The Fascinating Geology and Fossils of Redwall Cavern in the Grand Canyon

[Music] foreign [Music] hello and welcome to red wall Cavern in the Grand Canyon here on a river trip with some friends we're on our first day thanks for joining me I'm geology Professor Sean Wilson down here in the Grand Canyon one of the most amazing geologic places on Earth undoubtedly and we've come down about 30 or so miles down the river through different successive stratigraphic layers and now we're in this very thick unit known as the red wall limestone a Mississippian aged deposit from the bottom of the ocean that's around 320 or so million years old this all this material was deposited at the bottom of the ocean during of time in a position where we had very warm tropical Seas lots of biologic activity different exoskeletons of marine organisms yeah and so in the Grand Canyon this forms one of the most resistant layers one of the biggest Cliffs largely an impenetrable Cliff throughout the Grand Canyon and red wall Caverns is just immense huge overhang right along the riverbed and you can see a few things why it might have formed well one is that the red wall Limestone like a lot of limestones has caves in it and so it's dissolved by acidic groundwater and so in different places along the river you'll see a small little caves sometimes bigger ones as well there's a little bit of one right about over here across the river went a little bit above it right there and there there's two so this may have formed as initially a cave and then as the river eroded deeper and deeper the other reason we have this big cavern as you can see the river moving towards us but it actually takes a big bend right here and so periods of high water and flooding when the water is coming down this Corridor the water would rise up into this Cavern and you can see all the sand that's been deposited here and probably the sand and the boulders transported along the outer Bend of the river has eroded back some of the Limestone just the the sediment of braiding that over time gouging it out and undercutting this big cavern it's a little hard to fit it all in the camera so maybe I'll try coming down here to the river and looking back into it and then maybe we'll go look for a couple of the fossils that we often find in the red wall Limestone so again looking down the corridor just beyond the boats down here you can see another cave in the red wall and then we can see the the soup I group The Pennsylvanian kind of leggy Sands and mud Stones through here and then above that a light colored layer that's the Coconino Sandstone with their hermit Shale below it and then the kai baventura wheat formations up on the rim so here I guess the river is maybe about 2 000 feet deep a give or take and as we head further Downstream of course the the canyon will get deeper and deeper but hopefully a little better view of the immensity of red wall Cavern I would guess from the lip of the cave out here all the way to the back end where some of our crew is back there it might be um a good 100 feet or so maybe 80 feet maybe like uh what would that be like maybe 30 meters something like that so it's incredibly overhung you can see some of the layering here in the red wall itself as well and then one of the characteristic things in the red wall limestone is the shirt that it contains and so in and amongst all these limestone beds there's also hardened layers of chert which is a a silica Rich material and so that's what's forming a lot of the more Browns we're seeing down here everything down here has been really well sculpted and smoothed by high waters high water erosion and the sand that comes through here um but you get these chirp beds that were also accumulating out in the ocean at the time of the red wall deposition as well and again so we can see not just the big cave like we looked at down river um but even some of these smaller ones that are forming along the layering in the bedding the layering the depositional layering with the Limestone as well um some of these have been filled in by mineral material but in places we can see this again very hard brownish Church layer that's much harder than the limestone just really fantastic just another kind of up and over View of the red wall limestone let's walk to the back of it real quick give you a sense of how big it is from the back and then let's see if we can find some fossils or maybe spend a minute or two looking for some of the invertebrate fossils that are common in the red wall as you can see the high water level deposited a lot of the sand that's up in here which makes it a great place to stop get out of the Sun or maybe get out of the the rain as well looks like we also have some some calcite nodules so these are uh the mineral that mainly forms the calcite or excuse me the limestone is a mineral called calcite in this case we've got some calcite crystals precipitating in this kind of void here looks like there's another really nice one right in here um so yeah like a lot of diversity in and amongst the red wall limestone so this thing probably doesn't see as much water coming up this High anymore with the dams Upstream maybe only during the most high water years where they're releasing more water Downstream so it's probably been a number of years since this thing was actually flooded to this point but we'll head back here give our group a little bit of space and then maybe give you a sense of how big the cavern is looking back out towards the entrance so I'm just kind of backing up as far as I can I'm giving you a sense of it here so let's go see if we can find some fossils and then we'll um wrap up this little segment all right so we have a boulder here and the red wall Limestone with some of the fossils that we thought we could look at here so here we have a crinoid stem it looks like in this slab which was the best thing I found for fossils um the best or the most prevalent fossil are these sort of Cheerio shaped uh feature here these are invertebrates called crinoids These are you can look these up on the Internet they're sometimes called sea lilies but they're not plants they're actually invertebrates they grow on the bottom of the ocean a whole column of these segments here these stems and you can see in places here where there's a hole a whole stem put together is one up here another one here that's actually been cut open and then when they break apart into individual segments that's when they look like these little rounded washers or Cheerios in here but this slab nicely shows some of the fossils we see here in the red wall there's other invertebrates that are common as well there's another stem down here another section there but for me the only thing I'm seeing in this little slab is mostly crinoid fossils but sometimes we get horn corals brachiopods bryozoans this rock was all deposited in an environment where we had incredible productive shallow Marine biologic organisms so just imagine like a reef or Belize or some Great Barrier Reef in Australia some tropical setting where there's just all this marine life thriving and then those organisms dying and these shells and such kind of falling down to the sea floor and then accumulating so hopefully that was helpful thanks again for joining me we're going to head on down river Sean Wilson here from red wall Cavern in the Grand Canyon one last view was stand up of the river and the canyon

Channel: Shawn Willsey

Views: 15,232

Keywords: shawn willsey, colorado river, colorado river geology, northern arizona geology, grand canyon, grand canyon geology, redwall cavern, redwall limestone

Id: M3nXchFwEFo

Length: 9min 40sec (580 seconds)

Published: Wed Aug 16 2023