May 28. Antler Orogeny

Antler Orogeny

Toward the end of the Devonian Period and continuing into the Mississippian that followed, parts of western North America collided with something, perhaps a volcanic island arc like the modern West Indies.

It does not appear to have been the intense kind of mountain-building event that results from continents colliding. There isn’t much evidence of extensive metamorphism and igneous intrusion. The best evidence is found in the state of Nevada, where coarse conglomerates indicate that there was an uplift in what is now central to east-central Nevada. Something was nearby, and eroding to make those thick, coarse conglomerate layers.

To the east, the Devonian rocks are the typical Paleozoic carbonate shelf deposits, the same kinds of limestones we’ve heard about for much of the Ordovician, Silurian, and Devonian across the middle of North America. But to the west the rocks are more siliceous, those coarse conglomerates, and there are some interbedded volcanic rocks. The western rocks are thrust eastward on top of the eastern rocks in the Roberts Mountains Thrust Fault, a major feature which is additional evidence for the tectonic activity we call the Antler Orogeny.

Could an impact event have triggered some of this tectonic activity? About 367 million years ago, Late Devonian time and about the time the Antler Orogeny was getting underway, something did crash into the waters that were forming the Devonian Guilmette formation, near the town of Alamo, Nevada. There is no crater for this impact, but the rocks record the results of the impact. Smashed and deformed rocks, huge broken blocks forming a rock called breccia, shocked quartz grains, and high iridium levels. There are even deposits that can reasonably be interpreted as tsunami deposits. This is some relatively recent work, mostly in the 1980s and 1990s. John Warme was one of the first geologists to recognize the nature of these rocks and to interpret them as a bolide impact.

But one impact, even one that made impact breccias and the rest, really could not have made a mountain range. We really have to seek tectonic causes for something that’s hundreds of miles long.

I said earlier that it might have been a volcanic island arc that was colliding. But that’s definitely not certain. There are various theories to put the Antler Orogeny into a plate tectonic context, but there are problems with all of them. It’s just not clear what was west of what is now central Nevada back in the Devonian. One reason for that is that much later, a lot of stuff was added to western North America – including most of California. And that process obliterated some of the rocks and structures that were there previously.

As always, research continues.

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Glacial Lake Agassiz

Today’s birthday is Jean Louis Rodolphe Agassiz, born May 28, 1807, in Motier, Switzerland. Louis Agassiz worked extensively on fish, both modern and fossil, and his reports published in the 1830s and 1840s established him as a renowned scientist. But he is probably best known as the first to propose scientifically that the earth had had an ice age. He worked extensively on the glacial deposits of both Europe and the United States. Glacial Lake Agassiz, of which Lake Winnipeg in Manitoba is a remnant, is named for him.

—Richard I. Gibson

Glacial Lake Agassiz map from Upham, Warren, "The Glacial Lake Agassiz". Plate III. Monographs of the en:United States Geological Survey: Volume XXV, 1895. 

Antler orogeny map drawn by Richard Gibson.