How do we date sedimentary rocks?

[Atheos canadensis]

In the old uranium-lead dating thread there were some side-discussions involving the dating of sedimentary rock. Certain members espoused an unconventional view in that regard which I would like to investigate.

The conventional understanding is that one cannot directly radiometrically date a sedimentary rock like sandstone because the consitutent grains would all provide different ages. Thus we must resort to dating igneous or metamorphic rocks that bracket the sedimentary layer of interest.

It was stated in the old thread that it is in fact possible to radiometrically determine the age of a sandstone but elaboration on this point was not forthcoming. I hope therefore that this thread will provide a convenient place for such elaboration.

 

[Subduction Zone]

In the old uranium-lead dating thread there were some side-discussions involving the dating of sedimentary rock. Certain members espoused an unconventional view in that regard which I would like to investigate.

The conventional understanding is that one cannot directly radiometrically date a sedimentary rock like sandstone because the consitutent grains would all provide different ages. Thus we must resort to dating igneous or metamorphic rocks that bracket the sedimentary layer of interest.

It was stated in the old thread that it is in fact possible to radiometrically determine the age of a sandstone but elaboration on this point was not forthcoming. I hope therefore that this thread will provide a convenient place for such elaboration.

I remember an article that one person linked about a mineral that formed during the lithification of sandstone. That is not a dating of time of deposition, that is a dating of an event in the history of the rock itself.

There may be methods of directly dating of some sedimentary rocks. I have not been active for many years in that area. When I was taught there were no such methods.

 

[Atheos canadensis]

I remember an article that one person linked about a mineral that formed during the lithification of sandstone. That is not a dating of time of deposition, that is a dating of an event in the history of the rock itself.

There may be methods of directly dating of some sedimentary rocks. I have not been active for many years in that area. When I was taught there were no such methods.

That's what I was taught too. I would think dating the cementing mineral would give you an approximate date, but a very rough one. This paper talks about the difficulties of dating a quartz matrix.

Quartz is chemically very pure and does not contain tracers that might provide information on the source of the silica or that might allow radiometric dating. The 1So/~60 ratio of the mineral can be analysed, but all current techniques require several milligrams of sample. The physical and chemical properties of the quartz cement and sand grains are so similar that it is impossible to isolate a sample of pure quartz cement free from contamination by detrital quartz. Attempts to calculate end-member isotope ratios from analyses of mixtures of grains and cement suffer from a combination of poor precision and indeterminate accuracy.

It goes on to discuss ways of estimating the age that focus on the duration of cement formation but concludes that such techniques significantly underestimate the actual age of the sandstone. Granted the paper is from 1991. This paper from 2004 summarizes a variety of common ways of trying to date sedimentary rocks by the cementing minerals and for the most part they have problems, often returning both over- and underestimates of the depositional age. It does mention that "a number of studies on Precambrian limestones have yielded Pb/Pb isochron dates, variously interpreted to be early diagenetic, and a close approximation of the depositional age", though it goes on to say that there are problems but that "Despite the potential limitations, the technique may provide dates that approximate the age of deposition (Woodhead et al., 1998), and in many cases, yield diagenetic dates that can be treated as useful minimum ages."

It promotes the use of U-Pb dating of xenotime crystals because they are widespread in siliciclastic sedimentary rocks. They conclude that this method is reliable for determining the age of diagenetic events and thus solid minimum depositional ages for the rocks in question.


All this is cool, but nothing I have found seems to be talking about radiometrically dating a sandstone to directly measure its age. The best techniques seem to furnish a minimum estimate (though the oldest results are sometimes considered a good approximation of the maximum age). I'm not seeing anything like the direct dating that was mentioned in the old thread.

 

[Subduction Zone]

Most sedimentary rocks have an assemblage of fossils. Many of them will be widespread in extent. Each particular fossil will usually have an "earliest observed date" and sometimes a "latest observed date". A very good estimate can be achieved by using index fossils. Especially useful are microscopic index fossils. Microscopic life evolves quicker than macroscopic life so it allows a finer dating.

 

[TLK Valentine]

Very interesting material.

(Is it bad that I saw the thread title and my first thought was "dinner and a movie"?)

 

[Atheos canadensis]

Most sedimentary rocks have an assemblage of fossils. Many of them will be widespread in extent. Each particular fossil will usually have an "earliest observed date" and sometimes a "latest observed date". A very good estimate can be achieved by using index fossils. Especially useful are microscopic index fossils. Microscopic life evolves quicker than macroscopic life so it allows a finer dating.

True. Unfortunately that doesn't work very well in Precambrian strata where some of these techniques are focused. And of course it still doesn't get us any closer to an example of directly radiometrically dating a sedimentary rock.

 

[Subduction Zone]

True. Unfortunately that doesn't work very well in Precambrian strata where some of these techniques are focused. And of course it still doesn't get us any closer to an example of directly radiometrically dating a sedimentary rock.

Another problem with Precambrian strata is that they are relatively rare. The further back we go the more likely that in anyone location that in some time of the last 600 million of years it was eroded down to the igneous or metamorphic basement rocks.

 

[juvenissun]

In the old uranium-lead dating thread there were some side-discussions involving the dating of sedimentary rock. Certain members espoused an unconventional view in that regard which I would like to investigate.

The conventional understanding is that one cannot directly radiometrically date a sedimentary rock like sandstone because the consitutent grains would all provide different ages. Thus we must resort to dating igneous or metamorphic rocks that bracket the sedimentary layer of interest.

It was stated in the old thread that it is in fact possible to radiometrically determine the age of a sandstone but elaboration on this point was not forthcoming. I hope therefore that this thread will provide a convenient place for such elaboration.

The key idea is to define the "age" of a sedimentary rock. Once that is defined, then proper dating method could be considered.

From the petrological point of view, the time of deposition is NOT the beginning of a sedimentary rock.

 

[Loudmouth]

The key idea is to define the "age" of a sedimentary rock. Once that is defined, then proper dating method could be considered.

From the petrological point of view, the time of deposition is NOT the beginning of a sedimentary rock.

The problem is that you reject methodologies because they give you dates you don't like.

 

[Atheos canadensis]

The key idea is to define the "age" of a sedimentary rock. Once that is defined, then proper dating method could be considered.

From the petrological point of view, the time of deposition is NOT the beginning of a sedimentary rock.

Ah good, this is more or less where we left off. So what is your definition of the age of a sedimentary rock? By "from the petrological point of view" are you saying that the age of a sedimentary rock should be properly defined as the time when cementation occurs?

 

[juvenissun]

Ah good, this is more or less where we left off. So what is your definition of the age of a sedimentary rock? By "from the petrological point of view" are you saying that the age of a sedimentary rock should be properly defined as the time when cementation occurs?

Is a piece of hardened mud a rock?
Has the mud been "cemented" together by drying?

 

[Atheos canadensis]

Is a piece of hardened mud a rock?
Has the mud been "cemented" together by drying?

No. Dried mud is not a rock. Cementation is a process of minerals precipitating out of pore water between the grains, thus binding them together. Are you suggesting that the time of cementation is the proper measure of a rock's age or not?

 

[juvenissun]

No. Dried mud is not a rock. Cementation is a process of minerals precipitating out of pore water between the grains, thus binding them together. Are you suggesting that the time of cementation is the proper measure of a rock's age or not?

Yes. But it is one of the options.
But, not all (sedimentary) "rocks" are cemented. So, what about those "rocks" not cemented? Do they have age?

 

[Loudmouth]

So, what about those "rocks" not cemented? Do they have age?

Obviously, they do have an age. The question is whether or not we have a methodology capable of measuring that age. Index fossils are useful. U/Th dating of shells or coral in the sediments could be used. Carbon dating of terrestrial organic matter was used to date the sediments in lake varves as another example.

 

[Subduction Zone]

Yes. But it is one of the options.
But, not all (sedimentary) "rocks" are cemented. So, what about those "rocks" not cemented? Do they have age?

The problem with a "cementation" age is that it is of very little use.

On the farm where I grew up on there are outcrops of the St. Peter Sandstone. It is a very loosely, if at all, cemented sandstone. When exposed to the elements it forms a crust of loosely cemented sand. Once you get through that crust the tool that is used to sample the St. Peter Sandstone is a shovel. Yet there are cemented strata found below and above the St. Peter. According to your "cementation" date those are both older than the St. Pete since they were cemented longer ago in the past.

The best and most useful date is when the sediments were deposited. That is what scientists usually want to know. When was it laid down? What flora and fauna were in existence at that time?

That is a much more consistent method of dating sedimentary rocks.

 

[Atheos canadensis]

Yes. But it is one of the options.
But, not all (sedimentary) "rocks" are cemented. So, what about those "rocks" not cemented? Do they have age?

As has been pointed out, of course they have an age. But not one that can be radiometrically determined with any precision. So how do you radiometrically date such uncemented rocks?

 

[That Guy 11200]

There's optically stimulated luminescence dating, which only works up to about 100,000 years, for mineral grains that were sufficiently exposed to sunlight and have been exposed to ionising radiation from radioactive elements in the sediment, or to cosmic rays. Structural flaws in the minerals contain an electric charge after exposure to the radiation, which can be measured when the sample is exposed to bright light of a certain wavelength.

There's also thermoluminescence dating which is similar, except the sample is heated instead of being exposed to a bright light.

Edit: Wow, I didn't realise I'd been lurking for that long.

 

[juvenissun]

As has been pointed out, of course they have an age. But not one that can be radiometrically determined with any precision. So how do you radiometrically date such uncemented rocks?

The same way as dating any other material.

 

[Atheos canadensis]

The same way as dating any other material.

Please expand on that response. The way we radiometrically date other materials cannot be applied to sedimentary rocks becuase the constituent grains will have different ages. So if there is no cement to radiometrically date, how do you propose it is done?

 

[Loudmouth]

The same way as dating any other material.

Does U/Pb age of the zircons in a granite kitchen countertop tell you how old the house is?