Marine Geology, 84 (1988) 19-30 19 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
MIGRATION OF MUD BANKS AND CHANGING COASTLINE IN FRENCH GUIANA
J.M. FROIDEFOND, M. PUJOS and X. ANDRE
Département de Géologie et Océanographie (I.G.B.A.), U.A. 197, C.N.R.S., Université de Bordeaux I, 33405 Talence (France)
(Received November 9, 1987; revised and accepted May 31, 1988)
Abstract
Froidefond, J.M., Pujos, M. and André, X., 1988. Migration of mud banks and changing coastline in French Guiana. Mar. Geol., 84: 19-30.
The French Guiana coast is characterized by migrating mud banks which act not only as markers of hydrodynamic
and climatic processes but also have an important impact on coastal evolution. The role of mud banks was documented by vertical aerial photographs taken in 1979, 1982 and 1984. These allow the delineation of the intertidal parts of the banks at low tide and at identical tidal ranges. The following results were obtained: (1) Six mud banks separated by interbank eroding areas are present along the French Guiana coast. The intertidal parts of three of these banks are cut by river mouths. (2) Between 1979 and 1984 the intertidal parts of the banks migrated to the west-northwest along the coast at rates ranging from 320 to 1220 m/yr (mean rate = 900 m/yr). (3) In the same period the extent of the intertidal areas colonized by new mangroves (60 km2) was nearly the same as that of the areas bearing older mangroves (58 km2) that were lost by erosion, while the nature of the surface covered by the intertidal parts of the banks remained nearly the same in spite of the migration of these banks. These results suggest the existence of hydrodynamic circulation cells linked to large coastal mud banks along northeastern South America. Additionally, historical data (1955-1972) show long-term variations in the velocity of mud-bank migration.
Introduction
The northeastern South American coast is bordered in the west by the mouth of the Orinoco River, and in the east by the mouth of the Amazon. Sediment discharged into the sea by the Amazon is estimated to be 731 million tons per year (499 million tons of suspended material and 232 million tons of dissolved material (Gibbs, 1967, 1976). The plume of fine suspended sediments from the Amazon trans-ported by the Guiana current flows along the coast in a zone 20–40 km wide as far as the mouth of the Orinoco River which is situated 1600 km from the mouth of the Amazon (Fig.1). As well as being transported in suspension (150 million m3/yr) this fine sediment is carried in
mud banks (100 million m3/yr) which migrate from east to west (Nedeco, 1968). In 1962 21 mud banks were counted between Cayenne in French Guiana and the Waini River in Guyana (Delft Hydraulics Laboratory, 1962) (Fig.1). The sedimentological and hydrodynamic pro-cesses in the Surinam mud banks have been studied (Nedeco, 1968; Augustinus, 1978; Wells and Coleman, 1981; Rine and Ginsburg, 1985) and the Guyana mud banks have been investigated by the Delft Hydraulics Laboratory (1962), Eisma and Van der Marel (1971) and Barreto et al. (1975). Additionally, in French Guiana mud-bank migration was described by Yayer (1948) and Lemiere (1953). No significant study of the French Guiana mud banks has ever been published; only internai reports
Fig.1. The plume derived from the Amazon River runs along the coast along a zone 20-40 km wide up to the mouth of the Orinoco River situated 1600 km farther west. A proportion of the fine sediment is transported as mud banks. (From Delft Hydraulics Laboratory, 1962). (L.C.H.F., 1976, 1977a, b, 1978) or short notes (Choubert and Boye, 1959; S.H.O.M., 1975; Turenne, 1978) have mentioned their existence.
In this study photographs and the available maps have been studied in order to identify major coastal variations. The obtained results concern (1) the morphology of the sectors of accretion and erosion, (2) displacement of the intertidal part of the mud banks, (3) comparison of surfaces which have been eroded and accreted, and (4) the displacement processes of the mud banks. Setting
The French Guiana coast extends over a distance of 350 km between Brazil and Surinam, and is associated with a 10 km wide coastal plain. The formation of the coastal plain is closely related to its relative position with respect to mean sea level, and it has been rising since the last glacial period until 6000 yrs ago when it reached its present level (Brinkman and Pons, 1968). The uppermost part of a Cretaceous—Quaternary clastic wedge forms the Holocene muddy coast and covers the northern margin of the Guiana shield (Precambrian igneous and metamorphic rocks). Tectonic movements appear to have
been relatively minor. The coastal plain is characterized by mangroves near the coastline and by grassy marshes covered with a network of linear relit beach ridges (cheniers) (Price, 1955). In French Guiana, these generally con-sist of a medium—coarse-grained fluviatile sili-ceous sand. The mud covering the inner shelf (from – 20 m to the shoreline) prevents any input of sand from the outer shelf. The continental shelf north of the Guianas has an average width of 150 km and a smooth slope (averaging 1/1600) towards the outer edge which lies in 90—100 m of water. The inner part is covered with fine sediment, at least down to – 20 m, which, on average, is 40 km from the coastline (Nota, 1958; Bouysse et al., 1977; Pujos and Odin, 1986). The clay composition of the Cayenne and Kourou mudflats is usually 40% illite, 30% kaolinite, 17% smectite and 13% chlorite, comparable with other Guiana coasts. Several outcrops and small islands of crystalline rock do occur along the coast of French Guiana however. Morphology of the accretion and erosion sectors
After an initial flight over the region and a field survey in 1984 (Froidefond et al., 1985) a short final investigation in 1986 was made to collect additional data. Vertical aerial photo-graphs (altitude =4000—4500 m) were taken by the Regnam Society and the whole coastline was covered in the course of three flights as follows: from October 5 to 9, 1979 (between 10.30 a.m. and 2 p.m.; 114 photographs), on September 5, 1982 (from 12.15 a.m. to 2.45 p.m.; 155 photographs), and on October 24, 1984 (from 9.50 a.m. to 11.05 a.m.; 110 photographs). All photo-graphs (monochrome) were taken using a Wild RC8 9 (f 52) camera at low tide, at the sanie tidal ranges in order to permit comparisons.
The photographs show alternations of accre-tion sectors (the intertidal part of the mud banks connected to the coast and partly covered by mangroves) and erosion sectors. The intertidal areas of the mudflats are 20—40 km long and up to 3—5 km wide at low tide,
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while the erosion sectors range from 8 to 26 km in length. The intertidal mudflats and erosion sectors are generally bordered by white mangroves (Avicennia nitida) and locally by cheniers with sand beaches. Between 1979 and 1984, nine intertidal mudflats alternated with erosion sectors or were separated by river mouths (Fig.2). These mudflats are part of six
large mud banks, three of these straddling the mouths of the rivers. Bach bank includes two intertidal mudflats. From west to east the observed sectors in this area are as follows: (1) The Cape Isere erosion sector just to the east of the Maroni; (2) the Magnan intertidal mudflat facing the Magnan Canal (mud bank 1, Fig.3); (3) the Iracompapi erosion sector, (4) the
Fig.2. The six major coastal mud banks of the coast of French Guiana. The emerged parts of the banks form intertidal mudflats ("slikkes").
Fig.3. Magnan mudflat (mud bank 1) exposed at low tide over a width of 2 km and a length of 21 km (1982). In the west, this mudflat was covered by numerous narrow braided channels, while in the east it was crossed by very few, but much more distinct channels.
\ O l l MAGNAN b k (1 )
BRAZIL
Coastal plain , , +u+u Melamorphlc & F_ I g n e o u s rocks (Gulana shield ) 0 10 20 30 40 50 km I ____ r ________ r ______ r_______r _ ,
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Organabo intertidal mudflat (mud bank 2, Fig.4) in front of the Organabo River, (5) the West Iracoubo erosion sector, (6) the Iracoubo intertidal mudflat (mud bank 3, Fig.5) between the Iracoubo and Sinnamary rivers, (7) the East Sinnamary erosion sector, (8) the Kourou intertidal mudflat (mud bank 4, Fig.6) cut by the Kourou River at its eastern end, (9) the East
Macouria erosion sector, (10) the Cayenne-Mahury intertidal mudflat (mud bank 5, Fig.7) cut by the Mahury River, (11) the erosion sector east of the Kaw River, and (12) the Approuague-Pointe Behague intertidal mudflat (mud bank 6, Fig.8) cut by the Approuague River, and bordered by the Oyapock River near the Brazilian border. The delineation of these
Fig.4. Organabo mudflat (mud bank 2), 26.6 km long and about 3 km wide (1982) at low tide. In the central area where the damping of waves in the fluid mud is important (Wells and Coleman, 1981), the extent of the mangrove locally increased by over 1500 m during the period of measurement.
Fig.5. Iracoubo mudflat (mud bank 3) between the mouths of the Iracoubo and Sinnamary rivers reached a length of 18.5 km and a width of 4 km (1982).
Fig.6. Kourou mudflat (mud bank 4). This mudflat was 4.3 km wide in front of the Kourou Space Center while its length exceeded 40 km at low tide. In the west, tidal channels were ubiquitous and braided, in the east they were fewer in number and wider.
ti
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Fig.7. Cayenne—Mahury mudflat (mud bank 5), colonized by mangrove. This mudflat is part of a vast bank cut by the Mahury estuary and necessitates dredging of the entrance to the Degrad des Cannes harbor.
Fig.S. Approuague—Pointe Behague mudflat (mud bank 6), situated on both sides of the Approuague estuary may be considered as one mud bank, cut in the middle by the estuary. The Pointe Behague mudflat is surrounded by two large rivers (Approuague and Oyapock).
alternating sectors agrees with the results of Augustinus (1978) in Surinam. Accretion sectors
Each intertidal mudflat is bordered to the west by the front slope of the mud bank, to the east by its back slope, to the north (ocean side) by the subtidal part of the bank, and to the south by mangrove swamps or by landlocked cheniers (Fig.9). In contrast to the Surinam case (Augustinus, 1978), in French Guiana the morphology of the subtidal parts of the mud banks is not known because there are no detailed bathymetric charts. In Surinam the form of the mud banks becomes indistinct
20-40 km from the coast at depths of about 20 m. Mud banks are generally orientated N24° to the coastline (Nedeco, 1968), and a typical bank is asymmetric with the northwest side (leeward) steeper (1/500) than the southeast (windward) side (1/3000) (Nedeco, 1968; L.C.H.F., 1978). The maximum thickness of a mud bank is approximately 5-10 m with a volume varying between 2 and 6 billion cubic meters (average = 3 billion cubic meters). The sediment type is unconsolidated fine fluid mud (locally called "sling mud") (Nedeco, 1968). In French Guiana and probably also in the other Guianas, the intertidal mudflats are crossed by tidal channels which show two major aspects: In the west and close to the
1
APPROUAGUE - BEHAGUE bank 6
Approuague mudflat
Fig.9. Schematic morphology of an intertidal mudflat.
front slope the channels are small, numerous and interconnected (incipient morphology), while in the eastern part, close to the back slope, the channels are much less numerous, better developed, and large (mature morphology). Mature tidal channels may persist in-land when the mangroves encroach on the intertidal mudflats. Another feature of the mature channels is that they frequently run parallel to the shoreline for several kilometers before they turn to the ocean. Erosion sectors
In the sectors that are undergoing erosion, small embayments, 200-500 m wide create a sawtooth pattern in the coastline (Fig.10). Wave activity along this coastline causes the
uprooting and accumulation of mangrove trunks along the edge of the mangrove. At low tide, intertidal mud is exposed along a width of 100-300 m and between banks. the bottom consists of consolidated clay, while fluid mud is absent (Wells and Coleman, 1981). In the course of erosion the length of these sectors varies (8.4 km at West Iracoubo and 26.8 km at East Sinnamary). In 1984, the sectors undergoing accretion were longer (160 km) than the sectors undergoing erosion (100 km). Displacement of the intertidal parts of mud banks
All the intertidal (mudflat) parts of mud banks are migrating in a west-northwest direc-tion. The mouths of the rivers do not prevent
Fig.10. Schematic morphology of an erosion sector (interbank).
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)
this movement. The displacement of the front slopes (facing west) and of the back slope (facing east) has been measured, taking the points of inversion between erosion and deposition as a datum (Fig.11). In two cases (Cayenne—Mahury mudflat and Iracoubo mudflat), the front slope point is masked by the mouth of a river, in another case, the back slope point is
25 masked (Approuague—Pointe Behague). The error in this measurement is in the order of 200 m on average. The shifts which took place between 1979 and 1984 are shown in Fig.12.
After adding the average velocities (Fig.12) of the migration of the front slope and back slope and dividing by two, we found the overall average velocity of each intertidal mudflat as
Fig.11. Example of the westward migration of an intertidal mudflat showing the inversion points (the change from accretion to erosion).
Fig.12. Displacement (km) of the inversion points of front slopes and back slopes from 1979 to 1984.
2 6
having been as follows: Magnan, 970 m/yr, Organabo, 1220 m/yr, Iracoubo, 1000 m/yr, Kourou, 910 m/yr, Cayenne–Mahury, 1100 m/yr, and Approuague–Behague, 320 m/yr. These rates are lower than the average migration rate of 1500 m/yr for the Surinam and Guiana mud banks which was calculated from historical records (Delft Hydraulics Laboratory, 1962; Nedeco, 1968; Allersma, 1971). It should also be noted that differential displacement of the front and back slopes can cause a lengthening or shortening of the intertidal mudflats. For example, between 1979 and 1984 the Kourou mudflat has grown by 4 km and the Magnan mudflat has been reduced by 3 km. As a first approximation, we can assume, as Rine and Ginsburg (1985) did, that the westward mi-gration of the mudflats and mud banks results from deposition of soft mud on the west side where the tidal channels are interconnected and simultaneous erosion on the east side where the mud is more consolidated.
On the aerial photographs we noted a time lag of 7–12 yrs between the beginning of mudflat progradation and the beginning of significant mangrove (Avicennia nitida) coloni-zation. The mangrove colonies generally ex-tend seaward along the higher tidal channel levees. By the time the back slope of the mudflat reaches it the mangrove zone may be relatively wide. The mangroves are, however, still destroyed by the breaking waves. The sectors ûndergoing erosion (interbanks) between the intertidal mudflats migrate in the same way and at the same rate.
In the light of the initial results, two questions arise: (1) what is the surface area of each inter-tidal mudflat, and does this vary in time? and (2) is the land-gain oceanwards greater or smaller than the land-loss between 1979 and 1984?
The balance between eroded and accreted areas
Variation in the areas covered by the intertidal mudflats from 1979 to 1984
Measurements were made with a planimeter using maps at a scale of 1:86,000 drawn from
the aerial photographs. Bach intertidal mudflat was divided into several units and the area of each unit was measured three times. The absolute error is approximately 1 km2 for each value.
These results are illustrated in Fig.13. It appears that the mudflats and possibly the entire mud banks do not have uniform sizes. For example, the Kourou mudflat has twice the area of the Iracoubo mudflat, and three times that of the Magnan mudflat. Moreover, differ-ences in area between 1979 and 1984 are relatively small for the Magnan (1), Organabo (2), Iracoubo (3) and Kourou (4) mudflats and a little larger for the Cayenne–Mahury and Approuague–Behague mudflats (Fig.13). We believe that the larger size of the latter two mudflats is due to the tidal influence of river mouths. Determination of the areal extent of land-gain (mangrove colonization) and land-loss (erosion between mud banks) between 1979 and 1984
Measurements of surface area were made using the method just mentioned. The results shown in Fig.14 indicate land-accretion for each mudflat as determined by the growth in areal extent of the mangroves, and supratidal land-loss caused by erosion in areas between mud banks (generally involving destruction of the mangroves). In spite of a wide range of losses and gains between intertidal mudflat and interbank areas (Fig.14), there is almost a balance between net loss and gain for the whole of the studied coastline (gains = 60 km2 and losses = 58 km2). It is apparent from Fig.14 that the greatest variations concern the east coast of French Guiana where sandy cheniers are absent. Discussion — displacement processes in mud banks
The migration of the mud banks, which causes comigration of the interbanks, may be caused by three factors:
(1) Longshore drift caused by waves: Waves
f
1
Fig.13. Variations in the surface covered (km2) by the intertidal mudflats from 1979 to 1984.
Fig.14. Areas of land-gain and land-loss between 1979 and 1984. In spite of wide variations, the overall total is balanced during this period.
on the shelf, arriving from the northeast induced by the oblique approach and by the quadrant throughout the year, are 1–2 m high deformation of wave profiles damping in the on average and exceed 4 m during less than 1% fluid mud (solitary waves) can explain long- of the year (Nedeco, 1968). Longshore currents shore transport (Wells and Coleman, 1981).
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These authors suggest that a large energy potential exists for sediment transport in solitary waves. Between mud banks, where fluid mud is absent, waves are more sinusoidal.
(2) Tidal currents: Tidal current speeds in a Surinam mud bank intertidal zone are only in the order of 10—30 cm/s at 1 m above the bottom (Wells and Coleman, 1981). In contrast, tidal currents which create ebb and flow at the entrance to the river mouths perpendicular to the coast are very strong. Migration can, therefore, be slowed or accelerated by tidal currents near these river mouths.
(3) The Guiana current: The Guiana current originates in the southeast and transports the mud from the mouth of the Amazon to the northwest. In the surface waters on the inner shelf its speed may reach 1 m/s at 20 km. off the coast (Abbes et al., 1972). At 2 m above the sea floor, the speeds are much slower, at around 10—35 cm/s (Castaing and Pujos, 1976). Accord-ing to Wells and Coleman (1981), the residual current in Surinam, flowing to the northwest, reaches its maximum surface speeds of 10—20 cm/s near the coastline.
We know that the westward migration of the mudflats results from deposition of soft mud on their west side (incipient morphology) and simultaneous erosion on the east side (mature morphology) (Fig.15). The question of the most important dynamic force in mud-bank displace-ment can now be discussed.
According to Wells and Coleman (1981), this migration is governed by solitary waves and associated longitudinal currents, although when we compare the location of mud banks, this interpretation is not evident. First, the orientation of the northeastern South American coastline changes from the Amazon area (N—S) to the Orinoco area (E—W). Consequently, if the longshore current was the only factor in mud migration, the mud banks would migrate between the mouth of the Amazon and French Guiana in a direction opposite to that of the mud banks along the Surinam and Guyana coasts. In addition, the changing location of mud banks in 1955, 1972 and 1982 (Fig.16) indicates the variety of migration
Fig.15. According to Rine and Ginsburg (1985) stratification of the deposits suggests a migration movement similar to that of linear sand ridges in shelf environments. These banks (5-10 m thick) overlie consolidated muds. The migration may be caused by three types of currents: currents induced by solitary waves, tidal currents or the Guiana Current.
Fig.16. Location of mud banks along the French Guiana coast in 1955, 1972 and 1982.
velocities and the probable combination of the processes involved. The migration velocity was higher (1400—2000 m/yr) than the 1979—1984 value (900 m/yr). Further, the probable division of a giant mud bank located at Iracoubo into two parts in 1955 can be seen in Fig.16. This
Intertidal mudflot
M d b kP rog rad a tion
Inter.. bank
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observation points to a displacement of mud in suspension at a rate faster than the migration rate of the bank itself. As a consequence, a set of factors seems probable: As each wave passes on the mudflat, a cloud of sediment is put into suspension and near-bottom density increases (Wells and Coleman, 1981). The tidal current and the Guiana current, combined with the action of waves and winds displace the sus-pended mud. Indeed, according to Nedeco (1968), most mud movement occurs during annual periods of sea-level rise when wind and wave activity is greater than normal (Choubert and Boye, 1959). The boundaries between the initiation and cessation of these annual move-ments are recognizable on seismic profiles as inclined reflectors within the mud banks (Rive and Ginsburg, 1985). Conclusion
The coast of French Guiana is presently bordered by six major intertidal mudflats ranging from 20 to 40 km in length, and migrating along the coast to the west-northwest towards Surinam (mean rate of 900 m/yr between 1979 and 1984). These intertidal mud-flats, which are parts of mud banks, are separated by sectors undergoing erosion (inter-banks) and these interbanks are displaced in the same way as the intertidal mudflats.
Over the entire study area there is a net balance between land-gain, seen in the new colonization of mangroves on the mudflats, and land-loss caused by wave erosion between the mud banks. (Land gain reaches an area of 60 km2 and land-loss, 58 km2.) These results, combined with small variations in the surface area of the mudflats between 1979 and 1984, point to the mud banks being volumetrically stable and suggest the presence of circulation cells centered on each mud bank. However, a comparable study of the location of the mud banks between 1955 and 1984 reveals a more rapid displacement (1500–2000 m/yr) than that seen in the above-mentioned case, and the probable division of a giant mud bank into two parts occurred under this regime.
There are probably three simultaneous causes of displacement: (1) a higher than normal sea level, (2) strong waves that generate suspension processes, and (3) the tidal currents and the Guiana current which gener-ally transport the suspension from the east side to the west side of the mud banks. Acknowledgements
This research was carried out with a grant from the French "Ministère de la Recherche et de l'Industrie" (contrat CORDET/IGBA). Co-operation and field support were provided by the Direction Départementale de l'Equipement (DDE) and Dr. M.T. Prost in French Guiana and by Drs. R. Prud'homme and R. Griboulard (Université de Bordeaux I) and J.M. Rine (New York State University).
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