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Author Topic:   Exploring the Grand Canyon, from the bottom up.
roxrkool
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Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 136 of 260 (297337)
03-22-2006 2:43 PM
Reply to: Message 135 by jar
03-22-2006 1:26 PM


Re: Time to play stump the chump.
Correct. If you go back to the link you provided and click forward through the images, you'll see it's believed the channel was stream-cut... I think.

It's a shame those images are not individually annotated.

This message has been edited by roxrkool, 03-22-2006 02:46 PM


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jar
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Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 137 of 260 (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
This message is a reply to:
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roxrkool
Member
Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 138 of 260 (297567)
03-23-2006 12:03 PM
Reply to: Message 137 by jar
03-22-2006 3:05 PM


Re: On Angular Unconformities and other wonders
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.


Basically yes, although the sediments were likely somewhat consolidated to fully lithified when they were eroded. However, without knowing more about that particular photo, I hesitate to state any sort of conclusion.

Abe: I now think the are probably turbidite deposits, which means the Bouma Sequences (see below) were deposited simultaneously.

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?


Well, like I said above, it's a bit difficult for me to know what's going in that picture since I don't really know what the lithologic units are.

I think they are part of the Brushy Canyon Formation down somewhere in Texas because if you click through the other images in that slide 'presentation' it discusses the Brushy Canyon.

If so, those rocks are sandstones, siltstones, and limestones, and the the actual depositional setting appears to be the submarine environment. The channel, instead of being stream-cut, may instead be the result of underwater gravity-induced 'slides' - similar to a landslide. These sediments are not likely to have been lithified, but they were somewhat consolidated/compacted prior to being eroded.

From the photo alone, I really can't tell if those layers are turbidite-deposited, but they do have the look of it, and the other images appear to suggest they are.

That means some of the layers were deposited simultaneously. The heavy stuff (gravels, etc.) will settle out first and the finer muds last. The layers are graded and the sequence of layers is called a Bouma Sequence. In some settings, these sorts of deposits can be found one atop another for several hundred feet, possibly more.

Below are some images that might help.

How turbidites develop (SOURCE):


Click to enlarge

The resulting Bouma Sequence (SOURCE):


Click to enlarge

Turbidite images:

Folded turbidites

Cretaceous turbidites

Underwater turbidites

This message has been edited by roxrkool, 03-23-2006 12:04 PM


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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 139 of 260 (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
  
Minnemooseus
Member
Posts: 3251
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001


Message 140 of 260 (297597)
03-23-2006 2:06 PM
Reply to: Message 137 by jar
03-22-2006 3:05 PM


Re: On Angular Unconformities and other wonders
Your photo is from http://strata.geol.sc.edu/PermianBasinTxNMx/pages/049-West-Face.html

Clicking on that photo gets you to http://strata.geol.sc.edu/PermianBasinTxNMx/pages/050-Guadalupe%20Mts-Delaware-Group.html, which seems to be an attempt to explain the geology shown by the photo.

The process shown is that of a dropping sea level. My impression is that the non-horizontalness of some of the strata relative to other strata may be a result of some of the strata actually being deposited on a slope. In other words, an exception to the general rule that sediments deposition results in horizontal layers.

There is also the possibility that there was some soft sediment deformation, such as from the slumping of whole multiple layers.

Perhaps what looks to be the major angular unconformity in the photo is actually just a larger version of a channel scouring and filling.

It is hard to do a great analysis from a photo. But, of course in the context of this forum, that is what we have to work with.

Again, getting pretty remote from things Grand Canyon.

Moose


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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 141 of 260 (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
This message is a reply to:
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roxrkool
Member
Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 142 of 260 (297786)
03-24-2006 11:11 AM
Reply to: Message 141 by jar
03-24-2006 12:09 AM


Re: But what is shale?
Here are a few good descriptions:

shale 1
shale 2
oil shale
Oilfield definition of shale

shale photos with clickable images
more shale images from Black Shale Research
.
.
.
Hakatai Shale from Grand Hikes website
.
.
.
.
.

Good descriptions of other sedimentary rocks.


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 Message 141 by jar, posted 03-24-2006 12:09 AM jar has responded

Replies to this message:
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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 143 of 260 (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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roxrkool
Member
Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 144 of 260 (297902)
03-24-2006 6:26 PM
Reply to: Message 143 by jar
03-24-2006 12:22 PM


Re: Summary up to the second layer of the Supergroup.
That was a pretty good summary. The only thing I guess I would mention or suggest is to start thinking or practicing thinking of the rocks from bottom to top - the order of deposition. It takes some getting used to, but once you get the hang of it, the processes might start making more sense.

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?

I believe the first signs of life in the Grand Canyon are visible in the Bass Limestone. I recall reading about stromatolites in the Bass.

This message has been edited by roxrkool, 03-24-2006 06:26 PM


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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 145 of 260 (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
This message is a reply to:
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roxrkool
Member
Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 146 of 260 (297955)
03-24-2006 9:47 PM
Reply to: Message 145 by jar
03-24-2006 9:04 PM


Re: Great, glad it passed. Now we can move on.
I'll have to read a little about this one first. I think this one is a pretty interesting unit. Generally, 'quartzite' is a metamorphic term, but the Shinumo is underlain by a shale (not a slate, which is metamorphosed shale) and the Dox Formation, which is also unmetamorphosed (as far as I know) sedimentary rocks. It's pretty hard to sandwich a metamorphic layer between two unmetamorphosed units.

My guess is that the Shinumo is not the result of metamorphic processes, but simply a well to strongly cemented sandstone. I'm going to go read now. :)


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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 147 of 260 (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
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 148 of 260 (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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roxrkool
Member
Posts: 1489
From: Nevada
Joined: 03-23-2003


Message 149 of 260 (298718)
03-27-2006 2:11 PM
Reply to: Message 148 by jar
03-27-2006 11:29 AM


Re: Where we are?
Jar, I can't seem to find anything (easily accessible) that discusses the Shinumo in more detail; however, in the Grand Hikes link it states:

Geologists have recognized four or five poorly defined members based on varying lithologies of the sandstones within the Shinumo. These consist of a bottommost member of conglomeratic subarkose and subminiature quartz sandstone, overlain by a mature quartz sandstone, followed by a brown quartz sandstone containing clay galls and fossilized mud cracks, followed by a hard, fine grained sandstone with rounded and well sorted quartz grains lithified with a siliceous cement.

The lithology and structure of sediments making up the Shinumo sandstones suggest a shallow marine environment with stream and river born sediments deposited in near coastal delta environments. The contact between the Shinumo and overlying Dox Formation is regarded as generally conformable, with the end of Shinumo time marked by a rapid transgression of the Dox sea and ensuing deposition of the lower shaley and mudstone members of the Dox.


It's pretty clear that in this case, the term 'quarzite' refers to a rock of sedimentary origin that has been strongly cemented - in this case with silica cement. (Apparently I did not read this link very well since I missed the reference to silica cement.)

Without reading a more detailed description of the Shinumo, I can't offer more on the presence of the 'silica cement.'


This message is a reply to:
 Message 148 by jar, posted 03-27-2006 11:29 AM jar has responded

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jar
Member
Posts: 24784
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 150 of 260 (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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