#turbidity current

Beautiful Bouma This annotated rock shows 4 of the classic layers of the Bouma Sequence, a type of sedimentary rock produced by a deposit called a turbidite. Turbidites or turbidity currents are produced in submarine landslides. In canyons or places where lots of sediment is deposited as in river deltas, occasionally sediment offshore becomes oversteepened and collapses. The sediment will then pour down the steep continental shelf and re-deposit at lower elevations.

The Grand Banks Earthquake 1929

Many people haven’t heard about the Grand Banks Earthquake and it is hardly surprising. The earthquake itself did very little damage and the tsunami produced pales in comparison to those of Indonesia or Japan. However, the Grand Banks Earthquake is very important to geologists because it led to the discovery of a brand new phenomenon: the turbidity current.

I want you to think of a landslide. Now imagine that landslide was underwater. This is effectively what a turbidity current is, a load of sediment rich water hurtling down the continental slope towards the basin floor. The current is driven by gravity as the sediment within it is far denser than the surrounding water and the turbid nature of the flow allows it to propagate over large distances.

In 1929 no-one had ever heard about a turbidity current mainly because no-one realised they existed. This is hardly surprising considering they occur so far off the coast and happen to be underwater. At 5:02pm on November 18th a 7.2 magnitude earthquake occurred 250km of the coast of Newfoundland with the effects being felt as far away as New York. Luckily damage was limited to Cape Breton Island where a few chimney stacks fell over and some roads became blocked by landslides.

It wasn’t until 2.5 hours later that a tsunami up to 13 metres high struck the eastern seaboard killing 28 people and laying waste to over 40 villages in southern Newfoundland (newspaper reports all mention the loss of 280,000lbs of salt cod which I’m sure was heavily mourned too). The wave even reached the coast of Portugal, albeit several hours later.

The tsunami was so powerful that it lifted some buildings clean off their foundations, depositing them some distance from where they had first stood. There was even a general merchandise store that is said to have been moved 60 metres inland and unceremoniously deposited in a meadow. When the owner reclaimed his store he found that despite its epic voyage all of the stock was undamaged and had remained stacked on the shelves!

When scientists began pouring over the data they noticed several strange reports of broken transatlantic cables. These cables had been placed on the seabed in the Grand Banks area, and appeared to be in a similar position to the earthquake epicentre. Not only that, but the cables closest to the epicentre had broken first while those further away didn’t break till later. Overall 23 cables had been broken over a 12 hour period giving an average speed of movement of 55km/hr.

It took scientists over 20 years of modelling, hypothesising and generally scratching their heads before they managed to work out what had happened. What they discovered was that the earthquake itself was not responsible for the Tsunami. It had merely made sediment on the continental shelf unstable, causing it to slide downhill. This process, named a turbidity current, had not only broken the cables but had displaced enough water to generate a tsunami!

So why should we care about turbidity currents? Well firstly they can generate tsunamis and destroy any form of seabed based cable unfortunate enough to be in their way. Secondly they are fantastic hydrocarbon reservoirs and are exploited in the Forties Field in the North Sea to the Wilcox Formation in offshore Gulf of Mexico.

Watson

References:

http://bit.ly/1H1aNw7

http://bit.ly/1QFvHbf

Further Reading:

http://bit.ly/1csXJaS

http://1.usa.gov/1G2VA1F

Image credit:

Houses - photograph by H. M. Mosdell from the collection of W. M. Chisholm

Taiwanese Typhoons Trigger Turbidites

These sedimentary layers are turbidites, the remnant of ancient debris flows underwater. Turbidites are the main way that sediment deposited in the shallow water near a continent makes its way into the deep ocean. Rivers and streams bring sediment to the ocean and deposit it near the shore, and then eventually so much sediment is deposited near the shore that it becomes unstable and slides down to the deeper part of the ocean, often in submarine channels.

Turbidites can be recognized in sedimentary layers by what is known as the Bouma sequence – a classic pattern where the coarsest grained sediment is at the bottom and the sediment gets finer to the top. In this sequence, the big brown layers are sandy, while they thinner dark layers are silt to clay. These turbidites started as flows of sand and silt moving cascading down from shallow water to deep water. The heavier sand stayed at the bottom of the flow, forming the sandy layer, while the silt and clay were suspended above the flow and only settled out into layers after the flow stopped.

Scientists at Tongji University in Shanghai just published an interesting study regarding turbidites in Taiwan. Turbidites can be triggered by many things; weather on shore, earthquakes, or just random events. Over a 3.5 year period from 2013 to 2016, an area of Taiwan that feeds into a submarine canyon known as the Gaoping Canyon received heavy rainfall from 16 typhoons. The scientists tracked turbidites in this area and found that 72% of the sediment delivered to this canyon as turbidites occurred during these storms. While this is not the case everywhere, at that particular spot, typhoons are therefore the main trigger of sediment moving into the deeper ocean.

-JBB

Image credit: https://flic.kr/p/UgSo

Beautiful Bouma

This annotated rock shows 4 of the classic layers of the Bouma Sequence, a type of sedimentary rock produced by a deposit called a turbidite.

Turbidites or turbidity currents are produced in submarine landslides. In canyons or places where lots of sediment is deposited as in river deltas, occasionally sediment offshore becomes oversteepened and collapses. The sediment will then pour down the steep continental shelf and re-deposit at lower elevations.

In the process, the sediment will sort itself by size as different mass grains behave differently. At the bottom of a turbidite, there is often a scoured surface where the momentum of the moving sediment picked up and entrained whatever was there beforehand. On top of that is a layer of coarse-grained sediment shown here as layer A– the heaviest particles settle out right on top of the scoured surface. Sometimes, pieces of the scoured chunks from the layer below are included with this layer as well.

On top of that, the grain sizes become finer. Layer B is composed of finer-grained sandstone, separated into various layers depending in part on composition.

Layer C is still sandstone, but the texture has changed. In the lower layers, energy is high enough that the sand and pebbles are carried by the water; in layer C the grains saltate or bounce off each other as they move. Saltating sand grains tend to form ripples and ripples form cross beds and climbing patterns like you see here.

Finally, on top of the ripples you get layers that settle out slowly from the water as the energy drops. Layer D is composed of silt-sized grains that settle out in distinct beds, and on top there can be a layer E composed of even finer-grained clay.

The combination of these different layers forms all sorts of interesting geologic features. In folded rocks, they can give an “up” direction since the coarse grains are at the bottom. The clay layers can be sources of oil and gas and the sandy layers can serve as traps for that oil and gas, making them solid reservoirs. They can even be recognized after metamorphism since the composition changes as they’re deposited. Not all of these layers occur in every spot and they aren’t always exposed as well as they are here, making this rock a nice shot.

-JBB

Image credit: http://bit.ly/1GKEkP2

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