Summary: Today I’m tacking the Laramide Orogeny of western United States. It’ll be a long one, in part because I’ve done work on it myself, both for general academic understanding and for oil exploration, and because it's a complex system of mountain building. You recall that ‘orogeny’ is just Greek for mountain building, and over the course of these podcasts I’ve talked about enough complications that you know such an ‘event’ is anything but instantaneous, nor is it always the same in all places. This one is even more complicated, perhaps in part because it is young enough that we see a lot of its effects and therefore understand it a little better. First let’s try to deal with some nomenclature issues. What do I mean when I say “Laramide Orogeny”? The question has to be asked because sometimes geologists use the phrase to refer to a particular time interval – in this case, mostly late Cretaceous into the early Cenozoic, around 75 to 40 million years ago – but some use the name Laramide to refer to a general geographic location, from Canada to Mexico, but generally in the eastern section of what we call the Rocky Mountains today, and extending as far east as the Black Hills of South Dakota. The expression is probably best in Wyoming, and the orogeny is named for the Laramie Mountains of southeastern Wyoming. Double Lake, in the Precambrian of the Wind River Mountains, a Laramide uplift. Photo by Richard Gibson (1982)Yet another meaning for “Laramide Orogeny,” and the one that I’ll try to use, is the structural style, the nature of the folding and faulting, because it is distinctly different from typical collisions that produce mountain ranges. What happened that’s unusual is that the continental crust, the strong, granitic craton well inboard from the point of collision and action to the west, that crust broke in gargantuan brittle faults. This kind of faulting is also called “thick-skinned” because it affected the thick continental crust, in contrast to the thin sheets of sedimentary and other rocks that are typically pushed over each other in thrust faults as a result of many types of collisions. In fact, to some extent overlapping in both time and space, that kind of action – “thin-skinned” – was happening at the same time as the Laramide Orogeny. To make things more confusing, that aspect of the event is called the Sevier Orogeny – and it isn’t spelled ‘severe’, but “Sevier”, named for a location in Utah where it’s expressed pretty well. I’m going to talk a bit more specifically about the Sevier Orogeny in a few days, but it really isn’t separate from the Laramide in many ways, and the two are both the rather different results of the same tectonic action. Map of Laramide structures by Hamilton (1981), USGS. Contributions to Geology - University of Wyoming, V. 19, no. 2. So let’s focus on Wyoming, where the Laramide Orogeny is probably best expressed. Most of the big mountain ranges in and near Wyoming are the result of this unusual intense breaking of the continental crust. The Uinta in Utah, the Wind River, the small Owl Creek Range, the Beartooth in Wyoming and Montana, the Bighorn, the Sierra Madre, the Laramie Range, and the Black Hills of South Dakota, all are the result of huge reverse faults that bring the deep Precambrian rocks not just up to the surface, but high above the present surface. In the Wind Rivers at Gannett Peak, the Precambrian is at the surface almost 14,000 feet above sea level, and equivalent rocks in the Green River Basin west of the Wind Rivers are about 32,000 feet below sea level. That’s a difference of more than 45,000 feet – almost nine miles. The break that separates them is one relatively narrow fault zone, the Wind River Fault, along the west side of the mountains. That’s really a huge amount of offset along a fault. The Wind River Fault slopes off to the east, under the Wind River Mountains, so the Precambrian rocks have been brought not just up, but up and over much younger rocks, including roc
