We’re in the Silurian, but today we’re going to talk about the absence of Silurian rocks. Where I live here in western Montana, and across much of the west, there are no Silurian rocks present. In fact, in much of the area, there are no Ordovician rocks either. You probably recall that when there is a break in the rock record, we call that an unconformity, and unconformities can form in one of two common ways: either the rocks were never laid down, or they were laid down but were eroded away later. Sometimes that “later” is after a lot of tectonic activity, so the rocks below the unconformity might be tilted, making an angular unconformity with the younger deposits above.
Out here in Montana, there doesn’t seem to be an angular relationship between the older Cambrian rocks and the younger Devonian rocks that are on top of them. The two packages of rock are parallel to each other, just as if sedimentation had proceeded continuously from the Pilgrim formation, near the top of the Cambrian, into the Jefferson formation in the Devonian. But there is still a gap there, a gap that spans the entire Ordovician and Silurian Periods, a gap that represents at least 75 million years – more than the time span since the dinosaurs died off. What happened?
Well, again, there are two choices. Nothing was deposited, or it was deposited but eroded away later. How can we tell which it was?
If you go to the last places where Silurian or Ordovician rocks do crop out, you can look at the nature of those rocks. If they were at the edge of the basin in which they were deposited, it’s pretty likely that we’d see changes in the rocks to tell us that we reached the edge of deposition. For example, you might see a gradual transition, over many miles, from the kinds of sediments and fossils that are found in deep sea, progressing to a shallow sea environment, to increasing amounts of silt and sand as we get closer and closer to shore, to a good beach deposit at the former water’s edge, and maybe even really coarse pebbles in a rock we’d call conglomerate, close to the uplifted source area, dumped there because they couldn’t be carried far offshore. You might also see changes in the types of fossils, reflecting the changes in the environment that would mean we were approaching the edge of deposition.
On the other hand, if we have a wide expanse of sediments and fossils that indicate a huge area of deposition, something grand in scale like the entire Florida shelf, extending for hundreds of miles, and it comes to an abrupt end without any of those things that might indicate an expectable transition to land where there was either no deposition, or deposition of a very different kind, then we might conclude that the rock had gone on much further at one time but had since been eroded.
When we look at the edges of Ordovician and Silurian rocks in Montana, Wyoming, and Nevada, the whole northern Rocky Mountain region, it does seem like the Ordovician and Silurian rocks must have once been much more extensive than they are today – meaning they were laid down and then eroded.
To erode vast areas of rocks, you pretty much have to raise the rocks up above sea level. So a big chunk of land, two-thirds of Montana and beyond, was gently uplifted while holding the older geologic layers perfectly horizontal, eroded for 75 million years down to one precise layer in the Cambrian, then gently lowered back down so that the surface was still perfectly horizontal, so that the Devonian sediments could be laid down essentially perfectly parallel to the Cambrian rocks that were not eroded away.
Sound good? Well, I don’t buy it. There are way too may “perfectlys” in there. It’s not impossible for something like that to happen, but geologic events are typically a lot more chaotic than they are “perfect.” So let’s look more closely – or in this case, more broadly. If you look at the rocks that are under the Devonian, under that unconformity, across a really wide area, 500 miles or more, we find that they change. Here in western Montana, it’s Cambrian rocks under the Devonian. In central Montana and northern Wyoming, it’s the Ordovician Big Horn Dolomite that we talked about March 19. And even further east, deep in the subsurface of the Williston Basin of eastern Montana and western North Dakota, there’s the Silurian, in its proper, expectable position beneath the Devonian.
What it amounts to is this unconformity, which seems like it’s between perfectly parallel strata in the Cambrian and the Devonian out here where I know it well, is really an angular unconformity after all – a very very low angle angular unconformity. The angle is so tiny that you don’t see it until you look at a really large swath of territory.
What this all seems to indicate is that sometime after Ordovician and Silurian rocks were deposited in what was generally a wide, shallow, warm sea- after that, there was a very gentle titling of the continent, a slight uplift – nothing to make mountain ranges, but enough to raise what is now western Montana above sea level, so it could be eroded. Think of the Cincinnati Arch, that we discussed March 10, but on a broader, gentler scale. And that low uplifted area was eroded, slowly, gently, over 75 or more million years. And then slowly, gently, it all subsided below sea level so the Devonian could be deposited.
So is that the answer? I think it’s probably the best overall explanation, but don’t take it as gospel. There’s almost too many slowlys and gentlys in there for me. And we can only examine the details in the rocks where they are still present – the edges, the erosional zero edge is what we call it – of the Ordovician and Silurian rocks. Where they are gone, they are gone – and we can’t look at the details to figure out what exactly was going on. So I’m not about to say that it was absolutely positively a case of sediments deposited and lithified and uplifted and eroded. There might have been some places out there where the Silurian was never deposited. We’d need a time machine to tell with absolute certainty – but that doesn’t mean we’re guessing. The total body of information we do have points to the idea of deposition and later erosion as really pretty likely, at least for the best overview of what happened.
At least that’s what I think, until someone tells me they’ve found some rocks that have information to the contrary. That’s how science works.
—Richard I. Gibson
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