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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.
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