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Rift, Rock, and Revolution - The Beginnings of the Gettysburg Basin · PlanetGeo: The Geology Podcast

0:00
SPEAKER_00: The scale of these mountains.
0:01
SPEAKER_00: These were estimated to be 5,000 to 10,000 meters tall.
0:05
SPEAKER_00: Um, in other words, we're talking about the scale of the Andes Mountains and even maybe exceeding the scale of the modern-day Himalayas.
0:16
SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it worked, and why it matters to you.
0:29
SPEAKER_01: Your shirt.
0:30
SPEAKER_01: Uh I've seen that shirt before.
0:31
SPEAKER_01: Last time I saw it, it was soaking wet in Gettysburg, I think.
0:34
SPEAKER_01: Have you I wore it there?
0:36
SPEAKER_00: I didn't know that.
0:36
SPEAKER_00: Okay, it's dried out.
0:38
SPEAKER_00: Yeah, yeah.
0:39
SPEAKER_01: It's dried out from our field excursion that we got rained on uh a ton.
0:43
SPEAKER_00: Yes, we did a lot of it.
0:44
SPEAKER_00: It was a ton, and then you guys got hit again, big time.
0:47
SPEAKER_00: Oh man, yeah, totally slammed.
0:49
SPEAKER_00: Yeah.
0:50
SPEAKER_01: Um hey Chris, we looked at the field trip, we did that overview thing.
0:54
SPEAKER_01: Now we're we decided to take this and go deep, like take some deep dives into sometimes individual rocks, sometimes just regional geology.
1:03
SPEAKER_01: But I wanna maybe we start out with we've talked about the Appalachians, learning Appalachian geology a lot, and we've had listener you know requests for for episodes about the Appalachians.
1:15
SPEAKER_01: Um it is such a big topic, and this whole series just reinforced for me, and especially an episode like this where we're talking about the rift basins on the East Coast, just reinforced for me how complicated the geology of the East Coast in the United States is and really how long it's gonna take to do it justice in a full way.
1:32
SPEAKER_00: A hundred percent.
1:33
SPEAKER_00: Uh, this is really, really complicated.
1:36
SPEAKER_00: So we're we're gonna kind of we like you said we're gonna get into the weeds, but we want to stay with the rocks that are tied somewhat loosely, sometimes intimately, to the Battle of Gettysburg in the Gettysburg area.
1:48
SPEAKER_00: That's right.
1:48
SPEAKER_00: So our geologic descriptions are gonna be focused in more on that than the geology of the Appalachian Mountains.
1:58
SPEAKER_01: That's right, but I agree, yeah, that's exactly right.
2:02
SPEAKER_01: And we're gonna kind of, I don't know, we do this in an hourglass form a little bit here as we go through these episodes.
2:07
SPEAKER_01: We're gonna kind of be zoomed out this episode a little bit.
2:09
SPEAKER_01: We're gonna go deep on the zoom out.
2:10
SPEAKER_01: We're gonna be talking about these rift basins, these Mesozoic rift basins on the East Coast, which Roger Fale, a Pennsylvania geologist, has published papers that we've used a lot of, and some of these images are modified from from sort of write-ups, you know, schematics that that he put forward that kind of loop these all together, the stratigraphy of these sedimentary basins that form all along the east coast that are major players in the geology here and the geology of Gettysburg.
2:36
SPEAKER_01: We're gonna kind of start zoomed out from that topic this episode.
2:39
SPEAKER_01: But we kind of have to, we can't just start from there, right?
2:42
SPEAKER_01: Like rifting.
2:43
SPEAKER_01: You know, rift basins, we can't just what happened before that?
2:47
SPEAKER_00: You know, that's like I I think you and I were a little, we came at this a little different.
2:51
unknown: Yeah.
2:52
SPEAKER_00: You know, you wanted to start right.
2:53
SPEAKER_00: Let's talk about the riffs.
2:54
SPEAKER_00: I'm like, well, what happened before the riffs, Jesse?
2:56
SPEAKER_00: Come on now.
2:56
SPEAKER_00: Let's we gotta we gotta do this.
2:58
SPEAKER_00: But it's like an endless little thread.
2:59
SPEAKER_01: You keep pulling on that thread, and pretty sure pretty soon, you know, we're gonna hours and hours deep into what came before in the Appalachian region.
3:06
SPEAKER_01: So that's right.
3:07
SPEAKER_01: What makes sense for you here?
3:09
SPEAKER_01: I mean, what where does it make sense to kind of when should we hit start the geological story?
3:14
SPEAKER_00: Today we're gonna just talk about the basins that happened because of the rifting.
3:17
SPEAKER_00: That's the focus of this episode.
3:20
SPEAKER_00: But to me, we have to go a little bit further back in time and talk about what happened before the basins.
3:27
SPEAKER_00: And so you and I were talking about this.
3:30
SPEAKER_00: Like, what's the best way to describe what happened?
3:32
SPEAKER_00: Well, if you if you kind of like we got to talk about the formation of Pangaea.
3:37
SPEAKER_00: We have to bring these continents together.
3:38
SPEAKER_00: So the analogy that you and I came up with is like if if you look at the kind of S shape of the mid-ocean ridge in the in the Atlantic Ocean, it resembles a zipper.
3:48
SPEAKER_00: So let's just start at the bottom of that, let's start at the top of it, doesn't matter which way, let's zipper that up.
3:55
SPEAKER_00: And if you do that, you bring these continents together.
3:58
SPEAKER_00: And and you know, I know that we're really simplifying it, but that's that's really essentially Pangaea coming together.
4:04
SPEAKER_00: And so it's this zippering up of the Atlantic Ocean, mashing these continents together.
4:11
SPEAKER_00: And that formed this collision, is is what we call the Allegheny orogeny.
4:16
SPEAKER_00: And this is basically the closing phase or the last part of the Appalachian Mountain Building event.
4:25
SPEAKER_01: And so long and torturous, and you know, took a long time, but this is kind of the final you're saying, this is like the final collisional thing.
4:32
SPEAKER_01: Like in the way Grizzly, I kind of think about this is let's say if you just wanted to close the Pacific Ocean, let's say, let's say we take the continents, and you know, all the continents, Asia collides with North America, and you get Australia colliding with South America, there's gonna be a whole bunch of stuff in between there.
4:48
SPEAKER_01: Like Hawaii's got to go somewhere, Japan's gotta go somewhere.
4:51
SPEAKER_01: There's all these like little blocks that have to end up before you collide the big dogs together, you gotta tidy up, sweep up all the little stuff.
4:59
SPEAKER_01: And that's that's what adds to the complexity of the appellation orgenic event, which was very long.
5:04
SPEAKER_01: And we're zooming in a little bit on the, as you said, the Allegheny orogeny, the end stage, the last part.
5:09
SPEAKER_00: So this would have been like what would this have looked like?
5:12
SPEAKER_00: You know, we would have had intense folding of continental rocks, faulting like crazy, intense metamorphism, and and all of the grades of metamorphism, which is a part of this story too.
5:25
SPEAKER_00: The scale of these mountains, these were estimated to be 5,000 to 10,000 meters tall.
5:31
SPEAKER_00: In other words, we're talking about the scale of the Andes mountains and even maybe exceeding the scale of the modern day Himalayas.
5:39
SPEAKER_01: And that's a good, I think that that's Chris.
5:41
SPEAKER_01: When we think about supercontinent building, when we think about continent-continent collision, this is in the intro classes, that's what we say.
5:48
SPEAKER_01: The Himalayas are continent-continent collision.
5:50
SPEAKER_01: That's our like mental model for what happens when continents collide together.
5:54
SPEAKER_01: That's what we look at.
5:55
SPEAKER_01: And so that's a good one that fits, except this is you know, not just the the sort of microcontinent of India smashing into Asia, but we've got all of these continents colliding together over a long period of time.
6:06
SPEAKER_01: That zipping does not happen all at once, right?
6:08
SPEAKER_01: It takes a little bit of time to go from the the the bottom of Africa and South America all the way up to you know northern Europe up into Greenland.
6:15
SPEAKER_01: So this takes a while, this is a complicated process, but the scale is enormous.
6:20
SPEAKER_01: What'd you say?
6:20
SPEAKER_00: I said this the zippers had hinges, it didn't all it wasn't just direct angles, you know, these were oblique angles that these were things were coming in at and all that.
6:29
SPEAKER_00: So but I want to I want to talk about this a second, Jesse.
6:32
SPEAKER_00: And I'm I'm interested in some of your input on this, so feel free to jump in here.
6:36
SPEAKER_00: But I want to talk about how we would come up with this estimate of 5,000 to 10,000 meters.
6:42
SPEAKER_00: And so, you know, we're looking at things like how much the crust was thickened and shortened.
6:48
SPEAKER_00: We're looking at something that is near and dear to your heart too, these metamorphic faces.
6:53
SPEAKER_00: You know, these uh these rocks that indicate these recipes of temperatures and pressures.
6:59
SPEAKER_00: You know, we're talking like green schist faces and amphibolites and granulites and even eclogites.
7:05
SPEAKER_00: And all of these rocks give us this kind of this recipe of of temperatures, pressures, in other words, then depth of burial that was involved here.
7:15
SPEAKER_00: Excuse me.
7:15
SPEAKER_00: And so those give us like really good estimates on on like how tall these mountains must have been to be able to achieve these temperatures and pressures.
7:26
SPEAKER_00: And then also just the vast volume, and we're gonna touch on this in the in episodes to come, the sediment accumulation.
7:33
SPEAKER_00: And we're not talking about just a couple of thousand feet, we're talking about massive sediment accumulation here.
7:40
SPEAKER_01: Yes, so there's two ways to kind of, I mean, there's multiple ways to adjust height of mountains, but you're right.
7:44
SPEAKER_01: You can look at them, the interior, and say how thick did it used to be above that?
7:49
SPEAKER_01: So in North America, the Piedmont region is mostly if you go out into east, you know, the Chesapeake Bay area, eastern Pennsylvania, Philadelphia, New Jersey, that's the Piedmont metamorphic rocks that were caught up in this mountain building event.
8:02
SPEAKER_01: You can look at the pressures and temperatures of those and say, hey, how thick did the crust used to be above this stuff?
8:09
SPEAKER_01: How deep was this buried?
8:10
SPEAKER_01: And that gives you an estimate of mountain building height.
8:12
SPEAKER_01: The other way is to look at the sediment, what was eroded off of it?
8:15
SPEAKER_01: And there's some really interesting calculations that go back, actually.
8:18
SPEAKER_01: Our department, people in our department, have done this going back into the 80s, where basically you look at the thickness of sediments next to the origin.
8:27
SPEAKER_01: So, you know, most of the sediments in eastern Pennsylvania, Appalachian Plateau, those were all weathered off, and you can do some basic calculations.
8:35
SPEAKER_01: Well, they're kind of intricate calculations, but you know, these were done.
8:38
SPEAKER_00: There are intricate calculations, but they have massive variables, which is why we're talking about 5,000 to 10,000 meters.
8:45
SPEAKER_00: That's a huge safe to say these mountains were big.
8:48
SPEAKER_00: We just don't know exactly like how big, you know.
8:50
SPEAKER_01: And Chris, a cool little thing is it's not it's not just like looking at the volume of sediments in the basin.
8:55
SPEAKER_01: That's not actually the calculation.
8:57
SPEAKER_01: It's actually looking at that down warping.
8:59
SPEAKER_01: So you talk often about you know deduction zones like as kind of rumpling the carpet.
9:03
SPEAKER_01: You know, you get this down warping and up warping that kind of extends into the crust away from the actual collisional zone.
9:10
SPEAKER_01: And that's what you're looking at.
9:11
SPEAKER_01: The down warping, that flexing of the crust can be related to the height of the mountains nearby.
9:17
SPEAKER_01: And so there's a bunch of different calculations that you can get at.
9:20
SPEAKER_01: But Chris, let's turn to image number one real quick, because I think this is a reconstruction, plate reconstruction, and it's based on again, we have to talk about this.
9:27
SPEAKER_01: This is a view of the world.
9:28
SPEAKER_01: This is not how the world looked, but this is a view of the world that's based on the rock records.
9:33
SPEAKER_01: All the rocks in this the story we're talking about here feeds back into this image, if that makes sense.
9:39
SPEAKER_01: But I think it shows it pretty well.
9:41
SPEAKER_01: Like this is what Pangaea looked like.
9:43
SPEAKER_01: Pennsylvania, we're zooming in here.
9:44
SPEAKER_01: Pennsylvania, we've got to the what is now the southeast, we have this high relief mountain belt that goes all the way up the east coast of the United States, but also, you know, all up and down this zippering of the Atlantic, this Allegheny orogeny.
10:00
SPEAKER_01: So we're on the northwest side of that when we're looking at these basins, these Mesozoic basins, what was grouped together as the Birdsboro basins.
10:10
SPEAKER_00: Right on.
10:11
SPEAKER_00: So let's put some timestamps on that first.
10:13
SPEAKER_00: The Allegheny Orogeny, the closing end phase of the Appalachian mountain building event, this zippering up of you know the Atlantic Ocean.
10:21
SPEAKER_00: We're talking like 325 to 260 million years ago.
10:26
SPEAKER_00: And then later it starts to rift.
10:29
SPEAKER_00: And this is about 30 million years later.
10:32
SPEAKER_00: So we're talking 230 million years ago, in the middle of the Triassic, basically, the Atlantic Ocean begins its like infancy.
10:41
SPEAKER_00: And so you it begins though as this continental rifting, right?
10:45
SPEAKER_00: And and so this is a good thing.
10:47
SPEAKER_01: Yeah, because you've got the supercontinent.
10:48
SPEAKER_01: Like we're not we're not breaking, you're not breaking off some edge, we're breaking it kind of down the middle, maybe not right down the middle, but we're breaking up.
10:53
SPEAKER_00: You're not breaking it exactly where it zippered together, you're breaking it in a in a different place.
10:58
SPEAKER_01: I Chris, I just want to hit pause here a minute because this is the scale of these things is so impressive.
11:04
SPEAKER_01: I mean, we're talking about a mountain building event there that you just said took 65 million years to kind of put this thing together.
11:11
SPEAKER_01: And then that's a long time.
11:13
SPEAKER_01: To me, only 30 million, it feels like only 30 million years later, you're breaking it apart.
11:17
SPEAKER_01: Took so long to put this thing together, it's kind of like a puzzle.
11:19
SPEAKER_01: It takes you forever to put it together, really quick to break it apart.
11:22
SPEAKER_00: But Jesse, 30 million years is roughly half the time since the dinosaurs became extinct.
11:28
SPEAKER_00: So we're not talking about an insignificant amount of time here.
11:31
SPEAKER_00: No, no, that's true.
11:33
SPEAKER_00: It's a ton of time.
11:34
SPEAKER_00: Yes, uh exactly.
11:36
SPEAKER_00: So anyway, this this kind of rifting that began roughly 230 million years ago, this is this is what kind of, and we're gonna like I'm gonna air quote that, kind of starts the story near Gettysburg.
11:49
SPEAKER_00: So anyway, this was continental rifting.
11:51
SPEAKER_00: I always think of, I think you have a little bit of a different like go-to with me, but or with this uh thing, but my go-to with this is the East African Rift Valley.
12:00
SPEAKER_00: That's like the modern analog for what's happening here.
12:03
SPEAKER_01: I mean, that's the go to that you think of Death Valley, maybe or in the Well, no, I I I I think I mean they're very different tectonics in a way, but I think that you know the the East African Rift is is the like it's the Himalayan equivalent for it, you know.
12:17
SPEAKER_01: If Himalayas are when you put continents together, East African Rift is what's happening as you kind of are breaking it apart a little bit.
12:23
SPEAKER_01: Um and there's all sorts of failed and successful rifts throughout the geological record, right, that contain sediments that look a lot like what we see here.
12:32
SPEAKER_01: So I think this is a pretty from a sedimentological perspective.
12:35
SPEAKER_01: Again, the sediments are telling us this story, but this is very robust conclusions based on what the world is.
12:40
SPEAKER_00: If we think about this that's right.
12:42
SPEAKER_00: If we think about this sequentially, like what will happen if it doesn't fail, if it continues to do what it does.
12:48
SPEAKER_00: The East African Rift is this continental rifting phase.
12:51
SPEAKER_00: Then it turns into something that looks like the Red Sea.
12:54
SPEAKER_00: This, you know, really kind of narrow arm of the ocean.
12:57
SPEAKER_00: And you think of, I think of the if we're gonna say put this on like human analogy kind of thing timeline, you're born, that's the East African Rift, and then you spend a little bit of your life in your infancy and toddler stages, and that's like the Red Sea, and then you kind of become mature and you live a whole adult life.
13:16
SPEAKER_00: That's like the Atlantic Ocean, where we are right now.
13:19
SPEAKER_00: So in terms of three stages, if it continues, I'm I'm near death.
13:26
SPEAKER_00: What happens at the end?
13:27
SPEAKER_00: You're like when you're the last gasp of the ocean as it's getting crushed together by two superiors.
13:32
SPEAKER_00: Actually, maybe I'm like Benjamin Button and and you just zipper me back up and I I start all over again.
13:38
SPEAKER_01: And then you go again.
13:39
SPEAKER_01: Okay, perfect.
13:40
SPEAKER_01: I like that.
13:40
SPEAKER_00: All right, that's a good one.
13:42
SPEAKER_00: You become young again.
13:44
SPEAKER_00: I like that.
13:44
SPEAKER_00: Yes, yes, absolutely.
13:46
SPEAKER_00: So, all right, let's back, let's get back to the story.
13:49
SPEAKER_01: Yeah, I hold on, I like the analogy, Chris, but but uh you know, we we have there's rifting, but it doesn't just go like you're describing there, it doesn't just go from a continent breaking apart into deep oceans all of a sudden.
13:59
SPEAKER_01: We have this long time period where you're getting sedimentation, but the sedimentation is happening on land, right?
14:05
SPEAKER_01: It's kind of terrestrial sedimentation.
14:07
SPEAKER_01: There's water flowing off the mountains, it's flowing into these basins, but it's not flowing into the deep ocean.
14:12
SPEAKER_01: We kind of tend to think in those binaries of land or deep ocean, but there's this transitional period that you're describing where it gets this shallow inland sea, maybe eventually, and then it becomes an ocean.
14:22
SPEAKER_01: But most of the the rocks we're gonna talk about here and focus in on, these basins, the Birdsboro basins, all of these Mesozoic rift basins all up and down the east coast, they are mostly like terrestrial or what we call lacustrian.
14:37
SPEAKER_01: So they're they're either lake sediments or river sediments, sediments deposited on land, and there's some clear evidence why that is, or how we tell that rally rather.
14:45
SPEAKER_01: And that to me, there's many analogies for how this happens and what this looks like.
14:49
SPEAKER_00: But so I think too, it's it's important.
14:52
SPEAKER_00: Why did we begin by talking about the Allegheny orogeny?
14:55
SPEAKER_00: Because in order to provide the sediment for these basins, you needed to have these mountains, and then you have this rifting which formed these kind of these just downdropped, sunken Birdsboro basin kind of things, with and then you had a bunch of sub-basins that are a part of that.
15:13
SPEAKER_00: The Birdsboro is this kind of all-encompassing term or name that we've given to these to all these sub-basins that we're gonna refer to.
15:20
SPEAKER_01: So, Chris, this is a I'm not sure how well accepted.
15:24
SPEAKER_01: I'm gonna go to image number two here, real quick.
15:26
SPEAKER_01: I'm not it's hard to kind of tease out how well accepted this idea of one big basin that includes them all.
15:33
SPEAKER_01: I think pretty well.
15:34
SPEAKER_01: I mean, I think this is uh uh a decent idea, let's put it that way.
15:38
SPEAKER_01: But here's a map showing the east coast of the United States with the states labeled outlined there, and all of the things that we're seeing here, basically from North Carolina, maybe a little bit in South Carolina, but all the way North Carolina up through Virginia, Maryland, Pennsylvania, New Jersey, and then up into Massachusetts, we have these basins.
15:55
SPEAKER_01: They have what are called remnants, they have sediment preserved still on the ground from this depositional event 230 million to about 190 million years ago.
16:06
SPEAKER_01: These are the remnants, and they can be broken apart into four kind of major ones.
16:11
SPEAKER_01: Basically, we have the Barbersville one down in Virginia, the Culpepper Basin that's up in northern Virginia and Maryland, then the Gettysburg Basin, which we will zoom in on in the next episode here, and then the Newark Basin, which is maybe the most famous one, I think.
16:25
SPEAKER_01: It's one of the large ones.
16:26
SPEAKER_01: Why is that?
16:27
SPEAKER_01: It's just been studied a lot.
16:28
SPEAKER_01: I'm not sure if it is or not, but it's been studied a lot.
16:31
SPEAKER_01: And it kind of is a template.
16:32
SPEAKER_01: It's one of the bigger ones that's preserved, so it kind of becomes a template for sort of extrapolating those ideas down to the the some of the smaller remnants down south.
16:40
SPEAKER_01: But lumping those all together, there's some consistency in the sedimentation that we see there.
16:45
SPEAKER_01: There's some dramatic differences as well, but it kind of helps paint this picture of what the world might have looked like if I go back to image number one here.
16:52
SPEAKER_01: It helps lead to the ability to paint a picture like this that shows big mountain chain, sediment going up to the northeast, flowing away from the high terrain to the southwest and flowing to the northeast.
17:04
SPEAKER_00: So, Jesse, so the sediment was being shed off the high, the higher ground, the mountains of the Allegheny Neurogeny from earlier.
17:12
SPEAKER_00: Which way was it draining then into the basins?
17:16
SPEAKER_00: And then which way did the basins drain themselves?
17:19
SPEAKER_01: Yeah, so that I think that's a good, it's a really good point, Chris.
17:22
SPEAKER_01: That you have to you kind of have to imagine this.
17:24
SPEAKER_01: I don't know if there's a good template, there's not one that a good template that kind of comes to mind immediately for me for this, but they're generally we're draining from the southeast.
17:33
SPEAKER_01: Things are draining to the northwest in the current look of the landscape.
17:37
SPEAKER_00: The current orientation, which it was not the orientation 230 million years ago.
17:42
SPEAKER_00: But that's where we're currently at.
17:44
SPEAKER_01: Yep.
17:44
SPEAKER_01: Kind of standing at the equator 230 million years ago.
17:47
SPEAKER_01: But basically, if you're standing in in Gettysburg now, the mountains existed to the southeast from you, and they were draining to the northwest broadly.
17:56
SPEAKER_01: But what's a good example, Chris?
17:58
SPEAKER_01: I mean, I'm trying to think of places that have big mountain chains that are eroding off into lakes, you know, forming these kind of playa lakes and forming these drainage networks.
18:08
SPEAKER_01: Because, like you said, you have to drain off into the basin, but then the basin has to drain itself too.
18:13
SPEAKER_01: And some parts of the basin have river networks that were easier to drain, other ones did not, and so they ended up forming these playa lakes at certain climate levels and then dried out.
18:22
SPEAKER_00: So Yeah, Jesse, let's but we should define what a playa lake is, though.
18:28
SPEAKER_00: And let me just say introduce that.
18:30
SPEAKER_01: Let me hold on, just real quick, Chris.
18:31
SPEAKER_01: Let me interrupt to just point image number three here, where we have a schematic.
18:34
SPEAKER_01: Again, this is built off a lot of the work that linked together a lot of these basins, but there's some generally accepted features to this that mountains to the southeast, off in the Chesapeake Bay area, eastern Maryland, eastern New Jersey, those mountains were draining to the northwest.
18:50
SPEAKER_01: We have alluvial fans forming in those mountains that are deposited.
18:54
SPEAKER_01: Then we also have playa lakes forming in between those.
18:58
SPEAKER_01: So differences between the different basins can be explained by whether you're on an alluvial fan deposit or a playa lake deposit.
19:04
SPEAKER_01: Okay, so playa lakes.
19:06
SPEAKER_00: These are just the I let me grab a crack at it, Jesse.
19:09
SPEAKER_00: Playa lakes are sometimes called ephemeral lakes.
19:12
SPEAKER_00: These are lakes that dry up a lot.
19:14
SPEAKER_00: So the the lake will form, sediments being dumped off in them, and then they will dry up because they're they're usually in a restricted basin where the water gets where it's going, then it has nowhere to go, really.
19:25
SPEAKER_00: And then you combine that with kind of this arid, warm climate, and they just dry up.
19:30
SPEAKER_00: And so that's what you mean by a playa lake.
19:33
SPEAKER_00: You also introduced the term these alluvial fans.
19:35
SPEAKER_00: Hold on.
19:36
SPEAKER_00: Alluvial fans.
19:37
SPEAKER_01: Chris, let me let me just j jump in there.
19:39
SPEAKER_01: This is where the Death Valley analogy comes to mind for me, or the racetrack playa, the southwestern United States, down in Mexico, in these big extensional environments, we have these arid basins where the lakes will fill up, especially in if you have more wetter climate cycles and drier climate cycles.
19:55
SPEAKER_01: You know, all that water is either evaporating or draining out.
19:58
SPEAKER_01: Sometimes it'll drain out when it's really wet for a couple, you know, decades or millennia, and then other times that it'll just dry out and form these salt lakes, basically.
20:06
SPEAKER_01: So that's the mental model for these ply lakes.
20:09
SPEAKER_01: All right, sorry.
20:09
SPEAKER_01: Um alluvial fans.
20:11
SPEAKER_00: Alluvial fans, you introduced that term real quick.
20:13
SPEAKER_00: We're gonna get into that in future episodes in great detail because it's super, super interesting.
20:19
SPEAKER_00: Um we saw on the field trip are hundreds of 100%.
20:26
SPEAKER_00: Beautiful.
20:27
SPEAKER_00: Um alluvial fans basically uh they're like a delta, essentially.
20:30
SPEAKER_00: Uh a river empties into an ocean and it forms a delta.
20:33
SPEAKER_00: And water slows down, deposition occurs.
20:35
SPEAKER_00: Same thing, except when you have water coming out of the mountains and then it hits relatively flat ground at the base of the mountains, like in the valleys, sediment drops off rapidly then, and you get it's called an alluvial fan, though.
20:48
SPEAKER_00: The river's not emptying into the ocean, it's just coming off this kind of high energy, high relief area to rather suddenly much flatter ground, and it forms this kind of fan-shaped deposit.
20:59
SPEAKER_00: We call it an alluvial fan.
21:01
SPEAKER_00: Alluvium in geology means river related.
21:03
SPEAKER_00: So this is a river deposit.
21:06
SPEAKER_01: And if we link those together, here's another term.
21:09
SPEAKER_01: If we link together a region that has a lot of alluvial fans, we can form what's called a bahata, which is a long sort of linked-up group of alluvial fans that are all dumping off.
21:19
SPEAKER_01: And Chris, we didn't see these rocks, the alluvial fan deposit rocks, but they're beautiful.
21:24
SPEAKER_01: I've seen them when I lived in southeastern Pennsylvania.
21:27
SPEAKER_01: In this sediment sequence, there are conglomerates, these beautiful quartz pebble conglomerate things that are all over this region of various ages.
21:35
SPEAKER_01: Some are really old, previous rift event, some are this age, but they're they're really beautiful, and that's kind of what helps define where these alluvial fan deposits were as opposed to the playa lake deposits.
21:47
SPEAKER_01: And you got to think like if you have a big alluvial fan system or a big bajada system, and then a playa lake, and then another big fan system, another bajada system, some of that sediment from those alluvial fan systems will flow into the playa lake.
22:00
SPEAKER_01: Lake system or the Playa Lake system could, in a wet cycle, get really high and start to flow out towards the the same exits that that all the rest of the water has as it goes out these alluvial fence.
22:11
SPEAKER_01: So this basin's restricted, water gets distributed differently in different climate cycles, but there's regional systematics to it.
22:17
SPEAKER_01: And then the water continues to flow out to the northeast.
22:20
SPEAKER_01: The sediment's still kind of flowing out to the northeast.
22:22
SPEAKER_01: And Chris, one thing that that I that really struck me is the lack of really fine-grain sediment in these basins is indicative of there had to be another outlet.
22:32
SPEAKER_01: They weren't true completely restricted basins because there's a lot of fine sediment that's missing.
22:38
SPEAKER_01: Like we have a lot of the sand and the silty stuff and the conglomerates, but a lot of the really, really muddy organic rich mud stuff is not here.
22:45
SPEAKER_01: That's right.
22:46
SPEAKER_00: And what you're what you mean by that, Jesse, is you're saying that there was still flow of water and it had enough energy to carry these finer grain sediments away.
22:55
SPEAKER_00: So we're left with the sand and up to like gravel size, and even bigger than that, actually, these higher energy deposits, but the fine grain stuff, there was enough energy, enough momentum, enough like kinetic to to move the stuff somewhere else.
23:08
SPEAKER_00: Right.
23:09
SPEAKER_00: So Jesse, let's move into then the geology of the basins themselves.
23:15
SPEAKER_00: These basins, they're often described as half grobbins.
23:19
SPEAKER_00: Um well, first of all, then I think we need to define what a groben is, and then a half groben becomes very easy.
23:26
SPEAKER_00: You know, take this.
23:27
SPEAKER_00: Uh how about you take groben and then I'll transition to the half grobin?
23:31
SPEAKER_00: Okay.
23:32
SPEAKER_01: Um I'm sure you have a much better analogy than I do for that.
23:35
SPEAKER_00: I I really don't.
23:36
SPEAKER_00: I mean, uh a groben is when you have area being pulled apart, tensional stress, the ground is then gonna sink unevenly.
23:46
SPEAKER_00: But tensional stress creates what we call normal faulting.
23:50
SPEAKER_00: And so basically a graben is when you have a mountain that's bound on either side by normal faults.
23:57
SPEAKER_00: Any geoscientist, we you know, we see that normal fault where the hanging wall drops relative to the foot wall of this inclined fault surface.
24:05
SPEAKER_00: Then we know that to us that means, oh, this area is being pulled apart.
24:09
SPEAKER_00: It's uh it's under tension, it's under tensional stress.
24:12
SPEAKER_00: So then a half groben is hold on.
24:14
SPEAKER_01: In the in the textbook way of a grobin, you're describing, okay, you pull something apart, we have a block that drops down.
24:20
SPEAKER_01: And in the textbook example, this rarely happens, but it drops down homogenously or uniformly.
24:25
SPEAKER_01: Yes, the whole block drops down.
24:27
SPEAKER_01: And what that means is you have high elevation on both sides, and so the sediments getting dumped in, it means that your sediment will be deposited uniformly in each layer.
24:38
SPEAKER_01: Like each layer will be the same thickness as you continue to drop the block down, you continue to throw sediment on top, it'll be uniform thickness of layers as you go up because both sides of this grobin were going down at the same time.
24:49
SPEAKER_01: That separates it from, and that becomes important into this question of whether this is a true half groben or not here in these basins.
24:57
SPEAKER_01: Because in a half groben, so let's move into describing, I'll take a crack at this.
25:02
SPEAKER_01: This is the Teton fault.
25:05
SPEAKER_01: This is you describe it very nicely as a I think a saloon door hinging downwards, right?
25:10
SPEAKER_01: So you take a door, a saloon door, and you turn it on its side and then open it downwards.
25:15
SPEAKER_01: That door, there's a hinge, it's on one side is locked in place.
25:19
SPEAKER_01: The fault is a curved fault surface that hinges downward, then on one side.
25:24
SPEAKER_01: And you have to think about how this happens over millions of years.
25:27
SPEAKER_01: If that fault keeps moving over a long time period, the sediment thickness will get way thicker on the fault side rather than the hinge side.
25:37
SPEAKER_01: The hinge side has no space because it's hardly going down at all.
25:41
SPEAKER_01: The fault side can have tens of kilometers of movement, and so the sediment layers, you can still trace those, they'll just kind of be little triangles.
25:50
SPEAKER_01: You can find a thick sediment near the fault.
25:52
SPEAKER_01: That same sediment layer will get thinner as you go towards the hinge side.
25:56
SPEAKER_01: And that's what's a normal, well, a textbook half graben.
26:00
SPEAKER_01: Um, this, Chris, I think these are used as let's zoom in now on these these basins, these Mesozoic basins all up and down the east coast of the United States.
26:09
SPEAKER_01: They often have almost all of them have a what's called a border fault, which is a fault on the northwest side.
26:15
SPEAKER_01: That is a normal fault that drops them down.
26:18
SPEAKER_01: We kind of have this hinging fault sequence to them, the the hinge point.
26:24
SPEAKER_01: So it's kind of a listric normal fault, which just means it's slow, it's like a sloped normal fault.
26:28
SPEAKER_00: Look at you dropping big words.
26:30
SPEAKER_00: Oof.
26:31
SPEAKER_00: Yeah, yeah.
26:32
SPEAKER_00: You gotta get pretentious up in here, man.
26:34
SPEAKER_00: Um Lystric is a good word though.
26:35
SPEAKER_00: I like it.
26:36
SPEAKER_01: It's uh it's a good word, it's kind of like coi, you know, it's like an underused word.
26:41
SPEAKER_01: Lystric.
26:42
SPEAKER_01: Koi.
26:43
SPEAKER_01: That's good.
26:44
SPEAKER_01: Um, so they look like structurally, they kind of look like this.
26:47
SPEAKER_01: There's a down-drop block, one side's hinging down, the other side's not.
26:50
SPEAKER_01: However, many of the sediment layers are not thicker across the deposit, which is one of, you know, I think in the field guides, there was like seven.
26:58
SPEAKER_01: Why is that important now?
27:00
SPEAKER_01: Well, there's like seven or eight points that people have brought up to say this is not a normal half graben.
27:06
SPEAKER_01: And the argument is that the sediment that we see today was deposited before the faulting.
27:12
SPEAKER_01: That's a key difference.
27:14
SPEAKER_01: So this is not a true half graben.
27:16
SPEAKER_01: It has a fault system like that, but the sediment was not deposited because there was a fault happening there.
27:21
SPEAKER_01: Like the sediment got layered in, then the faulting happened.
27:24
SPEAKER_01: That's the argument, at least.
27:26
SPEAKER_00: Um that makes sense.
27:28
SPEAKER_00: So I moved to this.
27:31
SPEAKER_01: Yeah, sorry, go ahead.
27:32
SPEAKER_01: Should I point to this image now or should or should we hold off a minute before looking at any?
27:36
SPEAKER_00: No, uh keep it keep it on that image.
27:38
SPEAKER_00: I like I like that because you can see the kind of listric, that kind of curved nature of the half groben fault there on the left-hand part of the diagram.
27:45
SPEAKER_00: Let me just describe it real quick.
27:48
SPEAKER_01: This is a cross-section of the Gettysburg Basin now, where we have on the left side is South Mountain, which we we kind of looked at.
27:54
SPEAKER_01: That's northwest.
27:55
SPEAKER_01: Then we have this fault just kind of dipping down into the right.
27:58
SPEAKER_01: Then we have tilted sediments that are all tilting towards the northwest.
28:02
SPEAKER_01: They're dipping kind of down to the left.
28:04
SPEAKER_01: And then as we move across that, there are a bunch of these little Lystric normal faults and a bunch of different sediments, and then we have the sills and dykes and all of it all.
28:11
SPEAKER_00: But to your point, you can see the sedimentary layers there, those kind of those brownish different colored layers that are uniform thickness across laterally.
28:21
SPEAKER_00: Okay, and so that's what you're talking about is that because they are uniform thickness, you wouldn't get that in a typical half grabin if the graben controlled the sediment accumulation.
28:32
SPEAKER_00: In other words, you had the faulting, you had the downdrop block, sediment then gets shed off the up thrown block down into the basin, and you'd have this kind of wedge shape that would thin to the southeast.
28:43
SPEAKER_00: And we don't see that in this, and that's where the debate comes in.
28:45
SPEAKER_00: Jesse, this begs the question then, to me anyway, uh, all right, if this basin isn't controlled by half groben, then why the basin in the first place then?
28:57
SPEAKER_00: Because you we had a basin Chris.
29:00
SPEAKER_01: Great question, man.
29:01
SPEAKER_01: Uh great question.
29:02
SPEAKER_01: Um, I was not expecting this question, but it's such a key one.
29:06
SPEAKER_01: Like, why the basin, if not the fault, that's sitting right there, right?
29:10
SPEAKER_01: And the answer, I think, to me, this is a bit of a a bit of conjecture here, but we see this in a lot of different places where When I'm big, especially really high mountain building events, super continent mountain building, you build up mountains that are not stable.
29:28
SPEAKER_01: And so we have this thing that is you know lithospheric collapse or orogenic collapse, where the mountain range gets heated up because you've stacked continents on top of each other, you smash this together, made super thick crust, there's lots of heat-producing elements in there, cranking up the heat.
29:43
SPEAKER_01: It cannot remain stable.
29:45
SPEAKER_01: And as soon as you know, magmas move that heat to the surface, the thing becomes weak and it can kind of collapse down in on itself.
29:52
SPEAKER_01: It basically just spreads back out, I guess is the way you kind of think about it.
29:55
SPEAKER_01: Like the basin range, as we've talked about this before, has spread out twice its thickness, so it's been stretched by two times.
30:03
SPEAKER_01: Like it's take your rubber band and stretching it twice as long as it was before.
30:07
SPEAKER_01: That's a lot of stretching.
30:08
SPEAKER_01: And some of that is accommodated by half grobbins, some of that is accommodated by double-sided grobbins where the whole block is drown down dropping.
30:15
SPEAKER_01: So they need not be that's an easy way to say we built these big mountains, we would almost expect them to collapse on itself, and then you're going to be forming these basins in that area.
30:24
SPEAKER_01: Even if it's not this fault that was happening, there was probably some normal faulting that was going on.
30:31
SPEAKER_00: That's right.
30:32
SPEAKER_00: So, in other words, everyone, in geology, things rarely play out like you see in a textbook.
30:38
SPEAKER_00: You know, you said it before.
30:41
SPEAKER_01: Just another example of how the intro level textbook's lying to you in some way or oversimplifying for the purposes of getting the point across, right?
30:50
SPEAKER_01: So which is okay.
30:51
SPEAKER_00: It's okay to do that.
30:54
SPEAKER_01: Chris, I think one uh other aspect of this is we are missing a huge amount of the story here.
31:00
SPEAKER_01: Uh uh we're preserving you know seven kilometers or something like that of sediments across these basins.
31:05
SPEAKER_01: But there's a lot more that we don't have, right?
31:08
SPEAKER_01: We there's a lot more that we are missing for sure.
31:11
SPEAKER_01: So we're missing a huge chunk of the story here.
31:13
SPEAKER_01: And it's been overprinted and it's overprinting previous faulting events.
31:17
SPEAKER_01: So, you know, the interplay between those, it's really hard to figure out when exactly a fault has moved, especially if it's moved a bunch of different times in different directions.
31:24
SPEAKER_01: It's complicated.
31:25
SPEAKER_01: We're missing huge chapters of this book have been ripped out that we don't have access to.
31:30
SPEAKER_01: So hence the debate, I think, or hence the you know, uh, the existence of the previous debate that, yeah, oh look, these look like half grobbins, and then wait, no, they don't.
31:38
SPEAKER_01: They look not like half grobbins in that the sediments are deposited before the faulting happened.
31:43
SPEAKER_01: So it just adds a little bit of additional complexity.
31:46
SPEAKER_01: I don't know if it's that much complexity though.
31:48
SPEAKER_01: Do you do you feel like this like dramatically changes our interpretation of this stuff?
31:52
SPEAKER_01: I kind of think a slight tweak, really.
31:54
SPEAKER_00: No, I don't.
31:55
SPEAKER_00: I I think that um I think that that's just you know, what level of weeds do you want to get into?
32:02
SPEAKER_00: How high do you want the weeds to be?
32:04
SPEAKER_01: Uh the jelly weeds are the thick, thick jelly weeds.
32:07
SPEAKER_00: What kind of tall grass are we talking about here?
32:10
SPEAKER_00: Um so no, I don't think it changes the story at all.
32:13
SPEAKER_00: I mean, we had the Allegheny orogeny, we zipped up the Atlantic Ocean, and then it began to break apart in a different area, and it formed these basins.
32:23
SPEAKER_00: Sediment got shed off from the Alleghenies into these basins and formed the larger structure that is that's the Birdsboro.
32:33
SPEAKER_00: And that's the story in a nutshell.
32:36
SPEAKER_00: And and so, you know, you did a good job explaining this, and then in the gift that you created, they're showing that they're prevalent along the eastern part of the United States, um, going all the way up into New York, you know, from basically South Carolina, the northern part of South Carolina, up to New York.
32:51
SPEAKER_00: You know, that's an extensive that's a lot.
32:54
SPEAKER_00: It's huge.
32:55
SPEAKER_01: It's five over 500 kilometers long, yes, you know, sequence that that have some similarities to them, to each other, uh, and some key differences as well.
33:03
SPEAKER_01: But it's a really, you know, the the sedimentation here, it's kind of impressive, right?
33:06
SPEAKER_01: That you had 30 million years-ish of sedimentation that occurred in these regions that filled up seven kilometers or more often of sediment that is uh of various compositions.
33:18
SPEAKER_01: We have arco sandstones, we have red mudstones, we have all sorts of cool features.
33:21
SPEAKER_01: And we're gonna next episode uh in our series, Chris, we're gonna zoom in on the Gettysburg Basin to talk about the specific rocks that make up the Gettysburg Basin.
33:29
SPEAKER_01: But just while we're doing that, remember, this is just one of these remnants.
33:32
SPEAKER_01: The Gettysburg Basin is one of the remnants.
33:35
SPEAKER_01: There's one up to the north, there's a couple down to the south that kind of linked together we can interpret as this larger basin that we're only getting pieces of.
33:43
SPEAKER_01: We only see these remnants of now.
33:45
SPEAKER_00: That's right.
33:46
SPEAKER_00: But we have to dive into the Gettysburg Basin because this whole thing started because we got it in our heads that we wanted to explain how the geology of the Gettysburg area.
33:58
SPEAKER_01: He called me up and said, Hey, we could go to Gettysburg.
34:01
SPEAKER_01: Uh I I I finished school in like two weeks.
34:04
SPEAKER_01: What do you think?
34:04
SPEAKER_01: It's like, okay, let's do it, man.
34:06
SPEAKER_01: Let's so it's been uh it's been a I would say this has been a cram session, it but it's been so fun.
34:11
SPEAKER_01: I mean, I had I've had some of the funnest days of just sitting down and be like, okay, I am going to read a bunch of papers about the East Coast geology.
34:20
SPEAKER_01: I've been wanting to do this for a long, long time.
34:22
SPEAKER_01: I've been wanting to think about South Mountain for a long, long time.
34:25
SPEAKER_01: And this was just a really good opportunity to kind of do that, to you know, force yourself to sit down and read a bunch of papers.
34:31
SPEAKER_00: One of the things I love about this podcast, Jesse, is it forces us to learn about things that we've wanted to learn about forever.
34:39
SPEAKER_01: That and you get to see my face way more frequently than you otherwise would have.
34:44
SPEAKER_01: So that's gotta be a benefit.
34:46
SPEAKER_01: That's true.
34:46
SPEAKER_01: Yeah, yeah.
34:47
SPEAKER_01: Hey, uh, this is episode two in you think this is a wrap, Chris, on this?
34:51
SPEAKER_01: We come up with the Birdsboro Basins.
34:54
SPEAKER_01: All right.
34:54
SPEAKER_01: Um, shallow weeds, medium weeds, I think.
34:57
SPEAKER_01: Medium weeds, probably.
34:59
SPEAKER_01: Medium weeds, medium weeds, medium weeds.
35:01
SPEAKER_01: Yep.
35:01
SPEAKER_01: Um, this is episode two.
35:03
SPEAKER_01: You can find all the other episodes on our Camp Geo Mobile app.
35:06
SPEAKER_01: They're there now live in our Gettysburg Visual Podcast series there.
35:09
SPEAKER_01: So you can download the Camp Geo Mobile app first link in your show notes.
35:12
SPEAKER_01: Check that out.
35:13
SPEAKER_01: Leave us a like, a subscribe, a follow, leave us a comment.
35:17
SPEAKER_01: If you're on Spotify or YouTube, we love hearing about that and and hearing what you think.
35:21
SPEAKER_01: Are these weeds too thick?
35:22
SPEAKER_01: Not thick enough.
35:23
SPEAKER_01: What do you want to know more of?
35:24
SPEAKER_01: We love hearing stuff like that.
35:25
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35:27
SPEAKER_01: Shoot us an email.
35:28
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35:32
SPEAKER_01: Cheers.