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) 

September 20. Late Triassic cratering

About 214 million years ago, during the Norian stage of the late Triassic, a large astronomical body broke apart above the earth. Fragments of it that impacted the surface may – or may not - have left at least five significant craters across the globe.  

Manicouagan Crater, Quebec (NASA photo)

The five craters are Rochechouart (France), Manicouagan and Saint Martin (Canada), Obolon (Ukraine) and Red Wing (North Dakota, USA). See the links below for a map.

The craters are widely separated today, but when the continents are reconstructed in terms of their plate tectonic motion, the crater positions are consistent with multiple impacts that occurred within hours of each other, ending up in a straight line across the earth. That’s the interpretation of researchers who published the idea in Nature in 1998. There are some problems with the interpretation, including more recent age determinations that indicate that there’s millions of years between some of the impacts. For example, Rochechouart is now dated to about 201 million years ago, Saint Martin is estimated at 227 million years, and Obolon may be as recent as 169 million years, in the Jurassic. None of the age dates can be considered absolutely definitive, and most of them have pretty wide error bars, so I don’t think they completely destroy the multiple-impact hypothesis.

Manicouagan, in northern Quebec, is one of the oldest impact structures whose expression is still visible on the surface. It’s well dated to 214 million years ago, plus or minus 1 million years. A circular lake, a reservoir, occupies the 70-kilometer-diameter inner ring of the crater. The entire crater is about 100 kilometers across, making it the 6th largest known impact feature on earth. St. Martin, in Manitoba, is about 40 km in diameter, and the craters in France and Ukraine are about 20 to 23 km across. The crater at Red Wing, North Dakota, is buried beneath thousands of feet of later sediments, but seismic data define it nicely at about 9 km across, and oil well samples show the shock metamorphism that is typical of impacts.

The Manicouagan impact was clearly a big one. But at about 214 million years ago, it’s about 13 million years before the end Triassic extinction event, and not even obviously associated with smaller global extinction in the Late Triassic, although there’s a regional extinction in North America at about 215 million years ago that might be connected to Manicouagan. The linear alignment of these craters is pretty much inarguable in a reconstructed earth globe, but the age discrepancies are real problems for a multiple impact scenario. Put this in the category of intriguing, but unproven.

—Richard I. Gibson

Links:
Multiple Impact hypothesis

Map of multiple impact hypothesis 

Photo from NASA (public domain)

May 9. A Devonian cratering event?

In the History of the Earth book, written back in 1994, I had a page for a possible Devonian cratering event when the earth-moon system might have undergone a greater-than-usual bombardment from meteorites. In 1994, Copernicus and other young craters on the moon were thought to have formed as recently as 350 to 400 million years ago, the Devonian. Now age dates put the formation of Copernicus at more like 800 million years ago, or even as old as a billion years ago – still young, as the moon goes, but back in the Precambrian in terms of earth’s time scales. 

Copernicus (NASA photo)

So was there an impact period during the Devonian on earth? It doesn’t look like it. There are a dozen or more known large features that represent Devonian impacts. Some, such as the Siljan Ring in Sweden, are pretty big – at 52 km or 32 miles across, it’s the largest known crater in Europe. Even after much erosion, the edge of the crater is still evident in the topography – a lake fills part of the crater rim. It formed about 377 million years ago, during the Late Devonian.

Some of the Devonian craters have fairly reliable age dates, ranging from about 396 million years ago to about 360 million years ago. Nine well-dated events in 36 million years. Given the error bars in the dating, three of the impacts, including the Siljan Ring, were about 380 million years ago, plus or minus five million years or so. So I don’t think we can call this anything like a “cratering event,” and we probably really can’t say the Devonian has a greater incidence of impacts than any other time in earth history.

We talked about one likely global impact event, 480 million years ago, during the Ordovician. That one was based on multiple fossil meteorites in many locations around the world and it seems reasonable to call that a specific event. 

There’s plenty of research going on to investigate the possibility that there may be some predictable periodicity to impacts on earth – obviously we have a vested interest in knowing that, even if it is on scales of many thousands of years or more. Check out a 2005 paper on this topic. This is challenging work in part because there’s a bias toward information about younger craters, simply because younger craters and associated information are better preserved and often better exposed than the old ones.

For now, I’m going to say that there was no Devonian cratering event, nor even any noteworthy increase in impacts then. But you have to realize two things – first, I’m not an expert on this, and even though I reviewed a lot of papers before making that statement, I could have missed something significant. The other thing to realize – to always realize – is the problem of sampling – the total number of impact craters known on earth is only a couple hundred. That’s really not enough to make any confident, far-reaching conclusions about periodicity or increased occurrence. And remember that the older you get, the more likely that all the evidence of an impact might be eroded away or subducted into the earth.

—Richard I. Gibson

Image credit: NASA