Dating by Stratigraphic Position

quote:

---

quote:

---

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

---

​

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?

---

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...

quote:

---

We expect layers to be horizontal.

---

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

quote:

---

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.

---

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.

quote:

---

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

---

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

quote:

---

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

---

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.

quote:

---

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.

---

This reminds me of something a YEC said on another board recently. It was something to the effect that there are chalk beds in Australia that correlate with the Cretaceous chalk of western Europe. Do you have any information on this? I have looked at my correlation charts and can find no such thing. Maybe it's time to go googling...

quote:

---

If there are chalk bed laid down at the same time is that the corrolation? Or are they chalk that is as similar as the chalk in England and norther france?

---

Yes. The is the "Chalk." I expect it has some fossil similarities, but many differences (such as thickness) as well, from other Cretaceous chalk beds.Sure, they correlate, but are they the same continuous deposit? Not many would say so.

quote:

---

To answer a very much earlier question, Ned, I would say that since organic bone is a very different material than a mineralized fossil, it should be easy to tell whether a fossil was eroded out of the original material and transported after fossilization. But, then, I am not a geologist.

---

It would certainly be possible to have transported fossils that look pristine, but usually the process of erosion and transport is quite destructive of rock materials compared to the depositional environment of fossil remains.