Welcome back to Our Earth, Its Climate, History, and Processes. I'm David Schultz. What we want to talk about in this lecture, is what the layers of rock tell us, and how their relationship from one layer to another tell us about the geologic history of that particular area. This whole discipline is called stratigraphy. That is, how we put the geologic record in place. In other words, how we know which events occurred either before or after another. And so there are three different ways that we can place these rocks in this type of chronologic order. First, we can use lithostratigraphy. These are the relationships between the different rock layers. Second, we could use fossils to date geologic events. But for certain types of fossils, we may be only able to do this in a relative sense, so we can just say which one is older or younger than the other, based on the type of fossils that were present. Then the third way is chronostratigraphy. This is the only way that we can apply an absolute date to the age in a rock. This is where we're using the radioactive decay of isotopes to place a precise date on a layer of rock. Nicolaus Steno was a Danish physician, and in 1669 he developed the principles of stratigraphy. These four principles tell us about the relationships of rocks and how we might use them to date their relative ages. The first one is called the Principle of Superposition and this simply says, that younger rocks lie on top of older rocks. So here we have a picture from the Grand Canyon. You can see the layers in the rocks up through here and the result is that the oldest rocks have been deposited first, these are sedimentary rocks. And as we go up through the column, we see progressively younger formations. We recognize that these sequences, because these beds have been laid down, they have parallel contacts. We consider them to be a period where the deposition of these sediments occurred, more or less, in a continuous manner, without any breaks. And what I mean by that, is that there were no periods where the sediments came up out of the water, were lifted, or the water retreated, and there was erosion of these sediments. We term this relationship, where there are no major breaks that are visible from the stratigraphic record as conformable, and we're going to use this term later here. The second principle is that of original horizontality. This means that rocks that have been tilted or folded would had have to have suffered this disturbance after they were laid down. We recognize that these rocks are horizontal when they are laid down. So here's an example. Here, you can see this u-shaped bend fold in these rocks that have been laid down. So we recognize that this fold must have happened after the rocks, after the sediments had been laid down, the rocks had been formed and then later deformed. Here's another example. Here you can see a nearly right angle in these rocks, indicating a rather severe deformation of these rocks, this fold, this angular fold. Again, we recognize that these being sediments, they must have been laid down horizontally and then later deformed. The third principle is that of cross-cutting discontinuities. This means that if we have horizontal beds, and then they get traversed by a lava dyke or a fault, then the bed must be older than the event that led to this cross-cutting. So for instance, here is a satellite image, and you can see some brightly colored rocks on the surface here, and then a similar set of brightly colored rocks over there. And we recognize that there is a fault here that has displaced one section of this towards the north, relative to this other section. So again, using the principle of cross-cutting discontinuities, we recognize that this, these sediments, these rocks must have been laid down first, and then the fault led to the northward relative movement of one to the other. The fourth principle is that of lateral continuity. This basically says that sediments form in continuous layers, and if we see an abrupt ending to the layer, then that means that something must have happened either during or after the deposition of that sediment. So for instance, there was a fault that led to an abrupt ending. A shoreline that meant, that of course, the sediment would not be deposited, or the sediment was exposed to the air, and then eroded away. And these breaks in the geologic record caused by the exposure of the sediment, erosion away of some of the rock layer, and then leaving a gap in geologic history, is called an unconformity. And here is perhaps one of the most famous unconformities in the world. This is at Siccar Point in Scotland, and what you can see here are horizontal beds extending in this direction, meeting at almost right angles, beds to below at a different orientation. And, so to give you some perspective, what you're seeing here is a horizontal bed here extending up towards there. And then below this yellow line, represents the older bed at a different orientation. We know that the beds above this unconformity are Devonian sandstones of an age of 345 million years ago. But these beds that are oriented almost vertically are Silurian greywackes, which are essentially sandstones, 425 million years ago. And so you can see that these rocks must have been laid down, folded up, deformed into an upward direction, eroded away. These new sandstones were then laid down on top of this unconformity, which represents a gap of 80 million years. And then these sandstones were later lifted up into this angular non-horizontal position. The second famous unconformity is that of the Great Unconformity in the Grand Canyon of North America. So here I've indicated a close up shot of the unconformity and I want you to identify where that unconformity is in the following quiz question. So hopefully you got the quiz question right. Here, we can see the answer, I've indicated the basement rocks, in this case, part of the Vishnu complex and you can see the nice, highly deformed metamorphic rocks here in the bottom. Above that, we see the Tapeats sandstone which lie almost horizontally, and then between them is a gap of over a billion years. We can also see some other kind of unconformity, a smaller one here between these horizontal sediments that were laid down, and these ones as well. But the main Great Unconformity is this region here, separating the Vishnu basement rocks from what comes above. So, to summarize today's lecture, stratigraphy is used to determine the relative or absolute ages of geologic events. In this lecture, we dealt primarily with lithostratigraphy, and we looked at the principles of lithostratigraphy. These say that layered rocks are laid down horizontally, with the oldest at the bottom, that they extend horizontally. And where they end, they must be ending due to something that happened either at the time they were deposited, or later. We recognise that cross-cutting relationships such as dykes or faults can help date the relative ages of these types of events. And then finally, we saw that unconformities represent missing geologic information.
