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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 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


(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
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


(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
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


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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edge
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 66 of 88 (790825)
09-06-2016 9:59 AM
Reply to: Message 61 by jar
09-04-2016 7:34 PM


Re: landscapes come and landscapes go?
Just to elaborate a little on Pressie's responses...

And these different types are recognizable ...

Yes, in fact a child could recognize the differences. However, there is a whole science behind coal analysis at the analytical level.

... and each results from a different history and process?

The degree of process is the most important factor. But again, it's not just simple burial, but other factors can contribute such as deformation.

Do the different types point to different initial origins or only to the formation process?

They can, but the primary difference is in the degree of post-burial processes.

When I went to school, there were entire laboratories dedicated to the detailed study of coal and numerous PhD's were cranked out over the years. This is not a trivial topic no matter how YECs present it.


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edge
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 67 of 88 (790826)
09-06-2016 10:15 AM
Reply to: Message 65 by jar
09-06-2016 9:01 AM


Re: landscapes come and landscapes go?
Okay, but then can you walk us through the cycle from some surface landscape to each of the resulting coal types?

Any time you can accumulate a pure organic sediment, you can ultimate end up with a coal bed.

Here is a peat deposit in the bogs of Northern Ireland.

There is almost no input of clastic material at this lcoation, just an ages long build up of organic debris. Even the streams in this immediate area have no rocks, sand, boulders, etc.

Just dead grasses, some animals and (once upon a time) trees and roots.

This material is, even today, harvested and used as fuel. In fact, that's why scotch tastes the smokey way it does.

Interestingly, there is sheep skull in the picture. While it is younger than the peat itself, geologically speaking, it is of the same age. With very little imagination, one could see it being buried eventually and become part of the geological record. However, since we see erosion occurring here, it's not too likely in this case.

Certainly, you can imagine other depositional settings, such as swamps or ponds. I understand that there are even some Precambrian coal deposits, though I'm not really sure how they formed, but basically, it's just 'pure' organic carbon residue.


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edge
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 69 of 88 (790833)
09-06-2016 11:50 AM
Reply to: Message 68 by jar
09-06-2016 10:47 AM


Re: Spy vs Spy
So in the Mad tradition of spy vs spy can we make a general statement that the various forms of coal products are more the result of processing than original source. All begin as marshy wetlands that remain marshy wetlands long enough for deep deposits of dead plant material to accumulate?

And for the other Spy, can we say chalk results from long term deposition of dead micro-organisms with calcite shells in moderately deep water while other forms of limestone result from different original environments?


As a general rule.

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edge
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 83 of 88 (791670)
09-19-2016 7:04 PM
Reply to: Message 82 by Pressie
09-09-2016 6:27 AM


Re: Spy vs Spy
It's all subjective in the field. It comes from experience and a hand held magnifiers.

Ya got that right!

Some things don't change, no matter which hemisphere.


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edge
Member (Idle past 943 days)
Posts: 4696
From: Colorado, USA
Joined: 01-09-2002


Message 88 of 88 (796400)
12-29-2016 2:38 PM
Reply to: Message 85 by jar
12-28-2016 8:54 PM


Re: So what is in this picture?
What can be said about how the stuff in this picture was created?

Having been fooled by nature too many times to count, I'd say, not a whole lot. Certainly these are boulders of thinly bedded material, partially covered by transported (see the small ripples) sand(?), of a different composition. Possibly mud cracks are present, but without direct observation it could be some other things. The best possibility is water lain, fine sand and silt or some kind of eolian deposit (possible large-scale cross-beds on the left side of image). But that's pretty uncertain.

To me, all of the Mars pictures are 'flat', without a lot of definition. I'd like to see colors, too.


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