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  Stop 6. Slope Facies in the Guadalupe Mountains: a) Carbonate-rich turbidites b) Seismic-Scale Clinoforms c) Seismic Expression of Reefs d) Forereef and Backreef

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Transcript

- [Ali Jaffri] We are in McKittrick Canyon and we're starting to approach the upper slope, which you can tell from the series of these clinoforms if you look at in detail later on. And we know we are in the upper slope, not just from the clinoforms but by the lithofacies that you can see in the rocks here. If you zoom in, you can see a normally graded bed and above it, thosethat you see, they may appear to be inversely graded but those are still normally graded beds. The lighter parts right there representing the corson grain fractions. And if you get even closer, you can actually see increasing flow, unsteadiness in the flows. Right here you've got some sand. You can even see some burrowing into those sandy tops. Now we keep seeing sand, but of course these are all carbonate grains. You can see the normal grading in there. If you focus on the grains themselves, I'm gonna zoom in a little bit, you can see that some of those grains are hexacorals or horn corals, and then we also have quite a few brachiopods in there, some, some gastropods, but all organisms that you see are actually being brought down here as a series of sediment gravity flows from the Capitan Reef itself. Earlier in the series, we saw sandstones of the Brushy Canyon that were brought as sediment gravity flows during low stand times, whereas these sediment gravity flows are coming during high stand times. So it's a good example of high stand shedding off carbonate platforms. We are in McKittrick Canyon, and you can see the reef up there, and the material that is shed from the reef helps its progradation and here you can see a progradating wedge that's coming off of the reef. The clinoforms that you see here are oblique clinoforms, which tells us that they're correction dominated, or some people like to call them advection-dominated and the reason for that is the clinoforms are mainly composed of sediment gravity flow pulses interrupted by pelagic delimit pelagic sedimentation of wacky stones and mudstones. We are in McKittrick Canyon, starting to approach the reef. We've been walking on the upper slope for awhile and it's covered with debrites, but very large pieces of the reef. And now we're looking at the reef itself. And what I want you to note is the lack of bedding within the reef. The vertical lines that you see are, of course, fractures. The holes are later karstification. So no bedding in the reef, but when we move a little bit towards the left, you can see that those layers I'm gonna zoom in on, those arebedded. Those are platform interior layers, so these are cycles that were deposited behind the reef. This is very important, because once we get closer, these beds are material that have been shed off the reef, so they're on lapping onto this structureless feature right here, which is the reef itself, the reef core. And if you were looking at this in seismic, the reef would appear as chaotic, because it's just boundstones, or boundstones, there's no impedance contrasts, whereas the package behind the reef is gonna be bedded, and sometimes it can even have a wedge-like morphology on lapping against the reef. The reef happens to be very porus here, but that's not because of primary porosity is a secondary porosity karstification, but it's water wet. We are on the Permian Reef itself, and all around are sponges, different species of sponges, and bryozoans. And looking from the Reef down slope, which was you can see these clinoforms. There's the material that will shed from the Reef during the high stand. The Reef itself is massive, which is the stuff that I'm currently standing on right now. So these are framestones off the reef. If you do see any sort of holes in them, these are mainly from karstification. reef cavities are pretty much filled up. Now if you look behind the Reef, you actually start looking at a platform interior or back-reef cycles right there, okay. This is a great analog for rim detached platforms, whether they're in China, Southeast Asia, and people that actually use the Permian Reef as an analog for the carbonate platforms, even in Central Asia