Geologic Timeline

[TheFossilHunter]

Friends,

As a collector of 5 years, I am ashamed to say this, but I've just realized I am not sure of when certain geologic periods started and ended. This should be the first thing one learns, so Ive probably just lost like 150 cool points just by posting this, but there is so much conflicting data about this.

For example. Jurassic period started 199-200 million years ago according to Britannica and Encarta and many other sources. According to a bunch of other sources it started 206 or 208 million years ago. I understand there are revisions and such. But do you know what is the currently accepted number?

Same with when it ended. Some sources say 144, some 146 or even 148 or even 151. I ve always thought it was 144. Same with when Triassic started - a lot of confusion. As ashamed as I am to post this, I do want to get to the thruth.

let me know what you think.

Thanks

[MOROPUS]

...and same with Caenozoic (Pliocene example; when does it starts-more or less is 5 or 5.5 million years- and fisnishes?)

[Metopocetus]

http://www.stratigraphy.org/upload/ISChart2008.pdf

[Guest solius symbiosus]

I still use the old time scale, but I have been learning the revisions. Really though, for me, as long as I generally know the age, it is the relative position of the series and stages that are most important.

[TheFossilHunter]

I still use the old time scale, but I have been learning the revisions. Really though, for me, as long as I generally know the age, it is the relative position of the series and stages that are most important.

you are right Solius, the relative position of strata is what's important. this had been known for a long while before the absolute ages were determined. That's why when the were determined , geologists almost " didnt care" what the actual age was as long as the sequence of events stays the same.

But it still would be good to know what those absolute ages are!!

[TheFossilHunter]

http://www.stratigraphy.org/upload/ISChart2008.pdf

International comission on stratigraphy 2008 sounds solid. i am also inclined to this that these are the correct accepted ages. Does anyone have another opinion?

[Guest Nicholas]

I've found this scale to be very helpful.

[Guest solius symbiosus]

International comission on stratigraphy 2008 sounds solid. i am also inclined to this that these are the correct accepted ages. Does anyone have another opinion?

For my area, conodonts and graptolites aren't as specific as other faunal assemblages, so I will stick with older interpretations. The assemblages here are probably controlled more by tectonics than time, but hey, it works for me.

[Auspex]

The "divisions" were originally assigned to mark obvious faunal changes; with radiometric dating, we are now narrowing-down on the absolute "when".

[TheFossilHunter]

I've found this scale to be very helpful.

well this one doesnt tell you presicely when an age starts or ends. it's a very "rough" one. can only be used for approximate reference.

[ashcraft]

Ultimately, does it really matter "exactly" when the change occurred? Unless you are talking about a specific event that may have ended a time period such as a possible meteor strike at the end of the Cretaceous, then the event may have taken several years, such as the Ordovician extinction, which was probably caused by lowering of sea levels. Geologic time periods are a man-made division, and hence are open to interpretation. I am pretty general when I cover time periods, I say the ice ages appeared to end 10,000 years ago, the cretaceous ended a little over 60 million years ago, and the Permian extinction took place 220 million-ish years ago. Exact, certainly not, in the ball park as a time reference, certainly.

Put another way, if you are off by 10 million years on a fossil that is 250 million years old, you are off by less then 5 percent, or you are greater then 95 percent correct. That rates an "A" in my class.

Brent Ashcraft

[TheFossilHunter]

Ultimately, does it really matter "exactly" when the change occurred? Unless you are talking about a specific event that may have ended a time period such as a possible meteor strike at the end of the Cretaceous, then the event may have taken several years, such as the Ordovician extinction, which was probably caused by lowering of sea levels. Geologic time periods are a man-made division, and hence are open to interpretation. I am pretty general when I cover time periods, I say the ice ages appeared to end 10,000 years ago, the cretaceous ended a little over 60 million years ago, and the Permian extinction took place 220 million-ish years ago. Exact, certainly not, in the ball park as a time reference, certainly.

Put another way, if you are off by 10 million years on a fossil that is 250 million years old, you are off by less then 5 percent, or you are greater then 95 percent correct. That rates an "A" in my class.

Brent Ashcraft

Brent,

it all depends on your the circumstance. I do think it's important for everyone to use the same scale. yes divisions are man made, but they are tied to major extinction events, so in a way we aretalking about the time when those events occured. I agree they lasted a while, but they did not last millions of years (in case of the meteor strike at the end of cretaceous, the extinctions were much more rapid, definitely under one million years in duration). In case of triassic-jurassic boundary, the extinctions were even more rapid. Since we are not just unsing the relative time scale, bt talking about absolute timescale, it becomes important for everyone to be on the same page. By the way in Triassic jurassic boundary, this is even more important as it is possible to narrow down the age oof the deposits to several thousand years using Van Houten and Milankonich cycles. This was done in the newark basin deposits - lake regression rates were studied based on atmospheric changes. This allows to fine tune the strata.

For general overview of how life on earth developed 5% error doesnt matter, but for other purposes and more specific studies 5% error is huge.

[Auspex]

I think that, if I am talking in "Big Picture" scale, periods and epochs are appropriate, but if I want to be definitive time-wise, I would use "X million years ago". There really are two disciplines here, each best served by their own method.

[raptorclaws]

We usually use stages. such as 'uppermost Tournaision'...sometimes broad sweeps such as 'Lower Carboniferous. Never 'x' millions of years ago.....as observed wisely above, when studying paleozoic rocks 'what's a few millions of years. More important is the stratigraphic succession and as Solius points out, the relation of stages to each other.

There is a very interesting paper from about 25 years ago. I can't recall the publication. It involved a half dozen or so researchers who concentrated on Devonian paleontology. Each gave their rationale for what actual age they would put on the Devonian era. The difference among oldest and youngest was about 20 million years. Did a particular type of rugose coral make it's debut 380 or 360 mya?...largely irrelevent and speculation. What's useful is the evolutionary line of that coral and its use in biostratigraphy.

Unless someone is teaching geology or paleontology, they would probably know more the stages of 'the Jurassic' or 'whatever' than anyone on this forum. They tend to know their own age of research inside out and that's it.

[TheFossilHunter]

We usually use stages. such as 'uppermost Tournaision'...sometimes broad sweeps such as 'Lower Carboniferous. Never 'x' millions of years ago.....as observed wisely above, when studying paleozoic rocks 'what's a few millions of years. More important is the stratigraphic succession and as Solius points out, the relation of stages to each other.

There is a very interesting paper from about 25 years ago. I can't recall the publication. It involved a half dozen or so researchers who concentrated on Devonian paleontology. Each gave their rationale for what actual age they would put on the Devonian era. The difference among oldest and youngest was about 20 million years. Did a particular type of rugose coral make it's debut 380 or 360 mya?...largely irrelevent and speculation. What's useful is the evolutionary line of that coral and its use in biostratigraphy.

Unless someone is teaching geology or paleontology, they would probably know more the stages of 'the Jurassic' or 'whatever' than anyone on this forum. They tend to know their own age of research inside out and that's it.

Well, the stages are just a more detailed subdivision. the same question will apply to them.

The question of this thread is not " which is more important" but it is simple "when" expressed in absolute time.

While succession of events is important to many, when it actually happened is definetely interesting to know, whether its important or not. It may not matter how many million years ago something started as long as everyone agrees on it, but what is important is that there should be some agreement. while certain extinctions may be slightly shifted in time in different areas of the earth, such a major milestone as the jurassic period cannot be considered ok to be off by some 6 million years.

[Guest solius symbiosus]

... then the event may have taken several years, such as the Ordovician extinction, which was probably caused by lowering of sea levels.

HERE is a thread that illustrates the change in depositional environments, and resulting lithologies, that resulted from the Ordovician glaciation in far away Gondwana.

[Frank Menser]

Friends,

As a collector of 5 years, I am ashamed to say this, but I've just realized I am not sure of when certain geologic periods started and ended. This should be the first thing one learns, so Ive probably just lost like 150 cool points just by posting this, but there is so much conflicting data about this.

For example. Jurassic period started 199-200 million years ago according to Britannica and Encarta and many other sources. According to a bunch of other sources it started 206 or 208 million years ago. I understand there are revisions and such. But do you know what is the currently accepted number?

Same with when it ended. Some sources say 144, some 146 or even 148 or even 151. I ve always thought it was 144. Same with when Triassic started - a lot of confusion. As ashamed as I am to post this, I do want to get to the thruth.

let me know what you think.

Thanks

I've ben collecting for over 40 years and I still have trouble remembering my wife's birthday. Don't sweat the small stuff - that is what books, journals and updates are all about, to keep us from going nuts trying to remember all this stuff...

[TheFossilHunter]

I've ben collecting for over 40 years and I still have trouble remembering my wife's birthday. Don't sweat the small stuff - that is what books, journals and updates are all about, to keep us from going nuts trying to remember all this stuff...

Thanks Frank. Well this is a good candidate for another thread on this forum- you know you are a fossil collector if....you can't remember your wifes birthday but know by heart when all geologic periods started..

[Auspex]

Radiometric dating is slowly refining our understanding of the absolute dates of deposits. For example, the Yixian Formation ("feathered dinosaur" deposit) in China was long thought to be from the Late Jurassic-Early Cretaceous boundary (145 mya), but has recently been shown to be Barremian to early Aptian (125-121 mya). This makes a big, and important, difference in the study of its fauna. Maybe someday your cellphone will have a probe that runs a radiometric scan on whatever it is touching, but for now we're stuck with the "horseshoes and hand grenades" method of stratigraphy. The periods were originally named for deposits, with no idea at all of the actual time frame.

[Guest solius symbiosus]

... but for now we're stuck with the "horseshoes and hand grenades" method of stratigraphy.

??? Stratigraphy is a very precise science, and one of the oldest sub-disciplines of Geology.

http://www.uga.edu/~strata/

http://strata.geol.sc.edu/MapSite.html

[Auspex]

??? Stratigraphy is a very precise science, and one of the oldest sub-disciplines of Geology.

Amazingly so! I was speaking in contrast to the kind of "absolute dating" being sought by the topic's starter.

[ashcraft]

Brent,

it all depends on your the circumstance. I do think it's important for everyone to use the same scale. yes divisions are man made, but they are tied to major extinction events, so in a way we aretalking about the time when those events occured. I agree they lasted a while, but they did not last millions of years (in case of the meteor strike at the end of cretaceous, the extinctions were much more rapid, definitely under one million years in duration). In case of triassic-jurassic boundary, the extinctions were even more rapid. Since we are not just unsing the relative time scale, bt talking about absolute timescale, it becomes important for everyone to be on the same page. By the way in Triassic jurassic boundary, this is even more important as it is possible to narrow down the age oof the deposits to several thousand years using Van Houten and Milankonich cycles. This was done in the newark basin deposits - lake regression rates were studied based on atmospheric changes. This allows to fine tune the strata.

For general overview of how life on earth developed 5% error doesnt matter, but for other purposes and more specific studies 5% error is huge.

As somebody who has done much analytical work, I can tell you that 5% error on an analysis compared to a "true" value is pretty good, especially in an environmental sample where there are many problems with matrix, contamination, etc. If you have had chemistry, think back to significant digits. Your analytical results can be no more accurate then your least accurate component. If your analysis is off by 5 percent, then your conclusions are off by 5 percent.

Whenever you look at a study representing age, always look at the +/- values associated with the date that they give, it will give you a feel for how accurate the analytical results were to themselves (they should be testing multiple samples from the same matrix and using an average), but you also have to consider what results would be obtained from a different laboratory using the same technique, or even a different technique that can give similar conclusions.

I agree that some dates have been tested enough to get below the 5% error platform (which is statistically important when you are analyzing data for correctness), like the k/t boundary which has been tested ad-nauseum with many different methods. However, most events have not, and if somebody is saying this occurred at exactly at that time, give him the stink-eye.

Brent Ashcraft

[Auspex]

...and if somebody is saying this occurred at exactly at that time, give him the stink-eye.

<sorry, that just caught me off guard; I have tears on my cheeks!>

[JohnJ]

<... I have tears on my cheeks!>

Must be a symptom of "stink eye"; watch out for it.

[siteseer]

As somebody who has done much analytical work, I can tell you that 5% error on an analysis compared to a "true" value is pretty good, especially in an environmental sample where there are many problems with matrix, contamination, etc. If you have had chemistry, think back to significant digits. Your analytical results can be no more accurate then your least accurate component. If your analysis is off by 5 percent, then your conclusions are off by 5 percent.

Whenever you look at a study representing age, always look at the +/- values associated with the date that they give, it will give you a feel for how accurate the analytical results were to themselves (they should be testing multiple samples from the same matrix and using an average), but you also have to consider what results would be obtained from a different laboratory using the same technique, or even a different technique that can give similar conclusions.

I agree that some dates have been tested enough to get below the 5% error platform (which is statistically important when you are analyzing data for correctness), like the k/t boundary which has been tested ad-nauseum with many different methods. However, most events have not, and if somebody is saying this occurred at exactly at that time, give him the stink-eye.

Brent Ashcraft

Also, consider that the dates for the Pleistocene Epoch have a smaller "fudge factor" than the Paleocene. The use of mammal ages or foram stages and other "ages" shoot for more more precision within the epochs and having the magnetic reversal scale as another overlay has brought an incredible level of resolution in our lifetimes. I heard a paleontologist say that the absolute dates going back perhaps into the Cretaceous could sharpen to within a few centuries rather than millennia.

I think if you are really interested in fossils from a particular time unit, then you should try to stay on top of the latest dates but it's tough to follow it all, especially when experts quibble over a million years one way or the other in the Paleozoic Era within articles that can be elusive even when you know the authors and the title. The original question was a legitimate one. No one here should fear to ask an honest question like that.