A few days ago we talked about the Laramide Orogeny, the brittle breaking of the continental crust into huge uplifts along relatively high-angle faults with miles and miles of throw. And I mentioned a different style of mountain building that took place partially at the same time and in some areas, in the same places. That aspect of the deformation is called the Sevier Orogeny – and again that’s not “severe” but “Sevier,” from a place in Utah. Just to keep things confusing.
|Sevier vs Laramide (source)|
The basic difference between the Sevier part of the activity and the Laramide part is that instead of those big brittle breaks in the crust, in the Sevier we had much thinner slices of rock – mostly the bedded sedimentary rock – being pushed over each other in generally low-angle thrust faults with often only a few thousand feet of displacement, but sometimes more. This is the fairly typical result of collisions.
Think of a short pile of carpets and sheets and bedspreads all nicely on top of each other in horizontal layers. That’s the sedimentary package of rocks in western North America, ranging in age from Precambrian to the early Cretaceous. There’s been some bumping and breaking and so on, but on the whole, those carpets and sheets are still more or less intact and relatively horizontal. Now set a file cabinet on one side of the pile, and start pushing. The fabrics will fold and push up and over each other. In the real world of rocks, they are brittle enough to eventually break and slide over each other, and those breaks are called thrust faults. But the floor, the crystalline granitic rocks underneath the sediments, does not break. Well, it did in the Laramide Orogeny that we talked about the other day. But not in this more straightforward pushing we had during the Sevier Orogeny.
In the real world, as the rocks that are the equivalent of our carpets and sheets piled up on top of each other, two things happened. First, the tops start eroding, with the eroded sediments shedding off to the east of the uplifted, thrusted mountains. And second, the weight of the thrust sheets, plus the sediment, was enough to bow down the crystalline granitic floor. Not really enough to break it, but to make it sag.
We’ve just created a classic foreland basin. The foreland is the territory toward the craton, in this case the North American continent, inboard from the mountains created by the collision off to the west. The deepest part was in the west closest to the collision zone, where the thrust sheets and sediment piles were thickest, then it gets shallower and shallower, really pretty quickly, as you head east onto the strong craton.
We can see the evidence for this in changes in sedimentation from west to east. Here in Montana, where I live, along the Big Hole River, the Kootenai Formation of early Cretaceous age is something like 3,000 feet thick. Just 50 miles to the east, the Kootenai is about 400 feet thick. That change in thickness reflects the bottom falling out – the crust subsiding – in the western part of the foreland basin. To an extent it’s a self-perpetuating event: as more and more sediment and thrust sheets come in, the crust bows down more and more, allowing for more and more sediment to pile into the basin. Eventually, of course, it slows down or stops, once the collision has stopped and the mountains have been eroded to a level where they don’t contribute sediments in huge volume any more.
One important difference between the Laramide style and the Sevier style of mountain building is that the Laramide was pretty much a case of brittle breaking. The crust is thick, relatively uniform, and brittle, so it tends to break rather than bend. The sedimentary pile, on the other hand, was a package of diverse rocks that overall was a bit more plastic, and could fold before it broke. In detail, of course, there are brittle units and more plastic units – sandstones and limestones tend to be stronger and more brittle, and shales tend to be weaker and behave more plastically. Sometimes shales essentially even flow, because the rock is so weak and the particles are both small and mobile.
|Part of the Sevier Orogenic Belt in Montana (USGS SIM 2860)|
Anticlines in general are excellent traps for oil and gas, if the other requirements are present, and in the Sevier Orogeny of western Wyoming everything came together. It’s called the fold and thrust belt there, with long, gently sloping thrust faults underlying big rounded anticlines, many of which hold oil and gas fields.
In other places along the Sevier Orogenic belt, the structure wasn’t enough – something was missing to make a hydrocarbon province, either source rocks, a seal, or something particular in the burial history to mature the organic matter into oil or gas. Consequently, for example, in the Montana Disturbed Belt, part of the Sevier Orogeny, there’s very little oil and gas. But the action did help create some spectacular scenery. The jagged alternating mountain crests of the Bob Marshall Wilderness in many cases reflect alternating thrust sheets. And all of Glacier National Park rides on one of the thrust faults, a special kind called an overthrust because it is very low angle, maybe 5° or less, and the rocks have been pushed over underlying rocks a great distance, tens of miles. The Canadian Rockies also are underpinned by Sevier Orogeny thrust faults, although like Glacier National Park, the sculpted mountains are the more direct result of glaciation in the last 2 million years or so.
I’ve said that there was some overlap in both time and space between the Sevier and Laramide styles of mountain building, but it’s fair to generalize some and say that the Sevier was mostly somewhat earlier than the Laramide, and generally took place to the west of the places deformed by the Laramide Orogeny. This makes sense if we buy the idea that an initial subducting oceanic slab, off to the west in what is now California and Idaho, began somewhat conventionally, with a magmatic arc and thrusted pieces pushing off the collision belt, followed by that change in the angle of the descending, subducting oceanic crust so that the forces were transmitted further east where they created the brittle breaks of the Laramide.
In places where the two types of deformation overlap, it can be challenging to unravel the sequence. Was a strong crystalline basement cored block already uplifted when the weaker, thinner thrust faults impinged on it? If so, the Laramide block might serve as a buttress, with the Sevier faulting swinging around it rather than through it. But you can get essentially the same expression by having the Sevier faults present first, and the massive Laramide uplift coming second, folding the earlier faulted rocks and faults around it. In some places, I really think it is correct to think that the two different styles of deformation were actually happening at the same time.
That’s enough for today. Tomorrow, we’ll talk a bit about the igneous rocks associated with all this action in western North America.
—Richard I. Gibson
Cross-section from Geologic Map of the Canyon Ferry Dam 30´× 60´ Quadrangle, West-Central Montana, By Mitchell W. Reynolds and Theodore R. Brandt, 2005: USGS Scientific Investigations Map 2860.
Seaway map from USGS (public domain)
Green River Basin cross section by Richard Gibson (source)