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Author Topic:   How do geologist know what they are looking at really is what they say it is?
jar
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Message 46 of 88 (790619)
09-01-2016 7:49 PM
Reply to: Message 45 by edge
08-31-2016 6:43 PM


Re: What are examples of biological rocks and how are they identified?
Good info. Can limestone and similar products can also get incorporated into clastic sediments?

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Pressie
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Message 47 of 88 (790635)
09-02-2016 7:33 AM
Reply to: Message 46 by jar
09-01-2016 7:49 PM


Re: What are examples of biological rocks and how are they identified?
Yes, the Dwyka tillites (Late Carboniferous) towards the south west of Jo'burg consist, amongst others, of clays, sands, gravels and boulders obtained from the underlying Malmani Dolomites of the Transvaal Sequence (Palaeoproterozoic in age).
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jar
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Posts: 29449
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 48 of 88 (790642)
09-02-2016 9:02 AM
Reply to: Message 47 by Pressie
09-02-2016 7:33 AM


pause to sum up.
Thanks.

So let's return to the question asked in Message 42.

quote:
So from what has been said lithification is basically compaction and cementation.

From that I would guess that the deeper something is buried the greater the compaction. Also the longer something is buried the greater the compaction and cementation.

For cementation though I assume what is needed would be water and minerals leached from the overburden. Is that correct?


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?

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?)


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Dr Adequate
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(1)
Message 49 of 88 (790646)
09-02-2016 11:40 AM


Incidentally, can I have a quick question, I think I used to know it but I've forgotten. What do you call a conglomerate/breccia which is made of clasts from a previous conglomerate/breccia?
  
edge
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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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jar
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From: Texas!!
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Message 51 of 88 (790677)
09-03-2016 9:09 AM


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?

What about accumulations of diatoms? What do they produce?

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


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edge
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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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jar
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Posts: 29449
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 53 of 88 (790708)
09-03-2016 6:35 PM
Reply to: Message 52 by edge
09-03-2016 4:35 PM


Re: Before moving on ...
It seems that coal is often found between layers of sandstone or limestone or mudstone or shale. Why is that the case?

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Dr Adequate
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Message 54 of 88 (790719)
09-03-2016 8:08 PM
Reply to: Message 53 by jar
09-03-2016 6:35 PM


Re: Before moving on ...
'Cos it tended to form in swamps by the shore; it was always just a little variation in sea level away from being in a marine environment.
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jar
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Posts: 29449
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 55 of 88 (790721)
09-03-2016 8:11 PM
Reply to: Message 54 by Dr Adequate
09-03-2016 8:08 PM


landscapes come and landscapes go?
So it would be another indicator of environments and landscapes changing over time with one landscape being buried and replaced at the surface by a different landscape?

Edited by jar, : there's a "d" in lanscapes


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Dr Adequate
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Message 56 of 88 (790722)
09-03-2016 10:03 PM
Reply to: Message 55 by jar
09-03-2016 8:11 PM


Re: landscapes come and landscapes go?
Or, y'know, seascape. But yes, you get coal measures where you can see the same cycle over and over: paleosol (fossil soil) with root traces in it and tree stumps sticking out of it into coal which is then covered over by marine sediment, and then the same sequence over and over again.
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jar
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Posts: 29449
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 57 of 88 (790746)
09-04-2016 9:18 AM
Reply to: Message 56 by Dr Adequate
09-03-2016 10:03 PM


Re: landscapes come and landscapes go?
So are there different types of coal and if so what does each type tell us about the source and process that produced the end product?

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edge
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Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
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(2)
Message 58 of 88 (790763)
09-04-2016 5:18 PM
Reply to: Message 53 by jar
09-03-2016 6:35 PM


Re: Before moving on ...
It seems that coal is often found between layers of sandstone or limestone or mudstone or shale. Why is that the case?

Because the depositional environment is constantly changing, despite what Faith seems to think. What we see is a combination of highly organic swamps with little clastic input (to make nice coal seams) and low gradient streams with overbank deposits (mudstone) and channel deposits (sandstone) and often times, volcanic ash (look up 'tonstein' sometime).
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edge
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Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.7


(1)
Message 59 of 88 (790764)
09-04-2016 6:03 PM
Reply to: Message 56 by Dr Adequate
09-03-2016 10:03 PM


Re: landscapes come and landscapes go?
Or, y'know, seascape. But yes, you get coal measures where you can see the same cycle over and over: paleosol (fossil soil) with root traces in it and tree stumps sticking out of it into coal which is then covered over by marine sediment, and then the same sequence over and over again.

In drilling coal fields, you can find three different types of sandstone. There are the sand bars that indicate streams, and there are transgressive beach sands, and regressive beach sands. They all look different in shape and continuity.

Here is a huge sand bar overlying a coal seam in New Mexico.


You can tell by the shape (kind of a wedge) and the presence of large-scale cross-bedding.

Here is a section of drill core from a thick transgressive sandstone in a coal region which drilled directly through some kind of a shell fossil which is right where one would expect it to be:

Such transgressive sandstones tend to be thicker and with greater continuity across a basin.


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edge
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Posts: 4002
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.7


Message 60 of 88 (790765)
09-04-2016 6:15 PM
Reply to: Message 57 by jar
09-04-2016 9:18 AM


Re: landscapes come and landscapes go?
So are there different types of coal and if so what does each type tell us about the source and process that produced the end product?

Well, that could fill a book.

Basically, you've got various grades of coal which reflect the degree of compaction and thermal history (metamorphism). Anthracite is the highest grade of coal.

"As geological processes apply pressure to dead biotic material over time, under suitable conditions, its metamorphic grade increases successively," ...
(https://en.wikipedia.org/wiki/Coal)

... from peat to lignite, to bituminous to anthracite and eventually to graphite.


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