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  Stop 8. Platform interior Facies in the Guadalupe Mountains: a) Grayburg Shoaling Cycles b) Seven Rivers Shoaling Cycles Overview c) Seven Rivers Shoaling Cycles Details d) Grayburg Tidal Channels

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- [Instructor] We're looking at the Grayburg Formation and we're looking at a series of carbonate-siliciclastic cycles here. So what you have is a mixed system. And what's really important to learn from these particular cycles is that the sandstone is actually subtidal, whereas the carbonate happens to be supratidal. We'll zoom in, and you should be able to see these large cross-beds in these sandstones And those were deposited in the subtidal environment probably as a series of dunes. Now when I zoom out, the carbonates that you see they're interbedded with, those are supratidal, we know they're supratidal because they have pizoids in them. And pizoids are diagnostic of supratidal environments. Again, the traditional view when people do see interbedded carbonates with those clastics is that the clastics represent more proximal facies whereas the carbonates represent more distal facies. But in here, that is not the case at all, okay? So that's yet another way you could possibly be wrong when you kinda carry convential wisdom with you. These carbonates don't always behave in the way that we expect them to behave. So in this case, these are actually shoaling cycles where the base of the sandstone marks the beginning of the cycle and then it has a supratidal cap and that supratidal cap is represented by the carbonates that contain pizoids and sometimes they can even have TPs in them. And in, in a later video, we'll show you better examples of pizoids and TPs. You also get what are called birds-eye textures within these supratidal carbonates which we'll show you in the next video. We're looking at carbonate cyclicity here and what I want you to do is focus on the color changes. You've got orange layers and you've got red layers. The orange layers that are sticking out are dolostone layers. Whereas the red layers are actually claystones. Now, if you actually look at these, the way these cycles work is that each cycle actually has a supratidal cap so they're shoaling upwards cycles and what we're gonna do is we're gonna move in closer so you can see what their alternations are like and then we're gonna figure out which ones of these are actually subtidal and which ones are supratidal. So we're closer to the same cycles that I showed you from a distance and now when we look closer, we can see that these are actually dolomite stones. And within the dolomite stones, you can see these cavities. These cavities are where salt crystals used to be. They're often called hoppers, okay? Right above these dolomite stones, you can start seeing these clayridge beds that are red in color, okay? The dolomite stones most likely represent subtidal deposition possibly in some sort of lagoonal setting, whereas the red claystones that you see, these were most likely deposited in a supratidal environment. The red oxidation is, oxidation occurs in a supratidal environment and the red color is very common in what are called Wadi Plain Deposits. Very similar to the coast line of modern Western Australia where you've got continental red beds associated with subtidal lagoons. Now this is fairly interesting because in a well log, most people would interpret these differently. The claystones going up towards to dolomite stones that are sticking out, they would still interpret those as shoaling upward cycles, except the claystones they would interpret as marine shales whereas the dolomite stones would be sticking out more in your gamma ray, they'd have a cleaner signature, a lower API. So the cycle boundaries would be placed at the wrong spot. So instead of the cycle boundary being placed at the base of the dolomite stones, the cycle boundary, they would most likely place at the base of the claystones assuming those claystones were marine shales. So this is one way in which you an actually be wrong when trying to figure out shoaling upward cycles and carbonates using just well logs. We're still in the Grayburg Interval and I want you to focus on are these discontinuous sand bodies that have a concave downwards base. And if you notice, you'll see that the strata are actually truncating against those. These are interpreted as channels. I'm gonna zoom out and you'll see another channelized body right next to them. If we get close to these channels, this is what the fill looks like. As you can see, the fill is extremely coarse-grained and they're cutting into what are interpreted as subtidal sandstones. Unfortunately, due to weathering, you can't really see the internal structure. But the dimensions of these suggest that these are most likely carbonate-filled tidal channels. Carbonate-filled tidal channels are extremely common not only in a rim-detached platform, so if you were to visit modern rim-detach platforms, in a place like maybe Beliz or something, or if you went to a modern carbonate ramp such as Abu Dhabi, you could see tidal channels there as well, there the tidal channels are filled with Ooids. These ones are not filled with Ooids, they're filled with skeletal material. But carbonate tidal channels can be important reservoir components in the Bakken Formation of the Williston Basin, tidal channels have been sampled in core. And from producing wells. So they can make very high reservoir quality facies. The sorting of these sands is excellent because twice a day, tides are coming in and out and the reworking of the sediment, the agitation is constant such that fine-grain material is vetoed away leaving behind a nice, clean well-sorted sand with good reservoir qualities. Now I keep saying sand, but in this case when I say sand, I'm mainly referring to the grain-rich material within these channels, and that grain-rich material, in the case of the Bahamian Channels or the ones along the Persian Gulf, those tend to be filled with Ooids. These, as I said, are filled with skeletal material.