The Chesapeake Bay Impact Structure: A Buried Crater Still Shaping Virginia

The Chesapeake Bay impact structure, buried beneath eastern Virginia. USGS.

An ancient catastrophe

About 35 million years ago, in the late Eocene, an asteroid or comet about 2 miles wide struck the ocean where the lower Chesapeake Bay in Virginia lies today.

• It vaporized, melted, fractured or shoved aside the rocks and sediments it hit, and threw billions of tons of water, sediment and rock into the air.
• Glassy beads of melted rock rained down as far away as Texas and the Caribbean.
• Models suggest that even 50 miles away the searing air blast exceeded 1,500 miles per hour, and the ground shook like an earthquake above magnitude 8.0.
• Large tsunamis swept most of the North Atlantic basin.

It is among the 20 largest known impact structures on Earth.

How it was found

• 1940s–1950s: USGS hydrologists noticed unusually salty groundwater in the southern Chesapeake Bay area.
• Mid-1980s: coreholes brought up a chaotic mixture of sediments and fossils ranging from 135 to 35 million years old.
• Early 1990s: shocked quartz — quartz altered in a way found in nature only from impacts — turned up in this jumble (impact breccia). More drilling showed the breccia matched the salty groundwater's extent. Seismic surveys released by the petroleum industry revealed the structure's shape and boundaries.
• Late 1990s: the buried structure, its form and its effects on groundwater were well established.
• Since 2000: the USGS and partners drilled deep coreholes at six sites, culminating in a 1.1-mile-deep, continuously sampled hole in the central crater in 2005.

Shocked quartz: the crossing sets of parallel deformation features form only in an impact. USGS.

The Eyreville drill site near Cheriton, Northampton County, Virginia, in 2005, where the corehole reached 1.1 miles deep in the central crater. USGS.

How big is it?

The structure lies buried under hundreds of feet of Coastal Plain sediment; its size and shape come from drilling and geophysical surveys. The target had three layers: seawater, sediments, and igneous and metamorphic basement rock. The structure has three parts:

With the trough and fracture zone, the whole structure is about 83 miles across. Published diameters vary with which parts are counted.

Cross section of part of the structure (water not shown; vertical scale exaggerated five times). USGS.

Still affecting Virginia

• Salty groundwater. Groundwater in the structure is up to twice as salty as seawater — brine not found in deep sediments west of today's coast. Dated at 140 to 100 million years, the same age as the deepest disturbed sediments, it is ancient North Atlantic seawater trapped in place. Higher up it stays salty as brine seeps upward, though fresher groundwater dilutes it. Several communities above the structure have built desalination plants for brackish water.
• Where the bay is. Ongoing subsidence and compaction keep the area low, so rivers have drained toward it — helping decide where the Chesapeake Bay lies.

Questions people ask

• Did it kill the dinosaurs? No. That impact, on Mexico's Yucatán Peninsula, came about 30 million years earlier.
• Did it kill anything? Yes: all life on land and sea for tens of miles around. With other impacts of similar age, it may have contributed to moderate global extinctions and to the start of major global cooling at the end of the Eocene.

Partners: the International Continental Scientific Drilling Program, the Virginia Department of Environmental Quality, the Hampton Roads Planning District Commission, NASA's Langley Research Center and Science Mission Directorate, and DOSECC.

Sources

Based on David S. Powars, Lucy E. Edwards, Gregory S. Gohn and J. Wright Horton, Jr., "The Chesapeake Bay Impact Structure," U.S. Geological Survey fact sheet; a work of the United States government in the public domain. The map, cross section and photographs are taken from the fact sheet's PDF.