The Chesapeake Bay "Bolide" That Shaped the Groundwater in Southeastern Virginia

the bolide impact that occurred offshore in the ocean created a crater, generated a tsunami that swept across the mainland, and disrupted groundwater aquifers
the bolide impact that occurred offshore in the ocean created a crater, generated a tsunami that swept across the mainland, and disrupted groundwater aquifers
Source: National Science Foundation, Crumbling Walls of Ancient Chesapeake Bay Crater Threaten Regional Groundwater Supplies

About 35 million years ago, a "bolide" (meteor or comet) about 1-3 miles wide slammed into the ocean off the eastern edge of the North American continent. The bolide penetrated through 600-1,500 feet of water of the Atlantic Ocean until reaching the Continental Shelf. It then 1,800–3,000 feet of unconsolidated sediments before reaching the granite crust. In addition to vaporizing/displacing water and sediments and then excavating a hemispherical cavity in the crystalline basement, the impact created fractures that went as deep as 7 miles.

The plume of ejected bedrock that may have risen in a towering cloud 15-30 miles high. Some debris transformed by the impact was scattered as far as the Continental Shelf off the coast of New Jersey. Geologists exploring there for oil and gas discovered "tektites" (tiny glass beads) and "shocked quartz" in the 1980's, finding the critical clue that there was an impact crater nearby. The specific site of the Chesapeake Bay impact crater, near Cape Charles, was not discovered and described in the scientific literature until the 1990's.

After the bolide hit 35 million years ago, most debris fell back to earth near the site of the impact. Some filled the crater, but other material disrupted the sediments on the continental shelf. The fluidized sediments at the edge of the crater slumped, expanding the size of the crater far beyond the first "hole" in the crust created by the impact. The crater's initial width, two seconds after impact, was 17 miles, but it soon widened to 25 miles.

Seawater trapped between debris particles was extra salty (hypersaline), since heat had evaporated much of the water at the site of impact. The disrupted sediments inside and around the crater formed a new layer now known as the Exmore breccia.1

the bolide central crater and annular ring outside
the bolide central crater and "annular ring" outside
Source: US Geological Survey (USGS) - Professional Paper 1622, The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units of Southeastern Virginia, South of the James River

When the bolide hit, there was no Chesapeake Bay or Eastern Shore. The Chesapeake Bay itself did not form until after the Wisconsin glaciation ice sheet melted 18,000 years ago. The impact location 35 million years ago was in the Atlantic Ocean off the eastern edge of Virginia, when sea levels were higher and the shoreline was roughly the path of I-95 today.

A story in the Richmond Times Dispatch claims the impact created a massive tsunami:2

The waves nearly overlapped the Blue Ridge Mountains before washing back into the horrible gash, then covered the superheated water beneath a thick blanket of debris, rock and sediment. Over time, as this new geologic formation settled, it set the stage for Virginia's baffling coastal groundwater system, with its pockets of salty groundwater. USGS geologist Wylie Poag, another co-discoverer of the bay's ancient depression, has called it "probably the most dramatic geological event that ever took place on the Atlantic margin of North America."

Over the last 35 million years, gravel, sand, silt, and clay deposited by shifts in the path of the Susquehanna River have formed the Eastern Shore peninsula. Today the Exmore breccia is 1,500 feet deep below the surface of the Eastern Shore peninsula, and the center of the impact crater is underneath the southern tip of Northampton County.3

Impact craters are rare on the earth, in part because crust is recycled by tectonic forces and in part because so much of the Earth's surface is covered by water. Bolides must penetrate through the water column in order to carve out a crater. When they strike, tsunamis form in two stages. First, a "rim wave" is created when the water/earth ejected from the crater falls back into the ocean. Second, water rushes back into the hole created by the passage of the bolide. That "collapse wave" creates a jet of water erupting in the center. Several cycles of collapse waves may occur.4

Though the Chesapeake Bay impact was a dramatic event, but an even larger crater was formed 65 million years ago by an impactor perhaps 2-3 times larger. The Chicxulub crater was over 100 miles in diameter. The ejected debris altered the earth's climate and led to the extinction of the dinosaurs. As happened with the Chesapeake Bay crater, sediments also covered the site of the Chicxulub crater, burying it partially under the Yucatan peninsula in Mexico.5

sediments exposed on Coastal Plain in eastern Virginia were deposited primarily in Miocene/Pliocene epochs, after the bolide impact in the Eocene
sediments exposed on Coastal Plain in eastern Virginia were deposited primarily in Miocene/Pliocene epochs, after the bolide impact in the Eocene
sediments exposed on Coastal Plain in eastern Virginia were deposited primarily in Miocene/Pliocene epochs, after the bolide impact in the Eocene
Source: US Geological Survey (USGS) - Professional Paper 1680,
A Surficial Hydrogeologic Framework for the Mid-Atlantic Coastal Plain (Plate 2)

The Chesapeake Bay impact created a zone of weakened rock. That may have shaped the direction of the Susquehanna River and James River as they carved their channels to the Atlantic Ocean, ultimately affecting the location of the Chesapeake Bay. As noted by the Chesapeake Bay Impact Crater geologists:6

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.

However, the bolide did NOT blast out a hole and form the current Chesapeake Bay immediately. The impact was 35 million years ago, while the bay formed much more recently.

The Chesapeake Bay evolved over just the last 10,000 or so years, as sea levels rose and flooded the valleys of the Susquehanna and James rivers. By that time, several hundred feet of sediment had buried the crater.

The location of those river channels (and the modern Chesapeake Bay) may have been affected by the much-older depression created initially by the bolide. The impact was a one-day event, but subsidence may have continued gradually over the last 35 million years. If the sediments deposited in the crater continued to subside, then the low spot in the crust of the earth may have affected the route of the James River and Susquehanna River channels.

path of Susquehanna River and sharp changes in direction of James and York rivers at edge of crater show how modern river channels were shaped by the bolide impact 35 million years ago
path of Susquehanna River and sharp changes in direction of James and York rivers
at edge of crater show how modern river channels were shaped by the bolide impact 35 million years ago
Source: The Chesapeake Bay Bolide: Modern Consequences of an Ancient Cataclysm

Far more than the earth's surface above the crater, groundwater today is affected dramatically by the ancient impact. The U.S. Geological Survey drilled a test hole 2,699 feet deep into the impact crater near Cape Charles, Virginia, during May/June 2004. Ground-water salinity in the well was saltier than sea water (35 parts of salt per thousand parts of water), reaching 40 parts per thousand.7

Between September 2005 and May 2006, the Chesapeake Bay Impact Structure Deep Drilling Project drilled again into the crater at a location about 4 miles north of the town of Cape Charles, at Eyreville Farm. The geologists drilled through post-impact sediments (those less than 35 million years old), then through the "breccia" (shattered rock created when the bolide smashed through the water column and into the ocean bottom), and finally reached the crystalline basement rocks more than one mile below the surface.

The deepest of three wells dug there reached 1,766 meters (5,794 feet, over a mile deep into the ground). They ran into granite rock at a shallow depth, greatly complicating what they expected to be drilling through soft sediments of the Coastal Plain. After deciding that the hole was too shallow to have struck bedrock below the sediments, they kept drilling. After drilling through 900 feet of granite, they broke through it and encountered the expected breccia, the broken pieces of bedrock mixed with sediments.8

It appears the bolide tossed a big chunk of granite into the air at impact, 35 million years ago. The geologists had the misfortune to pick the location where that boulder had landed for their drilling project.

how the impact fractured the bedrock
how the impact fractured the bedrock
Source: USGS Fact Sheet 049-98:
The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution

The dramatic disruption of the bedrock layers "entrained" hypersaline water in some places underneath what is now the Eastern Shore and Hampton Roads. As the debris fell back from the impact, it sealed off aquifers where the heat of the impact had evaporated some of the seawater, trapping extra-salty pools of water underground. The "inland salt-water wedge" of unusually salty groundwater in the crater basin has a thin lens of freshwater on top, in the sediments deposited during the time after the impact.

aquifers that could supply groundwater in Newport News area
aquifers that could supply groundwater in Newport News area
Source: City of Poquoson Comprehensive Plan 2008-2028

The Hampton Roads region is limited to extracting a limited amount of fresh water from the more-recent sediments deposited over the last 35 million years. The disruption of the bedrock layers has greatly complicated efforts of Newport News to find a reliable supply of fresh drinking water. The city plans for substantial growth in demand for fresh water, but is unable to extract a sufficient supply from groundwater. The City of Poquoson gets its fresh water from Newport News, and notes in their Comprehensive Plan how the Lower, Middle, and Upper Potomac aquifers were affected:9

The impact of the bolide obliterated the deepest three aquifers beneath the Chesapeake Bay, the Eastern Shore, and portions of the Lower and Middle Peninsula.

The James River runs right next to Newport News. There is plenty of water, but at that location in Tidewater, the river is brackish. It is too salty to drink without expensive treatment. As a result, Newport News tried to build a surface reservoir in King William County, damming Cohoke Creek and (since the creek's watershed is so small) pumping fresh water from the Mattaponi River into that reservoir. Ultimately, after great political debate and multiple lawsuits, the proposed King William Reservoir was blocked by the Federal government.

The bolide that impacted 35 million years ago affected what is now the Eastern Shore, Chesapeake Bay, and Hampton Roads. However, Lake Drummond and other crater-like features on the Coastal Plain of southeastern Virginia were created by a totally different process within more-recent times. The Caolina Bay depressions are unrelated to the Chesapeake Bay bolide impact.

Chesapeake Bay

Chesapeake Bay Geology and Sea Level Rise

Groundwater in Virginia

King William Reservoir

Tsunamis in Virginia

a bolide struck over 300 million years ago near modern-day Cumberland Gap, and the town of Middlesboro (Kentucky) developed in that crater
a bolide struck over 300 million years ago near modern-day Cumberland Gap, and the town of Middlesboro (Kentucky) developed in that crater
Source: US Geological Survey (USGS), Geologic map of the Middlesboro and part of the Bristol 30 x 60 minute quadrangles, southeastern Kentucky (2004)

Links

References

1. "The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution," US Geological Survey, USGS Fact Sheet 049-98, 1998, https://pubs.usgs.gov/fs/fs49-98/; "Chesapeake Bay Crater Offers Clues to Ancient Cataclysm," National Geographic, November 13, 2001, http://news.nationalgeographic.com/news/2001/11/1113_chesapeakcrater.html; Gareth S. Collins, Kai Wunnemann, "How big was the Chesapeake Bay impact? Insight from numerical modeling," Geology, Volume 33, Number 12 (December 2005), https://pubs.geoscienceworld.org/geology/issue/33/12 (last checked September 28, 2017)
2. "Drill explores blast: Research seeks insight into explosion that carved huge crater under the Chesapeake," Richmond Times Dispatch, September 8, 2005, (no longer available online)
3. "Geologic Columns for the ICDP-USGS Eyreville A and C cores, Chesapeake Bay impact structure: Postimpact sediments, 444 to 0m depth," in The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes, Special Paper 458, The Geological Society of America, 2009
4. K. Wünnemann, R. Weiss, "The meteorite impact-induced tsunami hazard," em>Philosophical Transactions - Series A, Mathematical, Physical, and Engineering Sciences, 373, September 2015, http://dx.doi.org/10.1098/rsta.2014.0381 (last checked October 24, 2017)
5. Takafumi Matsui, F. Imamura, Eiichi Tajika, Y. Nakano, Y. Fujisawa, "Generation and propagation of a tsunami from the Cretaceous-Tertiary impact event," in Catastrophic events and mass extinctions: impacts and beyond, Christian Koeber, Kenneth G. MacLeod (editors), Geological Society of America Special Papers Volume 356, 2002, p.70, https://dx.doi.org/10.1130/0-8137-2356-6.69 (last checked September 28, 2017)
6. "The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution," US Geological Survey Fact Sheet 049-98, 1998, http://marine.er.usgs.gov/fact-sheets/fs49-98/ (last checked August 390, 2012)
7. "Site report for USGS test holes drilled at Cape Charles, Northampton County, Virginia, in 2004," Open File Report 2007–1094, p.12
8. "Chesapeake Bay Impact Structure Deep Drilling Project Completes Coring," Scientific Drilling, No. 3, September 2006, http://rockbox.rutgers.edu/p38-41_Chesapeake.pdf (last checked September 8, 2008)
9. "City of Poquoson Comprehensive Plan 2008-2028," CHAPTER 5- NATURAL RESOURCES ELEMENT, Section III: Ground Water Sub-element, p.5-42, http://www.ci.poquoson.va.us/sites/default/files/CompPlan03.22.10_0.pdf (last checked January 1, 2013)

location of 2004 core drilling into bolide crater
Source: Open-File Report 2007–1094, Site report for USGS test holes
drilled at Cape Charles, Northampton County, Virginia, in 2004

deep geologic structure of eastern Virginia
deep geologic structure of eastern Virginia
Source: US Geological Survey (USGS) - Professional Paper 1680,
A Surficial Hydrogeologic Framework for the Mid-Atlantic Coastal Plain (Plate 2)


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