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Friday Field Foto(s) #10: Turbidites in the French Alps

February 2, 2007

Instead of one photo this week, i’m gonna show a series of photos from a trip I took to southeastern France last June. Most of my research is involved with characterizing and understanding the deposits of turbidity currents, or turbidites. Essentially, picture an underwater ‘avalanche’ of sand and mud. The sediment travels down a slope into the deep water in fast-moving turbulent flows. Additionally, this very outcrop is where, in the late 1950s Arnold Bouma developed what has come to be known as the ‘Bouma sequence’.

The photo above and to the right is a succession of alternating sandstone beds and mudstone beds (note trees for scale).

A road snaked its way up this mountain so we were able to look the rocks in more detail in the well-exposed roadcuts.

This photo shows the character of bedding at a scale of a few meters. The thicker sand beds are typically a little coarser-grained and tend to be more resistant and stick out of the cliff. The finer-grained material is commonly in thinner beds and more recessive.

Statistics of bed thickness patterns have been done for many years on turbidites. The variability is so great from place to place that there doesn’t seem to be a very systematic pattern. Some workers have documented some interesting relationships (Talling, 2001 has a good summary of this).

A lot of turbidite sequences include intervals of chaotically deformed material. It may look like good ol’ tectonic deformation but it is the result of slumping and sliding of material on the sea floor. In steeper areas of the slope leading to the deep sea, material that was deposited may sluff
off as submarine landslides. The blocks can stay
relatively intact in many cases.

Here’s a larger-scale view of a famous outcrop called Chalufy (note trees for scale). This face is cool because you can see the sand-rich bodies pinching out into the blue-gray fine-grained rock. If you notice there are two distinct sand intervals that successively pinch out as wedge shapes from left to right.

This represents the edge of the deep-marine basin. A series of sand-laden turbidity currents came down the slope and incrementally deposited material that started filling in this hole. This kind of complex stratigraphy is more the rule than the exception. These are not simple layers. These flows both create and respond to sea-floor morphology with channels, levees, fans…you name it. The challenge is to take a preserved ancient succession like this and interpret the processes and morphology that existed on the sea floor. From there we build up and start to understand how the basin filled, what controlled the patterns, and relate it to the tectonic evolution.

I’ll be posting a bunch of web resources for turbidite research soon….still working on that.



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