Dating of sedimentary rocks

[GoodKat]

What methods are usually employed to date sedimentary rock? Wikipedia doesn't seem to have any information on the subject.

 

[scikidus]

What methods are usually employed to date sedimentary rock? Wikipedia doesn't seem to have any information on the subject.

I'd imagine you date things around the sedimentary rock and take it from there. Otherwise, no clue.

The first result on Google for "sedimentary rock dating" is a creationism website. :evil:

 

[GoodKat]

The first result on Google for "sedimentary rock dating" is a creationism website. :evil:

That's why I asked.

 

[Deleted member 499]

As far as I'm aware the only real way to do it is using relative techniques.

Generally a sedimentary rock will contain dateable fragments of its source rock in the form of zircons or garnets. Dating these will give an upper age. If it rests on a metamorphic or igneous basement then dating of that will also given an upper age. A similar principle can be used to get the lower age if it is subsequently overlain by (or contains) volcanic deposits.

Alternatively fossils can be used. There are charts around which give details on which species are found in which periods. A certain assemblage of fossils will correspond to a specific period in time when the sediment was deposited.

 

[orpiment99]

The upper limit is still pretty vague. The minerals that can be dated which may be present in a sedimentary rock can only be dated to their formation. It is often unknown what the source for the minerals was, thus leaving the amount of time those minerals were rolling around before being lithified into a sedimentary bed unknown. There are ways to generally tell how well worked a mineral in a sedimentary bed is, but this changes due to condition as well. That is why relative dating, if possible, or fossils are used. As fossil ages have been verified by radiometric dating, they provide the most accurate way to date a sedimentary bed. They typically are well studied, especially index fossils, and their morphological changes indicate the approximate age.

 

[e2iPi]

The first result on Google for "sedimentary rock dating" is a creationism website. :evil:

They are pervasive little fuckers aren't they?

Try scholar.google.com. I got several excellent results with those terms.

e2iPi

 

[GoodKat]

As fossil ages have been verified by radiometric dating, they provide the most accurate way to date a sedimentary bed.

Could you elaborate on that? What radiometric dating methods can be used on fossils?

 

[orpiment99]

We can't radiometrically date fossils. We can, however, date igneous layers that occur above or below the sedimentary layers the fossils occur in or, as FAJA pointed out, on ash layers. Do this in enough places and you have a very precise (to a geologist, anyway) age spread for the fossil.

Once we have some dates, we can use other methods to age the fossils that occur in between the ones we have dates for. Rates of chalk accumulation, for example.