The formation of alternating laminae of fine and coarse grains in
large-scale
sedimentary structures is a widespread phenomenon familiar to specialist
and nonspecialist alike.
However, the question
of how such periodic patterns are generated remains unanswered.
Previous attempts to explain the occurrence of stratified structures in
rocks have
been related to the existence of
periodic fluctuations in sedimentary condition, such as
oscillations in wind velocity.
Here we argue that stratification in rocks might occur in the absence of any
periodic external perturbation
[H. A. Makse, S. Havlin, P. R. King, and H. E. Stanley,
``Novel
Pattern Formation in Granular Matter'', in ``Lectures
on Stochastic Dynamics'', Series
``Lecture notes on Physics''
(Springer-Verlag, 1996);
H. A. Makse, S. Havlin, P. R. King, and
H. E. Stanley,
Experimental studies of stratification in
a granular Hele-Shaw cell,
Phil. Mag. B, 77 1341-1351 (1998)].
Specifically, we comment on the connection between
our
stratification
experiments and
the stratified patterns
observed in sedimentary structures.
A typical example of a stratification pattern in rocks is below
This is a photograph of a typical
Aeolian
sandstone taken from Petra, Jordan on March 31, 1997 by
H. A. Makse and H. Hlalat during a trip to Israel for the
Minerva Workshop and Bar-Ilan Conference, Israel.
The size of the
sample is 6 cm by 5 cm by 12 cm
tall.
The red (black) layers consist of
coarse grain material.
The white layers consist
of more exclusively fine grained material.
The wavelength of the layers is 0.9 cm.
The sedimentary rock
was formed predominantly by grainflow (avalanches) of windblown sand.
Other
processes, such as grainfall (``raining''),
also contribute to the formation of real
sand dunes.
However, the effects of grainfall
are known to insignificant at the
length scales of interest, i.e. from 10's of centimeters
to 10's of meters.
Indeed,
grainfall gives rise to the opposite size segregation
(small-scale ripple formations which are typically 1 cm in amplitude).
where large grains are
observed at the crests and small grains at the bottom,
as opposed to the size
segregation we study which involves large grains on the
bottom of the dune.
As unidirectional
wind moves sand along a bed,
a small sand accumulation
(incipient dune) is formed.
As the wind continues, sand
moves from the upstream side of the dune to the crest of the dune.
When the slope of the dune becomes
steeper, a downstream slip-face is developed
where
avalanches of sand begin.
As new material is brought
to the top of the dune, another avalanche occurs.
Since the actual geological system is translationally
invariant along the transverse direction (due to the unidirectional
flow of sand)
our experiment performed in a quasi-two-dimensional geometry
might be relevant for the avalanche dynamics in the
slip-face of the dune.
Our experiment on
stratification
shows a grading in a triplet of three consecutives sublayers of the form:
from bottom to top,
small-medium-large, small-medium-large, etc.
The same grading can be observed in the rock sample shown in the above
Figure
indicating that similar grading mechanisms
might be acting in the slip-face of the dune as in the experiments
presented
here.
According to Fineberg, the self-stratification
phenomenon
might be relevant to another puzzle: long-runout rock slides.
Such rock slides are known to destroy
entire towns as was the case of Frank, a town in Alberta, Canada
which was wiped out by a 74 million tonnes of rock crashed down from Turtle
Mountain
in April 29, 1903.
The rock slide
began when stones fell one kilometer down the mountain into the valley.
But when the rocks reached the bottom of the mountain, they kept moving for
4 kilometers across the valley sweeping out Frank and stopping at the
foot of another slope.
During avalanches, the rocks spontaneously form layers, with the
small rocks at the bottom of the layers; a phenomenon called
kinematic sieving.
These small rocks act as the best ``ball-bearing'' reducing friction
and providing an overall lubrication effect, so that the mass of
material
can continue to slide over for large distances after reaching the
ground.
