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Author Topic:   Exploring the Grand Canyon, from the bottom up.
IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 10 of 283 (295249)
03-14-2006 2:06 PM


This is a very interesting thread. Pity the technical stuff is making my head hurt. ><

I'm just about to leave work, I'll write up a better response when I get home.

IRH


  
IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 33 of 283 (295470)
03-15-2006 9:26 AM
Reply to: Message 29 by Faith
03-15-2006 7:27 AM


Re: Again a mixture of fact with imaginative speculation
*sighs*

Great, so we have to explain absolutely everything then.

quote:
From reading the technical paper I linked to earlier, it seems the lack of a coarser grained component (i.e., the conglomeratic sequence), points to a marine depositional setting rather than a terrestrial setting for the original proto-sediments; which is consistent with previous theories regarding the geologic history of that particular terrane.

I actually thought the same thing as soon as Rox mentioned that the conglomerates were absent. Conglomerates tend to be formed in terrestrial environments like river beds because the action of the river creates the rounded pebbles. So you tend to see conglomerates in river basins but not marine basins.

If we could see the original rock (i.e. unmetamorphosed) we'd know immediately if it was formed in a terrestrial environment or not, because terrestrial rock is red or orange, on account of the iron in the rock reacting with the air. Offhand I can't remember if you can still tell from the chemical composition of the metamorphosed rock (I haven't done metamorphism for years).

As well as that, any cross-bedding or similar structures not destroyed by the metamorphism are also indicative of how the rock originally formed.

quote:
In another thread, I briefly touched on how the U.S. grew via island arc accretion south from about the Wyoming/Montana area. Wyoming is located on the Archean Wyoming craton and everything south and basically west are progressively younger accreted terranes - generally considered to be island arc terranes (volcanic island chains similar to Japan and Indonesia that develop along subduction zones).

Look, Rox didn't pull this out of her ass or something. (Believe it or not, part of Ireland formed the same way.) An accreted terrane is a term describing a geological section that was formed from slices of rock formations; so I guess what you see is individual slices or areas, all crushed together, consisting of rock formations similar to what we see in Japan and Indonesia. We assume that, as the formations are similar and there is evidence of uplift and other tectonic action that squashed them together, they may have formed like Japan and Indonesia. Hence "progressively younger accreted terranes - generally considered to be island arc terranes" - because the evidence in the rock indicates that this is the most likely explanation.

quote:
According to the linked paper (Ilg et al., 1996), the protosediments which later became the Vishnu Schist, are remants of one of those island arc terranes that collided with the continent back in the Precambrian (early Proterozoic time?).

The Vishnu Schist is pretty old, but still - we see marine sediments, deformed and uplifted, which were intruded by igneous rock later. That's more or less consistent with what we expect to see in an island arc terrane.

quote:
Evidence is given, but still the interpretations have that aura of fact. No contrary interpretations are suggested. There is no way to verify or falsify such an interpretation. This is what I find so frustrating about both OE and TOE discussions.

The evidence suggests this is the most likely explanation. We verify or falsify it by examining the evidence - if we see what we expect to see if our explanation is correct, then the explanation is somewhat verified. If we see something unexpected, or something that is not possible for our explanation, it is falsified.

Call it interpretation if you will, but that is what science is. Period. There isn't another 'interpretation' in this case because we don't have another that fits the evidence as well or better than our current one. This is not imagination, or speculation - it's called scientific investigation.

Imagination and speculation are more the creationist's forte.


"Gradually it was disclosed to me that the line separating good and evil passes not through states, nor between classes, nor between political parties either, but right through every human heart, and through all human hearts. This line shifts. Inside us, it oscillates with the years. Even within hearts overwhelmed by evil, one small bridgehead of good is retained; and even in the best of all hearts, there remains a small corner of evil." --Alexander Solzhenitsyn, The Gulag Archipelago

"Those who fear the darkness have never seen what the light can do."


This message is a reply to:
 Message 29 by Faith, posted 03-15-2006 7:27 AM Faith has responded

Replies to this message:
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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 39 of 283 (295478)
03-15-2006 9:48 AM
Reply to: Message 32 by Faith
03-15-2006 9:18 AM


Too complex?
quote:
In fact the exact conditions of the rocks that lead to such conjectures as "marine environment" are often left out or blurred over. I'd simply appreciate it if you and the other geologists would be alert to this effect and try to fill in the descriptive material that supposedly explains the scenarios -- but of course first of all I'd appreciate it if the imaginative scenarios were clearly identified as such and set apart from the facts better.

It's not imagination. Please stop using that term, because it is entirely the wrong descriptive term to use for a scientific explanation based on research and extensive field studies.

Anyway - I'm wondering if we should start on a less complex formation. Otherwise it'll take a very long time to explain all the relevent reasoning and lines of evidence behind every conclusion.


"Gradually it was disclosed to me that the line separating good and evil passes not through states, nor between classes, nor between political parties either, but right through every human heart, and through all human hearts. This line shifts. Inside us, it oscillates with the years. Even within hearts overwhelmed by evil, one small bridgehead of good is retained; and even in the best of all hearts, there remains a small corner of evil." --Alexander Solzhenitsyn, The Gulag Archipelago

"Those who fear the darkness have never seen what the light can do."


This message is a reply to:
 Message 32 by Faith, posted 03-15-2006 9:18 AM Faith has not yet responded

  
IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 40 of 283 (295482)
03-15-2006 9:51 AM
Reply to: Message 35 by Faith
03-15-2006 9:36 AM


Re: Again a mixture of fact with imaginative speculation
I'm taking this to a new topic.

This message is a reply to:
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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 47 of 283 (295538)
03-15-2006 12:27 PM
Reply to: Message 45 by Jazzns
03-15-2006 11:44 AM


Re: Can we head back towards the topic.
Hey Jazz

Tough question there - why have only one kind of metamorphism? To be honest without examining the schist around the igneous intrusion we won't know how extensive the contact metamorphism is. Considering the size of the schist I would expect that most of it was formed by regular metamorphism.

So let's see:

1. Some silicate rock somewhere gets eroded into sand.
2. That sand is deposited which later becomes the Vishnu Sandstone.
3? 4? A magma body intrudes into the sandstone causing contact metamorphism.
3? 4? The formation is subjected to pressures and temperatures causing it to metamorphose further.
5. The magma never makes it to the surface and therefore cools as granite.

The timing of 3, 4, and 5 depend on information we don't have at the moment. Any help Rox?

IRH


This message is a reply to:
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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 55 of 283 (295609)
03-15-2006 3:29 PM
Reply to: Message 54 by roxrkool
03-15-2006 2:57 PM


Re: Can we head back towards the topic.
*bows to Rox*

You explained it better than I did. :)


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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 198 of 283 (435502)
11-21-2007 9:02 AM
Reply to: Message 197 by jar
11-16-2007 9:51 PM


Re: What is the layer above the Dox formation?
Above the Dox formation is the Cardenas Basalts. Volcanic rock, i.e. lava flows. Much of what you get from google on it conisists of demented creationist honking about how the dating for this series is inaccurate or whatnot.

Here's a quick description I managed to dig up:

"The Cardenas Lava is the name given to a series of basalt and basaltic andesite flows and sandstone interbeds that are above the Dox Formation, but below the Nankoweap Formation. The Cardenas Lava is only exposed in the eastern part of the canyon, where it ranges from 785 feet to 985 feet in thickness. The contact between the Cardenas Lava and the Nankoweap Formation is an unconformity. An unknown amount of the Cardenas Lava was removed before the deposition of the Nankoweap."

Sandstone and basalt, hmmm. Periodic coastal volcanic eruptions?


This message is a reply to:
 Message 197 by jar, posted 11-16-2007 9:51 PM jar has responded

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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 203 of 283 (435826)
11-23-2007 8:47 AM
Reply to: Message 202 by jar
11-22-2007 2:31 PM


Re: More questions on the Cardenas Lava flows.
That's about right, jar.

For reference, when we talk about a formation such as the Cardenas Basalt, we're not speaking of a single type of rock. We're actually talking of a whole series of layers of rock, representing a particular depositional environment, in this case sandstone being deposited in between volcanic eruptions. In general terms, formations are divided based on the change in depositional environment, and within the formation you will have several members or types of rock which define that environment. The Cardenas Basalt has several members such as a red sandstone layer and a basalt layer.

Formations are the basic units, I guess, of how you look at the rocks.


This message is a reply to:
 Message 202 by jar, posted 11-22-2007 2:31 PM jar has responded

Replies to this message:
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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 209 of 283 (436507)
11-26-2007 3:36 AM
Reply to: Message 205 by jar
11-23-2007 10:12 AM


Re: Read it and Nankoweap
quote:
From what little I have learned so far, several thing jump out at me. First, it mentions that the lower part of the formation has a finer grain composition than the upper. That seems to indicate that the lower layers were deposited in a less active environment than the upper. I understood that when material is transported by both water and air, the larger particles get deposited first and the finer material is held in suspension longer. If the Nankoweap had been a single event I would have expected the larger coarser material to be lower with the finer sediment towards the top.

Is that reasonable?


More or less, yes. I couldn't say 100% because I haven't actually examined the rock in detail.

quote:
If I remember correctly from earlier parts of this thread, sandstone, siltstone, shale and mudstone each form under different conditions and have different compositions.

Can you or another of our geologists give me a short description of each of those just as a reminder?


Wikipedia has a very concise description on shales and sandstones:

Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery and usually parallel to the often-indistinguishable bedding plane. This property is called fissility. Non-fissile rocks of similar composition but made of particles smaller than 1/16 mm are described as mudstones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones.

Just to explain: fissility is the way that shales break into thin sheets rather than into blocks like other rocks.

Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white.

So overall, the visual difference between sandstone, siltstone, and shale/mudstone is how 'gritty' or coarse they are. And mudstone and shale differ only in that shale is fissile and mudstone is not.

Edited by IrishRockhound, : fixed quote box


This message is a reply to:
 Message 205 by jar, posted 11-23-2007 10:12 AM jar has responded

Replies to this message:
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IrishRockhound
Member (Idle past 2751 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 211 of 283 (436773)
11-27-2007 1:55 PM
Reply to: Message 210 by jar
11-26-2007 10:56 AM


Re: Read it and Nankoweap
Once again, Wikipedia is our best friend.

quote:
Sedimentary rocks are formed because of the overburden pressure as particles of sediment are deposited out of air, ice, wind, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as lithification ('rock formation') and the original connate fluids are expelled.

Sediment gets dumped in a spot. As more and more gets dumped there, whether by air, ice, wind or water, the stuff on the bottom gets compacted. All the air/water gets squeezed out, and eventually after it gets reeeeeeally well crushed, it solidifies into rock.


This message is a reply to:
 Message 210 by jar, posted 11-26-2007 10:56 AM jar has responded

Replies to this message:
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