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Author Topic:   How do geologist know what they are looking at really is what they say it is?
edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


(1)
Message 4 of 88 (790263)
08-28-2016 4:03 PM
Reply to: Message 1 by jar
08-28-2016 10:54 AM


How do geologists really know what a sample is and how it came to be?

It all starts with some assumptions. The first one is 'uniformitarianism'. This principle was formulated by James Hutton the Scottish farmer, physician, and geologist. He was the first to realize that processes going on today could produce the effects that we see in the geological record and have been going on for a very long time.

While our idea of uniformitarianism has changed with time, it is an assumption that has worked well and allowed us to construct a meaningful geologic history for much of the earth's existence. Otherwise the pattern of rocks and fossils and minerals would make no sense.

No one has overturned the modern view of uniformitarianism.

Of course, some people have a hard time with this principle because the past cannot be directly observed. However, they can offer nothing in its place as far as interpretation of the geological record is concerned.

I'm going to leave this subject for discussion rather than get into the ramifications of uniformitarianism. Let's just say that it was based on Hutton's observations during his experience in Great Britain and is applicable to the Phanerozoic sedimentary record.


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 Message 1 by jar, posted 08-28-2016 10:54 AM jar has replied

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


(1)
Message 6 of 88 (790270)
08-28-2016 4:57 PM
Reply to: Message 5 by jar
08-28-2016 4:27 PM


Re: uniformitarianism is nice but...
...specifically are all sedimentary rocks produced the same way?

Not at all.

The kind we are familiar with is clastic sedimentation, meaning it consists of small rock and mineral fragments that have been eroded and transported to a depositional center.

There are also chemical sediments that precipitate out of water such as chert, or evaporites, or travertine, or various types of iron formations.

Some others are accumulations of biological materials such as coral reefs coal and other types of bioherms.

And that's only dealing with composition or source. There are a number of physical modes of deposition as well such as hot springs, turbidites, mudflows, deltas, clastic fans, eolian, etc., etc.

And then you can get into the transition between volcanic and sedimentary deposits.

It's not rocket science, but it can be complex, kind of difficult to explain in a forum such as this.

I offer all of this just by way of support for my initial statement: "No".


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 8 of 88 (790274)
08-28-2016 10:12 PM
Reply to: Message 7 by jar
08-28-2016 5:17 PM


Re: great start.
How can I tell if a sample is clastic instead of chemical or biological?
You would look at the grains that make up the rock. Clastic rocks would consist of rock and mineral fragments from older rocks.

Biogenic rocks would be mostly composed of recognizable fragments of organic remains, usually carbonate or silica.

There is also a gray area where biological remains can be transported and deposited as a clastic rock. It is also very common to have some amount of clastic material mixed in with the biological carbonate.


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 17 of 88 (790313)
08-29-2016 10:53 AM
Reply to: Message 14 by Percy
08-29-2016 7:56 AM


This would be interesting to understand better. What is you find that tells you it was "deltas resulting from rivers flowing out of U-formed glacial valleys into a lake"?

That is where data collection comes into play. We literally map rock bodies in three dimensions. We can actually see their shapes in space. We can see the paleotopography of a valley, the piles of glacial till, the lake beds and the great fans of river delta silts and sand channels.

I once modeled a hill and realized that it was really a lake back in the Miocene. In fact, I'm certain that it had hot springs feeding it. Others tried to model it as a big featureless blob, but I showed that the whole environment made sense and controlled mineralization.

Hey, it isn't easy and there's a lot of uncertainty that some people can't deal with, but as Pressie says, it all works.


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 18 of 88 (790314)
08-29-2016 10:56 AM
Reply to: Message 15 by jar
08-29-2016 8:39 AM


Re: can we hold on coal for just a little while.
Before we move on to the biological sedimentary rocks can we spend a little more time at clastics? I know I'm slow but I still have a few more questions about identifying clastics before we move on to chemical and biological sedimentary rocks.
Hopefully we will get there though.

Well, I have to say that learning from books and lectures is one thing, but there is nothing like seeing the field relationships.

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 19 of 88 (790315)
08-29-2016 11:06 AM
Reply to: Message 16 by jar
08-29-2016 8:57 AM


Re: fragments from older rocks
I'm glad you mentioned mud.
So clastic sedimentary rocks begin as other rocks and before they can exist there needs to be time enough to first weather and erode other rocks to make the small rock and mineral fragments that have been eroded and transported to a depositional center that edge mentions in Message 6.

That brings up a couple other points.

First it seems from what edge said we need two processes. We need weathering and erosion and then the pieces parts need to be transported to some spot where the stop and accumulate. To end up as a layer in the geological column of a given location the pieces parts need to stay in that location long enough for the individual pieces parts to turn back into another big rock.


That sums it up pretty well.

And back towards mud. It seems that the size of pieces parts also plays some part in determining what the final clastic rock will become.

Clastic rock definition is mostly based on grain size.

Is that correct and if so what types of clastic rocks are made from the different sized pieces parts?

That's were it gets really technical. In order of decreasing grain size:

Gravel ---> conglomerate
Sand ----> sandstone
Silt ----> siltstone
Mud ----> mudstone
Clay ----> claystone

Yeah, I know, it's tough.

We can make it more difficult by giving them modifiers like 'calcareous' or 'organic'; or we can combine the terms to things like 'sandy carbonaceous mudstone' (which may not actually exist, but you get the idea).

Hey, people spend careers studying this stuff.

And we haven't gotten into carbonates or igneous rocks, etc. ...

How can someone tell the various resulting rocks apart?

The most definitive way would be microscopically, but in the field one gets a feel for it. Basically, if you can see the grains you are dealing with siltstone or coarser.

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 22 of 88 (790321)
08-29-2016 11:45 AM
Reply to: Message 20 by jar
08-29-2016 11:24 AM


Re: Great so far but I'm slow so humor me by expanding some.
I think so but as usual, a few questions. I've seen things described as mudstone and siltstone but also as shale.

Particle size is pretty clear but what produces the different sized particles. Why does something end up as silt or mud or clay?


Either chemical weathering to smaller mineral grains, or abrasion during transport.

Where does shale fit in? What is it and why is it different?

Shale is the traditional term for a mudstone. Here is the Wentworth scale for determining rock type:

The different gravel sizes become conglomerates.

Sands are self-explanatory.

Muds include clay and silt in this diagram. In my work, I reserved claystone for very soft material that had no silt-sized grains in it.

And then there is slate???????

Slate is a low-grad metamorphic rock derived from mudstones. They usually have a very strong cleavage (finely space parallel partings).

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 23 of 88 (790323)
08-29-2016 11:48 AM
Reply to: Message 21 by Dr Adequate
08-29-2016 11:45 AM


Re: Great so far but I'm slow so humor me by expanding some.
Shale has lots of thin layers.

Yes, shale is defined as a mudstone with a lot of irregular partings due to bedding.

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 32 of 88 (790343)
08-29-2016 3:51 PM
Reply to: Message 24 by jar
08-29-2016 11:56 AM


Re: Great so far but I'm slow so humor me by expanding some.
Okay but jargon alert. Help please. What does irregular partings mean and what does bedding mean?

Planes along which a material can more easily separate.

Irregular means not perfectly flat.


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 33 of 88 (790348)
08-29-2016 4:01 PM
Reply to: Message 30 by herebedragons
08-29-2016 1:06 PM


Re: Great so far but I'm slow so humor me by expanding some.
Anyway, I was thinking that since the topic is "How do geologists know..." the topics of grain shape (or roundness) and sorting should be covered, since those are major factors that help determine deposition environment.

Yes, but 'small steps' ...

Basically, the greater the distance of transport, the finer the grain size and rounding of rock fragments. But then, some materials will break down faster, so be careful.

Sediments will sort in a water medium according to grain size with finer materials being suspended longer (carried farther out to sea) or settling out last in a still column of water.

Also you mentioned chemical and mechanical weathering in the context of the production of the sediment; but don't those processes also occur during and after lithification? And don't those characteristics give us clues as to the depositional environment?

Yes, however, the breakdown of rock to producing clastic sediments is kind of the opposite to lithification.

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 43 of 88 (790557)
08-31-2016 6:30 PM
Reply to: Message 39 by Pressie
08-30-2016 7:07 AM


Re: so short summary so far.
Serpentinite, a very common mineral, is metamorphic, but also very soft. So, metamorphism is not really a good indicator of how hard a rock is.

Just think of talc, eh?

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 44 of 88 (790558)
08-31-2016 6:35 PM
Reply to: Message 40 by Percy
08-30-2016 7:49 AM


Re: Great so far but I'm slow so humor me by expanding some.
I guess I didn't understand the question. About chemical and mechanical weathering occurring during lithification, ...

Although I can think of exceptions, this is kind of a contradiction of terms.

... since lithification occurs after burial, how could there be any weathering?

As I said, if you want to get into details, I can think of something, I'm sure. The point is that for a general discussion like this, it would be confusing.

And about after lithification, can't weathering only occur after exposure?

Almost by definition, although some kinds of alteration can occur at depth.

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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 45 of 88 (790559)
08-31-2016 6:43 PM
Reply to: Message 41 by jar
08-30-2016 11:50 AM


Re: What are examples of biological rocks and how are they identified?
Back in Message 1 edge mentions a third type of sedimentary rocks and that was accumulations of biological materials such as coral reefs coal and other types of bioherms.

What are bioherms and how are they identified?


Basically, piles of shells or fossilized organic material.

What are other examples of biological material rocks?

Outside of limestone (including chalk)? How about coral reefs?

Or coal? (actually, according to strict definition, coal is not a rock).

Or radiolarite? (https://en.wikipedia.org/wiki/Radiolarite)

Or phosphorite? (https://en.wikipedia.org/wiki/Phosphorite)

I'm sure that I'm missing some more ...


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 50 of 88 (790660)
09-02-2016 11:14 PM
Reply to: Message 48 by jar
09-02-2016 9:02 AM


Re: pause to sum up.
So it seems that lithification can produce hard or soft rocks, cemented or compressed rocks, rocks that are compressed and cemented, rocks that can be dissolved by different chemicals and that even within one grouping (mudstones as an example) we can see a vast range of physical characteristics but in every case the resulting product is a direct indicator of how that sample was formed.

Is that correct?


Essentially, yes.

Next, it also seems that before any sedimentary rocks are formed there must exist the primary (or in many cases secondary or tertiary) sources of material and the earlier materials must first be weathered and eroded and transported to the location where they are found. (Is this the Party to whom I am speaking?)

As in clastic rocks, yes.

Edited by Adminnemooseus, : Fix 1 quote box.


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edge
Member (Idle past 942 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 52 of 88 (790699)
09-03-2016 4:35 PM
Reply to: Message 51 by jar
09-03-2016 9:09 AM


Re: Before moving on ...
Can someone explain the mechanisms for formation of a couple of the biological rock types for me?
First limestone.

I have heard that limestone can be formed is several ways, by accumulation of shell or coral and by direct precipitation or evaporation.

How can geologists tell which process was the primary one in a given sample?


The short answer is 'from fossils and rock textures'. Fossils are the main clue, but sometimes limestones are so fine-grained that you can't tell without a microscope and even then it's just a matter of naming it by grain size.

Carbonate geology is a whole separate area of the science. If Petrophysics were here, we could get a better idea...

The send big question I have is what different types of rocks get produced by buried marshes and wetlands?

Highly reduced, carbonaceous stuff, like coal ...

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