COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

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