The Organization of Geologic Time

[Exalt]

Hello, everyone. This is my first thread so please bear with me.

I would imagine that most people who would make it to a site like this are familiar with the fact that our understanding of "deep time" as I saw it referred to in my attempts to research this before asking, divides the Earth's history into parts, and parts within parts, etc.

Now, I don't know about rocks, for one thing, and the writing on this subject seems to be very, very technical. I don't disagree with how we do this, as of now, or with the configurations given, but I would like to understand why we have it the way we do, especially at the period level.

I understand, of course, that some of the distinctions, both in era and period, are marked by Big Five extinction events, and I won't disagree with that. What I'm looking for is the everything else: what does or doesn't qualify for some kind of shift, and why: for example, there was a mid-Permian extinction, but the before and after are both one period. I also am familiar with the fact that the Carboniferous is named due to an influx of, well, Carbon.

As far as I know, no single animal species survives a whole period. So for one, what makes the beginning and end of a period, the same period? Additionally, I've noticed that with the Cenozoic, we broadly tend to discuss epochs rather than periods. Is that one simply because we can see the changes in more recent time more clearly?

I hope I managed to make myself a bit clear here, it was a bit difficult trying to detail what all I am asking here.

Edit: I appeared to have used epoch at one point where I meant period, so I have adjusted it.

[theropod]

Aug 24, 2023 7:31:16 GMT 5 Exalt said:

Now, I don't know about rocks, for one thing, and the writing on this subject seems to be very, very technical. I don't disagree with how we do this, as of now, or with the configurations given, but I would like to understand why we have it the way we do, especially at the period level.

Yes it’s a little complicated, mostly because there are few definite, objective rules of the kind you seem to be asking for. You can always get the most recent official geological time scale on the website of the international commission on stratigraphy, which is the international authority that decides about this stuff: stratigraphy.org/chart#latest-version

Basically, most things about stratigraphy are simply a matter of tradition. There are of course changes and events that are used to define the borders of stratigraphic units, but which ones exactly are chosen for that simply breaks down to what’s commonly accepted. One relatively solid rule though is that the lower border of each unit of geological time should be defined by a type section (a so called GSSP), in which the transition into that unit can be observed and defined, usually biostratigraphically, i.e. by the first or last occurrence of some sort of index fossil.

I understand, of course, that some of the distinctions, both in era and period, are marked by Big Five extinction events, and I won't disagree with that. What I'm looking for is the everything else: what does or doesn't qualify for some kind of shift, and why: for example, there was a mid-Permian extinction, but the before and after are both one epoch. I also am familiar with the fact that the Carboniferous is named due to an influx of, well, Carbon.

Most of these geological units are very old, in fact far predating numerical dating. "Carboniferous" for example was first used towards the end of the 18th century. People were familiar with there being certain rock strata that were coal bearing (i.e. carboniferous), which had obvious relevance to them, long before they had a precise idea of when they were formed or even what else happened in that period. The official designation of the carboniferous as a period came much later.

Usually people simply started by grouping easily distinguishable rock units (Carboniferous, Cretaceous etc.) together, and these names sometimes stuck and were kept to denote geological periods later on. Other rock units weren’t as internationally applicable, and hence didn’t become the namesakes of international geologic periods, but only regional lithostratigraphic units (groups or formations). There’s no really cut and dry rule what makes something a period and what doesn’t though, it’s all historically grown.

for example, there was a mid-Permian extinction, but the before and after are both one epoch.

This is quite commonplace. There were also major extinctions in the Eocene, the lower Jurassic, the Turonian, the upper Devonian, several massive extinctions in the Cambrian and Ordovician etc., none of these represent the end of their respective periods. The general tendency is that the rules don’t really apply to the Cambrian (because of the Cambrian radiation and the very quick biotic changes that came with it, that would have to be split into a ridiculous number of ridiculously short periods otherwise), and for other periods, usually a big five extinction will mark an end of a period, but a "smaller" mass extinction might only mark the boundary of an epoch (e.g. Eocene) or a stage (e.g. Turonian).
Then we have eras, which tend to be characterized by a characteristic faunal (and floral) composition (which is also ultimately subjectively defined, but this still correlates well with how eras are separated by particularly large mass extinction events that caused particularly high levels of biotic change).

As far as I know, no single animal species survives a whole period. So for one, what makes the beginning and end of a period, the same period?

Basically just what I wrote, tradition. It would be terribly confusing if now that everybody is agreed on what the Jurassic or the Carboniferous is, we’d just redefine it somehow.

Additionally, I've noticed that with the Cenozoic, we broadly tend to discuss epochs rather than periods. Is that one simply because we can see the changes in more recent time more clearly.

Basically yes. Of course what’s the most useful depends on the context, but being more recent, and much shorter, and studied in excessive detail by very detail-oriented mammal-people, there tends to just be a higher temporal resolution for the cenozoic than there is for the mesozoic.

[Exalt]

Oh I'd probably never ask to change it, that would cause many future headaches. I'm merely trying to understand why, how, etc.

I certainly get the impression that some of these things predate our understanding of deep time entirely, which may explain some of the oddities.

Aug 24, 2023 22:43:43 GMT 5 theropod said:

This is quite commonplace. There were also major extinctions in the Eocene, the lower Jurassic, the Turonian, the upper Devonian, several massive extinctions in the Cambrian and Ordovician etc., none of these represent the end of their respective periods. The general tendency is that the rules don’t really apply to the Cambrian (because of the Cambrian radiation and the very quick biotic changes that came with it, that would have to be split into a ridiculous number of ridiculously short periods otherwise), and for other periods, usually a big five extinction will mark an end of a period, but a "smaller" mass extinction might only mark the boundary of an epoch (e.g. Eocene) or a stage (e.g. Turonian).
Then we have eras, which tend to be characterized by a characteristic faunal (and floral) composition (which is also ultimately subjectively defined, but this still correlates well with how eras are separated by particularly large mass extinction events that caused particularly high levels of biotic change).

So what causes the changes in the rock layers? All I know is that some places on Earth have a literal white line marking the KPG event.

And what defines the non-big five transitions? For example, I've not seen much discussion of the Silurian-Devonian and Jurassic-Cretaceous transitions. All I've heard is that sea levels rose during the latter.

[theropod]

Yes, they absolutely do predate our understanding of deep time. Terms such as "Carboniferous" are from a time when geologists were just beginning to realize that the earth must me much older than the bible claims, but still wouldn’t have an actual quantitative idea of how old that is for centuries to come.

Changes in (sedimentary) setting always bring with them changes in the rock layers. For example if you have a rising sea level, in the same place that was previously a beach, you may have deeper marine sediments, in the same place that was previously a desert, you might have coastal sediments and so on. So if you have a major environmental change, you are going to see that reflected in the rock layers, as well as (of course) the fossils you find in those layers. In the case of the K/pg event, there are additional demarcations, especially the iridium anomaly and tectite layer, that are directly tied to the Chicxulub impact.
As for how the beginning of the Devonian is defined, see here: stratigraphy.org/gssps/lochkovian
Apparently it is primarily defined by the earliest ocurrence of a species of graptolite.