Figure 1. Location of bolide impact and of

shoreline when bolide hit.

Coastal and Marine Geology Program > The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution

USGS Fact Sheet 049-98

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).

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.

Related Research

Projects:

The Chesapeake Bay

Bolide: Modern

Consequences of an

Ancient Cataclysm

USGS Coastal & Marine

Geology Program

Chesapeake Bay

Ecosystem History and

Change

USGS

Related Publication:

The Chesapeake Bay:

Geologic Product of

Rising Sea Level - USGS

Fact Sheet 102-98

USGS Coastal & Marine

Geology Program

Related Links:

Earth Impact Database

University of New

Brunswick Department

of Geology

National Aeronautics and

Space Administration

U.S. Federal

Government

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolu... https://pubs.usgs.gov/fs/fs49-98/

1 of 3 11/17/2017, 9:17 AM
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 contaminating 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 Chesapeake

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

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolu... https://pubs.usgs.gov/fs/fs49-98/

2 of 3 11/17/2017, 9:17 AM
1 meter (m) = 3.28 feet

4,000 cubic kilometers = 960 cubic miles

Contact Information

C. Wylie Poag

U.S. Geological Survey

384 Woods Hole Road

Woods Hole, MA 02543

Phone: (508) 457-2258

Email: wpoag@usgs.gov

Coastal and Marine Geology Program > The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution

U. S. Department of the Interior | U.S. Geological Survey

Center for Coastal & Watershed Studies

email Feedback | USGS privacy statement | Disclaimer | Accessibility

This page is http://coastal.er.usgs.gov/fs/fs49-98/index.html

Updated November 29, 2016 @ 05:58 PM (JSS)

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolu... https://pubs.usgs.gov/fs/fs49-98/

3 of 3 11/17/2017, 9:17 AM