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  Tuesday, April 25, 2017  
   
 

 
Living on the Rim

 

No one knows if the cataclysm arrived in daylight while life was stirring or in the dark of night while many slept. Traveling 60,000 miles per hour, it took just seconds from the time its shadow appeared above Earth until it plunged through the sea floor, blasting a hole over fifty miles in diameter, and spewing debris in a towering cloud thirty miles high.
Before the region’s inhabitants could register that something colossal had occurred, a crushing super-heated shock wave tore through the atmosphere. In its wake, untold tons of ejected debris roared across the sea floor, and a series of towering tsunamis raced outward from the impact zone. The blast wave alone destroyed all higher life forms within six hundred miles.
This was no manmade weapon of mass destruction, but a comet or asteroid measuring two to three miles across. A similar strike today would wipe out all the major East Coast cities, killing tens of millions. As it was, this 35 million year old celestial visitor claimed untold lives, rearranged the geological framework of what would become the Chesapeake Bay, and whose impacts we still experience today.
Over the ensuing days and months, debris from the impact fell back into the crater, filling it, and forming a breccia deposit. Local rivers flowed towards it, carrying their sediments. In the ensuing 35 million years, as glaciers advanced and retreated and sea levels rose and fell, the crater was gradually buried under a half-mile thick layer of sedimentary rocks.
Referred to by experts as the Chesapeake Bay Bolide, the impact took place during the late Eocene Epoch, when sea levels were higher, and lush tropical rain forests draped the Appalachians. The shoreline of Virginia was somewhere in the vicinity of Richmond, or even beyond, pressing up against the mountains. This event was no exotic phenomenon, but one of a series of impacts that has altered the Earth’s surface since its inception.
Bombarded with space debris daily, military satellite images have revealed that Earth averages one house-sized meteorite monthly, exploding and disintegrating in earth’s upper atmosphere. Occasionally, one greater than a half-mile in diameter survives, plunging into the oceans’ depths or striking the continents. Data suggests a meteorite in the range of 2-3 miles in diameter strikes the earth every 50-60 million years. A comforting thought perhaps when contemplating the arrival of the next NEO (Near-Earth Object) and its potential for mass extinction.
The crater in question is now buried under 1500 feet of gravel, sand, silt, and clay beneath the southern part of the Chesapeake Bay and the peninsulas of Southeastern Virginia. First identified by Dr. C. Wylie Poag and Senior Geologist David S. Powars of the United States Geological Survey, they assembled an impressive international team to investigate not only its characteristics but its consequences.
Discovery of the crater revised our understanding of the Atlantic coastal plain and revealed several consequences that have direct impact on the population bordering the bay today; the composition of coastal aquifers, continued land subsidence, river courses, ground instability, and the location of the bay itself.
For years, scientists and hydrologists had been intrigued by the presence of briny water found in various test wells; water that was one and half times saltier than seawater and unfit for use. As demand for more and more fresh water in Hampton Roads arose following World War II, geohydrologists tried to fathom the puzzling and random presence of briny water in what was supposed to be a huge fresh water aquifer. It was important to know why and where these pockets existed so as to prevent inadvertent drilling into them and contaminating the overlying freshwater.
Suspecting the presence of an ancient cataclysm, in 2004 the USGS drilled a test hole 2,699 feet deep into the impact crater near Cape Charles. What they discovered was an extremely old body of ancient seawater in the world lying underground; Early Cretaceous seawater 100 million to 150 million years old.
The structure and geometry of the crater was determined by profiling, which produced a two-dimensional cross-section that determined its size and depth. The completed profile revealed an impact crater twice the size of Rhode Island and as deep as the Grand Canyon. It’s the largest US crater and the sixth largest known on the planet.
Between September 2005 and May 2006 the Chesapeake Bay Impact Structure Deep Drilling project drilled into the crater four miles north of Cape Charles. They drilled through post-impact sediment, then through the breccia deposit, finally reaching impact melt rock lying above the crystalline granite basement rock. Drilling further, they broke through a large block of granite in the breccia and encountered a hodgepodge of rock and other sediment; a tossed salad of geologic material. It was here the highly saline water was trapped, overlaid by a thin lens of freshwater.
Surprisingly, the subsidence depression can be observed today on the crater’s western edge, and one of the best places to examine its curved steep slope and rim is the Suffolk Scarp. Dr. Poag explains:
“If you drive along Route 17 from Newport News to Gloucester, you travel right along a flat surface called the Newport News terrace. The Suffolk Scarp forms the eastern boundary of this terrace about a mile east of the highway. Here you are right on top of the crater rim. One of the best places to examine the scarp itself is along Virginia State Highway 14 between Gloucester Courthouse and James Store.”
I am quite familiar with this geologic feature since I travel this road almost daily. Here the highway undulates; rising, curving, and falling like a roller-coaster until it dips past James Store and flattens out again. When we began our search for land in Gloucester many years ago, my husband’s first requirement was good elevation, tidal and storm flooding foremost in his mind. Here on the crater’s rim, our home site sits seventy feet above sea level; unusually high elevation this close to the bay.
The continual slumping of rubble within the crater affected the flow of the region’s rivers and helped shape the Chesapeake Bay. Most of the rivers that feed the bay flow southeastward towards the Atlantic. In contrast, the York and James make sharp “L” shaped turns to the northeast near the outer rim of the crater. Without the crater, the port of Hampton Roads would not exist. The bolide did not form the Chesapeake Bay directly, whose age has been determined to be 10,000 to 18,000 years old. But as rivers and rising sea levels repeatedly flooded the river valleys, water flowed towards the depression, gradually creating an enormous basin—the bay.
Perhaps the most far reaching impact created by this bolide is land subsidence. “Such abrupt changes in topographic elevations and river courses seldom take place without a good geologic reason,” says Dr. Poag. “They indicate that the land surfaces, bay bottom, and seafloor inside the crater rim have continued to subside faster than areas outside the rim for nearly 35 million years. Such geological processes as greenhouse warming, sea-level rise, and glacial melting are growing concerns.
In many places we are not sure whether the sea is actually rising at an unusually high rate, or the land is actually subsiding more rapidly than elsewhere. The relative effect is the same. The fact is the Chesapeake Bay is one of the places being monitored closely as it is undergoing one of the most rapid rises in relative sea level of any place on earth.”
Finally, seismic profiles across the crater show numerous faults that cut through sedimentary beds, extending upward toward the bay floor. These regions of crustal weakness further exacerbate subsidence. They allow salty water to percolate upward through cracks and fissures, contaminating freshwater. There have been four recorded earthquakes aligned with the outer rim of the crater, the most recent in 1995 in York County.
As the various regions in southeastern Virginia grapple with the consequences of an event few of us can fathom, the Earth and its oceans continue the dynamic changes that static management will never overcome. Some municipalities are already contemplating retreat from the most vulnerable areas, abandoning infrastructure and leaving residents to cope with increased inundation.
Today, the Chesapeake Bay is 200 miles long and 30 miles across at its widest point. Its waters are fed by nineteen principal rivers and hundreds of lesser rivers, creeks, and streams, creating more than 11,684 miles of shoreline—more than the entire US west coast. It is home to more than 2000 species of marine animals and plants, some so rare they may be unique to the region. The Chesapeake Bay Watershed stretches from Cooperstown, NY to Norfolk, VA, including parts of six states and the District of Columbia. It holds more than 18 trillion gallons of water. All thanks to a chance encounter with a celestial visitor 35 million years ago.



Special thanks to Dr. C. Wylie Poag, Ret. Senior Research Scientist USGS Woods Hole, MA for his enlightening emails, images, and for his informative book “Chesapeake Invader” that first sparked an interest in the subject.