River and Beach - A Gravel Study
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| License | No Open Access License: German copyright law applies. This film may be used for your own use but it may not be distributed via the internet or passed on to external parties. This film contains music to which the collecting society GEMA holds the rights. | |
| Identifiers | 10.3203/IWF/W-1923eng (DOI) | |
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Transcript: English(auto-generated)
00:52
The Calabrian massif lies in the center of the Mediterranean Sea. The massif was lifted extremely rapidly and is being extremely rapidly eroded.
01:04
The weathering products are transported by short mountain rivers into the neighboring seas. The Ionian Sea to the east and the Tyrrhenian Sea to the west. On the Ionian side, the shelf is very narrow and is interrupted by deep submarine canyons.
01:23
The heads of these canyons are fed with debris from the river mouths and the coastal areas. The canyons transport this debris further out into deeper waters. We want to follow carefully the gravel and pebbles on their way to the canyon
01:40
and take an especially careful look at the grain size distributions. These can tell us much about the history of the coarse debris. Why? Grain size frequency distributions of sand tend to be composed of one or more log-normal or Gauss distributions.
02:04
With the typically S-shaped cumulative frequency curve, which forms a straight line if plotted on log-normal probability paper. Many gravel populations also fit this well-known pattern, but others do not.
02:22
They yield asymmetrical, positively skewed frequency distributions and curved instead of straight cumulative curves if plotted on log-normal probability paper. So these gravel distributions do not fit the Gauss distribution law.
02:43
But how can this kind of distribution be described best? In the 30s, the German coal engineers Rosin, Rammler, and Sperling found a function, the Rosin law, that controls the grain size distributions of coal powder and rock crusher products.
03:05
Strangely enough, our non-log-normal gravel distributions also fit very well to this rock crusher Rosin law. Here, such an ideal Rosin distribution is plotted on Rosin law probability paper.
03:25
So we have two different kinds of gravel distributions, symmetrical and non-symmetrical ones. The log-normal or Gauss type and the Rosin type. To clear this discrepancy, we formulate a hypothesis.
03:46
We consider the non-symmetrical Rosin distributed gravels as immature early stages and the symmetrical log-normal gravels as more mature and later stages of a transport development.
04:02
And we conclude from this that the degree of symmetry of a gravel deposit may be taken as a measure of its textural maturity. To test this hypothesis, we need a large set of data and an objective standardized procedure.
04:22
Let's take a granite, a jointed and weathered source rock from the outcrop as an example. The computer-executed goodness-of-fit test compares this gravel distribution with an ideal log-normal distribution, correlation coefficient 0.967, and an ideal Rosin distribution, correlation coefficient 0.992.
04:50
This Rosin fit is now simply plotted against the Gauss fit and in this way, each sample is characterized by a point in the Rosin-Gauss diagram.
05:02
Now, let's suppose a gravel population changes during transport from an immature non-symmetrical Rosin distribution to a more mature symmetrical Gauss distribution. The point cluster in the diagram should move accordingly from the yellow Rosin field into the green Gauss field.
05:28
To test this, we first have to get samples from the source areas and the river systems.
05:42
We developed a field sieving and splitting device and used it for many years. This made it possible to gain exact data on grain sizes, petrographic composition and particle forms of very large samples.
06:16
The source rocks of the Calabrian massif are intensively jointed and weathered.
06:21
They easily disintegrate into coarse debris. This debris is predominantly Rosin distributed. Granite, for example, or gneiss, schist, or pegmatites,
06:53
but also sandstone or jointed reef limestone.
07:05
All of these are easy to sieve and usually Rosin distributed. Here we have the correlation coefficients of the primary distributions of the jointed and weathered source rocks of almost all Calabrian rock types.
07:23
Grain sizes between 2 mm and 25 cm almost always show Rosin distributions rather than log normal distributions. Almost all are in the yellow field, but also regolith and rockfall debris are usually Rosin distributed.
07:53
Landslides, the most common slope-wasting products in Calabria, are usually Rosin distributed.
08:07
And the alluvial fans which feed the rivers are mostly Rosin distributed.
08:25
All these stages of early transportation yield correlation coefficients with a strong Rosin tendency. And so it is not surprising that most Calabrian river gravels are also Rosin distributed.
08:43
Obviously, the short fluvial transport is not sufficient to change the original Rosin distribution. This is some sediment transport in the oversupplied Milito system.
09:01
Almost 100 samples with a total of more than 30 tons of gravels from 19 Calabrian rivers show a dominance of the Rosin distribution type. Let's now compare these final products of the fluvial process with the starting conditions.
09:23
The average jointed and weathered source rock contains only 4% sand, whereas the average river mouth sediment contains 25% sand. These river sands, which we cannot discuss here, are log normally distributed.
09:43
Both gravel fractions, however, are Rosin distributed. And so we interpret the Rosin distribution of the source rocks, here yellow, as the factor which determines the river gravel's Rosin distribution, here blue.
10:02
This clearly means that no Rosin-Gaus transformation takes place during the short Calabrian river transport. From the wooded canyons of the headwaters to the braided stream,
11:00
from the fall line, Rosin distributions are produced and maintained along the beach.
11:49
The constant flow of the rivers, although with countless and long pauses, is replaced by an almost constant back and forth on the coast.
12:17
Is the Rosin distribution retained here, or is there a transformation to a log normal, a Gauss distribution?
12:29
Two rivers, La Verde and Buonamico, periodically dump enormous amounts of debris into the sea. The coast between the two rivers has a wide strip of gravels, both onshore and offshore.
12:54
The net longshore transport runs from the La Verde delta in the foreground to the Buonamico in the background.
13:02
This seems to be a good area and our last chance to find a Rosin-Gaus transformation. But how can we get tons of gravels out of the shore face when there are no tides and no low water?
13:21
Only heavy machines help here.
14:44
In this way, we obtained 25 large samples out of the 8 kilometres of coast between the two rivers. Using this underwater photo sled, drawn by hand from land and operated by divers, we did an extensive photo profiling of the shore face gravels.
15:21
These surface grain size distributions were calculated electronically. And the results? There is a transformation from Rosin-distributed flueville gravels to log normal-distributed coastal gravels.
15:45
Both excavator samples and photo samples indicated this. Let's use large balls to represent excavator samples and small balls to represent photo samples. Yellow indicates Rosin-controlled distributions and green indicates log normal distributions.
16:09
This model of the coastal area shows that the river delivers Rosin-distributed debris, marked yellow. On the coast, there is first a definite tendency towards yellow Rosin distributions,
16:23
which then fades into a definite tendency towards green log normal distributions. The yellow Rosin samples here signify the next river mouth. The small circles representing the photo samples are at 250 metre intervals
16:43
along the entire 8 kilometre section between the two rivers. Let's now summarise. Jointed and weathered source rocks in mountainous areas, yellow triangles, feed Rosin-distributed debris into the flueville system.
17:01
The short and only seldom active rivers are not able to modify this type of distribution. Thus, they also feed Rosin-distributed gravels to the coast. Only the almost constantly active longshore transport is able to symmetrize the distribution.
17:23
A Rosin-Gauss transformation takes place. Or in terms of our Rosin-Gauss diagram, the point cluster moves from the jointed and weathered source rocks over the early stages of transportation and the river transport
17:42
to the longshore transportation. This may be seen as a signal of increasing textural maturity and it provides support or non-falsification of our hypothesis. So what? The Rosin-Gauss fit gives us an additional measure of textural maturity
18:05
and makes it possible to distinguish between different gravel sources. For example, we know that the head of the Buonamico Canyon acts as an entry point to the West Ionian Basin.
18:20
This entry point is fed with Rosin-distributed gravels from the river mouth and mainly log-normal gravels from longshore transportation. At the same time, the canyon head acts as a point source for the canyon and fan sedimentation. We know this because we have morphological data
18:41
from the canyon head from 1876, over 100 years ago, and we know the present morphology. There are huge differences between them, marked by the colors on the map below. In these red areas of the canyon head, more than 50 meters of sediment have been eroded during the past 100 years.
19:06
And in the blue areas, more than 20 meters of sediment have accumulated during this time. This isn't surprising in view of the erosion rate of 0.5 millimeters per year,
19:21
catastrophic rainfalls about four times a century, and the many landslides which take place in this area. Most of the morphological changes in the canyon head during the last century must have been caused by the masses of gravel
19:40
which are discharged by the rivers and beaches and fed into the sea. Probably these differently distributed gravel deposits maintain their specific distributions as a provenance factor
20:01
during further marine mass movement. For example, we know from the geological record in the northern Apennines that marine channelized gravels tend to be rosin-distributed, while gravelly turbidites tend to be log-normally distributed.
20:20
This suggests different source areas, but not necessarily different entry points. Today and here in Calabria, only very large samples of the canyon and fan sediments
20:40
can tell us more about rosin and gauss problems and their geological significance down there.