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Chesapeake Bay Impact Crater

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Chesapeake Bay Impact Crater: A Cosmic Connection in Virginia

David Powars:  Quick summary of the talk. The three main points that I'm going to hit are when/where the Chesapeake impact crater, so that everybody can leave here being able to tell a seven-year-old what an impact crater is and what the Chesapeake Bay impact crater is. How did we discover it? I'll briefly cover that. It's a unique crater. It was discovered by cores. I'll just lead into that early.
And then, how does the crater still affect the region? That's probably the most important thing. I'm going to give a little bit more time to explaining how the Chesapeake Bay is related to it. It's a little difficult to understand.
You just can’t say . . . A lot of people out here think the crater is at the floor of the bay. The crater's buried quite a bit, thousands of feet below the surface. Discovering it was a little bit of a trick in detective work. It took some real hypotheses and then testing that hypothesis. Right off the bat...
I'm not sure. Is that me barking? I'll get away from that mic and stand over here.
What are impact craters? The first one that was ever proven...50 years ago, if you believed that these were craters, you were a heretic in the scientific community especially 100 years ago. Our leaders all believed that these were crypto-volcanic explosions -- meaning a volcano that blows up -- including the ones that were on the Moon. They all thought those were volcanoes that blew up.
Thanks to Eugene Shoemaker and a number of other scientists...Ed Chow was another one of our scientists here. They discovered shock minerals. I'll show you some of those a little bit further in the talk. Those are key indicators. We don't have a process on this planet that can produce those kinds of pressures, that we know of, to produce these shock minerals.
One of the terms you'll hear often in impact crater studies is "shock quartz." We're finding all these other minerals can get shocked, as well -- feldspars and a lot of other things. Something else that happens. These come down and land and hit cars, houses. If they hit and land on something, people don't realize that people will pay you a lot of money for that hole in your roof.
That guy, that doctor that had the one that landed over there in Lorton in 2010...
Too close. I talk too loud. I usually don't have a mic on. [inaudible 02:09] come down.
If they'd saved that instead of repairing it, they could've cut it out, and somebody would've paid $20,000 for it, easily. You'd be amazed. We now find many, many meteorites on the planet, thanks to Antarctica. Nice white surface. Anything black, probably going to be a meteorite. They've found over 20,000 since the [inaudible 02:30] in Antarctica. You can go on the Internet and buy these things. I have some on display up here. Please come up and look and feel free afterward.
One thing, going back. The reason [inaudible 02:42] doesn't look...We've got to fix this. Oh, I can't look down. I see. I'll just do this [inaudible 02:55]. OK. That'll be fine with me. These are hot. PK, that's all. Is this better? Oh, much better, yeah.
[laughter]
David:  All right. We're happy, anyway. We don't see our Earth looking like the Moon or a lot of other planets because the Earth is so dynamic. The plate tectonics are always moving. The oceans are going down underneath the continents. And the weather. There's not a meteorite that, since it lands on the planet, it starts weathering. These craters erode and get washed away. As you can imagine, a lot of different things disappear. So we don't see them.
I can look down because I need to look at these buttons and move forward.
Real quickly, lesson number one. Simple impact crater, a bowl like the meteorite crater. One piece of rock comes in or a piece of stony metal or nickel or iron. Something comes in and hits it. Blows out ejecta there. It's a simple hole. The ejecta falls back in, and then lakes starts to form when the groundwater falls in.
The Chesapeake Bay impact crater is so large. The bigger the impactor...We believe the Chesapeake Bay was one to two kilometers in diameter. One to two miles, rather, in diameter. I don't want to be like NASA. Keep my miles and kilometers straight.
It has a complex crater with lots of different terraces, or annular troughs as we call them, slumping out here. A central uplift or often, as it collapses...like you can see in this example from the Moon, with these terraces and slumps. It's a much bigger impact crater with a lot more dynamics. That's what we have in the Chesapeake Bay impact crater.
Moving on. What are these things coming in? Sometimes they're small. Sometimes they're 10 miles in diameter, the one that killed off the dinosaurs. Chicxulub was 10 miles in diameter, it's estimated. This one was one to two miles in diameter.
This one, before we studied it and found it, this was considered big enough to be a global killer. It depends on what the target did. We threw a lot a silicon in the atmosphere. Chicxulub, it threw a lot of sulfur in the atmosphere, so it made acidic rain -- it's the leading theory right now -- so you don't see a bone bed across the whole world. Everything died off -- the trees, the plants. They went out to prove they were wrong. They proved even more that they were right.
We find this iridium, the smoking gun from the impactor. That's a chemical. It's much higher in meteorites than asteroids. When we see that, when we find that, we say, "Oh. We should be looking for something that has to do with an impact. We see it much higher than the normal amount that's found on the Earth."
Comets travel a lot faster. These travel slower. The size of the impact as to whether it's a comet or whether you're two trains coming together. Is it coming into the planet, or is it chasing the planet when it hits it? Those will all affect the size of the impact.
Most impacts are circular if you find them. The reason they're circular is because of the explosion. They explode like a nuclear bomb or whatever. Unless they're below five degrees, you don't see any kind of an angle at all. They blow up. The one that created the Chesapeake Bay impact makes our nuclear arsenal, if you put it together, look like a firecracker compared to a nuclear explosion. That's how big the explosions are.
This one here is a meteorite that fell in Namibia a couple thousand years ago. I have a 60-pounder over there for you. Pull, touch, or whatever. Came in a few thousand years ago. It's a big iron and nickel one. It blew up. It's now the national treasure over there. Worth coming up and feeling and touching.
This one actually fell. It's a stony one, the one that fell in Lorton. Luckily, I believe, it was given to the Smithsonian instead of arguing over who should own it. It came through a doctor's office just after they'd left the room.
So they fall all the time. I saw, honest to god, last night, driving. One broke up. I hit it. I was in my truck. Came right over. Three! It broke up into three, and I went, "I've got to go look for that." Not going to happen. Dark. Between here and wherever, another couple thousand...Maybe only 100 miles because it really broke up and then it just stopped. I was really excited. Who else has seen anything that's coming down?
Anyway, this is just to give you a little idea of what the meteorites look like and if they come in. I'm sure you're all aware if you've been watching any of the television recently, we have a real close call happening on February 15th. If you don't know, go home. Type in "JPL NEAT." Near earth asteroid tracking. They have great stuff on it. Or just type in "DA-14."
It will immediately bring you to all sorts of stuff. It shows you...It's a little bit dark. Here's the Earth. Here's the Moon's orbit. This is our geocentric satellites. When you're watching television and everything, that's where all of them are. We actually have over 20-some thousand, 23,000 satellites up there now. If you go on and look at an artist's illustration of it, you can't see the Earth from all the tracks. We have a little protective field of satellites all around us.
This thing is about 45 meters in diameter. They only discovered it in southern Spain less than a year ago. It was February 22nd, 2012. It's going to come in early in the morning. You can see it on the southern horizon with binoculars. It's up to luminescence of seven.
You really have to see it with your naked eye. You're only going to see it for four minutes right away. Then it's eclipsed by the shadow of the Earth for 18 minutes. Then it will pop out again and move on across the sky to the northern, northeastern part of it.
Notice here. Here's the shape-up on where it's coming in. Every day they'll tell you a different speed of it or how close it's going to going to come. When they first discovered it, they said it's going to hit us, if you look at the news releases. They said it's going to get captured by the gravity and then fall in. Well, the gravity is affecting it. It's changing its trajectory.
Something people don't think about when we look in space here is that they are constantly...We're going through new space all the time. Our whole galaxy is going through new space.
As we look out there, we're still seeing, like, one percent of what's out there. The last 10 years, they've been looking, they found maybe about eight or nine bigger than a kilometer, very dangerous that cross our pathway. They found hundreds of ones from 100 grams up to that size. They still think that even if they find all the ones they looked at in the next 10 years, they're still looking at a few percent of what may be happening.
I won't belabor that, but we will be hit again. We're actually getting larger every day. The lowest estimate is 40,000 tons a year, new space influx that lands on our planet. 14,000 that's larger than 100 grams land on the planet every year. The planet's getting bigger. That's going to change [inaudible 09:38].
Just quickly, this was a slide I really debated whether I'd keep in or not. I'll move through it very fast. The depositional history, so you get an idea of what happened before the event and then how the crater got buried. Here, 140 million years ago. There you can see the cross section line. It would be showing up above this north/south, from North Carolina up to the Delaware/Maryland line, basically.
You deposit all these deltas and rivers in the old Cretaceous. Then you start depositing what was preserved, some of the marine deposits in the Upper Cretaceous up to about 65 million years ago. I don't show how much...because we don't really know how much of that...There actually should have been some on the north side.
Then we deposit stuff up to the time...Most of it's marine. You can see it's mostly to the north of here. That was the tectonic low underneath Salisbury, Maryland. There's a structure down there on the south side. You can see that the bedrock arches up there.
Then you had this happen. A big impact. A lot of debris fell back into it but you still had a big hole, so it was much deeper water up in the shallow shelf. Then it got capped by a clay. That's one of reasons we believe it's the best preserved. Everywhere we core it and drill it, the top shows the final settling out of the water column from the great event. It's not eroded -- at least, in the crater.
Then we buried it for the last 35 million years with all these marine sediments and now rivers and shallow marine.
Now, I'm going to do this. This is actually a model. This is Gareth Collins' model that he did for the Chesapeake Bay, and you can go online. At the end of the talk, I'll share a webpage that you can go online that they developed, Jay Malosh and some of his students, and you can go fit in anything that you want, where you want it to hit, what sizes of impact, and they'll give you the consequences to a certain degree. It's very fun, very cool thing to do. You got a city you want to hit, go play with that.
[laughter]
David:  Whoop, I guess it's not playing my...Can't show it to you because now I remember I didn't move that video over to this...We just moved everything. I forget about that. Trust me. This is an image that is taken from it, and this is actually out of a Science article that Greg Gohn was the senior author on. Basically what you want to look at is you'll have seen a big hole that opened up down to about 10 kilometers deep, and then everything [inaudible 12:10] you would have seen this big wave to 50 kilometers away be over a kilometer tall, even 50 kilometers away. Shoreline being 150 kilometers away we know it was really tall still. Gravity's going to pull it down, but then the shoreline lifts it back up just like any normal wave.
That's six minutes. 10 minutes, it's already resurging back, moving all the stuff so the whole event happens very, very fast. Anyway that will shorten a little bit of what I wanted to talk about. Now quickly we're leaving the event now, and we're moving to the discovery, and I'll move through this quickly.
Let me place where we are, the Chesapeake Bay. This is a satellite image. Here's the eastern shore of Delmarva Peninsula, Potomac River, the James River, and right about here is the North Carolina border. Cape Charles is down the center where I was talking from, the ground zero. That's right here in the center.
Look, it's like a doughnut. Well, this is the hole that's the inverted sombrero. If you looked at an upside-down sombrero that's what this looks like. There's a deep hole in the center that's in the crystalline rock, and then there's a broad rim that actually formed in the wet sediments that were hundreds to thousands of feet thick.
I'll show you some cross-sections that show you how asymmetric the target was, and that had a major effect on how it in-filled, and probably the key thing here is a marine impact, because it hits these wet sediments, is much broader and wider than it is, than most any one that landed on land. If this landed on land, the crater might only have been out to about here. This is 56 miles in diameter.
This is showing a little bit of square, but we keep modifying where we believe the edges...As a matter of a fact, we recently cored a hole, and I have an example of it over here that was right over here. We were still in the breccia at 900 feet so where's the edge of the crater really? We're still not sure.
This extra...this line here is actually what we call the outer fracture zone. It makes the entire structure about 100 miles in diameter, and within this zone it's all fractured and faulted up, and you actually only find the debris from the impact in the zone like here and then in-between here. It doesn't even get to that particular core hole. Each one of these yellow dots is core holes.
It was eroded as the oceans came and went. I've got to make sure I'm back from that mike - or, actually, this mike's on.
We actually started in 1986 with a core right there, and it was from that core we ran into the breccia, and I've hardly slept since then, trying to figure out what all this means and then what's the significance of it and how does it affects the people in the region. Went through the water well data and figured out a hypothesis that we had a semicircular steep wall that's still right there, and a deeper hole in the middle. Came up with three hypotheses.
One is an earthquake, and that the shelf slope we didn't know about all dumped it all together, because we had all these things mixed together we'd never seen mixed together, and we had delicate clay minerals that would disappear if it went from this end of the building to that end of the building in a stream. How could you have that preserved in this chaotic mess? We knew it was something very catastrophic.
So we thought, well, maybe the shelf fell in. Possible. Maybe it was a big paleo-channel that, earthquake, and everything fell in, filled the whole channel. Had the idea even from the core site maybe it was an impact breccia. Well, that's the one that turned out to be the truth. It was the one that we laughed at the most originally. We thought, "Oh, that's not going to be it."
We got more cores to prove it, and thank goodness for the Virginia Department of Environmental Quality that helped out and the USGS. Without the USGS doing this, we'd still be wondering what it all meant, because one core hole isn't going to prove it.
Then we were able to - lots of years of begging - get seismic data from the oil companies. They were looking for oil in these truck structural basins, Triassic basins out here, and they said, "You'd never see it," but then I was very persistent, and I finally came up with the idea of asking for their overburden, which is the sediments, and nobody seemed to care about this.
They said, "Well, nobody's ever asked for that." Sure enough, they cut it right at the top of the basement, and then I talked to them, and they all agreed it was the only hole in the whole region they couldn't figure out what it was, and they agreed, "Oh, it must be a crater," once we put that hypothesis to them.
Real quickly without naming them, the cores are really what proved this crater, and the seismic data just backed it up, and everybody agreed. Finding those shock minerals was essential to winning the community over to it, and the one picture in the middle - I'll show you again - I think is the one where we got to drill the deep one.
We invited the international community to help us study it, and that was a very, very good move. It's always good to invite the experts. We always are learning from them.
There's a little bit of history to this. In 1913, a guy named Sanford actually did groundwater quality studies. They were pumping water a lot. All the artesian wells were dropping down low so he studied...he found out there was an inland saltwater wedge so he called it the Virginia Inland Saltwater Wedge, and nobody understood it. And so, even some of our best hydrologists here at the Survey worked on it and the ideas, and they had lots of theories, but none of them were satisfactorily.
For many years, from that time on, nobody understood it. Then this guy, D.J. Cederstrom, came along, and we had droughts in the late '30s, and he was hired by the state as well as he was working for the U.S. Geological Survey to study the water. His work, I've stood on his shoulders to figure out the puzzle, in the initial hypothesis.
He was allowed to publish through the state his description of stuff. He knew there was some structural basin there, and he knew that the groundwater quality was related to it somehow. He thought it was some kind of differential flushing. That's hard to prove, differential flushing, but that's part of the story here.
Then came along studies by the USGS to study the regional water quality. They came up with water quality maps and subsurface maps. Independently from, we studied at this time this one core and came up with this hypothesis. When we laid out the quality maps, it lined up right with where I was saying this hole was. So when we started...one more coincidence, and then we discovered the crater with everybody's help, and a lot of people working on it. It was no single effort.
Now, there's a major groundwater publication that shows how the impact has affected the groundwater and truncated many of the aquifers, and it's still affecting the region. But we now can tell people not to waste their money drilling into this area. There's not fresh water there. There's none, no matter how deep you drill.
Go back one. Quick picture of the cores. The main thing to see here is we were getting things from 650 million years old when we started dating it to 35 million years all mixed and right next to one another. And every time you drill, it it's a different pile.
Sometimes we lose our hole, and we deviate. We're less than a meter from where we were drilling before. It's completely different. The only place you'll see it the same is where you go through giant clasts. So any one core hole, or even all the cores holes, do not define what's really out there, and the water seems to be compartmentalized. We squeeze the water out of the cores and we study it.
We've also thrown the textbooks away and some of that. We've found freshwater sitting right next to salty water, brines, that have not equivalated in 35 million years. So the water's not moving. We think this may be the oldest water sitting still for the last 35 million years or at least since the hydrothermal activity ended, which was 100- 200,000 years after the event.
I put this in. We have a twin crater in northern Russia, up in northern Siberia. There's an aerial view of it. It's called Popigai, and it's actually the glaciers have scraped off a lot, and there's exposures to it. It's very difficult to get to it. There’s no roads. The military has to drop you in, and you hope they come back and get you, and I've only heard of one field trip there ever.
If we were to take a core of this, you could see what information you'd get out of compared to seeing a big outcrop, one little thing. We only know a little bit about this crater. Let's see. Here we quickly go...this was where we won the prize money thanks to a lot of work by a lot of scientists here, and the international community.
We did seismic beforehand, and we planned out this core. We'd hoped to go to 2.25 kilometers. Well, we were very happy to get down to 1.76 kilometers deep - 1776 meters. Post impact sediments, breccia, sediment breccias, a big granite block. I've got a piece of it over there. Everybody wanted us to stop drilling, thought we'd made a mistake. Our crater's very small. We were wrong. All the seismic data was wrong. We were wasting important scientific money.
There was a few of us that knew that, no, this was an unshock granite block. This could not be the floor of the crater. This was not the crater. And I kept telling them, "No, we're going to come back in the sand." I had a dream we were going to come back into the sand. Well, I was lucky.
We came back into sand and there it was, and then we finally hit the coolest stuff, the melt sheet, but it's not actually the melt sheet. My theory is that it's the vapor cloud. They say that. It collapses down. So we're way up in the crater still.
The floor is another couple kilometers below here, and our seismic data suggests that. It's down about three and half kilometers down where we were drilling. We went 1.5, 1.7, so we need to go much deeper, but very expensive. We're going to bring somebody else's pocketbook [laughs] to get that one. And you can see the names of many different agencies that helped us with that.
We actually ended in unshocked blocks down here below that melt so we believe we were not...we didn't get down deep enough. This is just an image of I wanted to show you one of the shock minerals. This is shock quartz. To produce it in our laboratories, we have to go to 300 to 600,000 pounds per square inch. Atmospheric pressure is 33 1/3 pounds. You don't produce this on the planet that we know of -- metamorphism, burial of rocks.
What's interesting is they're only found associated with impacts. They were found associated with the impacts on the moon. Same things we see there, we find associated with the impacts here. So that was the coup de grace. Unique to this crater for the first time we have a new indicator.
Lucy Edwards over here at the front will be showing you these rocks over here and maybe do a demonstration if you're real nice to her, about a simple little impact we do for students found these welded clumps, fused [inaudible]. These things are not melted completely. These are in the water column, but they're...If you find some stuff like this in your sediments, you know that there's an impact someplace. We don't know of other things that would produce that.
Something else that was very unique to this crater is before we discovered it,in Texas and Georgia, they had found something called what you see over here...I have some examples over there that are from Indo-China - black glass, dumbbell-shaped, teardrop-shaped, impacted, flat.
They use to be believed that they were meteorites, but the chemistry showed -- and it was a guy named Billy Glass from the University of Delaware that proved it. I thought it was unique that his name is Glass and he proved this, that these actually are from the earth, and there's only like six or seven of them associated with impacts, one of them being the Chesapeake Bay Impact Crater.
Well, they found them in the deep-sea sediments before we found the crater, and they had predicted that based on the size and distribution of them, there ought to be a crater somewhere in the mid-Atlantic, but most of them thought it was out in the ocean, and once we found this big crater, they all were happy to compare and say, "Yes, this is it."
Now quickly I'll just show you we did a lot of seismic. We got seismic from the oil companies. We ran marine seismic in here. We ran on land seismic at different places. Without belaboring it, let me just show you one of the oil company lines. This was helpful, very helpful, in us figuring out the geometry and the shape of the impact.
Very quickly, this is a seismic image, and the seismic images produced it's basically like a sonogram. It's based on sound and velocity, and so the stuff...You shoot off an explosion up here, the sound, the shockwave, travels down and hits different densities and bounces some of the energy back up to the geophones.
What's interesting is it's like logarithm scale. The distance from this to here, is less than the width of the line here. Down here you're down at least about a kilometer deep. This is the top of the basement. Here's the edge of the crater. Look how faulted up it is, and it rises up slowly, then really crazy near the edge, and then that's going into the central crater. That's it dropping off, and it keeps on going. Here's down 1.6. Here's a mile deep. Now we drilled below that into this thing.
This is the top of the debris, deformed. It's still compacting. This is the post-impact sediments. Out here this was all normal. This was before the crater. So everything is younger and dips into the crater. There's actually a geomorphic expression, which we call scarps, terraces, we’re at the edge of the crater.
The edge of the crater's very complex, because it's slumped, and it's moved, and things have turned this way so imagine a block that's like this, and it slides into the crater. Well, this part's deep than this part now. And so, it's not just a one-to-one correspondence. It's a little tricky and there are multiple terraces, but there is a little scarp.
If you look carefully at the satellite images you can actually see it, or look at any geological map. You'll see younger units on the surface inside the crater and older just outside.
This was a sketch I did of all the way down to six seconds. Right about here is 35,000 feet so when you're flying in an airplane, you look down when they're announcing it, that's how far this thing fractured the rocks, 35,000 feet down easily. We actually believe it fractured...It actually cracked the crust pretty big. I mean the whole crust may be cracked.
Some of our deep seismic, which we have not published on yet, shows that we may have cracked the entire crust. This is just the inner basin, and it raised the rim on this side, but it's truncated and dropped down there. This thing is so well preserved. It hasn't changed much. About the only thing it's done is the eastern side that goes out underneath the Atlantic shelf because of the subsidence has dropped down some. This image will show you that.
I could spend the whole lecture just talking about this one image, but I'll point out the succinct things. Our core hole that went deep all the way down to here, the central uplift we cored into that, found there is a peak underneath there, and there's different highly shocked rocks, and we actually had some shatter cones in the core from there.
There are a lot of people that want to go back and try and deepen that. They will get the high shock fractures, because we were far away from the high shock stuff that's down there. We just got bits and pieces of it in this vapor plume from the ejecta.
Way back up here near the fall zone hear Richmond, and you're going well offshore here. The two things to note here, this fuzzy gray line, that's where we think the surface of the basement was at the time of the impact, so it's subsided more here, and it's actually gone uplifted over here since the impact in the last 35 million years.
Then the other thing to note is right here is the top of the debris. Look how much it has sagged since it probably was up here. It's really compacting and really dropping down in the central crater, but it's also compacting anywhere in the crater. This was the surface of, we believe, the projected surface before the impact. And so, it knocked a really good hole in into it so all this...this debris filled it in, but there was still a good hole in it.
Now the biggest thing to notice is this is all crystalline wall over here, couple thousand feet of it, over here, couple thousand feet of sand. It's easy to get a granite block floating in on top of the sand here when this transient crater, which may have been where these black line collapse.
The other thing is that sea level was possibly 300 feet above where it is now. The main thing is very asymmetric target so very complex infilling and mixing of all these different rock types. The other great thing is these cores give us a great sampling tool for a large area of what rocks may be down there. Wasn't just one core...Wow, we've got all these rocks from over this whole region. As a geologist, that's just...What a great thing.
This is a picture. I have a box over there of this, but a fresh suevit. It's a German term. Or suevite. It means that it's a melt mixture. That's the bottom line. And lithic breccia. Again, that's a vapor cloud that actually fell down, and we actually find in that whole sediment breccia all the way up to the top little pieces of melt that are falling down as all that tsunamis and everything swashing around and moving things as it fell back into the ocean.
A quick picture of some of the melt up close - you can see that it froze fast. It cooled off quickly. Ocean water coming in not too far away, and you can see it looks like blood veins coming in. This is a quick view of what the crater might look like if you emptied it out. See the central peak, and this was a cross-section based on seismic, and you’ll see how everything dips in as it's younger.
Let me move on. I want to get to the...I think this summary's in the write-up. If you got the write-up these are just the things I've been saying. One thing I'll add is down here is the 30-mile splash. The geophysicists tell me this definitely splashed 30 miles up. And in the movie when I say the rocks rebounded, the top one-third excavated. The bottom two-thirds is depressed, and in the middle there's a central peak that shoots up, and that's this big mountain we have there.
How does the crater affect the region? I'm going to do this very quickly. I've got about five minutes to finish up so that you can have plenty of time for questions. If you were to look at the normal groundwater the way...and this is with the crater there, the water's flowing, recharging.
These are the aquifers, the blue or confining units, and so with the crater there the water wants to go around it. We've made a sort of a wall there of things. We've knocked off the aquifers. We now have a clay against where a bunch of gravel...I mean all the sands were. We also find that it seems to hit it and go up right behind it.
The point there is that you only have freshwater. This is chlorides as you go into the crater higher and higher and higher so that it's very quickly a brine and mostly, it comes up very high. And even the post-impact sediments that have filled in have an awful lot of salinity in them. Nothing's moving into that area to move stuff out.
We found that the crater coincides with this inland saltwater wedge, or bulge, as I like to call it. That was never explained for a long time. Coincides with the crater margin, as I said. If this works right - some of the arrows aren't working because this is on...Oh, there they are.
But the water's changed direction because of our pumping center, and now we have all these desalinization. There is a threat of saltwater intrusion into some of these major pumping centers and desalinization plants. If I get this right...Here's a nice image that shows where they're putting in these desalination plants. These are already up and running. The one in Gloucester I'm not sure if there's one running yet, but they're going to have to do one over here for the eastern shore for sure.
And so they're trying to do it, but what if we keep these cones of depressions keep growing, and they start trying to pull this stuff in? All of a sudden you've got a change. There's going to be more money spent on that to try to figure out how they're going to keep getting water for this major growth around the Chesapeake Bay.
Then there's structures. There's actual earthquakes that line up on the outside. This thing hasn't finished settling. It's still subsiding. That leads me into...We actually find faults in some of our cores. This is a seismic line...the oil company. It shows the faulting all the way out. Here's the edge of the crater. There's big faults that are on out there.
I want to get to this because this is the part that's very hard to get across to people -- how the Chesapeake Bay impact crater affects the mid-Atlantic drainage and therefore ultimately the Chesapeake Bay. To get to the story...and this is the end of the story, so we will be finishing in a minute or two.
The big story. Seven million years ago, we started getting glaciations. Well, we had glaciations before that, but the glaciations started affecting the modern landscape. We had a Hudson/Delaware River system. The Hudson didn't go out through New York City at this time. It come down to Trenton, New Brunswick, Princeton, all that, and built its first big delta sheet.
First it built it up here. Then it built here in the southern part of the Delmarva. This was all a tectonic embayment. A basement, low, it kept subsiding. This was all marine for a long time, all the way in here like this. Also underwater.
This built this big pile of gravel, here, and sand. Then it built one big one here, the backbone of the Delmarva Peninsula, a small one off the Potomac River. Nothing happening until the Pliocene, or just the last two, two and a half, three million years was one in James River. These other ones were quiet.
The key here is that this divides these sediments. In the subsequent sea level rises and falls, especially during the Quaternary and the Pliocene, they reworked those sediments along the coast. The rest of the Delmarva Peninsula, a lot of it's based on reworking of those sands southward into the open water.
Sure enough -- this is just an example -- the last 200,000 years, an oxygen isotope curve. They're still arguing over whether these are high sea level events or really they're low. Again, that may be related to a whole other story where I won't get into, if there's a glacial bulge or things. I don't want to go there. That's just to give you an idea. Here's an example of just a couple hundred thousand years ago with the shoreline. You were building spits southward. This is actually setting it up so you can have a bay.
Because the Bay is nothing but a drowned river system. That's the key. It's a drowned river system. As sea level rises and falls constantly, it's going to drown it. What's been nice is, sea level has stepped down over the last million years, so we have a nice sort of record of the terraces stepping down, or at least the last million years, or the last 700,000 years if you don't believe the million years. When sea level is down, the rivers come down here.
All of our data so far proves that the paleochannels of the Susquehanna River, which is the main tributary that even the James, all of them could be going into it. We don't really know enough down here to tie it all together, but we've got paleochannels that come to the crater and then go to the ocean. The crater has outdone the tectonic low. Also, because of this sediment, it came down from the north and filled in the [inaudible 35:09].
Indirectly, the crater is the cause of one of the largest estuaries in the world. It's controlling the drainage. The minute the rivers reach the edge of the crater, they turn and go into it, and then they coalesce. What's interesting is, they actually come to the crater.
This is an older channel. This is a new channel that we know about that may be even older. It's right on the edge of the outer fracture zone. This is just the inner edge of the crater here. This next, younger one is called the Exmore channel. The Eastville channel, east of the town of Eastville. This is the town of Exmore here. This one's Cape Charles. And then there's the modern channel. It keeps shifting.
Notice, here's the central uplift. It wraps around that. It doesn't want to go across the central uplift because that's not subsiding much. We have that record. Notice, here, there's only a little freshwater bubble in the top. Here's these channels cutting unto it. They add to the problem of is that water going to be contaminated.
Last but not least, you have brackish water [inaudible 36:14] here. This is what they'll have to tap and keep water going over there.
Last slide. [inaudible] should be happy because we'll get over to here and get some questions going. A lot of people don't understand what's going on with sea level rise in the mid-Atlantic. There's a lot of theories going on. The Survey has a lot of different ideas. We believe, especially the impact people, that...
Here's a tide gage at Hampton Roads, since 1927. These are meters. The average rise is 4.42 millimeters per year. The world average is little over one millimeter a year. What's going on? I ask the students this, sixth graders on up, fifth graders. They come up with the answer. I'm sure you can, too. What's going on? Is sea level actually rising faster here?
That's 1.5 feet per century. That's pretty fast. I want to be moving my house back. Actually, something a lot of people don't know. The Navy...Norfolk's spending millions, tens of millions maybe hundreds of millions -- I'm not even sure of the number -- on rebuilding their docks because of sea level rise right now. The military is not waiting. They're already on it.
What's really going on here? The compaction of the pudding that's in the crater. Subsidence. It's a no-brainer. With that, I'll take questions.
Let me go to this next slide. This is where to go. You can almost put in "impact" and you'll get it, or "impact simulation." Purdue.edu/ImpactEarth. Go create your own impact crater. Happy for questions. That was very important that I get to questions. I hope I'm not boring you to death or it's going too long.

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Title: Chesapeake Bay Impact Crater

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February 2013 public lecture, presented by David Powars

Location: Reston, VA, USA

Date Recorded: 2/6/2013

Audio Producer: Melanie Gade , U.S. Geological Survey


Usage: This audio file is public domain/of free use unless otherwise stated. Please refer to the USGS Copyright section for how to credit this audio.
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