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  Stop 5. Carbonate Frictional Debris Flows

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Transcript

- You can see my trekking pole for scale and I want you to notice the floating clasts. The clasts are all carbonate, the matrix is sand rich, and as we move higher into the section, the clasts are getting bigger and bigger. Now this is very different from what we observed in some of those earlier videos when the largest clasts were at the base, and the smallest clasts were towards the top. Here, you have just the opposite situation. Now that last video which you saw, the largest grains at the base and the smallest towards the top. I told you it was normal grading and it was that way because of waning flow, and we interpreted that as a turbidite. But this is the opposite situation, here, the largest grains are towards the top. And it's not because flow velocity is actually increasing over time. In this case, there's a few different processes that are going on. Think about it this way: If you've got a few M&M's and you pour the M&M's into water or skittles into water, they'll all sink to the bottom. But if you've got some cold honey and you pour the Skittles or M&M's onto the honey, they would actually float. The reason for that is an increased viscosity of the honey as compared to the water. So the honey is basically behaving as a plastic in fluid mechanics versus in Newtonian fluid. The reason why the Skittles and the M&M's are not sinking is because of this process called Hindered Settling and that's what's going on here, okay. So that tells you that this matrix was very viscous that prevented all these clasts from sinking to the bottom. In addition to this, there's another process going on. A lot of people colloquially call it The Brazil Nut Effect, in which, if you have a bowl with mixed nuts and you start shaking it up, the Brazil nuts will rise to the top. The technical term for that is Buoyant Lift. The larger the surface area of a grain, the higher it will rise. And kinetic sieving which is smaller grains will sink down through the pores of large grains. So all these sediment support mechanisms are typical for debris flows. And this is what you're looking at. You're looking at debris flows. And these grains can be fairly large. So if you look on the other side of the road, you can see some fairly large grains. There is a huge grain right there and here's my car for scale. I'm gonna zoom in a little bit on the car just so you can see that large clast right above it. Okay? So, very very large grains that are floating in this debris flow. Now more importantly, you can see that there's an upper contact, a very sharp contact. And then there is different strata that are sitting on top. Those are low density turbidites. Those are the kind of deposits we saw in an earlier video. But what I want you to focus on is that those low density turbidites are basically filling in the topography created by the debris flow. This is extremely common not just here, but I've seen the same process occur in the raw sandstone where you've got mass transport complexes, or mass transport deposits, if you will. And their upper surface is irregular which is often the case in these debris flows. And then subsequent sand rich turbidity currents will pond sands onto the pre-existing topography of the debris flow. Why else is this important? Well, here's the deal, This is the basinal deposit in the Permian basin. We're gonna look at another one here in a little bit. But the key is to remember that these debris flows do not form laterally continuous sheets. They have a distinct leaf-like pattern or low bay pattern and they're not laterally continuous, okay? That's something to bear in mind. But the distinguishing character here are these floating clasts, and these clasts in short pattern. Smallest clasts at the base, and the largest clasts towards the top and that is inverse grading. But inverse grading is not a function of increasing flow velocity, it's just kinetic sieving, buoyant lift and hindered settling.