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 a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, September 20, 2014

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
Multiple Impact hypothesis

Map of multiple impact hypothesis 

Photo from NASA (public domain)

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