- [Dr. Jaffri] Now as you can see, we have uncovered all our core boxes, and we're gonna make a series of observations, and we're gonna document our observations. Since we are dealing siliciclastic core first thing we're gonna do is we are going to look at our grain sizes, okay, and what I have on here is a grain size chart. This one happens to be from SEPM. You can get one anywhere, and they all pretty much work the same. Now, on the grain size chart are divisions and there's a graphic column. What we don't do is just take this guy, place it on the core, and then just eyeball it. What we're gonna do instead is use a hand lens. Here is our hand lens right here with a grain size chart. I'm gonna place the grain size chart directly on the core. I don't hold the hand lens like this. We're not looking at a butterfly, this is not a magnifying glass. What we wanna do is we wanna bring the lens super close to the core, and then, our eyeball as close to the hand lens as possible. We want to match the grains that we see in the core with what we have in this column towards the right, and that's gonna give us our grain size. Since these are classics, the grain size also gives us our lithology, for example, all these are sand size grains, so this is a sandstone. When we move here, these are clay sized grains, so that gives us a shale. Our grain size determines our lithology. The next thing you'll see on this grain size chart is sorting, which is how similar are the grains to one another. Let's look at a section that seems fairly homogenous. If you look at the sandstone here, you can see that, just by eyeballing it you can tell that's fairly homogenous, but we're not just gonna eyeball it. We're gonna take our hand lens to that part right there, and we're gonna look at a few things. Are the grains the same composition? Are they the same size? Are they the same shape? If they are, it's well sorted. If they're not, it's poorly sorted. So let's take a look at this section right here. You can see that there's grains of different sizes in there. Right off the bat, you can tell that not only are they different sizes, they're different shape. This large clast is made up of clay, so that's poorly sorted right there. Sorting depends on how long the sediment was reworked. The more it's reworked, the better the sorting. Next thing we want to do is we want to look at the thickness of individual beds. In sandstones, most of the time these beds are split by shale partings. Here's a shale parting right there, here's a shale parting right there. That right there is one single bed. I would measure the thickness of this guy going from the bottom towards the top. We go a little higher up, another shale parting, another shale parting, so that's another bed. From that shale parting to this one is yet another bed. We're noting the thicknesses of all these beds. Not only are we doing that, we are also gonna look at contacts. This is very important. What kind of contacts do we see? If we move back here, you can see we've got a lot of sand up here, a lot of clay towards the top, so somewhere between this column of sand and that shale on top is our contact, which is roughly in this zone right here. But you can really put your finger on it. If you do this, you can see that it's a nice gradational change going from sand into clay. In this case, maybe I place the contact right around there, and that would be a gradational contact. In contrast to that, let's take a look at this guy again. You can see how sharp that contact is. There can be two different kinds of sharp contacts, like this the contact is extremely sharp, but it's also fairly horizontal, it's fairly flat. In this case, the contact between this sandstone and this thin shale parting, that is sharp and it's fairly regular. If you look up here, that's yet another sharp contact, but that's undulating, and when it's undulating like that, that's an irregular contact. We can have gradational contacts, sharp contacts, sharp contacts can be straight, in which case we call them regular. If they're undulating, we call them irregular. Whenever you see a sharp contact, it represents a sharp change in depositional energy. Normally when you're describing a core, I would just note down sharp, sharp irregular, sharp regular, gradational. I'm not interpreting the core at the core facility. That's what you do when you go back home or back to your office. Every one of these things means something. Since I am making this video for you here, I'm gonna give you a little bit of interpretation as well, but that's just so you know, again, that's not what you do while you're describing core. You interpret it later, but we don't wanna make a separate video just for interpretation. When you do a see a sharp contact, it represents a change in depositional energy. If it's sharp and it's irregular, in most cases, that represents some sort of an erosional surface. That's why we're noting the contacts, we noted the sorting, we noted the grain size, we noted the thicknesses of the beds.