Crash and Burn

About 35 million years ago, a large meteor came crashing through Earth's atmosphere and hit an area in the vicinity of the southern Chesapeke Bay. While it's not possible to pinpoint just where it hit, the general location is in the area of where we are today, Onancock, VA. How was this discovered since there is but the fainest suggestion on the surface that such an impact was felt? What effect did the impact have? Did this impact have the same life-altering effect as, say, the Yucatan meteor impact 66 million years ago that seems to correlate with the end of the dinosaurs and the ascendency of the mammals?

It can only be guessed that the Chesapeake Bay meteor was smaller than the Yucatan meteor. Closing out the age of the dinosaur and the dominance of the reptiles was far more significant than any observable influence on the mammal evolution. But the mammals had only been evolving for 30 million years after the Yucatan impact at 66 millions years ago when the Chesapeake Bay meteor hit. Perhaps the mammal life forms were not as susceptible.

Geologists Love to Create New Names for Old Terms: Meteor Becomes Bolide

We are in Onancock, VA, about at the upper area of the described impact site. Note where the shoreline was during the Eocene. The major cities of the Coastal Plain, Norfolk to New York, were under water. The meteor is estimated to have been between 3 and 5 kilometers in diameter. Some researchers of this event suggest that a massive tsunami resulted from the impact with waves reaching up to the Blue Ridge Mountains of the Appalachians.

The sub-surface evidence is incontrovertible. From drill logs of the area, the disruption of the existing rock and sediments can be mapped. The breccia is the description of the destroyed existing rock, blown into fragments. The megaslump blocks are sediments that collapse into the crater left by the impact. Look at the ejecta blanket, which results from the debris thrown skyward and then being deposited over vast areas of the eastern U.S. In fact, this blanket has identifiable characteristics that allow it to be pinpointed in the earth's history globally and it becomes a useful time marker.

How does this matter today? Below the surface, we rely on groundwater to supply well water to fill municipal storage tanks and to pump water to residences. The aquifers that supply this water have been broken and are discontinuous in this region, meaning that such sources are much less reliable and useful. Saltwater intrusion of fresh water aquifers represents a real threat to water supplies.

Onancock is located approximately where "E" is located on the map.

The impact and the resulting debris caused the eventual drainage patters of causing streams and rivers to be influenced as well. The Rappahannock River flows southeastward to the Bay while the York and James Rivers make sharp turns to the northeast near the outer rim of the crater.

The existence of the crater helped define where the southern part of the Bay is located. A muted suggestion of the existing Bay's shoreline reflects where the crater underlies the Bay itself. The crater-related debris has caused land subsidence in the area, accelerating the threat of islands and shallow areas to sink beneath any encroaching seas. It is estimated, for example, that Tangier Island will disappear in a matter of decades even though the case for sea level rise may be debatable or insignificant.

The crater itelf formed a depression that determined the alignment of streams and rivers in the area that subsequently became flooded with post-glacial sea level rise perhaps 10,000 to 15,000 years ago.

Once again, Brio finds herself deep into geology. Just as with a bumpy crossing in big waves, she says, "It rocks."

Cheers,

Brio