December 14. Burial of the Rockies and Exhumation

During the Oligocene epoch, which lasted from 34 to 23 million years ago, the Rocky Mountains disappeared. Well, not really, but they were so eroded that to a great extent, they were buried in their own debris. The huge uplifts of deep crystalline rock that were forced upward by the Laramide Orogeny, starting in the late Cretaceous and continuing into the Paleocene, say from about 75 to 55 million years ago, of course began to be eroded as soon as they were above sea level. Because they were well within the continent, rather than on the margins like so many collisional mountain ranges are, the sediment that was eroded pretty much had to go into the basins between the mountain ranges and onto the mountain flanks. 

We’ve talked about some of that early Cenozoic sediment, in the Fort Union, Green River, and Wasatch Formations, mostly of Paleocene and early Eocene age. Erosion of course continued – that’s the way it works – so by Oligocene time, about 30 million years ago, the erosional debris, deposited on the flanks of the mountain ranges, had to a large extent buried the mountains. 

The sedimentary deposits on mountain flanks are called pediments – from words meaning foot of the mountain – and the pediments extended well up onto the flanks. In some places the erosional debris is more than 3000 meters thick, almost 10,000 feet, and it was deposited to elevations, in present-day topography, as high as 6,000 or 7,000 feet above sea level. So yes, there were still mountains, but not like the high rugged ranges we know today, in the Wind Rivers, Uinta, Big Horns, and others. It would have been more like a relatively smooth, sloping plain, with occasional high crags sticking out of it.

That’s how things would probably still be today, but in mid- to late Miocene time, about 5 to 10 million years ago, the whole region began to be actively eroded again. We call it the exhumation of the Rockies, which had been buried in their own debris. What happened to allow that? There are two likely possibilities, both of which may have happened at least partially simultaneously. First, the whole region might have been uplifted. Higher relief allows streams to erode more aggressively, removing things like the sediment in the pediment around the mountains. And second, if the climate changed from arid to wetter, there would have been more water to do its thing and erode stuff away. It’s likely that both happened. 

The gentle, regional uplift must have been related to some kind of tectonic activity, and it was at about this time, the Miocene, that western North America was beginning a different type of interaction with the oceanic plates to the west. Instead of simple subduction, the continent overrode the spreading ridge and the contact changed to one with plates sliding beside each other – the San Andreas Fault began to form in the Oligocene, about 30 million years ago, but much of it only became significant in the past 5 to 10 million years. And things were happening to the whole region too – it was pulling apart, forming the basin and range in Nevada and Utah. We’ll talk more about that in a few days, but for now, let’s assume that all this tectonic activity might be a factor in the regional uplift that allowed the erosion to strip the sedimentary cover off the Rockies, carving them into the craggy forms we see today. The recent glacial period put the final touches on that sculpturing, but the stage was set by the exhumation that began about 5 to 10 million years ago, and continues to this day.

We can figure this kind of thing out by looking at the sediments themselves – the grains in the rocks can tell us what they were derived from. Was it recycled Cretaceous sand? Or Mississippian limestone? Or was the erosion deep enough that even the Precambrian cores of the Laramide mountain uplifts were being eroded? The nature of the Oligocene sediments tells us a lot about the uplift and erosional history of the Rocky Mountains.

Tomorrow, we’ll talk about some of the sediment that was part of the burial of the Rockies.

—Richard I. Gibson

Burial and exhumation

See also Rising from the Plains, by John McPhee

Paradise Mountains drawing from GRANITIC DOMES OF THE MOHAVE DESERT, CALIFORNIA, by William Morris Davis (USGS)