Sediment accumulation and modern sedimentary sequences

Sediment accumulation and modern sedimentary sequences in coastal bays, sea basins, water reservoirs, and lakes

The sediment transported by rivers will mainly be deposited in front of the river mouths in lakes and in coastal areas, and on wetlands of floodplain type, where the decrease in flow velocity and the presence of vegetation promotes sedimentation. The effect of salt water in eustarine mixing is to further enhance sediment removal by flocculation of clay particles. Transfer of the sediment from the water column to the land surface has important consequences both for the quality of the water and the properties of the wetland. In those deltaic areas where the land surface is subsiding, sediment removal from inflowing river water is essential for maintaining the marsh surface.

Partly overlapping radiographs of the upper part of core 961 from a depth (1983-08-11) of 28 m in Byfjorden, a contaminated fjord situated at the west coast of Sweden. Several sediment layers in this core may be used as time markers, since they consist of dredged material from known dredging operations.

The character and activity level of physical, chemical, and biological processes regulate the deposition and redistribution of sediment in coastal bays, sea basins, water reservoirs, and lakes and thereby also the composition and structural organization of the sedimentary sequence. The vertical sequence of sedimentary structures reflects variations in processes and rates of sedimentation with time. Often the sequence consists of a combination of cyclic and event types of stratification, as illustrated above. However, the primary sedimentary structures may be obliterated or distorted by erosion, bioturbation, and gasturbation. The sedimentary sequence therefore often contains both primary and secondary sedimentary structures.

Radiograph of the upper part of sediment core 523 (with bottom water on top) from a depth of 74 m in the western part of the Åland Sea, showing a considerable time gap in the sedimentary sequence between the coarse-grained top layer and the laminated, clayey deposits below.

As pointed out in the file "erosion..." and as exemplified by the radiograph above time gaps in the sedimentary sequence due to erosion are characteristic of high-energy environments. Erosion and resuspension of sediment along shallow and steep parts of a basin may be rather considerable in connection with heavy storms and bottom-seeking currents. Therefore, in shallow water, repeated reworking and redeposition of sediments tends to obliterate primary sedimentary stuctures. In deep water, the sediments can be completely mixed by bioturbation if the rate of sediment accumulation is low. Annually laminated modern sediments which reflect seasonal variations in the sedimentation rate as well as in the composition of settled particles, as exemplified by the radiographs below, are therefore most likely to be found in low-energy environments, where the rate of sediment accumulation is high or where the bottom water is permanently or periodically anoxic.

Radiographic comparison between the uppermost part of sediment cores 894 and 995 from a depth of 53 and 54 m respectively in Baggensfjärden, a bay of the Swedish Baltic coast. The cores, with bottom water on top, were transported and stored in an upright position, and X-rayed, the upper parts in stereo, before extrusion of the sediment from the rectangular, 3 cm thick and 6 cm wide, transparent coring tubes.

I document modern sedimentary sequences by X-raying sediment cores (the upper parts in stereo) before they are extruded from the coring tubes, as exemplified above, and together with a seven-stepped aluminium wedge, used for density calibration. The qualitative information obtained from the radiographic image forms an optical database that is quantified by the use of X-ray densitometric methods. The radiographically calculated dry weights are reported on a salt-free basis. The vertical variation in density-related sedimentological parameters, calculated from the variations in the film density along the radiation images of a sediment core, is examplified by Table 558Xu (only for the uppermost cm in sediment core 558 from Lake Saggat).

The principal components of the X-ray radiographic and X-ray densitometric techniques I am using are illustrated in the flow diagram below.

Flow diagram of radiographically based sediment analysis. Modified after Axelsson 1983 (Hydrobiologia 103).

The use of X-ray radiographic techniques simplifies the determination of sedimentary properties and rates of sediment accumulation. The core-to-core correlation is facilitated as well as the identification of primary and secondary sedimentary structures, the dating of growing sedimentary sequences, and the monitoring of environmental changes.

The rate of sediment accumulation can be assumed to vary with sediment yield, distance from point, linear and areal sources, flow conditions, density stratification, bottom topography, and the character of the transported sediment. The void ratio of newly deposited, fine-grained sediments is often high, indicating that they may be eroded and resuspended by rather weak currents. Sediment focusing, as a result of erosion and resuspension and the transportation of sediment from shallower to deeper areas, is therefore of great importance for the spatial variation in sedimentation rate. In Edsviken Bay, more than 70 % of the variations in sedimentation rate are explained by the variations in water depth. For areas with marker layers mainly consisting of redeposited dredged material, as for instance in the Swedish fjord Byfjorden, it is important to distinguish between mean and median values when calculating sedimentation rates. It is also important to calculate mass and not only linear sedimentation rates, since the latter varies with the degree of compaction.

I use the method of radiographic core-to-core correlation to calculate spatial and temporal variations in sedimentation rates. The formation of cyclic and event types of stratification are identified by the correlation of sedimentation units between sediment cores, which were sampled on the same day during different seasons and years, as well as before and after specific events.

Radiographic comparison between the uppermost part of sediment cores 740, 981, and 982 from Bråviken, a bay of the Swedish Baltic coast.

Coring sites 740 and 981 were situated rather close to each other at water depths of 29 and 26 m respectively. The distance between coring sites 981 and 982 amounted to about 200 m, but the difference in water depth was only 0.2 m. As shown, core-to-core correlations, based on X-ray radiographs, improve the possibilities of calculating spatial and temporal variations in sedimentation rate.

Downcore variation in content of solids and calculated annual sedimentation rate for the upper part of sediment core 982 from the bay Bråviken. From Axelsson 2002, Geo-Marine Letters 21/4. (The uppermost part of this core is shown by a radiograph above.)

The part of the varves which had the lowest density was formed in winter and was in this case taken as the boundary between the given sedimentological years. This means that the period for formation of the given annual varves may be somewhat longer or shorter than 365 days. The rate of sedimentation was high at coring site 982, but only 7 % higher than at coring site 981 for the six-year period 1978 - 1983. For this period the annual mean mass sedimentation rate (salt-free) amounted to 8.6 kg/m² and the annual mean linear sedimentation rate to 27.7 mm at coring site 982. However, it must be stressed that given linear sedimentation rates for modern deposits will generaly decrease with time and especially with increasing overburden pressure due to gravitational compaction.

The use of X-ray radiographic techniques, especially stereo-radiography and X-ray densitometry simplifies the core-to-core correlation as well as the determination of sedimentary properties and the quantification of sedimentation rates. See further Axelsson, V., 2002: Monitoring sedimentation by radiographic core-to-core correlation, Geo-Marine Letters 21/4. (However, the tonal range of the published X-ray radiographs is bad in relation to that of the originals, and observe that the publisher has added a point after html in the source code of the link to my homepage in the online version of my paper. Delete that point in the address, the link will work, and you can study better X-ray radiographs.)

So called sedimentation-compression curves (showing void ratio in relation to effective overburden pressure) are used to illustrate and to calculate sediment compaction. I use the accumulated amount of solids in g/cm², down to a sediment depth of 10 cm, as a hardness index for the classification of lake- and sea-bottoms.

X-ray radiographic and X-ray densitometric methods have also been used for the determination of reservoir sedimentation in order to predict the loss of available storage as a function of time. Examples are given for some tropical water reservoirs.

The procedure during coring and X-raying is described in a manual, revised in 1994 and based on: Axelsson, V., 1992: X-ray radiographic techniques in studying sedimentary properties and reservoir sedimentation - A Manual. In Jansson, M. B. and Rodriguez, A. (Eds), Sedimentological studies in the Cachí reservoir, Costa Rica. UNGI, Rapport Nr 81. (Also in spanish.) See further the X-ray radiographic analyses of selected sediment cores.

File in Swedish.

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