Geologic Principles for Paleontological Discussions
Mar 14, 2021 23:02:30 GMT 5
Life and theropod like this
Post by creature386 on Mar 14, 2021 23:02:30 GMT 5
I made this thread as I thought a certain level of geology could be useful for the paleontologically inclined people. Moreover, we do have a thread about the Geological Time Scale, so, why not go into more depth? It's similar to theropod's "Basics" thread in a way.
As of now, this is a draft I wrote in a few hours today. It's gonna get be grammarly-read later.
Until now, a few questions remain.
Is this thread technical enough or too technical for its purposes?
Is any information factually incorrect (like I said, it was made a bit quickly)?
Is the information presented useful?
There are other relevant questions, of course, but these are all I could think off the top of my head.
Anyway, enjoy this little essay!
As a scientific discipline, paleontology is notoriously hard to classify. We are clearly most interested in the biological aspects. However, paleontology has undeniably geological aspects to it as well. Since I’m sure that a lot of people here are interested in paleontology without caring much for geology, this thread is intended as a crash course for any paleo-enthusiasts who might struggle with the geo parts.
Stratigraphy
Stratigraphy is the scientific study of rock layers (“strata”) and by far the most important field of geology for a paleontologist. A stratum is a rock layer with a uniform chemical, lithological or biological composition that is clearly distinct from other layers. What does that mean? Well, imagine you are standing in front of an outcrop/quarry and you look at the rock before you. You see that the bottom of it is brown and composed of homogenous carbonate mud with ammonite fossils. Above it is another type of rock that’s all yellow quartz sand with bivalves and cross-bedding sedimentary structures that you don’t find further below. Both these sections of rock are clearly distinct from one another and it’s clear what types of the outcrop belong to them. Since they are so different, they probably formed in different environments and thus at different times. For obvious reasons, you can assume that the bottom layer formed first (unless the layers got flipped which sometimes happens, but I won’t cover tectonics here). You can even determine their absolute age through radiometric dating, but I won’t cover that here, as that’s what the geochemists do, not the paleontologists.
What stratigraphers do is to correlate rock layers to find out which have been formed at the same time and which did not. The identification and naming of recognizable rock layers is the job of chronostratigraphy while geochronology gives these layers their ages.
At some point, a stratum is thick enough that it can be recognized all across the world, at which point we call it a stage which always has a corresponding age in geochronology. An example of such a stage is the Maastrichtian which corresponds to an age of 72 to 66 mya. Several stages form a series (corresponding to an epoch), and several series form a system (corresponding to a period) with the highest type of chronostratigraphic classification being called Eonothem (which corresponds to an Eon in geochronology).
Let’s look at an example:
Eonothem/Eon: Phanerozoic (541 mya - present)
Erathem/Era: Mesozoic (252 - 66 mya)
System/Period: Cenozoic (145 - 66 mya)
Series/Epoch: Upper Cretaceous (100 - 66 mya)
Stage/Age: Maastrichtian (72 - 66 mya)
Fortunately, you don’t have to memorize those as all important units of stratigraphic classification are recorded in the ICS’s Geological Time Scale:
theworldofanimals.proboards.com/thread/3598/geological-timescale
Out of breath?
I hope not, because before I close this section, I’d like to discuss various ways in which scientists can tell that two sections of rock found in different parts of the world are part of the same stratum.
There are a bunch of subdisciplines of stratigraphy dedicated to this question:
Biostratigraphy is the easiest to explain. It correlates rock layers based on so-called index fossils, that is, fossils that are abundant, globally distributed, easily recognizable and short-lived/characteristic of a very specific time. An example is the bivalve Argopecten gibbus which is characteristic of the Quaternary.
Here is a list of common index fossils:
This is an easy and reliable means of stratigraphic correlation.
Lithostratigraphy identifies and describes strata based on their mineral composition, their grain size and their crystal types. This probably isn’t a good idea for global correlation, but it’s fairly easy to do and popular in regional map-making.
Magnetostratigraphy is based on geomagnetic reversals. For a simple explanation, several minerals on the seafloor act like tiny compass needles that align themselves with the current magnetic field. The magnetic field changes its polarity every 50,000 - 800,000 years and through looking at those tiny ancient “compass needles”, we can determine if two layers got deposited during the same magnet field conditions or not.
Event stratigraphy compares rock layers based on global events specific to a certain time period. For example, strata at the K-Pg boundary can be identified by unusually high iridium concentrations which is a remnant of the Chicxulub impact.
Isotope stratigraphy compares the ratios of specific isotopes (like O18/O16 or C13/C12) which are normally indicative of global climate change events.
There are more, but these are the most important ones.
Sedimentology
I promise this section will be shorter than the last one.
Geology knows three types of rocks. Igneous rocks, metamorphic rocks and sedimentary rocks. Sedimentary rocks are all rocks formed through chemical/biological precipitation or through mechanical erosion and deposition. Why do we care? Because they are the only types of rocks that can preserve fossils.
What sedimentologists do is to classify and correlate sediments (in other words - stratigraphy; we covered that!) and study the conditions under which they form. The latter part is the most interesting one. If you find a dinosaur, you’ll want to know if it lived in a forest or a desert, don’t you? As it happens, forest-adjacent and arid environments also just so happen to form different sediments.
By far the most important concept for terrestrial sediments to understand is that of grain size and depositional energy. Here’s an intuitive example:
Suppose you have a river that flows really fast. All the small grains get washed away easily while only the largest sink to the bottom.
If the river flows more slowly, you won’t have so many big grains as they didn’t make it in the current to begin with (or they sank down much earlier)! Instead, you’ll see the small grains sink down. There are more principles, but this is one of the most important ones.
The most important grain sizes are gravel which is larger than sand which is larger than silt which is larger than mud/clay.
Here are some common terrestrial depositional environments:
Mountain environment = Normally, the grain sizes are all over the place here, but they tend towards the large end (e.g. lots of gravel).
River = Normally full of sand (and some silt) with so-called cross-beddings which are remnants of little dunes formed by the water current
Desert = Also sandy, but a bit smaller on average and with rounder grains than in a river and with a lot bigger dunes.
Lake = Normally full of mud arranged in laminar layers.
(Of course, you can cheat a bit by using characteristic fossils here as well.)
Marine environments are trickier. Generally, grain sizes are bigger the closer you are to the beach as the effect of waves is larger there.
Here, besides sand, silt and so on, you also have to deal with carbonates here. Carbonate rocks consist of a matrix of fine biological mud and of grains which are normally fossils trapped in the mud. Carbonates deposited close in high-energy marine environments (like beaches) have a lot of grains and very little mud while carbonates from low-energy marine environments (like lagoons) have a lot of mud and less grains.
Taphonomy
Finally, we get to taphonomy and the science of how fossils are formed. Generally, when an organism dies and turns into a fossil, three things happen.
First, the organism dies. The cause of its death and the processes leading to it are called necrotic processes.
Then, the organism is moved to the place of its burial and somehow has to get in the sediment which we call biostratinomy.
Finally, its preservable parts become turned into minerals which is called diagenesis.
The point behind taphonomy is threefold. First, we want to identify biases in the fossil record. For obvious reasons, vertebrate and echinoderm endoskeletons, arthropod exoskeletons, mollusk shells and plant pollen, spores or wood are overrepresented in the fossil record while organisms that lack those are found less frequently.
Secondly, learning about the cause of an organism's death helps us understand how it lived. It makes a difference whether an animal died of old age or whether it got killed by a predator.
Finally, it is important, especially for paleoenvironment reconstruction, to know where an organism died. Organisms that died in the same place in which they got preserved are autochthonous. They are allochthonous otherwise. Determining transport isn’t always easy, but there are a couple of ways we can determine such circumstances nonetheless. Sessile organisms like corals are normally preserved in-situ, so are organisms that used to live or bore in them at the time of their deaths. Likewise, several organisms (like bivalves) show very specific deposition patterns if they got transported by a storm, for example.
EDIT: Instead of this little lecture, maybe it'd be better to include a glossary of geological terms that paleontological papers sometimes use. Maybe I'll do both. It shouldn't be a surprise that this post is going to get heavily edited in the future.
As of now, this is a draft I wrote in a few hours today. It's gonna get be grammarly-read later.
Until now, a few questions remain.
Is this thread technical enough or too technical for its purposes?
Is any information factually incorrect (like I said, it was made a bit quickly)?
Is the information presented useful?
There are other relevant questions, of course, but these are all I could think off the top of my head.
Anyway, enjoy this little essay!
As a scientific discipline, paleontology is notoriously hard to classify. We are clearly most interested in the biological aspects. However, paleontology has undeniably geological aspects to it as well. Since I’m sure that a lot of people here are interested in paleontology without caring much for geology, this thread is intended as a crash course for any paleo-enthusiasts who might struggle with the geo parts.
Stratigraphy
Stratigraphy is the scientific study of rock layers (“strata”) and by far the most important field of geology for a paleontologist. A stratum is a rock layer with a uniform chemical, lithological or biological composition that is clearly distinct from other layers. What does that mean? Well, imagine you are standing in front of an outcrop/quarry and you look at the rock before you. You see that the bottom of it is brown and composed of homogenous carbonate mud with ammonite fossils. Above it is another type of rock that’s all yellow quartz sand with bivalves and cross-bedding sedimentary structures that you don’t find further below. Both these sections of rock are clearly distinct from one another and it’s clear what types of the outcrop belong to them. Since they are so different, they probably formed in different environments and thus at different times. For obvious reasons, you can assume that the bottom layer formed first (unless the layers got flipped which sometimes happens, but I won’t cover tectonics here). You can even determine their absolute age through radiometric dating, but I won’t cover that here, as that’s what the geochemists do, not the paleontologists.
What stratigraphers do is to correlate rock layers to find out which have been formed at the same time and which did not. The identification and naming of recognizable rock layers is the job of chronostratigraphy while geochronology gives these layers their ages.
At some point, a stratum is thick enough that it can be recognized all across the world, at which point we call it a stage which always has a corresponding age in geochronology. An example of such a stage is the Maastrichtian which corresponds to an age of 72 to 66 mya. Several stages form a series (corresponding to an epoch), and several series form a system (corresponding to a period) with the highest type of chronostratigraphic classification being called Eonothem (which corresponds to an Eon in geochronology).
Let’s look at an example:
Eonothem/Eon: Phanerozoic (541 mya - present)
Erathem/Era: Mesozoic (252 - 66 mya)
System/Period: Cenozoic (145 - 66 mya)
Series/Epoch: Upper Cretaceous (100 - 66 mya)
Stage/Age: Maastrichtian (72 - 66 mya)
Fortunately, you don’t have to memorize those as all important units of stratigraphic classification are recorded in the ICS’s Geological Time Scale:
theworldofanimals.proboards.com/thread/3598/geological-timescale
Out of breath?
I hope not, because before I close this section, I’d like to discuss various ways in which scientists can tell that two sections of rock found in different parts of the world are part of the same stratum.
There are a bunch of subdisciplines of stratigraphy dedicated to this question:
Biostratigraphy is the easiest to explain. It correlates rock layers based on so-called index fossils, that is, fossils that are abundant, globally distributed, easily recognizable and short-lived/characteristic of a very specific time. An example is the bivalve Argopecten gibbus which is characteristic of the Quaternary.
Here is a list of common index fossils:
This is an easy and reliable means of stratigraphic correlation.
Lithostratigraphy identifies and describes strata based on their mineral composition, their grain size and their crystal types. This probably isn’t a good idea for global correlation, but it’s fairly easy to do and popular in regional map-making.
Magnetostratigraphy is based on geomagnetic reversals. For a simple explanation, several minerals on the seafloor act like tiny compass needles that align themselves with the current magnetic field. The magnetic field changes its polarity every 50,000 - 800,000 years and through looking at those tiny ancient “compass needles”, we can determine if two layers got deposited during the same magnet field conditions or not.
Event stratigraphy compares rock layers based on global events specific to a certain time period. For example, strata at the K-Pg boundary can be identified by unusually high iridium concentrations which is a remnant of the Chicxulub impact.
Isotope stratigraphy compares the ratios of specific isotopes (like O18/O16 or C13/C12) which are normally indicative of global climate change events.
There are more, but these are the most important ones.
Sedimentology
I promise this section will be shorter than the last one.
Geology knows three types of rocks. Igneous rocks, metamorphic rocks and sedimentary rocks. Sedimentary rocks are all rocks formed through chemical/biological precipitation or through mechanical erosion and deposition. Why do we care? Because they are the only types of rocks that can preserve fossils.
What sedimentologists do is to classify and correlate sediments (in other words - stratigraphy; we covered that!) and study the conditions under which they form. The latter part is the most interesting one. If you find a dinosaur, you’ll want to know if it lived in a forest or a desert, don’t you? As it happens, forest-adjacent and arid environments also just so happen to form different sediments.
By far the most important concept for terrestrial sediments to understand is that of grain size and depositional energy. Here’s an intuitive example:
Suppose you have a river that flows really fast. All the small grains get washed away easily while only the largest sink to the bottom.
If the river flows more slowly, you won’t have so many big grains as they didn’t make it in the current to begin with (or they sank down much earlier)! Instead, you’ll see the small grains sink down. There are more principles, but this is one of the most important ones.
The most important grain sizes are gravel which is larger than sand which is larger than silt which is larger than mud/clay.
Here are some common terrestrial depositional environments:
Mountain environment = Normally, the grain sizes are all over the place here, but they tend towards the large end (e.g. lots of gravel).
River = Normally full of sand (and some silt) with so-called cross-beddings which are remnants of little dunes formed by the water current
Desert = Also sandy, but a bit smaller on average and with rounder grains than in a river and with a lot bigger dunes.
Lake = Normally full of mud arranged in laminar layers.
(Of course, you can cheat a bit by using characteristic fossils here as well.)
Marine environments are trickier. Generally, grain sizes are bigger the closer you are to the beach as the effect of waves is larger there.
Here, besides sand, silt and so on, you also have to deal with carbonates here. Carbonate rocks consist of a matrix of fine biological mud and of grains which are normally fossils trapped in the mud. Carbonates deposited close in high-energy marine environments (like beaches) have a lot of grains and very little mud while carbonates from low-energy marine environments (like lagoons) have a lot of mud and less grains.
Taphonomy
Finally, we get to taphonomy and the science of how fossils are formed. Generally, when an organism dies and turns into a fossil, three things happen.
First, the organism dies. The cause of its death and the processes leading to it are called necrotic processes.
Then, the organism is moved to the place of its burial and somehow has to get in the sediment which we call biostratinomy.
Finally, its preservable parts become turned into minerals which is called diagenesis.
The point behind taphonomy is threefold. First, we want to identify biases in the fossil record. For obvious reasons, vertebrate and echinoderm endoskeletons, arthropod exoskeletons, mollusk shells and plant pollen, spores or wood are overrepresented in the fossil record while organisms that lack those are found less frequently.
Secondly, learning about the cause of an organism's death helps us understand how it lived. It makes a difference whether an animal died of old age or whether it got killed by a predator.
Finally, it is important, especially for paleoenvironment reconstruction, to know where an organism died. Organisms that died in the same place in which they got preserved are autochthonous. They are allochthonous otherwise. Determining transport isn’t always easy, but there are a couple of ways we can determine such circumstances nonetheless. Sessile organisms like corals are normally preserved in-situ, so are organisms that used to live or bore in them at the time of their deaths. Likewise, several organisms (like bivalves) show very specific deposition patterns if they got transported by a storm, for example.
EDIT: Instead of this little lecture, maybe it'd be better to include a glossary of geological terms that paleontological papers sometimes use. Maybe I'll do both. It shouldn't be a surprise that this post is going to get heavily edited in the future.