Just a quick question for the geologists here --- has anyone ever actually directly observed a turbidite sediment being deposed by a turbidity current, or does the proposed origin of these sediments rest on inference alone?
If the latter, what do you see as the best evidence that turbidite sediments are, in fact, turbidite sediments?
I'm not sure of your specific intent, but my answer is yes, trubidite sediment formation has been observed. Examples are pyroclastic flows and avalanches.
I don't see how you make that out. Gravity currents, yes. In any case I'm thinking of marine turbidity currents caused by slope failure.
In fact, a 1929 earthquake in the Grand Banks produced such sediments. They are noteworthy because they broke transatlantic cables.
I'm aware of the Grand Banks incident, but the attribution of the breaking of the cables to a turbidity current is based on inference: no-one checked directly that whatever broke the cables also deposed turbidite sediments, did they? Let alone being down there with a submarine to actually observe the deposition, which I guess is what I'm asking.
I doubt someone was actually underwater when a slope failure event occured.
As a matter of fact, I have found a small amount of footage of slope failure and turbidity currents, at the start of this video:
Such footage might be easier to get than you'd think, since I gather that a submarine can trigger slope failure.
But watching deposition is another matter: you'd have to be lucky enough to be there when a turbidite current went by, watch it depose sediment, and then immediately take a core to look for a Bouma sequence.
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The 1996 Lake Brienz incident is nice, isn't it?
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The reason I want to know is that I want to write about turbidite formation, and I should feel a proper Charlie explaining the inferences by which we know that turbidite sediments are deposed by turbidity currents if in fact we know this from direct observation.
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Phillips Petroleum spared no expense to keep their exploration geologists current.
I bet they never spend a cent on teaching you about Noah's Flood though. Imagine how much more money you could have made if you'd based your work on God's Own Geology instead of wicked atheist lies, eh?
So, here's what I wrote on the subject. Any criticisms and suggestions will be gratefully received.
In order to see turbidite sediments being deposited, you would have to be in the right place at the right time, where the right place would be in a submarine and the right time would be when a turbidity current happens to go past. And then you would need to take a core sample to confirm that what it deposited had the patterns of sediment that are postulated to be the results of deposition by turbidity currents.
We have not been able to find that any such observation has been made. Some reasoning is therefore required.
Note first of all that turbidity currents themselves are not hypothetical. They can be produced in the laboratory, as seen in the video [i.e. the video in message #6 of this thread]. Slope failures are also not hypothetical, and can also be seen in the video, as can turbid flow. Furthermore, laboratory experiments confirm that the waning of a turbidity current does indeed result in graded sediments, as we would expect.
We know that whatever process forms these deposits must be happening in the present, because we can see freshly deposited turbidite sediments in the present day. But we also know that the process must be intermittent, partly because we can't see any continuous process forming these deposits on the sea floor, and partly because the grading from gravel up to mud would require a strong current, to transport the gravel, which must then wane (or the mud wouldn't settle). We know also that the initiation of the process must be fairly violent, to produce an impetus which will transport coarse sediments over distances of hundreds of kilometers. These events must also happen reasonably often, to judge by the number of turbidite deposits to be found on the ocean floor. The turbidity currents generated by slope failure would fit this bill.
Moreover, we know of no other cause that could transport such large clasts so far out to sea. This may seem like a mere argument from ignorance, but it gains force when combined with the following argument. We know that there are failures of the continental slope which are by the nature of their origin turbid. Therefore, these currents must transport sediment and deposit it in some form. If it is not deposited in the form of turbidity sediments, in what form is it deposited and where is it?
One frequently cited observation is the aftermath of the Grand Banks earthquake of 1929. In the hours following this, a number of transatlantic cables were severed. Their position was known, as were the exact times when they were cut. It is therefore possible to say that something capable of severing cables moved from near the epicenter of the earthquake at a speed of approximately 100 kilometers per hour.
Perhaps the closest anyone has got to direct observation of turbidite formation is the events in Lake Brienz in 1996. The lake showed distinct signs of an underwater landslip, including a sudden increase in the turbidity of the lake waters, a small (half-meter high) tsunami wave, and the release of a 200-year old corpse from the lake bed. Taking sediment cores from the lake revealed that an abnormal layer of sediment, 90cm thick at its thickest part, had been laid down concurrent with this event: the sediment graded vertically upwards from sand through silt to clay: that is, it looked just like turbidite sediment should. Further investigation suggested that the 1996 event was caused by accumulated sediment sliding down the slope of the Aare delta.
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