The Chesapeake Bay Bolide Impact:
A New View of Coastal Plain Evolution
Bolide Impact
A spectacular geological event took place on the Atlantic margin of
North America about 35 million years ago in the late part of the Eocene
Epoch. Sea level was unusually high everywhere on Earth, and the
ancient shoreline of the Virginia region was somewhere in the vicinity
of where Richmond is today (fig. 1). Tropical rain forests covered the
slopes of the Appalachians. To the east of a narrow coastal plain, a
broad, lime (calcium carbonate)-covered continental shelf lay beneath
the ocean. Suddenly, with an intense flash of light, that tranquil
scene was transformed into a hellish cauldron of mass destruction. From
the far reaches of space, a bolide (comet or asteroid), 3-5 kilometers
in diameter, swooped through the Earth's atmosphere and blasted an
enormous crater into the continental shelf. The crater is now approximately
200 km southeast of Washington, D.C., and is buried 300-500 meters beneath
the southern part of Chesapeake Bay and the peninsulas of southeastern
Virginia (fig. 1).
Figure 1. Location of bolide impact and of shoreline when bolide hit.
|
The entire bolide event, from initial impact to the termination of
breccia deposition, lasted only a few hours or days. The crater was
then buried by additional sedimentary beds, which accumulated during
the following 35 million years.
Bolide Crater
The Chesapeake Bay crater was recently identified by C. Wylie Poag
(U.S. Geological Survey, USGS), who has assembled an international team
to investigate its characteristics and consequences. Evidence of the
crater comes from two sources: (1) cores drilled by the USGS and the
Virginia State Water Control Board (fig. 2), and (2) marine
seismic-reflection profiles collected by Texaco, Inc., the USGS, and
the National Geographic Society.
|
Figure 2. Cross section showing main features of Chesapeake Bay
impact crater and three coreholes that provided data on these
features.
|
The cores sampled a sandy rubble bed, which contains hand-size to
person-size chunks (clasts) of clay, limestone, and sand. The clasts
change rapidly downcore in composition, size, color, orientation, and
age. The clasts are fragments ripped from previously deposited beds
that underlie southeastern Virginia. Small pieces of the deeply buried
granitic basement rocks also are scattered throughout the rubble and
contain shocked quartz and melted grains, which confirm an impact. The
structure and geometry of the crater were determined by seismic
profiling from ships in the bay. Seismic profiles are like
two-dimensional cross sections of the subsurface beds. Analysis of
1,200 km of profiles shows that the crater is 85 km in diameter and 1.3
km deep; an excavation twice the size of Rhode Island and as deep as
the Grand Canyon. It is three times larger than any other U.S. crater
and is the sixth largest crater known on the planet. 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.
Effects of the Bolide Impact
Discovery of the giant crater has completely revised our understanding
of Atlantic Coastal Plain evolution. In particular, studies by Poag's
project revealed several consequences of the ancient cataclysm that
still affect citizens around the bay today: land subsidence, river
diversion, disruption of coastal aquifers, ground instability, and
location of Chesapeake Bay. These are discussed below.
Land Subsidence
Evidence of accelerated land subsidence is reflected in the geology and
topography of the modern land surfaces around the crater. The breccia
is 1.3 km thick and was deposited as a water-saturated, sandy,
rubble-bearing slurry (like concrete before it hardens). The sediment
layers surrounding the crater, on the other hand, were already partly
consolidated, and so the mushy breccia compacted much more rapidly
under its subsequent sediment load than the surrounding strata. The
compaction differences produced a subsidence differential, causing the
land surface over the breccia to remain lower than the land surface
over sediments outside the crater.
Poag's team observed that the boundary between older surface rocks and
younger surface rocks coincides with the position and orientation of
the crater rim on all three peninsulas that cross the rim. The older
beds have sagged over the subsiding breccia, and the younger rocks have
been deposited in the resulting topographic depression. The topography
also reflects the differential subsidence. The Suffolk Scarp and the
Ames Ridge are elevated landforms (10-15 meters high) located at, and
oriented parallel to, the crater rim.
Crater-related ground subsidence also may play a role in the high rate
of relative sea-level rise documented for the Chesapeake Bay region.
One of the locations of highest relative sea-level rise is at Hampton
Roads (the lower part of the James River), located over the crater
rim.
River Diversion
Even the courses of the modern rivers in the lower bay region point to
the continued influence of differential subsidence over the crater.
Most of the rivers, like the Rappahannock, flow southeastward to the
Atlantic. In contrast, the York and James Rivers make sharp turns to
the northeast near the outer rim of the crater.
Disrupted Coastal Aquifers
Another consequence of the bolide is that all ground-water aquifers
were truncated and excavated by the impact. In place of those aquifers,
there is now a single huge reservoir with a volume of 4,000 cubic
kilometers. That's enough breccia to cover all of Virginia and
Maryland with a layer 30 m thick. In this huge new reservoir, pore
spaces are filled with briny water that is 1.5 times saltier than
normal seawater. This water is too salty to drink or to use in
industry. Geohydrologists have known of this salty water for decades,
but only now are we beginning to grasp the true nature of its origin
and distribution.
The presence of this hypersaline aquifer has some practical
implications for ground-water management in the lower bay region. For
example, we need to know how deeply buried the breccia is in order to
avoid drilling into it inadvertently and con-taminating the overlying
freshwater aquifers. Its presence also limits the availability of
freshwater. On the Delmarva Peninsula, over the deepest part of the
crater, only the aquifers above the breccia are available for
freshwater. The crater investigation shows that we need to be
especially conservative of ground-water use in that area.
Ground Instability Due To Faulting
Seismic profiles across the crater show many faults that cut the
sedimentary beds above the breccia and extend upward toward the bay
floor (fig. 3). The resolution of our seismic profiles allows us to
trace the faults to within 10 m of the bay floor. These faults are
another result of the subsidence of the breccia. These faults are zones
of crustal weakness and have the potential for continued slow movement,
or sudden larger offsets if reactivated by earthquakes.
Figure 3.
Location of faults (red lines) where they cross seismic profiles. The
large circle shows the extent of the buried crater. The brick pattern
shows the three main cities of the lower Chesapeake Bay. Red capital
letters mark the locations of the Newport News, Windmill Point,
Exmore, and Kiptopeke coreholes.
|
Some of the faults appear to completely breach the confining unit over
the saltwater reservoir. They could allow the salty water to flow
upward and contaminate the freshwater supply. Poag is mapping the
location, orientation, and amount of offset of these compaction
faults.
Location of Chesapeake Bay
Did the Chesapeake Bay bolide affect the location of Chesapeake Bay
itself? We know that the bay is nowhere near 35 million years old. In
fact, as late as 18,000 years ago, the bay region was dry land; the
last great ice sheet was at its maximum over North America, and sea
level was about 200 m lower than at present. This sea level exposed the
area that now is the bay bottom and continental shelf. With sea level
this low, the major east coast rivers had to cut narrow valleys across
the region all the way to the shelf edge. About 10,000 years ago,
however, the ice sheets began to melt rapidly, causing sea level to
rise and flood the shelf and the coastal river valleys. The flooded
valleys became the major modern estuaries, like Delaware Bay and
Chesapeake Bay. The rivers of the Chesa-peake region converged at a
location directly over the buried crater. In short, the impact crater
created a long-lasting topographic depression, which helped determine
the eventual location of Chesapeake Bay.
Metric Units Explained
1 kilometer (km) = 0.621 mile
1 meter (m) = 3.28 feet
4,000 cubic kilometers = 960 cubic miles
U.S. Department of the Interior
U.S. Geological Survey
|
USGS Fact Sheet 049-98
May 1998
|
This page is http://marine.usgs.gov/fact-sheets/fs49-98/
Maintained by Jim Robb
Last revised 7-1-98