Dating by Stratigraphic Position

The river deposit would easily be recognized as consisting of detritus derived from older rock.Although the individual clasts could be of a wide range of ages, collectively the sediment would be younger than what it overlies.Moose

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The river deposit would easily be recognized as consisting of detritus derived from older rock.

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Out of curiosity, how is this done. Is it the mineral makeup of the actual fossil (difference in permineralization between origination and deposition, silt vs. carbonate) or the overall condition of the fossil?

We're not quite at fossils yet even though they seem to be creeping in, we are still trying to make sense of the geology part. So if it's okay with you, can you hold that thought until later and then I think it would be a great issue.

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Aslan is not a Tame Lion

OK, back to layers. We agree that an undisturbed higher layer is younger than it's underlying layers. How do I tell if it is undisturbed?

Could you tell from consistency, what it is made of, whether it is regular and distinct from the layers above and below?

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Aslan is not a Tame Lion

We expect layers to be horizontal. If they are not, then we could probably conclude that something has disturbed them after the lithified. In fact, by tracing the layers over a large area we can sometimes trace out that the layers have been pushed together to give a folded appearance.We can also check to see if the minerals in the rocks are the type to have been formed under heat and pressure - metamorphic rocks, for those in the know.

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The river deposit would easily be recognized as consisting of detritus derived from older rock.

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Out of curiosity, how is this done. Is it the mineral makeup of the actual fossil (difference in permineralization between origination and deposition, silt vs. carbonate) or the overall condition of the fossil?

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Actually, there are probably more ways than can be mentioned in a short space. The shape of the deposit, the composition and heterogeneity of its constituent grains, the primary textures of the deposit, the cross-cutting nature of the contacts and the absolute grain sizes all come to mind. Fossil evidence could easily fit in as well. Basically, these are things that geologists are trained to do and it is a pretty basic task. Of course, we are all brainwashed...

I think that this is the crux of the matter. Large areas, looking for correlations between layers.

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We expect layers to be horizontal.

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Actually, not in all cases, but for a first approximation this is okay.

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If they are not, then we could probably conclude that something has disturbed them after the lithified. In fact, by tracing the layers over a large area we can sometimes trace out that the layers have been pushed together to give a folded appearance.
We can also check to see if the minerals in the rocks are the type to have been formed under heat and pressure - metamorphic rocks, for those in the know.

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I think what you are saying is that you would expect the usual primary depositional characteristics to be deformed or eradicated and a secondary structure/texture would be imposed. Again this is normally a pretty basic task for geologists. The only issue might be if there were, for some reason, no original primary features in the rock anyway. In that case, we would use surrounding information to interpret some kind of disturbance. It is always on the list of possiblities.

How big an area are these layers? Do they cross major divisions like an ocean?

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Aslan is not a Tame Lion

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I think that this is the crux of the matter. Large areas, looking for correlations between layers.

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It's called geological mapping, a basic tool that is often negelected in the age of mass-spec, microprobes, 3D seismics, and isotopic geology.

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How big an area are these layers? Do they cross major divisions like an ocean?

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Usually not. Plate tectonics forbids it. Some similar depositional environments can be correlated on a continental scale and some even farther. For instance the Cretaceous rise in sea level is global, so it may look like some units can be traced from continent to continent.

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Actually, not in all cases, but for a first approximation this is okay.

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Hi, edge.Yeah, I thought the intent of this thread was to start with a simple model of statigraphy, ala Steno, and then add complexities as we go along. Notice that I didn't mention the use of index fossils to correlate layers.And I knew we wouldn't expect all "layers" to be horizontal, but I couldn't think of an example. But your posts reminded me of one: alluvial fans!

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How big an area are these layers? Do they cross major divisions like an ocean?

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Some do - for a period in time (actually there were at least two periods) all the continents were joined together, so the continents were connected together.

Does anyone else have the feeling we are all just talking to ourselves once again?

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Aslan is not a Tame Lion