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Author Topic:   Exploring the Grand Canyon, from the bottom up.
jar
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Message 128 of 283 (297151)
03-21-2006 10:10 PM
Reply to: Message 127 by edge
03-21-2006 10:01 PM


Re: Got yet another question for you.
Yes, at the left below the smoother layer.


Aslan is not a Tame Lion

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jar
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From: Texas!!
Joined: 04-20-2004
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Message 130 of 283 (297160)
03-21-2006 10:40 PM
Reply to: Message 129 by roxrkool
03-21-2006 10:22 PM


Okay, more important points.
Under compression (increased pressure), rocks behave plastically, but under extension, rocks are brittle. So compression results in folding, deformation, metamorphosis, etc., but because it becomes easier for the crust to move laterally instead of straight up, they start extending in the orthogonal direction, forming horsts and grabens.

Okay, great info.

So you are saying that under pressure rocks will fold and deform. I assume this has been tested in the lab and that under pressure you can actually bend rock that normally would break.

Is that correct? So when geologists see something like this or this you good folk know that it happened after the rock had hardened and under pressure and not as some folk claim while the rock was soft and muddy.


Aslan is not a Tame Lion

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 Message 129 by roxrkool, posted 03-21-2006 10:22 PM roxrkool has responded

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 132 of 283 (297299)
03-22-2006 12:15 PM
Reply to: Message 131 by roxrkool
03-22-2006 12:01 PM


Time to play stump the chump.
Actually, jar, I think that second image may be an angular unconformity, but the first one is nice.

Great. That tells me that there really are things you see that us amateurs don't unless it is pointed out to us. I would like to see if these old eyes of mine can be taught to dance.

Is this an example of an angular unconformity, in fact a few of them?

At least two angular unconformities one with the point to the left, the other pointing right and lower on the face.


Aslan is not a Tame Lion

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 Message 131 by roxrkool, posted 03-22-2006 12:01 PM roxrkool has responded

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 133 of 283 (297304)
03-22-2006 12:31 PM


Pointing out where we are now...
and where we are going.

I'd like to pause for a moment (and will repeat this periodically throughout this discussion) so folk can visualize where we are in the Grand Canyon and get a feel for what will be coming.

Here is a drawing of the Grand Canyon I'll use as our position indicator as we move from John Wesley Powell's view, down in the bottom, up towards the top.

Right now we have covered 1a and 1b, and we are looking at the very first layers of group 2.

As you can see, we have a ways to go to get to the top. I hope that in the remaining posts of this thread that we can get through all of group 2 and get a summary of the wonderous story to be found just in these bottom sections.

Note that the layers in Group 2 are tilted. I don't want to deal with that issue until we've first covered all of them as they were created. Let's first build the layer cake, and then hopefully still have enough posts in this thread to discuss how they became tilted, and what we can learn both from what is seen, and what is missing.


Aslan is not a Tame Lion

  
jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 135 of 283 (297320)
03-22-2006 1:26 PM
Reply to: Message 134 by roxrkool
03-22-2006 1:18 PM


Re: Time to play stump the chump.
The upper one shows a nice erosional channel, too (just above center).

Great, that was going to be the next question I asked you.

So another new term to define.

Erosional channel.

Let's see if I understand what you are saying.

What we see here is a channel that was cut in the surface, perhaps by a stream or wind or glacier. It was then filled in over time until the surface was level again. We can tell it was an erosional channel because layers are interrupted on the right and left, missing in the channel itself, and new material deposited there.

Is that what we are seeing?


Aslan is not a Tame Lion

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 137 of 283 (297338)
03-22-2006 3:05 PM
Reply to: Message 136 by roxrkool
03-22-2006 2:43 PM


On Angular Unconformities and other wonders
I just want to make sure once again that I understand what is shown in the record left in the rock.

The evidence for the unconformity is that layers simply stop, are cut off. That means that sometime after they were laid down, the surface was eroded away. Then there is another layer that is continuous above the parts that was worn away.

So we are looking at time and events here.

In the picture we are discussing there are five main features, two angular unconformities where erosion happened and then a layer was laid down on top, the two layers, one separating the angular unconformities and the other above the higher unconformity and an erosional channel.

But there are also many, many layers in the picture.

What do those layers tell us? Is each one of those layers a change in the local environment or deposition?


Aslan is not a Tame Lion

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 139 of 283 (297596)
03-23-2006 2:05 PM


Okay, moving on to The Hakatai Shale
The Hakatai Shale is the next level in the Grand Canyon Supergroup. I realize that these layers are not homogenous and all one substance, but Shale is a new term

What is shale and how is it produced?


Aslan is not a Tame Lion

  
jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 141 of 283 (297692)
03-24-2006 12:09 AM
Reply to: Message 140 by Minnemooseus
03-23-2006 2:06 PM


But what is shale?
Thanks moose, but our next layer is called shale, Hakatai Shale. Can you explain how shale is different from, say sandstone or schist?


Aslan is not a Tame Lion

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 143 of 283 (297793)
03-24-2006 12:22 PM
Reply to: Message 142 by roxrkool
03-24-2006 11:11 AM


Summary up to the second layer of the Supergroup.
We began with the Vishnu Schist because it is the lowest exposed layer. It is schist, so originally it was sandstone. To get sandstone there first had to be some higher rock source which was weathered, accumulated in a basin, was later compressed and metamorphed into schist. There was clay, which is also a product of weathering mixed in with the sand.

So to this point, rock was created, weathered into smaller and smaller particles until we had sand and clays, washed or blown downhill (eroded) to collect in a basin, settled, compressed and over time, under pressure and heat became schist.

At sometime during this process, magama pushed through the sandstone or schist and over long periods of time cooled to become granite.

The whole structure, Vishnu Schist and Zoroaster Granite was pushed upward or the whole area was eroded down to where everything was at about the same level. We know this happened because accumulations of the sandstone stopped and the area began to erode. We know that because there is a non-conformity between the Vishnu Schist/Zoroaster Granite and the layers above them. Another indicator that that was what happened is that the Zoroaster Granite does not intrude into the layers above.

Then we began to look at the next level.

The Bass Formation is actually a a composite of several types of material including limestone, conglomerates, ash and other materials. At the very bottom of the Bass Formation is a layer of Ash. This would indicate a period of volcanic activity somewhere and the ash was brought in, likely by wind, and layered over the eroded surface of the Vishnu Schist.

One of the layers is the Houtata Conglomerate which again layers over the Vishnu Schist and likely simply filled eroded valleys in the Vishnu Schist and so is seen to be pinched out, or gradually disappear, in some areas. Conglomerates, as we learned earlier, are made up of rocks that have been worn down and weathered, have rounded edges, that are then cemented into some matrix under pressure, temperature and over time. We can see an example of a conglomerate here.

The important issues are that the Vishnu Schist being eroded at the top shows that biulding up had ceased. It was no longer accumulating things but was higher than the surrounding areas and so was wearing away.

The Bass Formation shows an intruding, and then a retreating sea. We can tell that by looking at the composition of the formation as we move from top to bottom as outlined in Message 115.

This means we see the land lowering to allow the sea in, and then later in the process gradually rising again to make the sea retreat or that sea level rose and later fell.

The next layer, the Hakatai Shale, points to another change in the environment. Shale is much finer grained than sandstone or schist. This tells us that the environment was one of deposits in fairly calm water. From the colors of the shale we can get some idea of the depth of the water where it was deposited, the darker colors being laid down in deeper, oxygen lacking water while the redder ones show greater reaction with iron and a higher oxygen content, most likely from shallower water.

The fact that we see these in bands shows us that the sea level rose and fell cyclicly over and extended period. The overlying sandstone shows a return to a more energetic flowing water.

So, here are the steps so far.

Intially rock is produced somewere. It is weathered and then eroded to sand mixed with the smaller, finer clay, compressed, heated and over time turned into schist.

At some point during that process, magma intrudes into the sandstone or schist.

The whole body is later weathered and eroded which tells us it was exposed for sometime and higher than the surrounding areas. Accumulation stopped and erosion began.

At some time it was once again lower than the surrounding areas and we find a layer of conglomerate, larger weathered rocks cemented together in a matrix. We also find a layer of ash which indicates volcanic activity.

Above that we find a layer of limestone and conglomerate, which from the record left show a sea intruding and then regressing. The sea intrusion could be because sea levels rose and later fell, or becaise the land lowered and later was raised, or a combination of both.

Finally (so far) we find a layer that was laid down in what must have been a coastal swamp with slow moving water that allowed the finest particles to settle out forming shale. The changes in coloring of the shale indicated that the water level rose and fell over time. And capping that we find more sandstone and conglomerate indicating a return of more active water flow.

Now for the big question. It seems that this last layer is the first to actually show signs of live, but only of cyanobacteria. Is this correct?


Aslan is not a Tame Lion

This message is a reply to:
 Message 142 by roxrkool, posted 03-24-2006 11:11 AM roxrkool has responded

Replies to this message:
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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 145 of 283 (297943)
03-24-2006 9:04 PM
Reply to: Message 144 by roxrkool
03-24-2006 6:26 PM


Great, glad it passed. Now we can move on.
The next layer is the Shinumo Quartzite.

Quartzite has been mentioned once before, you said that the sandstone layer that became schist would have become quartzite if it had not contained clay.

So before we go too far into this layer can you tell us a little about quartzite, how it differs from schist and shale? Also generally can you remind us what they tell us about differing environments?


Aslan is not a Tame Lion

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 147 of 283 (298470)
03-26-2006 11:13 PM
Reply to: Message 146 by roxrkool
03-24-2006 9:47 PM


Shinumo
Well that's a question that will come up later anyway as there are several layers that include slate IIRC.


Aslan is not a Tame Lion

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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 148 of 283 (298646)
03-27-2006 11:29 AM


Where we are?
Just a short post to keep us aware of where we are in our climb from the bottom of the Grand Canyon.

We are now into looking at the third layer of Group 2.


Aslan is not a Tame Lion

Replies to this message:
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jar
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Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 150 of 283 (298723)
03-27-2006 2:19 PM
Reply to: Message 149 by roxrkool
03-27-2006 2:11 PM


Silica Cement
Okay, more new terms.

Let me see if I understand this because if I have it right, maybe we can make another summary and move on. Sounds like we may have found that express elevator I mentioned earlier.

As I understand it, Silica Cement is the product of diatoms, the carcasses of microscopic shelled critters, that have disolved in water and then precipitated out to form the cement. Is that correct?

AbE: The point I want to emphasize is that what we are looking at in this case is the product of several processes. The main part is reprocessed rock, rock that has been weathered and eroded and that was then cemented together by a cement that is also an example of reprocessing, diatom carcasses that have been disolved and formed the cement.

This message has been edited by jar, 03-27-2006 02:09 PM


Aslan is not a Tame Lion

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 Message 149 by roxrkool, posted 03-27-2006 2:11 PM roxrkool has responded

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jar
Member
Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 152 of 283 (298883)
03-28-2006 12:40 AM
Reply to: Message 151 by roxrkool
03-28-2006 12:26 AM


radiolaria??????
Okay, one more question then. I thought radiolaria were another living critter, like diatoms, common in the marine environments from earliest times on. So regardless of whether it was radiolaria or diatoms, we are looking at a source that began life as something living in the oceans.

Is that correct?


Aslan is not a Tame Lion

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jar
Member
Posts: 32019
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 154 of 283 (298963)
03-28-2006 10:15 AM
Reply to: Message 153 by roxrkool
03-28-2006 1:54 AM


Okay, trying another summary through the Shinumo Quartzite layer..
We began with the Vishnu Schist because it is the lowest exposed layer. It is schist, so originally it was sandstone. To get sandstone there first had to be some higher rock source which was weathered, accumulated in a basin, was later compressed and metamorphed into schist. There was clay, which is also a product of weathering mixed in with the sand.

So to this point, rock was created, weathered into smaller and smaller particles until we had sand and clays, washed or blown downhill (eroded) to collect in a basin, settled, compressed and over time, under pressure and heat became schist.

At sometime during this process, magama pushed through the sandstone or schist and over long periods of time cooled to become granite.

The whole structure, Vishnu Schist and Zoroaster Granite was pushed upward or the whole area was eroded down to where everything was at about the same level. We know this happened because accumulations of the sandstone stopped and the area began to erode. We know that because there is a non-conformity between the Vishnu Schist/Zoroaster Granite and the layers above them. Another indicator that that was what happened is that the Zoroaster Granite does not intrude into the layers above.

Then we began to look at the next level.

The Bass Formation is actually a a composite of several types of material including limestone, conglomerates, ash and other materials. At the very bottom of the Bass Formation is a layer of Ash. This would indicate a period of volcanic activity somewhere and the ash was brought in, likely by wind, and layered over the eroded surface of the Vishnu Schist.

One of the layers is the Houtata Conglomerate which again layers over the Vishnu Schist and likely simply filled eroded valleys in the Vishnu Schist and so is seen to be pinched out, or gradually disappear, in some areas. Conglomerates, as we learned earlier, are made up of rocks that have been worn down and weathered, have rounded edges, that are then cemented into some matrix under pressure, temperature and over time. We can see an example of a conglomerate here.

The important issues are that the Vishnu Schist being eroded at the top shows that biulding up had ceased. It was no longer accumulating things but was higher than the surrounding areas and so was wearing away.

The Bass Formation shows an intruding, and then a retreating sea. We can tell that by looking at the composition of the formation as we move from top to bottom as outlined in Re: To the Bass Formation. (Message 115).

This means we see the land lowering to allow the sea in, and then later in the process gradually rising again to make the sea retreat or that sea level rose and later fell.

The next layer, the Hakatai Shale, points to another change in the environment. Shale is much finer grained than sandstone or schist. This tells us that the environment was one of deposits in fairly calm water. From the colors of the shale we can get some idea of the depth of the water where it was deposited, the darker colors being laid down in deeper, oxygen lacking water while the redder ones show greater reaction with iron and a higher oxygen content, most likely from shallower water.

The fact that we see these in bands shows us that the sea level rose and fell cyclicly over and extended period. The overlying sandstone shows a return to a more energetic flowing water.

Now we look at the Shnumo Quartzite layer. As I understand it, it is one of the thicker remaining layers in the Unkar Group of the Grand Canyon Supergroup, with well over a 1000 feet remaining. There is evidence that the Underlying layer was partially eroded away before the Shinumo layer was laid down.

That is important because it tells us that the underlying Hakatai Shale group must have stopped forming, been pushed up above the surface of the water to become dry land, weathered and part of it was eroded away. In the same way, the evidence is that the Shinumo Quartzite was also produced in a shallow marine environment, meaning that the land was either eroded back down to where it was once again under water, that sea levels once again rose, that the land itself once again was lowered or some combination of all of those. There are remains of streams and river borne sediment and it looks like it was once a near coastal delta like the mouth of the Mississippi.

So, here are the steps so far.

Intially rock is produced somewere. It is weathered and then eroded to sand mixed with the smaller, finer clay, compressed, heated and over time turned into schist.

At some point during that process, magma intrudes into the sandstone or schist.

The whole body is later weathered and eroded which tells us it was exposed for sometime and higher than the surrounding areas. Accumulation stopped and erosion began.

At some time it was once again lower than the surrounding areas and we find a layer of conglomerate, larger weathered rocks cemented together in a matrix. We also find a layer of ash which indicates volcanic activity.

Above that we find a layer of limestone and conglomerate, which from the record left show a sea intruding and then regressing. The sea intrusion could be because sea levels rose and later fell, or becaise the land lowered and later was raised, or a combination of both.

Finally (so far) we find a layer that was laid down in what must have been a coastal swamp with slow moving water that allowed the finest particles to settle out forming shale. The changes in coloring of the shale indicated that the water level rose and fell over time. And capping that we find more sandstone and conglomerate indicating a return of more active water flow.

Either water levels dropped or the land rises because part of the layer gets weathered and worn away.

Next we find another return to a marine environment. The Shinumo Quartze layer is laid down in a shallow sea. It is made of cemented sand. The land continues to change and in the Sinumo layer we find the remains of streams and a river delta environment.

Through the Shinumo Quartzite layer including suggested changes from Roxrkool.

This message has been edited by jar, 03-28-2006 12:15 PM


Aslan is not a Tame Lion

This message is a reply to:
 Message 153 by roxrkool, posted 03-28-2006 1:54 AM roxrkool has responded

Replies to this message:
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