[meteorite-list] Chesapeake Bay Impact Crater (Part 1 of 7)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:14 2004
Message-ID: <200106261703.KAA21152_at_zagami.jpl.nasa.gov>

http://www.pilotonline.com/special/meteor/part1.html

A mystery, meteors and one man's quest for the truth
By DIANE TENNANT
The Virginian-Pilot
June 24, 2001

Part 1 of 7

The meteor, two miles wide, crossed paths with Earth at 76,000 miles per
hour, 1,266 miles per minute. Twenty-one miles per second.

It came in over the northwest horizon, a white-hot light that would have
blinded had it not killed before optic nerves could signal the brain. It
moved too fast for sound.

Before they saw it, before they heard it, the sharks and whales and tiny
camels of prehistoric Virginia were incinerated.

The meteor blasted into the shallow sea that covered the state from Cape
Henry to Richmond. It exploded with more force than the combined nuclear
arsenal of today's world powers. Millions of tons of water evaporated
instantly. Millions more were hurled 60 miles into the atmosphere.

The space rock, unimpeded by ocean, tore through nearly a mile of sand and
sediment. It hit the bedrock granite that had been laid down 900 million
years earlier and shredded it. Huge boulders, big chunks and tiny grains of
solid earth were launched upward. The front end of the meteor slowed down,
while the back end flew on at supersonic speeds. The bedrock, the front end,
the back end compressed like an accordion, and rebounded. The bedrock
fractured. Immense faults cracked open. In a maelstrom of heat and fury the
meteor vaporized, leaving a crater 56 miles wide. A network of fractures
spread 40 miles beyond its rim.

Friction from the air ignited rocks as they blasted out, sparking firestorms
for hundreds of miles. From the sky, boulders and water recalled by gravity
collapsed into the gaping crater. Like pebble rings in a pond, swells rolled
out in concentric circles, headed for Greenland, Europe and the East Coast.
They raced at unthinkable speeds across the ocean floor until they rose with
the land and erupted. Tsunamis thousands of feet high crashed into and over
the Blue Ridge Mountains.

Then the water rushed back, back to the sea and the enormous empty hole,
carrying dirt and trees and animals, all to be dumped into a crater as deep
as the Grand Canyon.

Then it was over.

The sea, steaming and barren, flowed back over the crater. Earthquakes
rumbled underwater as house-sized rocks slumped down the crater's sides. The
ripples faded, and there was no one to remember that the impact had happened
at all.

No one would ever know.

Not until 35 million years had passed, and David Powars dug a hole.

The hole burrowed deep into the past. What came out of it was a tube of mud
2 inches wide and more than a thousand feet long. Powars took off his
glasses, looked at a sample with a magnifier, puzzled over its oddities.

Then he announced to the world that he was dead certain that his boyhood
passions of astronomy and rocks had come together in one glorious megaton
explosion not three hours from his mountain home.

No one believed him.

It was pretty unbelievable.

A buried crater is a marvelous thing. You can't see it, can't feel it, can't
touch it, can't smell it. You will never stand on the edge and look down
into it, or walk across its bottom. It is completely, totally, unequivocally
lost under sand and rock and mud. Boats sail above the Chesapeake Bay crater
and crabs tiptoe across it and developers build on top of it.

In a million little ways, the crater touches them. In riddles that surprise
or stump them, annoy or just perplex them, the crater makes its presence
known.

Something that big, that old, that cataclysmic will not lie dormant and
forgotten. The crater shapes the future of Hampton Roads. For those who can
read the signs, the omens are there.

The sedimentary beds of Virginia's coastal plain were laid down in orderly
fashion, one on top of the other, as sea level rose and fell over time.
Every geologist knew that. They even knew where the aquifers ran, like a
subterranean layer cake, from the fall line to the sea.

The state's system of granting permits to drill wells into these aquifers
depended on the assumption that there were layers of impenetrable clay
topped by groundwater, topped by more clay, topped by another layer of
water, and so on. Nine separate layers where groundwater could be tapped and
pumped to the surface.

Overpumping in the 1950s and '60s drained so much water that by the
mid-'70s, the state was concerned enough to begin sinking research wells
throughout southeastern Virginia. Scott Bruce was on a team that, in 1983,
began drilling in Newport News Park.

Bruce and his colleagues collected core samples at various depths as they
drilled, to check on the permeability of different layers and to look for
fossils. Paleontologists -- scientists specializing in fossil identification
-- can date the sediments on the basis of what creatures were living when
the bed was laid down.

But the Newport News samples frustrated them. They complained that the
layers had been contaminated during collection. Older fossils lay alongside
and above younger ones. Impossible to work with. The core samples were set
aside, and Bruce looked eastward for his next well.

Around the same time, Powars was part of a U.S. Geological Survey team,
studying subsurface layers across Virginia's coastal plain. The USGS needed
hard evidence for its maps, so as the team worked its way across the plain
from Fredericksburg to the Atlantic, its members also took core samples.
They drilled continuous cores through all layers instead of sampling from
here and there. Their deepest hole would be near Exmore, on the Eastern
Shore.

Bruce's team needed a hole. Powars' team needed the core from a hole. They
joined forces in 1986 and, late on a sultry August night, they pulled up a
core like none they had seen before.

A core sample looks like a giant gray Tootsie Roll. To a layman, analyzing a
core sample is analogous to unwrapping that Tootsie Roll and being able to
see lying in your hand the sugar and cocoa and partially hydrogenated oil,
right down to whether it's soybean or canola.

Rocks and soil work the same way. Each new rock or new layer of dirt or new
fossil tells of climate change and evolution, to those who can read it.

This explanation requires much arm waving from Powars, standing in his home
office in Stephens City before a spectacular view of the Blue Ridge, the
horse pasture and teetering piles of reference books and maps. He is
uniquely qualified to explain this, as he has two arms and exercises them as
he talks, which is pretty much nonstop. ``I'm working on this right now,''
he will say. ``I'll tell you about it and then I'll be quiet,'' which is a
nice offer but a complete lie, although he says it sincerely. The core
sample takes a lot of explanation.

``We do mineralogy on these cores and start seeing maybe here was renewed
uplift or here was where the weather changed and we got fresh rocks eroded
into the system,'' Powars says, rising on his toes for emphasis. ``You can
see, well, heck, the shoreline had to be somewhere east of the present fall
line. You're getting this much wood in here, well, the shoreline was
probably a delta rather than a rocky coast, things like that.''

Powars had never before seen things like he saw in the Exmore core sample.
The abnormalities were as easy to spot as raisins in what should be a
smooth, unbroken cylinder of rubbery chocolate. The ingredients of a coastal
plain, below a certain depth, were jumbled like lottery balls. The logical,
geological order of the Piney Point, Nanjemoy, Marlboro, Aquia and
Brightseat aquifers was no longer youngest to oldest. Some pieces of the
core looked as though they had been twisted and squeezed like toothpaste.

It was the most exciting thing he had ever seen.

Powars thought he had read of this jumbled layer before. He turned to the
library and the careful records Samuel Sanford and John Cederstrom had kept
earlier in the century.

In 1913, Sanford published the first report on groundwater in Virginia's
coastal plain. He charted the known wells on a map and drew contour lines
connecting those that contained equal concentrations of salt. The result was
an inland bulge around the lower Chesapeake Bay, centered around Cape
Charles on the Eastern Shore.

In the 1940s, the USGS sent geologist Cederstrom to hunt for water to
support the region's military buildup before World War II, given new urgency
by lingering drought that had depleted surface water.

In those days, the technology to take core samples was too expensive to use
often. Instead, wells were cut and chips of dirt or rock flushed out.
Cederstrom sat by the wells and cataloged the layers as they came up. He
noticed that the orderly sediment layers at one point became mangled. He
called this strange formation the Mattaponi, a local Indian name. He also
saw something else -- a low spot, perhaps a fault, where southeastern
Virginia's subsurface layers seemed to drop away.

What he didn't find was good water. Where everyone expected freshwater
aquifers flowing down from the west, Cederstrom found salt. At each of the
52 wells, he noted the salinity, or the amount of salt and minerals
dissolved in the water. He connected the dots, too, and called the resulting
contour an ``inland saltwater wedge.''

His work was scorned. While no one could deny that the wells were salty, no
one accepted his hypothesis that the jumbled Mattaponi Formation was somehow
involved, or that the structural low prevented the freshwater aquifers from
flushing out the saltwater.

Sedimentary rocks didn't get mixed up; everyone knew that. Cederstrom was
accused of confusing his specimens, of contaminating the samples, of being
misled by the method of drilling. He lost his job at one point, was rehired,
and finally retracted his published work in the 1950s, saying under pressure
that he must have been wrong.

Saltwater aquifers had been known in Virginia since the Civil War. Union
soldiers stationed at Fort Monroe were plagued by cisterns that dried up
during long, hot summers. In 1864, the fort began drilling a well.

For five years drillers labored, dropping a weighted bit repeatedly down the
shaft to chip away at rock and dirt. They finally reached water, but it was
too salty to drink. Deeper they drilled, then deeper, until at 907 feet they
gave up, still inexplicably drawing briny water.

No one knew why the groundwater wasn't fresh, any more than they knew why
the James River took a sudden 90-degree turn to the northeast near Fort
Monroe. Neither the well nor the river was acting right.

All along the East Coast, rivers flow from mountain to sea, most taking
gentle inclines to the southeast until they reach the Atlantic. The James,
the York and the Rappahannock rivers all flow this way until they near the
coast, where they bend abruptly and turn their open mouths north and east to
face the tiny village of Cape Charles. It is an anomaly. Just a few miles
down the coast, the Northwest River flows as it should, southeast.

Thousands of feet below the thirsty soldiers lay the answer. The crater
hungrily sucked in whatever would fill its enormous chasm. The weight of 35
million years' worth of sediment compacted the original crater debris.

No matter how much dirt went in, the crater continued to be a low spot in
the floor of what would become the Chesapeake Bay.

Water flows to the lowest ground. The rivers were no exception. When the
lowest ground became the sinking fill of the crater, the James, the York and
the Rappahannock turned toward it, as they continue to do today.

Without the crater, the port of Hampton Roads would not exist. Without it,
the shores of the world's greatest military harbor would have been cut
through by the relentless drain of the James on its way to the sea.

And without it, John Cederstrom's Mattaponi Formation would never have been
found, and his reputation lost.

Powars looked at the core sample in his hand and knew that Cederstrom was
right. Now to prove it.

Reach Diane Tennant at 446-2478 or dianet_at_pilotonline.com
Received on Tue 26 Jun 2001 01:03:15 PM PDT