How the geo strata are identified as time periods

I've been realizing I really want to know how geologists are so sure that a particular stratum found anywhere on earth belongs to a particular time period. What are the methods for determining this? Fossil content? Relation to other layers? Are there some areas where it is hard to determine?I'd like to sit back, ask questions and otherwise watch on this one.August 29 at 5 pm Eastern This thread is not answering my question but has become a General Geology thread. Edited by Faith, : No reason given.

Thread moved here from the Proposed New Topics forum.

Just a few points I hope you will respond to.

1. unless there are signs that the area has been disturbed, a layer under another layer is likely older than the layer above?
2. by looking at the structure of a layer it is possible to get an idea of how it formed?
3. generally, unless there is some obvious other explanation, a thicker layer took longer to build up than a thinner layer of similar composition?
4. that layers which are made up of sediment require some other higher source be eroded to provide the material?
5. that obvious signs of missing layers indicate that material was worn away?

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Aslan is not a Tame Lion

Of course, this is obvious. Sounds reasonable, depends on what this means in practice. Not sure this is necessarily true. It seems to me that differing amounts of sediments could take quite different time periods to accumulate -- no reason the process has to be uniform. A deep layer might have been quite rapidly laid down and a thin layer could have taken much longer. I see no necessary formula here. I don't know what this means. This is very problematic, since what is often described as obvious signs are not at all obvious, among other questions about the idea.But I don't want my ideas to become the focus of this thread. I'd just like to hear some discussion of how layers are determined to belong in particular time periods of the Geologic Time Table.

Hello Everyone:I like Faith's interest in learning and Jar's slow, step by step approach.Please try to leave most of it to Jar other than some corrections or small suggestions about explanations so it doesn't move too fast or become overwhelming.

to which Faith replied:

quote:

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Sounds reasonable, depends on what this means in practice.

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So let's look at that in some detail.If you found a layer that was made of small individual parts that were somehow conglomerated into rock, what conclusions do you think you could draw?What questions might you ask?

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Aslan is not a Tame Lion

You are correct faith for the most part. A thicker layer can be deposited quicker than a thinner although what constitutes a layer gives us some evidence as to comparitive speed of deposition. I think jar's question was more appropriate assuming a constant rate of deposition. I would think this is obviously true. In order to have sandstone, you have to have sand. That sand had to come from somewhere either created by God or eroded from some other larger grained silicate material. For the purposes of this thread we can ignore the former.We don't have this kind of question concerning non-sedimentary rocks like volcanic rocks. There source there is always magma or lava. Although there you do have another relative dating principle which is how long it took the magma to cool. In the case of rocks formed from magma, sometimes there are large crystals of consolidated material that can only form if the magma cools slowly enough to allow the crystals to grow. This can give us an indication of the time it took to get from liquid to solid but not the age of the rock, but it helps in some relative dating.{As an aside, unless there was something fundamentally different about the laws of nature, the creation of the above mentioned crystals actually by itself falsifies a young earth as some of them require a magma body to cool slowly over millions of years. If the magma body cools faster then the crystals dont form or dont form of a given size. This can be shown directly. Another thread if you are curious. }The main way layers get dated is by association to layers that CAN BE dated absolutly. Anything igneous can be radioisotope dated and sometimes volcanoes are nice enough to leave a very large continuous layer of ash. The ash can be dated and then it gives one notch on the ruler from which you can start using relative dating above and below it to fill in the gaps.So if you have layers like:Lava flow
Sedimentary rock type 1
Sedimentary rock type 2
Sedimentary rock type 3
Ash layerThe lava and ash are dated absolutly {another aside, they always give dates that match older is deeper which is good evidence for the correctness of the method}. Then you can put other evidence into play to sandwhich the 3 sedimentary layers inbetween the two absolute dates.Other things that come into play are cross cutting relationships, index fossils, and unconformities. These again are used to get evidence based best guess dates for things that we cannot date absolutly.It would be good for you to read up on unconformities. It seems like you dont think they exist or something. This seems to be an opportunity for you to learn something that may help you when talking about this stuff. It is hard to discuss the geo column if you deny the existence of unconformities just like it would be hard to discuss genetics of the ToE if you denied the existence of mutations. These things DO exist and it would be wise not to deny it. Their existence is problematic for a scheme of global deposition because they represent a clear stoppage of deposition, removal of material, and continuance of deposition.How unconformities help in dating is that if you identify one, they can speak to the relative newness of the layers above and the oldness of the layers below. This depends on how extensive the unconformity is and if we can find the column in a surrounding area where the unconfirmity is different to compare. For example, in another area the unconfirmity might not exist and you can see the layers that had been removed. All this helps fine tune the picture created by relative dating.The main thing though is the cornerstone dates created by absolute dating. If I could use a ruler as an analogy, absolute dates are the inch marks. The relative dating would be the smaller subsections of it although my anaolgy breaks when you consider that the subsections in the geo column are not going to be regular like they are on a rule. That should be obvious though. Without absolute dating things get a lot fuzzier as to assigning a date to a rock.

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Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

Fine by me, so I'll just suggest that radiometric dating should be included.--Percy

What I'd really like to see on this thread is discussion of some very specific locations and formations and how they were assigned to particular time periods, actually rather than hypothetically. The question I had was really about how you determine at a given location whether and how the various layers represent the established time periods of the Geologic Time Table.That is, you come across these strata in a new location. How do you determine that one is Jurassic, another is Carboniferous etc etc.I mean, I'm aware that the sedimentary composition is not always the same from one location to the next. That pretty much leaves fossils as the indicator of the time period. But in some places it seems that while the order of the fossils may stay predictable things may change in the appearance of the layers or something like that.Also, using the fossils as an indicator would seem to beg the question in some cases, since you have defined in advance that a particular kind of fossil only occurs at such and such a depth. Edited by Faith, : No reason given.

Which I hope is where we will get, if not in this thread then in the followup thread.AbE:The idea of the fossils and all will come later, just as it did in real life, as well as well as absolute dating. I'd like us to start off with some of the basics just as the folk did who initially determined first relative ages and then absolute ages. The absolute age question though can a long long time after the relative ages was determined.For example your issue with thickness of a layer vs time is a great question and we WILL get to that.Edited by jar, : add material

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Aslan is not a Tame Lion

This will get out of hand if you try to jump ahead too fast.How about we let Jar pace it?

I hope I am not being too pushy by providing you one example out of thousands for purposes of potential further discussion.In the paper Geology and Coal Resources of the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico and Colorado by James E. Fassett, several methods are used to date a sequence of formations immediately near and including the Fruitland Formation in Northwest New Mexico. These methods include: correlation to type fossils, particularly ammonites; radiometric dating on included ash beds using the 40Ar/39Ar method; magnetic polarity reversal data, and rates of sediment accumulation/subsidence (although in this last case, not as a precise determinant of age but rather as examined rates that make sense under the model presented). Please pay particular attention to the section on "Summary of Chronostratigraphy."Sorry if the paper is technical in nature but geology is after all, a physical science that takes years to completely absorb (I somewhat know). Perhaps if you look at the paper, it may lead to questions that myself or others here may help answering.I hope that in answering this request, I am not attempting to move the thread too quickly.PS-There are more ways to date geologic formations than have been mentioned up to this point, but the above may be information overload enough for now as per admin. suggestion. Link to paper cited added by AdminJarEdited by AdminJar, : add link to cite

OK, guys, I can see you want me to work hard for this information; might as well go get me a degree in geology. I could look up all the articles about geology on the internet myself, you know, and struggle through them all to see if they discuss how strata were named at particular locations, which would take years. I've actually spent a fair amount of time in that sort of pursuit in my sojourn at EvC. So. Well. I don't know. I guess we can let this thread take it's good ol' time and I'll answer when I get around to studying the stuff. Or not.

I didn't realize Jar was an expert in these matters.

even if he's not, his first post seemed really well done. And it works for what Faith wants to do. So unless there does happen to be a geologist on the forum, that we know of or who will make himself known, I say let Jar handle this, eh?not that I have that much weight around here.

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All a man's knowledge comes from his experiences