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Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 111 The effect of seismic vibrations on clay may be assumed to be small because the impulse per clay particle, which tends to displace the particle with reference to its neighbors, is very small compared to the adhesion between the clay particles (Terzaghi 1956, p. 7). Therefore, the clay may be assumed to act as a rigid body during vibrations. On the other hand, larger particles on a clayey surface might be thrown out of their former holes thus reaching a higher position (cf. Richter 1958, Figs. 5-4). Particles laid down on the surface or larger particles may be moved away by the friction against the water during the vibration or by currents initiated by earthquakes. All this might help nodules, slabs, etc. to remain at the surface for longer periods. Silty deposits have the character of forming a metastable structure which may collapse on slight provocation. The collapse is termed spontaneous liquefaction (Terzaghi 1956, p. 3). This problem will not be further dealt with here. Some irregularities in the distribution of particles of different size and density will be discussed in the following in the light of our experiences. INVERSE GRADING The foregoing discussion suggests that during jarring smaller grains on and near the sediment surface have a better opportunity of travelling downwards than larger ones. The larger grains will thus have a tendency to be at the surface, thus giving rise to an inverse grading. If, after additional sedimentation, new vibrations take place, this new layer will probably be inversely graded so that two layers with inversed grading may be found atop each other. Apart of such a pile may give the impression of being normally graded. In the interpretation of silty and sandy sequences from, e. g., deep-sea trenches, this possibility should be borne in mind. THE DISTRIBUTION OF NODULES AND MAGNETIC SPHERULES The concentration of the manganese nodules varies both regionally and locally (Mero 1960). Judging from photographs and corings, they occur mainly in a layer at the surface one nodule thick; however, they occur also further down but the concentration seems to be considerably less. The bulk of the manganese nodules seems therefore to occur mainly at the sediment-water interface. The nodules must be in contact with the water to accrete. Should the sedimentation rate exceed the rate of growth of the nodules, the nodules will be buried and cease to form. Since the rate of growth of the nodules is estimated to be of the order of 1 mm per 1 000 years
112 Bulletin de 1a Commission geologique de Finlande N: 0 212. (Pettersson 1945, Buttler and Houtermans 1950) or stilliess (Goldberg and Arrhenius 1958) while the reported rate of sedimentation for red clays and oozes ranges from 1 mm to several cm per 1 000 years, it raises the question as to why the manganese nodules are still at the interface. Shipek (1960) has discussed the problems regarding the mechanisms of the nodule formations and the factors involved. Here, only an additional factor, the influence of vibrations, will be discussed - this without denying the role of other factors. The uplifting effect on large particles caused by vibrations may be an additional force helping the nodules to remain at the surface for longer times and giving rise to high concentrations of nodules at the sediment-water interface also in regions with a comparatively high rate of sedimentation. In regions of major seismicity the top layer of the sediment carpet can be involved in seismic vibrations several times before reaching a depth of some centimeters. Therefore, the possibility of aredistribution of the grains must be considered. Also the presence of non-horizontal discontinuities may produce so me kind of turbulent cells during the vibrations which disturb the structure and texture of adjacent sediments. Coarse grains could accumulate in convergences at the surface. The density of the solids in foraminiferal oozes (and muds) is 2.69 g/cm 3 according to Hamilton (1960, p. 372). The density of manganese nodules is about 2.4 gjcm 3 but ranges between 2.0-3.1 gjcm 3 according to Mero (1960), the density of phosphoritic nodules is 2.62 gjcm 3 (Mero 1961, p. 5). Considering that the experiments are carried out with quartz sands (d = 2.65 gjcm 3 ) and glass balls (d = 2.53 g/cm 3 ) the results obtained from our investigations can be applied to foraminiferal deposits containing the nodules here mentioned. However, the comparatively high rate of sedimentation in such areas (cf. Olausson 1961) limits the influence of earthquake vibrations to the regions of major seismicity, where the uplifting effect may be able to keep pace with the sedimentation. In the oligotrophie regions where coccolithophorids are responsible for most of the carbonates, the rate of sedimentation is lower and, therefore, also seismic vibrations with a lower frequency and intensity may have an influence on the redistribution of the grains. The density of the solids in siliceous oozes is 2.1-2.3 gjcm 3 (for diatomite; Baas Becking and Moore 1959). Since manganese nodules may have an equally low density, it is our conclusion that the influence of vibrations on, for example, radiolarian ooze (muds) might be ab out the same as for foraminiferal deposits. Larger phillipsite crystals (d ~ 2.2 gjcm 3 ) and consolidated slabs (sensu Menard 1960 a) may be expected to remain at the surface as a result of vibrations. The slabs are assumed to have been exposed on the sea floor for 10 4 -10 5 years (op. eit., p . 35).
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