#turbidity current

[earthstory]

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 References: http://bit.ly/1FJqOJn http://booksite.elsevier.com/9780444521613/Chapter_08.pdf http://www.sepmstrata.org/page.aspx?pageid=37 http://soundwaves.usgs.gov/2012/02/staff.html http://bit.ly/1C8g87p http://bit.ly/1HYh9Q1

[earthstory]

View this post on Instagram

Slow motion laboratory simulation of a turbidity current moving across topography in a desktop box

[earthstory]

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

References: http://bit.ly/1FJqOJn http://booksite.elsevier.com/9780444521613/Chapter_08.pdf http://www.sepmstrata.org/page.aspx?pageid=37 http://soundwaves.usgs.gov/2012/02/staff.html http://bit.ly/1C8g87p http://bit.ly/1HYh9Q1

[deepseanews]

Imagine you are yodeling whilst snowshoeing in a majestic mountain range. All of a sudden you hear a rumble. You look up to see a white wall of snow heading towards you. You, unfortunately, are about to be engulfed by an avalanche. Now imagine you are an ROV sampling whilst exploring in majestic ...

http://deepseanews.com/2015/01/rov-gets-caught-in-turbidity-current-lives-to-tell-the-tale/