The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Thursday, December 11, 2014

December 11. Chesapeake Bay impact crater

Let’s take a break today from mammals and talk about an impact in Chesapeake Bay about 35 million years ago, a few million years before the end of the Eocene Epoch. The area was offshore then as it is now, but the climate was probably at least sub-tropical if not tropical.

Map from USGS
There is no doubt that this impact devastated the coastal region of the mid-Atlantic states. There’s some speculation that the impact-generated tsunami, which affected hundreds of miles of coastline, even overtopped the Appalachian Mountains in what is now Virginia. The center of the crater is pretty near the southern tip of the Delmarva Peninsula, but it has a diameter of 85 kilometers, 53 miles, making it the largest known impact structure in the United States and among the largest in the world.

The crater was unknown until 1983, because it is entirely in the subsurface, buried by later sediments. It was suspected because of the discovery of fused glass shards and shocked quartz grains in an exploratory oil well in New Jersey, and it was confirmed and defined in the 1990s though additional drilling. Once it was known, it became clear that its presence in the subsurface actually affects the courses of some modern rivers, including the York and James Rivers, which turn sharply at the buried rim of the crater. It also affects modern aquifer systems in the area, and the crater region is subsiding at a faster rate than the rest of the coastal zone, about 6 inches per hundred years. The crater also shows up in detailed gravity and magnetic surveys of the area. 

Cross-section from USGS
What was it? We don’t really know, so it is referred to as a bolide, a generic term for an extraterrestrial body, but it was probably a comet or asteroid about 4 km across. There’s another impact feature of about the same age offshore New Jersey that might be part of the same event. It is dated to Late Eocene time as well, but detailed studies have not been done on that one.
—Richard I. Gibson

LINKS and References
East-coast aquifers
The crater 
Gravity and Magnetic expression 

Crater map from USGS (public domain) 
Cross-section from USGS (public domain) 

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