Turbidites are sedimentary rocks caused by the lithification of turbidite sediments, that is, sediments deposited by turbidity currents. In this article we shall review what is known of their sedimentology, and discuss how we know their mode of deposition.
When a denser fluid flows through a lighter one, the difference in density prevents them from mixing, so that the denser fluid forms a current within the less dense fluid. In particular, turbidity currents in water are currents which are denser than the surrounding water as a consequence of being turbid (loaded with sediment). Because the turbidity current only mixes gradually with the surrounding water, its energy only dissipates very gradually into the larger body of water. This means that a turbidity current can flow for great distances (hundreds of kilometers) as a distinct current within the clearer water. Being denser than the surrounding water, it will flow downhill and along the bottom of the surrounding fluid: one might think of such a current as a sort of underwater river, although the analogy is not quite exact in that a turbidity current can flow up and over obstacles in its path.
The turbidity currents of interest to us in this article are those caused by slope failure, where sediment on the continental slope begins to slide down it, either as a result of a submarine earthquake or simply as a result of sediment accumulating on the slope until gravity alone is sufficient to start it sliding. This initiates a turbidity current, which flows down the slope accelerating as it goes: also, as it flows down the slope, it churns up more turbidity, increasing the difference in density between the current and the surrounding water.
By the time such a current reaches the ocean floor, it can be traveling at upwards of 100 km/h. As we have noted, the dynamics of a turbidity current ensure that it only loses energy very slowly, and so such a current can travel hundreds of kilometers before giving out.
Because these currents carry their loads of sediment at such high speeds, they must surely have a powerful erosional effect: they are thought to be the main cause of many underwater canyons. However, we are more concerned here with their role in the deposition of sediment, which will be discussed in the next section of this article.
Turbidity sediments and turbidites
The sediments deposited by turbidity currents are known as turbidity sediments. The rocks formed from these sediments on lithification are known as turbidites.
At any particular point over which a turbidity current passes, it will start off strong and gradually weaken until its energy is entirely dissipated. The consequence of this will be that the sediment will grade upwards from coarser to finer sediments. How coarse the sediment at the bottom is will depend on the source of the sediment: it may be as coarse as boulders and cobbles, or as fine as sand. The thickness of the deposit is also variable, from meters to centimeters in scale.
Note that the current fails not only over time, but also spatially, as it loses energy the further it gets from its origin. So at the extreme distance from the origin, only mud will be deposited; closer to the origin than that, we would see silt overlain by mud; and so forth.
After the deposition of the turbidity sediments, there will usually be a more tranquil regime of deposition, during which ordinary marine clay-sized articles will be deposited on top of the turbidity sediments proper. The entire sequence of sediments produced by these two mechanisms is known as a Bouma sequence. Note that although the top of the Bouma sequence is not deposited by turbidity currents, the term "turbidite" is used to include the whole Bouma sequence and not just the part of it so deposited.
While the ordinary marine clay in the Bouma sequence will contain organic remains from the deep waters in which they were deposited, the turbidity sediments will typically contain remains from the shallower waters in which they originated, and these remains will typically be fragmented by the violence of the process which transported them. The current-deposited sediments will often display sedimentary structures associated with flow, such as ripple marks. When a fresh sequence is deposited on top of the previous one, the force of the turbidity current will erode the layers of fine clay at the top of the previous sequence, producing what are known as sole marks.
The typical place to find a Bouma sequence is underneath one Bouma sequence and on top of another; although slope failures are intermittent, they are plentiful, and over a sufficiently long period of time great stacks of them will be deposited. The picture to the right shows part of one of these stacks, in lithified form.
Turbidites: how do we know?
How can we recognize the origin of the sediment in these rocks, and conclude that it really was desposited by turbidity currents?
To begin with, offshore drilling on the continental margin finds sequences of unlithified sediments which look just like the sequences of lithified sediment found on dry land. To identify the latter as the lithified counterpart of the former is trivial; and so we can be confident that the lithified sediments were marine in origin and were formed by the same processes as the marine sediments sampled from the sea floor.
But how do we know what those processes were? So far as I know, at the time of writing no-one has ever been at the right place at the right time to see a turbidity current depositing its load of sediment; this is unsurprising, since the phenomenon is intermittent and unpredictable, so no-one knows what the right time is; and the right place is at the bottom of the sea.
For this reason turbidites were for a long time a puzzle for geologists. But when they started taking turbidity currents into consideration, suddenly everything became clear.
Note first of all that turbidity currents themselves are not hypothetical. They can be produced in the laboratory in tanks of water and their action observed. Furthermore, laboratory experiments confirm that the waning of a turbidity current does indeed result in graded sediments, as we would expect. Slope failures are also not hypothetical, and turbidity currents have been observed flowing down the continental slope through marine canyons; it is only the actual deposition of the sediments that has so far gone unrecorded.
We know that whatever leaves these sediments flows along the bottom of the sea, because it leaves ripple marks in the sediment and because it leaves sole marks gouged out of the previous layer of sediment. In order for something to flow at the bottom of the sea it has to be denser than seawater — like a turbidity current is by definition.
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, and that it moved along the sea floor where the cables were laid. A turbidity current with its abrasive load of sediment would be a highly plausible candidate.
We know that whatever process forms the deposits that we're trying to explain 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 sedimentology shows the effects of a high-energy current waning to a low-energy current followed by a period of ordinary marine deposition, followed by the same thing happened over and over again. 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 causing currents 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?
The fossils found in turbidites are another important point. The alternation of shallow-water with deep-water fossils was once a baffling mystery. The theory of turbidity currents makes everything clear: the shallow-water fossils are carried by the turbidity current from shallow to deep water, and what was an inexplicable anomaly becomes an expected consequence of the theory.
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 turbidity sediment should, apart from not being marine in nature. Further investigation suggested that the 1996 event was caused by accumulated sediment sliding down the slope of the Aare delta.
In the light of all these facts, it seems to be a safe bet that turbidity sediments are indeed caused by turbidity currents.