Matanuska Bedrock - Alaska Geology Series

We're along the Matanuska River here. Thank you. And to our right you can see these high peaks here. These are all granitic rocks and these are mainly late Cretaceous plutons. And between us and those plutons is a major fault we call the Castle Mountain Fault. In the mid-ground is the formation called the Matanuska Formation, which is a late Cretaceous unit that's mainly submarine fan deposits. Yeah, so this morning we've come east from Palmer and we're getting into the Matanuska Valley now. Some of the structures and landmarks that Ken pointed out is we're along the Matanuska River, which comes here through the Map Valley. And this would be the Castle Mountain Fault, which is at the base of the granitic rocks. And the Castle Mountain Fault is basically running parallel to the Border Ranges Fault here. Got it. And so one of the things we're going to look at at this outcrop and throughout the Map Valley is with the Border Ranges Fault, this originates as an older structure. So it's interpreted as the megathrust related to subduction along this margin. And from there it starts to get overprinted by strike-slip faulting. And in this area, as we have strike-slip faulting through here along the Border Ranges Fault, a lot of that strike-slip starts to get moved off of the Border Ranges into the Castle Mountain Fault. This also has implications for looking at the map further to the southwest. And so the Castle Mountain Fault has interpretation of about 130 kilometers of displacement. That's by Jeff Tropp and others. And one important thing there is that that'll offset a fault here called the Broome Bay Fault, which is bringing up plutonic rocks and exhuming deeper crustal levels on the upper Alaska Peninsula out here. And there's another fault that's a little bit, you know, smaller from the scale of a map like this, but called the Little O'Shetna Fault, which is doing the same thing to the north of the Castle Mountain Fault. So this is the Matanuska Formation. And of the Matanuska Formation, but it's basically late Cretaceous in age, Campanian, Mestrictian. And these are four arc basin deposits. We'll put that in context when we see them a little bit less deformed. But these strata have some offset markers that we can use within this outcrop. So we have some more beige and tan colored sandstones. Yep, sandstones mostly here. There's conglomerates in this formation we'll look at. And then the darker gray, those are mudstones. Well, the low ridge to the right is still part of the Matanuska Formation. Okay. And so it's really part of the Foreland Basin. It's this stuff. It's this, absolutely. But a high peak in the background is on the other side of the Castle Mountain Fault. And the Castle Mountain Fault divides the arc rocks from the arc basin. And we know that even in modern floor arcs, like the Sumatra example, the Sundra Arc, there's always a lot of displacement between, there's often a strike slip fault between the arc and the basin itself to accommodate obliqueness. Yeah, we're on the road between the Palmer area and Glen Allen. So we're going through the Matanuska Valley. So this runs west east. Wonderful. This is all the Talkeetna Formation, which is part of the Oceanic Arc that we've been talking about. And as we come across the river, this is the Matanuska River, we're going to move into that Late Cretaceous Fore Arc Basin, the Continental Basin, and that basin you can really trace from Alaska all the way down to the Great Valley Sequence. And so by the Late Cretaceous, everything is pinned, in our opinion. We're going to slide things on strike slip faults, but the arc is pinned itself. And these rocks provide some of the evidence for that Cretaceous Arc. Oh yeah. Oh yeah. Oh my god. You want to talk about some of these class types, Brandon? This is a nice face here. Yeah, it is. So when we talk about this deposit or these strata being related to Cretaceous continental arcs, some of the class we can see that help with that is we're seeing a lot of exhumed plutonic class. Now at this point, these are going to be derived from both Jurassic and Cretaceous plutons that we're seeing. You know, we sometimes are pointing today up to the north in the Talkeetna Mountains. So we have plutons exposed of those ages, and we're finding those class types throughout here throughout this. You're also going to find reworked sedimentary class, volcanic class. And these conglomerates, you can see they're pretty, you know, they're nice rounded class, but overall this is massive. You know, we're not seeing a lot of clear stratification, but these beds are pretty steeply dipping where we can find that. Okay. And these are interpreted as mass flows coming down into fan deltas. So in an environment like this, this is basically coming into the Forak Basin down pretty high gradient slopes. And Jeff Tropp has done a lot of work throughout the Matanuska Valley on these rocks, interpreting these types of depositional environments. So again, coming from the arc into a marine environment, there's marine fossils throughout these rocks on high gradient slopes and building out these deltas into the basin. But so that Wrangelia composite terrain, which across the road was out in the water, is now docked. Yeah. And, and that collision and that docking is going to do a lot of the work in, in building topography, zooming plutons that provide the sources for some of these class. Okay. And so we're younger than a hundred million years ago now, suddenly if you've docked the thing. Yeah. So these are Campanian Mestrictian. So now we're getting into say 85, 90 to, to the end of the Cretaceous. Huh. Okay. Mike, are you surprised by this? You were surprised by the size of those class yesterday in the McHugh within the Chugach. This is more tame and more what you expected. You know, this unit a decent amount. I guess I am less surprised by this because of its sort of depositional location. We're in a Forak Basin. We're very proximal to that sort of topography that's going to produce all these rounded class. Whereas when we were out in the, out in the accretionary prism, I was a little bit more surprised that we had giant refrigerator sized boulders coming down into that. Yeah. Yeah. It just gets better and better. Come on now, Brandon, get us started. What's the name of this mountain over here? All right, this pyramid shaped mountain, this is King Mountain. This is a 58 million year intrusion. This is one of the near trench intrusions we'll be talking about. Thank you. Sticking right up basically in the middle of the Matanuska Valley. Wow. Behind that to the left, you see some more reddish rocks. Those red and gray kind of knobbly rocks are the Talkeetna formation. So those are the early Jurassic volcanic rocks of the Talkeetna arc. Doing great. Behind them, these more craggy gray rocks with snow on them. That's where we're going to start to get into more mafic plutons associated with the Talkeetna arc and eventually into the Chugach complex. Okay. Yeah. This is the Chikaloon formation. This is Paleocene and coming right up to the boundary with the Eocene. And we're going to look at this for a couple reasons. This is a nice example of a fluvial system where the four arc basin that we're talking about here in the Matanuska Valley has now transitioned from a marine to a non-marine depot center. This is also getting right up around the PETM, the Paleocene-Eocene Thermal Maximum. Okay. So there's a bit of a climate story there too. So we'll look at some evidence of vegetation within this overbank system. Sweet. Trees are up there. Yeah, our trees are down here. We're right around 56 million years ago-ish. Yeah. That's when we're starting Siletzia, Mike. Yeah, so this is, I've always been struck by the similarity in the stratigraphy here for the Paleogene and the Mat Valley, the Matanuska Valley, compared to like the Swak formation back down in Washington. Oh. The timing and the general tectonic story is very, very similar. But we're not talking about translating this stuff north, are we? It's just a coincidence? Or is this just a kind of strange? Well, so I do think that, you know, southern Alaska has probably been transported a fair distance. Now, I don't think that you're going to restore this like right next to the Swak formation. But they would have both been occupying a similar position along the same margin at the same time. And so they're recording very similar processes. But this would be very similar to what your viewers back in Washington would see if they were going over like Blewett Pass, you know. Heard of it. Yeah, I'm sure you have. No, alternating tan sandstones and dark shales. Is that what we have here? Or is there coal here? Yeah, there are some thin coals here. They're kind of, you know, a little bit less developed than some coals you might have seen. But kind of between that thicker tan sandstone bed here and then you come down a little bit where it's darkest, we can see some of the float coming down in the outcrop. And that's kind of, you know, part way to coal, most of the way to coal. Over here on the left, we have something cool, which is that here we can look at overbank deposits with some trees preserved. And the trees are, you know, they're almost a little shiny up in here. And those are basically coal. So I'll get up in here a little bit. That's awesome. Mike Eddy's impressed, everybody. Jesus. So here we have some of the sandstone. This is beige sandstone. We have the light gray. This is very fine sandstone and siltstone. And so we have overbank deposits in the siltstone. So this is on the floodplain of a river system. We have some of the sandstones. One of them up here is fairly thick, but most of them you see are really thin. And so along river systems, you often have places where when it floods or gets close to flooding, sand will overtop the banks and spread out pretty unconfined. So that's what we're seeing in a lot of the sandstones. But this tree here, if I were to trace the edge of it, we can go from here. I see it down in here. This is basically coal. And I've seen trees like this in a few places. And sometimes you get them replaced by sand and things. But this is neat because this is like, you know, makes you really feel like you're just seeing a tree that got coalified right here in the sedimentary sequence. One other thing, coming up here for the first time, you know, this close, I feel like you can basically see the texture and the bark along the edge, just like a tree. Maybe I'll bring some down. This is a new phase. So we used to work around the bend on the Chickaloon River on this outcrop. And we just came through here, Brandon and I doing some recon. And I almost cried like a baby when I saw this. And what strikes me, Nick, is from a dynamics point is that we just walked through those deep water submarine fan deposits with channels. And then all of a sudden, we're in the non-marine rocks with trees growing in it. And it's a pretty abrupt transition. Those rocks, as Brandon said, are Campanian Restricting. So they stop at about 65. And these rocks go and start with the Paleocene. So this is a very dynamic system. And as, you know, we were talking about Kings Mountain over there behind you, at the same time, you have Plutons, Neanderthal Plutons that are coming in. So this Eocene is a really interesting time in the Northern Cordillera for reconfiguring the whole margin. And now we have these non-marine rocks that you and Mike were discussing that you see parallels with the Washington geology. Thank you for that. This is our bark. Yeah, this was the outer edge up in there, right there. Wow. What's it smell like? Smells like the Eocene. Like a tree. So how abrupt is abrupt if we're going from marine to non-marine? Is there an event here? I would suggest that there is event and other people have worked on this as well. And I think it's related to ridge subduction, Nick. And so now what we're going to have is a hot blow torch underneath that marine floor arc basin and the accretionary prism. And as you know, mid-oceanic ridges, spreading ridges, have a lot of topography. So if it hits obliquely and moves across the floor arc basin, it's uplifting those marine rocks. And there's a beautiful inconformity between those submarine fan deposits that we showed you and these non-marine deposits. And we think that's related to the ridge coming through, but it's also related to these big plutons like you have across the road. There we go. So am I hearing that possibly, well, first of all, we've looked at near-trench magmas before in these programs and they're just these blotches on a map. That's one of the blotches? Basically a topographic blotch? That's a blotch, brother. I mean, that is freaking sweet. That's an Alaskan-scale blotch. But then I think I'm hearing you say that that blotch, that near-trench magma coming up, is a major player in changing the sedimentation from marine to non-marine. So Mike, I was introduced to these near-trench magmas in Washington, and it was from your work. Can you remind us of a few near-trench magmas that are near and dear to your heart in the is not as extensive. Probably the best ones would be Mount Pilchuck and the Bald Mountain plutons that are just east of Seattle, basically. They are really rich in aluminum, which is a hallmark of melting sedimentary rock. And so if you go to those plutons, you'll probably find places where they crystallized igneous garnet. And so you could go and you'll look and it'll be a granitoid with some red little blotches in them. And so basically what's happening there is you subduct a spreading ridge, and it continues to spread. The mantle will upwell to fill in that space, but unlike at the surface where that upwelling mantle melts and erupts and forms basalt, now you're creating melts that are going right into an accretionary prism or the forearc. And there's one right over your shoulder. There's one right over my shoulder, yeah. We don't have that down in Washington. We don't have a coherent landmark, really. I mean Mount Pilchuck, but there's other units. The record's not as extensive, yeah. But do you think you understand these magmas? Yeah, so the beautiful thing up here is, so you can imagine if you're spreading ridge is coming into a subduction zone and you're spreading and you've got magma coming up into the forearc, the spatial extent of that melting is fairly limited. It's just right where the ridge is coming in. And so up here in Alaska, if we go, I mean, well over a thousand kilometers that way, all the way out to Sanak Island, the age of these things will be like 60, 61 M.A. I can't remember the exact date. And as you swing through here towards the east along the margin, they just get progressively younger and younger and younger. And so this one is, how old is this one guys? 58. So this one's 58. If we go that way, they're going to get younger. All right. And so if you get all the way down to Baranoff Island, that's sort of the other end of that suite, they're about 51, 50 M.A. And that matches up in time with the stuff in Washington. And so I think that has always been a compelling link to me for bringing down the southern part of Alaska during the Paleogene to link up Baranoff Island with Vancouver Island and the Puget Sound region. So just to finish that thought, why couldn't we bring this down in the neighborhood of Washington? This stuff that looks Swakish. Am I missing, is the time? Well, Baranoff Island is pretty far that way. So if we take Baranoff Island and we put it down next to Puget Sound, this is still somewhere off of British Columbia. So that's what I mean by, I think you can move this down, but I don't think this was in the vicinity of Leavenworth. And yet how broad was that swamp, that Swak swamp? Did it continue halfway up through British Columbia? I think that is an excellent question that I don't know if we know that answer. But the similarity between the Paleocene and Eocene rocks up here in Alaska is very striking with the Swak formation down in Washington. Margin of North America. This is the Aleutians out here. I guess I'll draw Alaska. So the Sanak Baranoff Belt intrudes the Chugach all along like this. And there's a classic paper, Dwight Bradley was on it, USGS folks, that outlined that there's an age progression from, Bradley this would be Baranoff Island. All right, so if you go down here, it's a little Vancouver island, and all the way down here there's Washington. So there are also near trench intrusions right here. These are like 52 to let's say 49 MA. And so the two options are, we slide this down so that Baranoff Island is right next to the stuff off of Vancouver Island. So you just close this gap because there's, there are younger near trench intrusions along here, but there are not other Eocene ones in this in this belt. All right, and so that's something that Daryl Cowan has advocated for for a long time. Yes, so you slide all these down and then there would have been a triple junction down here, so we would have had a ridge coming in. And on the northern arm of the ridge you are moving everything up, right? So these end up getting transported to the north. All right, the problem with that is that you got to identify the structures along which it occurred. Yeah, so we've got a couple options. I think Daryl's original idea was doing most of it on the border range's fault. So the, all right, now I'm really worried that I'm not remembering things correctly, but the gap here, I think the number to get is 1,500 kilometers between Baranoff Island and the southern tip of Vancouver Island. Okay. Okay, and so border range's fault is the one that's outboard here, and the problem with that idea is that there are near trench intrusions that are just on the inboard side of the border range's fault that are very similar in age to the ones that are on the outboard side. And so that would imply that we don't have 1,500 kilometers on the border range's fault since these intrusions were in place. All right, so if we go a 52. Since 52. So that gets us down closer to being off of British Columbia, but not all the way there. We're a thousand kilometers short, and so that's sort of where we're at. So if you take that, and it's sort of like a minimal transport distance, then you end up with a situation where you may have had a ridge here, and a ridge here, and a very narrow plate that folks call the resurrection plate. Personally, I'm skeptical of that because as you start bringing these things down, the distance between these two ridge segments gets smaller and smaller and smaller. And so I could see maybe a situation where at the macro scale, we're dealing with two plates, Kula and Farallon, but as their ridge gets closer to the continent, you break into small micro plates. And so maybe the resurrection was a small micro plate. But I have a big problem, or it's hard for me to envision that the resurrection plate was a major player. You know, thousands of kilometers wide piece of ocean crust. The other way, if you want to keep doing the Sanak-Baranoff, or the Baranoff-Vancouver Island connection, would be that you distribute a thousand kilometers of slip on the other strike slip faults throughout here. So not on any individual strand, but across all of them, they all cumulatively get you to the thousand kilometers you need here.