Skip to comments.Drilling Finds Crater Beneath Va. Bay
Posted on 06/01/2004 4:21:15 PM PDT by Rebelbase
CAPE CHARLES, Va. - Geologists drilling half a mile below Virginia's Eastern Shore say they have uncovered more signs of a space rock's impact 35 million years ago.
For more than two weeks, scientists drilled around the clock alongside a parking lot across the harbor from Cape Charles. They stopped at 2,700 feet.
From the depths came jumbled, mixed bits of crystalline and melted rock that can be dated, as well as marine deposits, brine and other evidence of an ancient comet or asteroid that slammed into once-shallow waters near the Delmarva Peninsula.
Cape Charles is considered Ground Zero for the resulting 56-mile-wide depression below what's now the Chesapeake Bay. The drilling project marks the first time the geologists explored the inner portion of the inverted-sombrero-shaped crater.
"We expected to see some pretty strange rocks because of the extreme pressure and temperatures that occurred" approximately 35 million years ago, said geologist Greg Gohn, who led the $180,000 project for the U.S. Geological Survey (news - web sites).
Over the past decade, USGS (news - web sites) and Virginia scientists have investigated indications that a 2-mile-wide brilliant ball traveling tens of thousands of miles per hour crashed off the Virginia coast, burrowing thousands of feet and depressing and fracturing the bedrock.
Billions of tons of ocean water vaporized. Millions of tons of debris spewed 30 miles high before collapsing back into the excavation. A train of giant waves inundated the land. The waves then dragged debris as they washed back into the crater, preserving it beneath a blanket of rock and sediment.
It probably took just a few minutes to create the largest crater in the United States and sixth-largest known on the planet, according to computer simulations.
The catastrophe squeezed freshwater from many of the aquifers of southeastern Virginia and filled others with briny water. Its legacy is well-known to residents who try to drill for drinkable groundwater and encounter the saltwater "wedge," pockets of brine nestled in an arc from the lower Eastern Shore to the Hampton Roads-Newport News area.
Geological research off the coast of New Jersey and in Virginia, begun in 1983, led to the crater's discovery a decade later. Drilling and further study of seismic data narrowed the location in the Chesapeake Bay.
"We're getting evidence about how hot this thing (was) and what was the energy," said USGS hydrologist David Powars, one of those credited with the crater's discovery.
More clues to the space rock's identity will come from cores taken in the drill's final 280 feet.
A $1.2 million proposal to dig 7,000 feet not far from Cape Charles is before the International Continental Scientific Drilling Program, which would then assist the USGS with funding.
It has been known for a long time, but this confirmation is rather interesting.
Thanks for the post. I love science!
Do you think Bill Clinton knows about the "jumbled, mixed bits of crystalline and melted rock that can be dated"?
I'm waiting for Ted Kennedy to blame the asteroid impact on neo-conservatives and Halliburton...
There are also huge natural gas deposits under the Continental shelf offshore from Atlantic City NJ to Fla.
Congress stopped exploration prior to drilling back in 1983. I know because I was one of the many drillers who had to be certified with the USGS for surface and sub-sea well control techniques prior to the industry being allowed to develop this field. Needless to say, I remember hearing a small blurb shortly thereafter stating Congress had placed the area "off limits" to offshore drilling. Guess it was the NIMBY syndrome. Let 'em produce it out West and pipe it to us. Then NE congressmen can gripe about the high cost. As I recall it was a who's who of Eastern liberals who killed this important clean energy source. Anyone else recall this event or know of any records of the vote? I wonder. Hmmm. Kerry again?
I wonder if this eventhad anything to do with the Carolina Bays.
The Chesapeake Bay Bolide: Modern Consequences of an Ancient Cataclysm - U.S. Geological Survey - Coastal and Marine Geology - Woods Hole Field Center
During the late Eocene, the formerly quiescent geological regime of the Virginia Coastal Plain was dramatically transformed when a bolide struck in the vicinity of the Delmarva Peninsula, and produced the following principal consequences:
- The bolide carved a roughly circular crater twice the size of the state of Rhode Island (~6400 km2), and nearly as deep as the Grand Canyon (1.3 km deep).
- The excavation truncated all existing ground water aquifers in the impact area by gouging ~4300 km3 of rock from the upper lithosphere, including Proterozoic and Paleozoic crystalline basement rocks and Middle Jurassic to upper Eocene sedimentary rocks.
- A structural and topographic low formed over the crater.
- The impact crater may have predetermined the present-day location of Chesapeake Bay.
- A porous breccia lens, 600-1200 m thick, replaced local aquifers, resulting in ground water ~1.5 times saltier than normal sea water.
- Long-term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the area, which are potential hazards for local population centers in the Chesapeake Bay area.
I love this stuff...
Thanks. Nice maps.
And a third of the waters were made bitter...
First I heard of this discovery, also, although I was living in Virginia at the time and for years afterward.
But actually, quite interesting. Thanks!
Does anyone know if they have an estimate for how far inland the waves came?
fault...hahaha (geologist humor)
New and fascinating to me, too.
Thanks for locating those maps. They were what I had remembered seeing and was unable to locate tonight.
That is interesting.
I was aiming for Washington DC, but close enough for government work. I think I jumped on the trigger a bit early, but I blame that on coffee jitters.
hills of central KY
Hills are formed then the ground is uplifted by geological forces. What you see today as hills were once the floor of an ocean.
That's cool by me, I like finding the surprise fossil. I found them all over N. Texas, but I guess I never realized KY might have been under water as well.
Doesn't it suck when you sneeze as you're trying to aim?
crinoids came from the shallow ocean period about 310-350 million years ago.
The hills you you see today in KY were created when the European and American continental plates slammed into each other.
So, in a way, you could blame it on the French!
Ditto on the uplifted seafloor answer.
I've seen fossilized seashells in the rock up around 8000 ft. of elevation in the mountains east of Salt Lake City.
You were living in VA 35 million years ago? ;^)
Sacre Bleu!! Your hills, they have slammed into mine! Watch where you're steering your continent, you filthy pig!
Best place in the world to go for the fossils was the "New Cut" on the Ohio upstream of Louisville.
Before the last glaciation (of the current Ice Age) the Ohio flowed to the East. A "new cut" was made at the end of the glaciation from the Miami river to the vicinity of Louisville, and voila, a gazillion fossils.
My understanding is the state of Indiana now prohibits digging for fossils in the area since the digs had gotten so extensive they endangered the safety of roads in the region.
It's cool when you find a really good one in a limestone formation. We had a quarry nearby when I lived in Texas, and everytime they put down gravel somewhere, I'd find a ton of new fossils. Was a great way to waste time as a kid.
LOL, When did Bush know about this and what is he going to do about it?
I left as soon as the glaciers retreated.
Remember, the glaciers are known to retreat for only brief periods (geologically speaking). They will return and then the Kyoto Accords crowd are going to be in some real trouble.
Up around Jamestown, New York, in the hills, the fossil shells are so thick they form the soil. I can't say if they were freshwater or saltwater, but they are small, fairly intact, and have a primitive look to them.
Hi-Diddly-O neighbor! That's proof of Noah's flood. We all know the earth is only 6000 years old! :-)
they found Pelosis mouth?
Virginia was as well. It is interesting to find sandstone and limestone at the tops of sections of the Appalachians in Virginia.
Tonight, it was nice to see someone exploring the geological history of his area. The more you learn, the more interesting the view outside your own home becomes.
Where I live in Minnesota, almost everything was scraped away by the glaciers from our most recent ice-age. However, there is one cut through a hill that has exposed the bedrock granite.
The age and chemical composition of that granite, was found to be a very close analog to the rocks found on the Moon.
When you follow the Mississippi river South of St. Paul, it spreads out into a large lake around the city of Redwing Minnesota. The depression which forms the lake has also been identified as an ancient meteor impact crater.
In order to fully appreciate the consequences of the Chesapeake Bay impact, we need to understand what the crater is like, and how we know it is there. It is the larger of two craters recently discovered on the US East Coast by Wylie Poag and his colleagues. Both were formed 35 million years ago in the late Eocene epoch of geological time. That's about half as old as the dinosaur extinction. The crater is located approximately 200 km southeast of Washington, D.C., and is buried 300-500 meters beneath the lower part of Chesapeake Bay, its surrounding peninsulas, and the inner continental shelf of the Atlantic Ocean. There is, however, much telltale geological evidence of the impact.
The first evidence of a bolide impact on the East Coast came to light in 1983. Wylie Poag was serving as Co-Chief Scientist on the drill ship Glomar Challenger during Leg 95 of the National Science Foundation's Deep Sea Drilling Project. At an offshore drill site 120 km east of Atlantic City, NJ, the scientific party of Leg 95 recovered a core containing sedimentary debris diagnostic of a bolide impact. This figure focuses on that discovery, and introduces some key terminology. Shown here in great exaggeration, is the Glomar Challenger drilling into the sedimentary beds that make up the seaward edge of the continental shelf. The continental shelf is represented as a stack of sedimentary beds, displayed on a seismic reflection profile. The seismic profile is a type of sea floor sonogram. The survey ship sends a series of sound waves into the sea floor. As each wave encounters the boundaries between individual beds, part of the wave is reflected back to a recording instrument. These reflections are digitized and processed by computer to produce the seismic profile. The profile shows the thickness, depth, and spatial orientation of each bed, and allows one to determine the best drill site for solving a particular geological problem. For example, we see here that the yellow bed is tilted seaward, and has been fractured. The eastern block has moved downward along the fracture plane relative to the western block. This fracture plane is called a fault. At the lower end of the drill pipe, the drill bit is located near the crest of a folded bed.
The drill bit has a hole in its center, about the diameter of a tennis ball. So as it grinds down through the sediments, a cylindrical core of sediment protrudes through that opening and up into the hollow drill pipe. From there, it can be recovered and sampled. A core from the red bed contains a 20-cm-thick layer, which includes diagnostic evidence of a bolide impact. The evidence consists of certain minerals, whose physical properties have been altered by the tremendous force of the impact shock, which can be tens of thousands of times greater than atmospheric pressure. Two of the most common alteration products are shown in the yellow circle. Tektites are millimeter-to-centimeter-size glass beads derived from sediment melted by the impact. Shocked minerals, especially quartz, show several sets of closely spaced, intersecting dark stripes when viewed microscopically. The lines represent tiny fracture planes oriented at specific angles to the main optical axis of the quartz crystal. No natural mechanism other than a bolide impact produces tektites and shocked quartz.
The sediments containing the tektite also contain fossilized remains of microorganisms (microfossils) that lived in the ocean when the tektites were deposited. These photomicrographs illustrate a variety of these microfossils (note the scale bars). The microfossils indicated that the tektite layer at Site 612 was deposited in the late Eocene epoch, 35 million years ago. This age was confirmed by determining the ratio of two isotopes of argon gas contained in the tektite glass.
The second indication of an East Coast bolide impact came three years later (1986), from cores drilled onshore in southeastern Virginia. There, the U.S. Geological Survey and the Virginia State Water Control Board were investigating the composition, thickness, and geological age of subsurface sedimentary beds and evaluating their potential as sources of fresh groundwater. They drilled four cores, two on each side of the lower bay. Let's examine some of the core from the Windmill Point and Exmore sites.
Here are parts of two different cores, cut up into two-foot sections for ease of storage. We can call this rock material a sandy rubble bed. Mixed within the sand are larger hand-size to person-size chunks (clasts) of clay, limestone, and sand. The clasts in the rubble bed change rapidly downcore in composition, size, color, and orientation. No one had ever seen such a rubble bed before in the subsurface of Virginia, but it is present in all four of our cores. The strangest aspect of the bed is not visible to the naked eye, however. We didn't discover it until we analyzed the microfossils. The upper clay bed contained the normal stacked succession of microfossils... youngest on top, getting progressively older downcore. But that's not the case in the rubble bed. For example, the dark, fractured clay interval in the Windmill Point core differs by 20 million years in age from the white limestone below it. But the limestone is not older, as it should be; it's 20 million years younger. And we found a random mixture of ages among all the other clasts, too. The clasts turn out to be mainly fragments ripped from all the surrounding sedimentary beds that underlie southeast Virginia. Small pieces of the granitic basement are also scattered throughout the rubble. All these fragments were mixed together and redeposited in a layer that covers twice the area of Rhode Island. But most important of all, the youngest microfossils in the rubble bed are the same group of species we had seen in the tektite layer off New Jersey. Clearly, some terrific force had torn apart the normal horizontally stacked layers in Virginia, and scrambled them all together, at the same time a bolide impact had deposited the tektites off New Jersey.
This suggested a common origin for the rubble bed and the New Jersey tektite layer. So we looked for shock-altered minerals in the rubble bed. Sure enough, we found trace amounts of shocked quartz and bits of melt-rock in the rubble bed at each core site. Now we had diagnostic evidence that the rubble bed resulted from a bolide impact. But we still could not pinpoint the location of the source crater.
The final piece of the puzzle was provided in 1993 (ten years after the tektite discovery off New Jersey) by Texaco, Inc. and Exxon Exploration Co. These companies were exploring beneath Chesapeake Bay for structures that might contain oil and gas. And as part of that search, they collected a network of seismic reflection profiles in the bay. These profiles showed clearly that a huge peak-ring impact crater is buried beneath the bay and centered near the town of Cape Charles, on Virginia's eastern shore. The crater is 90 km in diameter and 1.3 km deep. It covers an area twice the size of Rhode Island, and is nearly as deep as the Grand Canyon. The rubble bed, which we now realize is an impact breccia, fills the crater and forms a thin halo around it, called an ejecta blanket. Inadvertently, we had drilled two of the core holes mentioned previously into the breccia inside the crater. The other two cores were drilled just outside the rim, into the ejecta blanket. The seismic profiles show that the breccia is much thicker than the cores indicated, however, reaching more than a kilometer.
Here is a seismic profile which shows, in cross section, the structure of the outer rim of the crater. Along the base of the profile is a prominent reflection separating the purple bed from the brown bed. The purple bed is composed of granite and granite-like rocks, which we call crystalline basement. The basement rocks are much denser than the sedimentary layers above it, and this produces the strong basement reflection. The stack of horizontal reflections to the right, between the purple and blue layers, represent the normal sedimentary beds that existed here when the bolide struck. The top of the blue bed represents the ancient sea-floor at the time of the impact. As we look to the left on this profile, however, these horizontal reflections are truncated by a series of faults, and the orderly stacking of beds is disrupted. The blue units are large blocks that have slumped off the crater's outer wall, and have slid to the left into the annular trough. We can still see some organized reflections in these blocks; some remain horizontal, but others are diagonal, indicating that the blocks have rotated. The pink breccia section is characterized by disorganized or chaotic reflections caused by the jumble of clasts it contains. On top of the breccia are horizontal reflections from the youngest beds, which accumulated during the past 35 million years since the bolide struck.
We can put all the core and seismic data together and produce a two-dimensional cross section across the entire crater. A map view at the upper right shows the location of the cross section relative to the crater outline and the core sites. Outside the crater we see a stack of gently dipping sedimentary beds lying on the granitic basement. The bolide punched a deep hole through the sediments and into the basement (the inner basin), fractured it to depths of 8 km, and raised the peak ring around it. The sedimentary walls of the crater progressively slumped in, widened the crater, and formed a layer of huge blocks on the floor of the annular trough. The slump blocks were then covered with the breccia. The entire bolide event, from initial impact to the termination of breccia deposition lasted only a few hours or days. In geological perspective, the 1.2 km-thick breccia is an instantaneous deposit. The crater was then buried by additional sedimentary beds, which accumulated during the following 35 million years. The white perpendicular columns beneath the drill derricks indicate the beds that we cored.
Outstanding information ckilmer and thanks for sharing it with us.
I did. Thanks for the ping.
Do you mean the Republican Governors of New York and New Jersey?
And since when is natural gas a clean energy source...maybe cleaner than the smell out of the Secaucus swamp, aka Giant Stadium.
It is Bush's fault!
If a meteor is hurling headlong into the earth, there might be a mighty tidal wave that drowns New York...& that's just how it's gonna be.
Excellent addition, thanks.
There are seashells & fossils (trilobites) in gravel pits in northern Ohio & fields in southern Michigan (same general area). Great Lakes or early ocean?
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