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|
—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.