When Observations Collide…A Grand Canyon Story
Two stories are out there about the Grand Canyon: one says the canyon is young (cut in the past 5 million years), one says it is old (cut by about 70 million years ago). Why is this? Fundamentally it is because one group is tied to one observation and another to another. This sort of thing happens in earth science (most intractably in the controversy over the Cretaceous location of British Columbia) and can lead to immense frustration.
Here, the idea that the canyon is young is fairly longstanding and largely based on the absence of detritus from the upper Colorado River in sediments deposited in the vicinity of Lake Mead (the Muddy Creek Formation, should you wish to look it up). There are workarounds that have been (and continue to be suggested), so let’s not focus on that particular problem now. There are two pieces of evidence that are close together and whose interpretations are mutually incompatible: a new-fangled radiometric age date and a classic old school geologic outcrop.
The first is a radiometric age (well, several ages) from basement rock in the western Grand Canyon. These particular ages look at how helium has escaped from apatite crystals containing uranium and thorium (which decay to produce the helium). When things are warm, you lose the helium easily, but cool down in the 120-20°C range and the helium starts to accumulate. The particularly recent variant on this technique (helium 3/4 dating) looks at the distribution of helium in the crystal to constrain the temperature history of that particular sample. What Becky Flowers and colleagues got from the western Grand Canyon is this:
Now it is really hard to be buried much when the temperature has to be below 30°C since 60 or 70 million years ago. So they interpret this to mean that the western canyon more or less existed to its present depth for a long time. And this isn’t as shocking as it might seem as there are paleocanyons nearby nearly as deep that were filled with sediment about that old.
OK, so the canyon is old? Well, consider this next item, which has been emphasized by Dick Young for awhile (he’s worked in the canyon since long before it was the object of such scrutiny). Perched near the rim of the inner part of the western Grand Canyon, not far from where Flowers and company got their samples, you find this little hill:
That butte on the right has a basalt flow at the top and some light colored sedimentary rocks just below. There is no way all that relief nearby existed when those rocks were created, and they are about 20 million years old. So there is no way there was a deep open western Grand Canyon during this time, right?
Now what is in the literature is skepticism aimed at one set of observations or the other. And your first instinct might be that this is the most likely cause of the discrepancy: somebody screwed up. Such challenges can be healthy, but from where GG sits, it is unlikely either group really messed up. Is there a solution? GG likes to think so; we’ll have to see if the folks actually doing the work end up with the same conclusion… The key is that you have to honor the observations, but you can ditch any interpretations you don’t like…
Well, we can see that the paleocanyons that were nearly as deep as the modern canyon filled with sediment (pay the Hualapai Nation and drive down from Peach Springs to see for yourself–it is pretty breathtaking). How could this happen if the western Grand Canyon remained open? It seems hard to figure, and when you throw in exposures like that butte above, it seems impossible. So what has to happen if we fill the canyon (not the whole way, but up to about the level in the photo above)? To match the helium ages, we need the rocks at the canyon bottom to stay cool. Is this possible? Well, maybe. The common geochronological assumption is to equate temperature with depth at something like 20 degrees C per kilometer, but this is a very imperfect description of reality. For instance, in South Dakota there is an area where things are surprisingly cold in the subsurface in one area and surprisingly warm in another, but on the surface you would have no reason to suspect any such silliness. In the South Dakota case it is hydrology doing the dirty work: cold surface water descending into sediments keep the rock refrigerated, but as that water moves laterally it warms as it absorbs that heat, and when it comes back up it is quite toasty. Similar things happen with the Snake River aquifer and, probably, the Coconino aquifer on much of the Colorado Plateau. Could something like that be happening here? Could we have had cold water finding its way down the sediments that filled the ancient Grand Canyon? The sediments filling the canyon were fairly coarse grained and so excellent aquifers: if there was a downhill for the water to go, it seems quite possible to get the cold water down there (this would mean the area had to stay kind of high for a long time, which makes this potentially a more far-reaching result). Could we really fill and then reexhume a canyon? It turns out that we don’t have to speculate too much on that: the Salt River Canyon just south of the Colorado Plateau looks to have done just that, but enough of the sedimentary record was preserved there to make the story clear. (This canyon also reversed direction, which looks to also be the case with the western Grand Canyon).
While it may well turn out that this idea is utter hogwash, it does suggest that maybe things aren’t as incompatible as it might seem. The basic point to note is that you want to focus on the observations and not as much on the interpretations in cases where interpretations collide.