Stop 4. Basinal Facies in the Guadalupe Mountains: a) Calciturbidites b) Turbidites vs Debrites c) Deepwater Carbonate Channels
- [Narrator] We're looking at a really good example, almost a textbook example, of a calciturbidite. My hand lens and grain size chart here are for scale. And how do we know that this indeed is a calciturbidite? Besides the grains being calcareous, fragments of different abiotic and biotic grain types from the reef and several extra class. You can see this sharp base. And then right above the sharp base, are the large grains. Now as a turbidity current is going through, velocity is waning, which means velocity is decreasing over time. And as flow velocity decreases, the competence of the flow decreases, which means it can't carry grains that are that large. And so the larger grains are left behind, whereas finer grains are deposited on top as flow velocities are decreasing. And eventually, the very top of the bed, which we're going to focus on now, it's beautiful that it's preserved almost in 3D. When you look at the top, you'll see almost no grains, and the reason you're not seeing any grains is because the top is mud ridge. So as the turbidity current is going through, the billows, which are the clouds, are depositing clay rich material at the very top. So there's a lot of density stratification in here. Higher density at the base, lower density on top, largest grains at the base, smallest grains on top. So if you are a fan of the Bowman sequence, this is a great example. We are on the basin floor right now, and we'll be looking at some carbonate rich sediment gravity flows. What I want you to focus on right are those ripples. If you look on top of the bed, and if you look in side view, that right there is a calciturbidite. It's not the best example and we'll show you something a lot nicer later on, but that is a sediment gravity flow. Besides just turbidites in here, which you can see all of them have sharp bases and gradational tops, you also have units such as this. And the grains, actually the grains used to be where the holes are now. So the holes represent wherever the grains used to be because of differential weathering, those have been dissolved. But in that particular case, let me zoom in on it, right there, those were actually not turbidites but debrites. You can see where the floating class used to be before they were weathered away. So, calciturbidites and calcareous debrites, but when you look above the stuff you'll see sandstones of the Cherry Canyon Formation. Now this again is a great example of reciprocal sedimentation, which means during low stands sand dunes would migrate all the way to the edge of the rimmed attached platform, and the sediment gravity flows would bring them down here during low stands. During high stands, when there was water over the carbonate platform, and rates of carbonate production were high, these calcite-rich sediment gravity flows were produced. Now just because we had sand dunes and the eolian system during low stands, that's not the only reason why you would get reciprocal sedimentation. There's other basins, such as the Kutai basin, in which you had shelf margin deltas that were directly debouching into the basin. They would prograde over the entire shelf, and then the deltas would reach where the reefs used to be, and then you would get siliciclastic input into the basin. So what we're gonna do now is get closer up and show you some features that are diagnostic of carbonate channels. In a very similar way as you can get deep water classic channels with developed levies, we get the same thing in carbonate systems. We're gonna take a look at that next. We're looking at a ephemeral stream in this canyon, a good example of a modern fluvial channel. Now the thing to remember about modern fluvial channels and how they're different from deep water channels is that modern fluvial channels have a low aspect ratio, whereas deep water channels have a high aspect ratio. By that, we mean the width to thickness. So fluvial channels tend to be fairly deep as compared to their width, whereas deep water channels can be several hundred meters across while they're only 10 to 15 meters deep. At that scale it's very difficult when you're looking at an outcrop, or if you're looking at a core or a borehole image log to tell whether you are in a deep water channel or a lobe. But there are a few things that we can use thanks to the work of Mike Gardner who is a deep water expert. One of the sedimentary structures that is fairly diagnostic of deep water channels is this thing called plow and fill. The process is very similar to what you get in fluvial channels, which is you have a high velocity current that erodes the base and creates a little depression. And then, subsequent flows are gonna fill in that depression. You can kinda see that right there, you can see a little bit of erosion right there at the base, and then beds have filled that. Now if you trace that same bed across, there is a much better example. So tracing that same bed, right here, and you can use my trekking pole right here for scale. I'm gonna move in a little bit closer to this thing, and this is an excellent example of plow and fill. So what you can see is that a turbidity current ran through, it eroded this section right here that almost looks channel like, but it's because we're looking at an oblique cut. And after excavating that channel like feature, which is the plowing process, the waning flow begins to fill it. In this case it's been filled with sponges, some of which are silicified. Other things like these you can ignore, these are actually just chert nodules that have developed in there because it's silica coming from the silicious sponges that were growing on the lower slope of the reef. But this feature right here, you can see that even if you have a core, which is this wide, you'll still capture this feature. I've seen this in deep water cores, deep water clastic cores from the North Sea, from the Taranaki basin, and other places. So highly diagnostic feature for deep water channels that you can see not only in core but in borehole image logs as well. It's one of the few features that you can use with quite a bit of confidence to differentiate deep water channels from deep water lobes. Otherwise, the sedimentary structures tend to be very, very similar. In this case, do bear in mind that we are looking at calciturbidites, and these are deep water sediment gravity flows that were coming from the Permian reef, which if you zoom out you'll be able to see up there. It's about 600 meters above us. So sediment gravity flows were sourced from that reef and they were brought down here as a series of turbidity currents, debris flows, and then of course transitional flows as well.