divided into nine facies (Fig. 15), described below. FACIES 1 (ACDL) The active delta-lobe facies occurs seaward of the main The distal lobe is situated seaward of the ebb delta lobe at a depth of > 15 m below mean sea level. This area is less influenced by waves and tidal inlet currents, but longshore tidal currents do influence the bottom sediments. Fine grained sediments transported in suspension through the inlet during ebb-flow are deposited because of the deceleration of currents when arriving in the open basin. Consequently, the sediment is dominated by muddy, fine grained sand, commonly showing current and wave structures, with interbedded clay layers (Fig. 16b). Sedimentary structures are slightly bioturbated, and escape structures indicate periods of high sediment input, probably related to storms. In contrast to examples of 'storm deposits' from other settings, the result of storms within this facies is an increased input and deposition of fine grained sand and mud from the ebb-tidal channel when, during the waning phases of storms, the accumulated water in the Wadden Sea returns to the North Sea Basin. FACIES 3 (ABDL) The abandoned ebb delta lobe facies occurs in front of the former main ebb channels, in the northeastern part of the three larger ebb-tidal deltas. The abandoned ebb-tidal delta lobe is subject to erosion, and the retreat is due to wave attack and lack of sediment supply. Parallel, hummocky or slightly inclined lamination with reworked, bioturbated, shelly sands are typical (Fig. 16c). Strong bioturbation occurs because of a low rate of sedimentation or erosion. FACIES 4 (SW) The swash-bar facies is found in the shallow, wavedominated zone, and develops where tidal currents are weak. It is generally shallower than 7 m below mean sea level. It consists of clean, fine grained, well-sorted sand which is regularly bedded, with thin, parallel, and finely graded laminae (Fig. 17a). FACIES 5 (SS) Figure 16. A. Active ebb delta lobe facies (ACDL). Megaripple crossbedded foresets with relatively coarse grained, poorly sorted, shelly sand erosionally overlying slightly inclined, parallel, finely laminated, thin, graded laminae with fine grained, well sorted, clean sand. Location: Texel Inlet. Scale in cm . B. Distal delta lobe facies (DSDL). Well preserved mud drapes are interlayered with ripple crosslaminated sand. An escape burrow is visible in the middle. Location: Texel Inlet. Scale in cm. C. Abandoned ebb delta facies (ABDL). This facies is characterized by strongly bioturbated sand. Location: Texel Inlet. Scale in cm. The supratidal shoal facies occurs on the surface of the shoals above the normal high water line (Fig. 14). Megaripples form during storm tides, and salt pans occur within depressions between aeolian dunes. Wind ripples and winddune foresets are common. The sand is fine grained and well sorted. Locally, shell layers are present. FACIES 6 (TCH) The tidal channel facies, found in active tidal channels usually shows thick, cross bedded foresets, sometimes with 210 L. P. SHA AND P. L. DE BOER Figure 17. A. Swash bar facies (SW). Well sorted sand with graded, thin laminae. Planar inclined bedding with dark and light laminae indicate heavy mineral concentrations formed by wave action. Location : Terschelling Inlet. Scale in cm. B. Tidal channel facies (TCH). Coarse grained sand with abundant mud balls, shell and shell fragments. Large-scale crossbedding shows topsets, foresets and bottom sets. Location: Ameland Inlet. C. Abandoned channel facies (ABCH). Dominance of eutrophic organic mud , and lenticular sand ripples. Location: Ameland Inlet. Scale in cm. D. Partly abandoned channel facies (ABCH). Sand dominates, but is strongly bioturbates. Location : Terschelling Inlet. Scale in cm. EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 211 toe-sets, topsets, current ripple sets, reactivation surfaces, and incomplete mud drapes (Fig. 17b). Sediments consist of reworked, poorly sorted, medium to coarse grained sand with many shells and shell fragments, mud pebbles and, in the thalweg, gravels. The proportion of coarse grained sand, shells and shell fragments, mud pebbles, gravels and large foresets is higher in the deepest channels. The orientation of the large foresets depends on the dominant current direction in the channels. FACIES 7 (ABCH) The abandoned channel facies is present in channels that are being abandoned in relation to attachment of shoals to the shoreline. The example in Figure 17c is from Ameland Inlet. The sediment consists of organic-rich mud with intercalated (lenticular) sandy ripples. This type of sediment is commonly bioturbated. In the early stage of abandonment, currents are still relatively strong, and prevent the accumulation of mud. As the tidal currents decrease, bioturbation becomes prevalent (Fig. 17c, d). FACIES 8 (LPT) The locally protected facies was found only in Texel Inlet on the landward side of the shoal 'Noorderhaaks', and does not seem to be a common facies. Mud is deposited because of low wave energy, and intercalated, lenticular, waverippled sands likely produced by storms, are also present. This facies is strongly bioturbated and in many respects is similar to the abandoned channel facies. FACIES 9 (lSTSS) The inner-shelf tidal sand-sheet facies is found seaward of the ebb-tidal deltas. Strictly speaking, it does not belong to the ebb-tidal delta setting. The sand is usually coarse grained, poorly-sorted, shelly, and reworked from underlying relict deposits. Megaripples foresets with topsets are common, but intensively bioturbated. SEQUENCE MODELS The facies analysis, in combination with morpho dynamic studies, provides a framework for constructing theoretical sedimentary sequences for ebb-tidal delta deposits. Three characteristic sequences are described, representing the effects of progradation and abandonment of ebb-tidal deltas. PROGRADATIONAL EBB DELTA LOBE SEQUENCE The active ebb-tidal delta lobe pro grades seaward over the inner-shelf tidal sand sheet, which consists mainly of reworked, underlying relict deposits. Thus a sequence of reworked inner shelf tidal sand, distal ebb delta lobe deposits and active ebb-tidal delta lobe sediments is produced. The part of the sequence produced by delta lobe progradation is upward coarsening (Fig. 18a). Bioturbation is fairly intense at the base, and diminishes toward the top. ABANDONED EBB-TIDAL DELTA LOBE SEQUENCE In the case of deposition within a subsiding basin (i.e.; under conditions of relative sea level rise), abandonment of an active ebb delta lobe will be followed by a transgression of the sea. In the present-day West Frisan Islands this is especially the case, because of the glacio-eustatic rise of sea level. Marginal flood channels and swash platform/swashbar facies form after abandonment. The marginal flood channels turn clockwise and landward, and simultaneously, shoals and swash bars migrate onshore. Thus a sequence representing the marginal flood channel facies, migrational swash bar and shoal facies is successively produced (Fig. 18b). The migrating shoal produces low angle foresets on the landward side, which are reflected in the sequence model. The sequence is truncated at the top by a ravinement surface formed during shoreface retreat. A reworked, inner-shelf tidal sand sheet covers the sequence, and if the rise of sea level continues, the series will be covered by shelf mud deposits. Below such a sequence, older deposits of the active ebb-tidal delta or of the main ebb channel may be present. In the distal area, a marginal flood channel facies may not develop, and a ravinement surface may directly truncate ebb delta foresets. INLET SEQUENCE In the proximal part of the ebb-tidal delta, migrating channels may produce a sequence with marginal flood channel deposits on top of a channel sequence produced by lateral migration of ebb-dominated channels (Fig. 18c). THREE-DIMENSIONAL GEOMETRY AND PRESERVATION POTENTIAL SAND BODY GEOMETRY OF EBB DELTA AT TEXEL INLET A three-dimensional picture of the ebb delta sand body in Texel Inlet was constructed based on boreholes and seismic profiles (Fig. 2; Sha, 1989d). The ebb-tidal delta sand body unconformably overlies deposits of the upper Pleistocene, that is, Weichselian, Eemian and Saalian. The sand body is up to 30 m thick in the central part of the ebb-tidal delta, and radially pinches out seaward. The internal geometry of the ebb-tidal delta lobe is revealed by seismic records. The stratigraphic succession at the Pleistocene-Holocene interface and the characteristics of the lower boundary (Fig. 2) of the ebb delta sand body are closely related to the evolution of the ebb-tidal delta. The high-lying Pleistocene surface was recently incised by deep tidal-delta channels. In the southern part of the ebb-tidal delta, Weichselian deposits have been preserved (Fig. 2). This indicates that the great depth of the tidal channels is a recent phenomenon. As described above (Fig. 9), the main inlet system was situated farther north in the past, and has deepened and shifted because of the increase of tidal prism. Thus, in the northern 212 L. P. SHA AND P. L. DE BOER A 4 I j Phi 2 0 ! Phi -2 !! BIoturbation Facies I ! B Top of the active delta lobe 2 0 -2 !! Itt Bioturbation ~% Facies 'IIW WWWM P Shelfrrud Inner-shelf tidal sand 800m Ravilement suiace E '" Distal lobe o Lobe front Swash bar and platform Migrational shoal - - Base of the ebb -delta lobe Inner-shelf tidal sand 800m (rewor1<ed old deposits) Marginal flood channel (fIood-<lrientedl Phi LEGEND 0 -2 I t c 4 I I 2 ! I Bioturbation Facies [ 'VWW MW M P T E 10 Eolian dune Beach ridge/spit (wave-dofriatedl (MI!7a tIonaO shoal Marginal flood channel (fk>od-orientedl -....:z. = §§§ sand mud drape wave ripple • mud small- scale ripple OOrizontai lamination low angle lamination mega ripple cross bedding large-scale cross bedding mud pebble stone pebble sheP and sheil fragment wood o /ljljj' Main inlet channel ~ " () 0 (ebb-or flood-otiel rtedl Inlet channel floor ~- d9 Figure 18. A. Hypothetical sedimentary sequence of a progradational active ebb delta lobe. B. Hypothetical sedimentary sequence of the abandoned ebb delta lobe facies. C. Hypothetical sedimentary sequence formed by an migrating inlet. Paleocurrent directions in the lower and upper part are more or less opposite. part of the ebb-tidal delta, Upper Pleistocene deposits have been eroded, and the depth of the lower boundary of the ebb-tidal delta sand body, and its thickness, increase southward (Figs. 2, 19). LOWER HOLOCENE EBB-TIDAL DELTA DEPOSITS OFFSHORE OF TERSCHELLING AND AMELAND A seismic survey offshore of Terschelling showed that ebb-tidal delta deposits can be preserved during transgression and shoreface retreat. It was shown (Sha, 1990a) that the preservation potential of ebb-tidal delta deposits depends mainly on the maximum depth of shoreface erosion and the depth of the seaward limit of the ebb-tidal deltas. These features are related to many factors, such as rate of relative sea level rise, rate of sediment supply, open-marine wave and current energy, and the tidal prism through the inlet. The morphological map of the Terschelling coast (Fig. 20) shows the configuration of a preserved ebb-tidal delta (20 m isobath), which is not relevant to the present-day dy- namic system (present-day depth of shoreface erosion is about 15 m; Postma and Kroon, 1986). Seismic lines show internal structures indicative of lateral migration of an inlet channel (Fig. 21). The angle of the accretion surfaces is similar to the slope angle of recent inlet channels. Borehole data reaveal that they must be Holocene shallow marine deposits (Sha, 1990a). A shore-transverse line (Fig. 21) shows very lowangle, seaward-dipping foresets of the ebb-tidal delta lobe and chaotic reflectors of swash platforms and ebb delta shoal deposits on the seaward side of large inlet-fill deposits (large tidal channel fill). A seismic line off Ameland even shows a sequence of large, migrational inlet channel fills (oblique reflectors) with a series of small channel fill features reflecting the cyclic migration and abandonment of small marginal flood channels (Fig. 22). REWORKING OF EBB-TIDAL DELTA SANDS Ebb-tidal delta sands can be preserved another way; that is, through reworking into shoreface-connected or noncon- EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 213 -6 4° 40' 12.5, 15, ',,_ \ N I I / / 17.5, \ 20 \ " I \ \ , \ '' TEXEL / \ \ 53° l/) C\J \ \ I I I -----depth (m) 10 I I I I below MSL I I \ \ , I I \ \ \ 3km \ Figure 19. Depth of the lower boundary of the ebb delta sand body of Texel Inlet (contours in metres below MSL). This boundary represents the Pleistocene-Holocene interface. The thin, continuous lines show the present-day water depth in metres below MSL. nected ridges (cj. Duane et al., 1972). This morphodynamic process has been reported from a tidal inlet in the southwest part of The Netherlands, which was closed in 1972 (Kohsiek, 1988). After the artificial closure of the tidal inlet, the ebb-tidal delta transformed into large, longshore bars in a period of about 15 years. Small ridges, commonly present offshore of former tidal inlets along the West Frisian Islands, documented by historical maps and geological data, also are suggestive of the reworking of ebb-tidal delta sands into sand ridges. DISCUSSION COMPARISONS WITH OTHER EXAMPLES (NORTH A'1ERICA, EAST FRISIAN ISLANDS) Morphological models described for the North American Atlantic coast inlets and ebb-tidal deltas mostly show an interaction of inlet tidal currents and offshore waves (Hayes, 1975, 1979, 1980; Oertel, 1975). Tidal currents through the inlets form the main inlet channels, which extend seaward. Waves dominate the ebb delta shoals, form swash platforms, swash bars (Oertel, 1972), and can produce an asymmetrical geometry of the ebb-tidal deltas (Oertel, 1975; Nummedal and Fischer, 1978) and of the inlets (downdrift offset; Goldsmith et al., 1975). The tidal current patterns around the inlets can also be strongly modified by waves and wave-induced longshore drift (Finley, 1978; Lynch-Blosse and Kumar, 1976; Davis and Fox, 1981). In these examples, wave influence is strong, and tidal influence, especially the interaction of offshore tidal currents and currents through the inlet, is weak compared to the West Frisian inlets and ebb-tidal deltas. Hypothetical sedimentary sequences constructed from data collected on one of the most tide-dominated ebb-tidal deltas along the North American Atlantic coast, (Edisto Inlet, Imperato et al., 1988) show a predominantly wavedominated sedimentary facies: swash platforms, swash bars, 214 L. P. SHA AND P. L. DE BOER 5° 20' E 5 0 30' 50 40' / / .... -- _--_ ---, ...... .... -------\ i I \..- ~ I I I i I (. \ __ ..... _3' .... _-_-,0#-------------- --- --'- . . . . . -----..:; . . -_ 53 0 30' N 20 m I \ \ , /--- ------------)".- _ .::::, ... L. (l 1,,\ , I +- \ \ ./ '- (.~ . .'-, .:.--.-...... l/'l.\. r-- . -.- .j 5rn . 1... V 1...- • • Ub'v'~'_ AMELAND . . . . • 't' 5 km L Figure 20. Morphological map of the Terschelling coast showing the locations of seismic lines of Figure 21. The ebb delta-like feature is shown by the 20 m contour (contours in metres below MSL). abandoned channels and shoreface deposits. A tidedominated facies was found only in the channels. Tideinfluenced, ebb delta lobe and distal lobe facies, and offshore tidal sands immediately adjacent to the ebb-tidal delta, as found in the North Sea (cf., Sha, 1990b), were not reported from North Edisto Inlet. Such differences again demonstrate the relative importance of the tides in the West Frisian tidal inlets. Seismic and other surveys along the North American Atlantic coast indicate a low preservation potential for inlet channel deposits during the Holocene transgression (Field, 1980; Belknap and Kraft, 1985; Hine and Snyder, 1985; Panageotou and Leatherman, 1986). No sub-Recent ebb-tidal delta deposits have been reported there. Strikingly, the depth of shoreface scour is about 10 m in that area (Belknap and Kraft, 1985), whereas Lower Holocene inlet and ebb delta deposits have been preserved offshore off Terschelling where the depth of shoreface scour is 15 m (Postma and Kroon, 1986). Therefore, the preservation potential of inlet and ebbtidal delta systems clearly does not only depend on the depth of shoreface erosion, but also on the depth of inlet channel scour and the seaward extension and depth of the seaward end of the ebb-tidal delta. These features, in turn, are related to the size of the tidal prism through the inlets. Therefore we speculate that the difference in tidal influence between the two areas is responsible for the difference in preservation potential. Tide-dominated inlet systems have a greater preservation potential than wave-dominated ones if other factors are similar, implying that inlet and ebb-tidal delta systems that have been preserved in the fossil record have experienced a relatively strong tide dominance. The difference in tidal influence between the West Frisian ebb-tidal deltas and the ebb-tidal deltas of the North American Atlantic coast is attributed to two causes. Offshore tidal currents in the North Sea are relatively strong as a result of the geometry of the North Sea basin. The strength of tidal currents through the inlets is, moreover, related to the tidal prism through the inlets, and the volume of the storage basin behind. The latter is much larger in the case of the West Frisian inlets (Wadden Sea) than in the tidal inlets of the North American Atlantic coast. For instance, the tidal prism of Texel Inlet exceeds 1000 million m\ whereas tidal EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 215 - 60mr--- - - - ------ - - -,- - --.- - - --.- - - 1050 m -- -- - , - - - -- - -,-I I -- '- - - --- -.. (Pleistocene) _ _ _ _ - - ----- _._- -------,-- -- ----- SE ~J O.-------------------------------------~--------------------------------~O K about 9 km A B NW I- 15 sea floor 20-1------ 4060ms -----~~~~~~~~~==~ ~~~~~~==~~~.~~========~t30 barrier island inlet channel --- main ebb channel delta shoal delta foresets , 1..J. -~----..::~ ~ ~~--- -----=-~ top of P1elS tocene' ' If t Pleistocene 1-45 m -:...,.-:i...:" (Holocene) top of Eernsn deposits . "--' (Pleistocene) --B 1 _- 1500 m -- - ...... - - - 1450m - - - - I~ -- =---, - Figure 21. (Opper) Seismic line (C-D) parallel to the coast of Terschelling showing a low angle, clinoformal (channel fill) reflection pattern indicating lateral migration of an Early Holocene inlet (for location, see Figure 20) . (Lower) Seismic line (K-L) perpendicular to the coast of Terschelling showing low angle seaward-dipping reflectors (shingled) and chaotic reflectors indicating relict, early Holocene ebb delta deposits (for location, see Figure 20). The inlet (channel fill reflection pattern) is landward of the preserved delta shoal deposits (chaotic reflection pattern)_ Compare with Figure 2. The two seismic records below correspond to the intervals indicated by horizontal lines at the right side, above the line interpretation. prisms of the North American Atlantic inlets seldom exceed 300 million m) (Jarrett, 1976). C ONCLU SIONS Similar sand transport patterns and morphologies are found in the ebb-tidal deltas of Texel, Terschelling and Ameland inlets. The sand transport patterns are strongly controlled by the interaction of tidal currents through the inlet and marine tidal currents parallel to the coast. This interac- tion enhances bidirectional tidal currents in the southwestern (updrift) part of the ebb-tidal deltas and results in a net seaward sediment transport there. In the northeastern (downdrift) part, it produces a rotational tidal current pattern and flood-dominated sand transport. The relatively great strength of the tidal currents through the inlets is the main reason for the aberrant morphology of the ebb-tidal deltas of the West Frisian Islands as compared to systems described from other places, for example the East Frisian Islands. The inlets 216 L. P. SHA AND P. L. DE BOER 1 1 -_...::..... West 30 pulse length sea floor m 31msl Figure 22. Seismic line off Ameland (parallel to the coast) showing a sequence of large migrational inlet channel fills (oblique reflectors) with a series of small channel fill features reflecting the cyclic migration and abandonment of small marginal flood channels. A small time delay was applied during graphic recording. Water depth is approximately 10 m. with large tidal prisms have large ebb-tidal deltas, which are dominated by the interaction of tidal currents through the inlets with tidal currents parallel to the coast. This causes the large ebb-tidal deltas of Texel, Terschelling and Ameland inlets to be asymmetrically directed to the southwest and west (updrift). The inlets with small tidal prisms have small ebbtidal deltas dominated by waves. Consequently wave attack and eastward-directed longshore drift drive the smaller ebbtidal deltas of the East Frisian Islands asymmetrically to the east (downdrift). The above model, which explains the differences in morphology between different modern ebb-tidal deltas, may also be applied to explain changes of ebb-tidal delta morphology through time. Indeed, the ebb-tidal delta of Texel Inlet was more wave- and less tide-influenced some centuries ago, and became more and more tide-dominated during the last centuries because of an increase of the tidal prism through the inlet. With respect to their morphological evolution, the three larger ebb-tidal deltas show another common feature; that is, a cyclical migration of shoals and channels in the downdrift (north to east) part. For the ebb-tidal delta of Texel inlet, four cycles have been recognized for the period since the 16th century. The cyclical pattern in morphological development is the result of a feedback response between the morphology and the dynamic processes. In this process the main, ebb-dominated channel is pushed downdrift by wave action and longshore drift; the pathway that tides must cover in order to pass through the main channel thus becomes longer, until a certain limit is reached, at which time another, up to then subordinate channel farther to the south takes over the role of main channel. The former, main, ebb-dominated channel degenerates into a marginal flood channel. In relation to these shifts and rotations of channels, ebb delta shoals attach to the island to the northeast. Nine sedimentary facies were recognized in the ebb-deltas of Texel, Terschelling and Ameland inlets. By combining the facies distribution of surface sediments with the morphodynamic processes, schematic models of sedimentary sequences can be constructed. These models represent hypothetical successions formed by prograding ebb-tidal delta lobes, by the abandonment of active ebb-delta lobes, and by migrating inlets. The lower bounding surface of the ebbtidal delta body of Texel Inlet is a stratigraphic unconformity situated some 50 m below mean sea level, and formed through erosion by migrating channels. The top of the sandy body is above mean sea level locally, and the thickness of the ebb delta sands may be up to 30 m. The three dimensional architecture of the ebb-tidal delta, reconstructed from drilling and seismic lines, shows that the sand body represents a volume of more than one cubic kilometre. Preserved ebb delta/ inlet deposits are recognized on seismic profiles off Terschelling and Ameland. These profiles shows low-angle, seaward-dipping foresets of the ebb delta lobe and low-angle, inclined beds formed by the inlet channel, migrating laterally, parallel to the coast. A prerequisite for preservation is that the maximum erosional depth of the regional shoreface is less than the depth of the lower bound- EBB-TIDAL DELTA DEPOSITS, WEST FRISIAN ISLANDS 217 ing surface and the base of the seaward extension of the ebbtidal delta. In other words, the ravinement surface formed during shoreface retreat should be positioned above the lower bounding surface of the ebb-tidal delta. The lower bounding surface is generally produced by migrating channels or is formed by the pre-existing sea bottom in the more distal parts. The relative positions of these bounding surfaces depend on rate of relative sea level rise, rate of sediment supply, wave energy, and the tidal prism through the inlet. We infer that tide-dominated inlet systems have a greater preservation potential than wave-dominated ones, if other factors are similar. 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