Composition and structure of recent marine sediments in the Netherlands

Ch. I. The environment of the Dutch Wadden Sea, as well as that of the estuaries in the southwestern part of the Netherlands can be divided into three sub-zones: (1) the channel floors (sensu lato), i.e. the areas below mean low tide level, (2) the tidal flats (sensu stricto), between the levels of mean low and mean high water and, (3) the salt marshes, above the level of mean high tide. The channel floors are composed, either of older sediments, which have been laid bare by erosion of the tidal currents, or by new deposits, formed in the channels themselves. The latter are predominantly of sandy character, but may show locally high contents of muddy material, especially in sheltered bays. The tidal flats consist for the greater part of sand. Slightly muddy sand is often encountered along the high tide lines and very muddy deposits, dissected by small ebb gullies, are frequently present along the sides of the channels. The marshes are usually composed of comparatively clayey deposits, which are cut through by creeks. Details are given concerning the morphology of these sub-zones, and on the processes of erosion and sedimentation, which are responsible both for the morphology and for the composition of the sediments. The chapter is based on own observations (most of them published before) and on the results of a great number of other investigators (see list of references). Ch. II. The chief sediments in the Wadden Sea are (1) sands, (2) mud and clay, (3) shell beds. “True” hard rock gravels are very rare. They occur in the vicinity of outcrops of older, psephitic deposits (glacial till). Special mention is made of the formation of gravelly sediments, composed of small, limonitic clay-rhizo-concretions (found at the base of marsh cliffs and in the mass of the marsh deposits themselves). Granulometrical analyses of Wadden sediments have been carried out by a number of authors. Several conclusions may be drawn from their work: The Wadden sands are rarely pure, but contain mostly a certain amount of material < 16 \u03bc; the average grain size of the sands decreases usually from the tidal inlets inwards; the material which has been transported in suspension, most of the time, shows a remarkably uniform grain size distribution along the whole length of the Dutch coast (Doeglas, Favejee, Hissink, Zuur) etc. Ch. III. The investigated sediments are mainly composed of psammitic and pelitic, elements of the following minerals: quartz, carbonates, micas and clay minerals, felspars, glauconites and heavy minerals. The quartz percentages decrease with diminishing of the grain size of the material under consideration. Micas and clay minerals show an increase in this same direction. Grains of carbonates and felspars have their maximum distribution in the siltfractions. From the heavy mineral composition of the Wadden Sea sands it may be deduced that the greater part of the material has been brought in from the North Sea, via the tidal inlets (Crommelin). The same conclusion is reached with regard to the silt fractions (Crommelin) and the clay material (Favejee). Ch. IV. The organic matter of the Wadden Sea sediments is partly derived from older peat beds which have been eroded; for another part it is produced by plants and animals living in the Wadden Sea area itself. The basic organic materials, required for the growth of the latter organisms, are probably chiefly supplied out of the North Sea (Verwey). The organic content of the Wadden Sea sediments may show a decrease from the surface downwards, at least in the first few decimeters. This is presumably due to decomposition, under the influence of bacterial activities, by enzymes and by purely chemical processes. A notable parallelism is observed between the percentage variations of the organic matter and those of the material < 16 \u03bc. Ch. V. A close relation exists also between the percentages of material < 16 \u03bc and the iron content. This element is present in various authigenous compounds: Hydroxides of iron are found in marsh deposits (above the ground water table) and in the uppermost few millimeters or centimeters of the tidal flat and channel sediments. FeS.nH2O is formed in anaerobic environment, under the surface of the tidal flats and the channel floors. This substance tends to take up additional sulphur, thereby changing into pyrite. The pyrite is normally distributed in very small elements. Comparatively large, more or less globular aggregates of pyrite crystals (up to 40 \u03bc diameter) are seen in brackish water sediments. The transformation of iron hydroxides into monosulphuric compounds takes place in a short time. That of the FeS . nH2O into FeS2 requires at least half a century. Where the vertical accretion has been continuous, a gradual change in colour is observed between the deep black monosulphuric sediments just below the surface and the greyish, pyritic material at greater depths. Ch. VI. The major part of the calcium carbonate material is (primarily) formed by calcareous organisms (foraminifera, echinoderms, molluscs etc.). A minor amount may have originated in other ways, e.g. by bacterial activities and by chemical processes. A relation is found between the increase of the carbonate percentages and the amounts of material < 16 \u03bc. A maximum is reached in the fraction 2—16ft. Marsh sediments are subject to decalcification processes. The velocity of the solution of the carbonates depends on many factors, which require still further investigation. No decalcification phenomena are known from the normal tidal flat and channel floor sediments in the Netherlands. They have been observed, however, in the Basin of Arcachon (France). It is thought that the solution in this area is caused, at least to an important extent, by organic acids, produced during the decomposition of the large masses of dead Zostera remains, which are embedded in the sediments. Ch. VII. Considerable quantities of silica are formed on the surface of the tidal flats by the skeletons of (living) benthonic diatoms. The numbers of dead skeletons which are encountered in the tidal flat deposits themselves are, however, mostly very small. In the marsh sediments a more normal relation seems to exist between the amounts of skeletons of living and of buried diatoms. It is supposed that, after the death of the organisms, a solution or at least a beginning peptization of the silica takes place, which is swifter in the tidal flat environment than in the marsh deposits, probably in consequence of the higher alkalinity. The relatively coarse and less soluble skeletons of a part of the planktonic diatoms are of much more even distribution and are found in all Wadden Sea sediments. Other sources of locally formed silica are: radiolarians, sponges and plants. Ch. VIII. This chapter gives some conclusions, to be drawn from the study of thin sections of Wadden Sea sediments and of various deposits formed in more brackish water environment. The minimum grain size of separate sand (and silt) laminae is about 40 \u03bc in the former and down to at least 20 \u03bc in the latter. The parallel orientation of mica flakes and clay minerals is often much more pronounced in brackish (and fresh) water muds than in muds of Wadden Sea origin. Another conclusion, following from thin section analysis, is that brackish water deposits often show a coating of the sand grains, which may be due to peptization of clay material. Ch. IX. Useful evidence regarding the circumstances of sediment formation can be gathered from the structures as seen in undisturbed core samples. Several types of laminations are described. The laminae of channel floor- and tidal flat deposits have comparatively even, smooth upper and lower sides. The sand may show current or wave ripple structures. The laminae are of a sublenticular character and cannot be traced over great horizontal distances. The marsh laminations are characterized by the somewhat undulating, nodular aspect of the lamiae. The structures of beach deposits differ from the channel floor- and tidal flat laminations in that their laminae are more strictly parallel (apart from ripple mark structures). Another difference between these two laminations is that the latter are normally free of mud material. The finest laminations, with the thinnest laminae, are found in (some) brackish water deposits. The laminations and other primary oppositional structures may be disturbed by secondary influences: the burrowing of bottom dwelling organisms, the penetration of the sediment by plant roots etc. The effect of the burrowing animals is in general most pronounced on the highest parts of the tidal flats, where the sedimentation tends to be slow, but continuous. A sharp limit is often found between the disturbed deposits of the high tidal flats and the overlying marsh sediments, the lower parts of which are scarcely inhabited by bottom dwelling animals at all. Among the other structures, which are dealt with in this chapter, special mention may be made of the fissures, developed in mud beds under a permanent cover of water. Some new data are presented concerning their distribution and character, but no satisfactory conclusion about the manner of their formation is reached. Ch. X. The sediment properties, described in the foregoing chapters, are summarized and arranged according to their distribution in the various environments of formation.