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  Stop 4. Phylloid Algal Mound: a) Overview b) Core Facies c) Flank Facies

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Transcript

- [Voiceover] The purpose of this video that was filmed in the Hueco Mountains is to help you differentiate between mound core and mound flank facies. You can see my car for scale, and we're standing in front of one of the more famous mounds in the area. And when you look in the distance, what you see when I'm zoomed in, are the mound core. And the first thing I want you to notice is the lack of bedding in the mound core. Everything looks super smooth. The vertical lines that you see, those are actually fractures. That is not layering. Now as I'm moving the camera towards the left, you see that the character is very different. Instead of looking massive as the stuff did on the right, this stuff is actually layered and you can see that there is at least three distinct beds that are in there. What I'm doing is I'm slowly moving the camera towards the right and you see that those beds have now disappeared. And if I move it some more, now we are getting back into that structureless or massive mound core facies. So you saw a transition going from distinctly layered flank beds into the mound core. Now, why this is important is because this lateral relationship can be lots of different kinds. In this particular case, it's just a lateral facies change. These beds are actually not onlapping onto the mound. Later on in this video series, you will see areas where beds are actually onlapping onto the mound core facies. So why is this important? Well this is important because the composition of these flank beds is very different from the composition of the mound core facies. And what we're gonna do here in a little bit is I'm gonna start going and climbing on these and I will show you a close up of what the mound core facies are made up of. And then I'm gonna hike to the flank beds and we're going to see what those flank beds are made up of as well. And then you should be able to tell the difference. What we've done is we've climbed on top of this phylloid algal mound. And what I'd like to show you are some of the details as to what this mound is actually made up of. Okay, so I'm gonna zoom in a little bit. And when I do you start beginning to see some of the grains that are in there. In particular, I'd like to - I'd like you to focus on these guys right here. Okay you see that little ring right there, right next to the ring is this thing right there. Further up, we've got those guys right there. And I'm gonna zoom in a little bit more so you can see what they're like. Okay, so. Right there. Those are crinoids, okay? Now, yes we call this a phylloid algal mound, but this is where you have to start thinking in a sense of carbonates and not classics. Carbonates are, it's a, this was a living community. So unlike a fluvial sandstone, which is just a fluvial sandstone, you have to think about these as like a tree in the rainforest today. You know a tree in the rainforest doesn't just have the tree itself. It's got monkeys that are living on the tree, there's different kinds of epiphytes, plants that are growing on the tree itself, there's poison arrow frogs, there's different sorts of birds that are nesting in the trees. Same thing this, what you're looking at right here is a paleo community. So yes it is a phylloid algal mound, but we call it that because the main constructor of the mound are phylloid algae, but the phylloid algae aren't alone. The reason they're even able to grow and collect sediment is mainly because of microbial activity. And on top of these phylloid algal mounds, you would often find crinoids, which is what you see right here. Those are stems of a crinoid that have disintegrated. Now I have poured water on this rock just so you can see the phylloid algal blades a bit more clearly. So all these squiggly black lines that you see, these are the leaves or calli of the phylloid algae. Now these phylloid algae used to look like fronds of lettuce. And when they disintegrated, they kinda packed together like potato chips. And that's what you're seeing right here. I'm gonna zoom in a little bit more so you can have a better look. But that's uh, that's pretty much what you've got going on right there, okay? All those, especially like that grain right there, okay, those are all broken aparts of algae. And what happens is, if you've got some sort of diagenesis, oftentimes either the entire algal filament will develop as a moldic pore, or sometimes I've seen in the paradox basin, you've got intraparticle porosity associated with where those algal filaments used to be. We're looking at flank beds of that phylloid algal mound. And what I'd like for you to notice is just how well-bedded this stuff is. Okay, we're gonna slowly start moving towards the mound itself, but again, it's super bedded until we look there. And you can see that that stuff is all massive. There's just no bedding in there, okay? That's the takeaway message here. Flank beds are well-bedded. So in seismic, you should expect divergent to parallel the subparallel reflections because between these beds of limestone you can get shales or finer grain units and you'll get enough of an impedance contrast. Whereas in the core there is none, which is why it appears seismically transparent.