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Author Topic:   Exploring the Grand Canyon, from the bottom up.
edge
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Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.6


Message 106 of 282 (296642)
03-19-2006 5:50 PM
Reply to: Message 105 by jar
03-19-2006 2:01 PM


Re: still on clay
This from Wikipedia:

"Clay is a generic term for an aggregate of hydrous silicate particles less than 2 μm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid, but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and high plasticity index."
(bold added)

Basically, most silicate minerals are susceptible to acid attack. This removes a bunch of cations and leaves behind a residue made up of clays.


This message is a reply to:
 Message 105 by jar, posted 03-19-2006 2:01 PM jar has responded

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jar
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Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 107 of 282 (296644)
03-19-2006 5:59 PM
Reply to: Message 106 by edge
03-19-2006 5:50 PM


Okay trying yet another final summary of the lowest two exposed layers.
Okay, great. So the proces that made our sand for the sandstone likely also produced the clay that went into making schist instead of quartzite.

So here is the layers to date.

Now for the summary.

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 Scist 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.

Is that a reasonable summary of the Grand Canyon so far?


Aslan is not a Tame Lion
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Replies to this message:
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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.6


Message 108 of 282 (296649)
03-19-2006 6:46 PM
Reply to: Message 107 by jar
03-19-2006 5:59 PM


Re: Okay trying yet another final summary of the lowest two exposed layers.
Well done. You realize this will be a very long thread. We'd better start training the next generation of geologists to finish it.
This message is a reply to:
 Message 107 by jar, posted 03-19-2006 5:59 PM jar has responded

Replies to this message:
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jar
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Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 109 of 282 (296652)
03-19-2006 6:59 PM
Reply to: Message 108 by edge
03-19-2006 6:46 PM


Thanks and moving on.
You realize this will be a very long thread.

Yes it will be, may even need a continuation or two, but people like Walt Brown and Steve Austin like to keep it simple so that they can con folk out of good money to pay for worthless books and videos. I think it's important to take whatever time is needed to explain things.

So next up, the Houtata Conglomerate.

Roxrkool brought this up and ao I'd like to deal with it before the Bass Limestone layer.

Does anyone have any pictures or information on this layer?

What does "pinch out" mean?

Why would that be significant?


Aslan is not a Tame Lion
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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.6


Message 110 of 282 (296654)
03-19-2006 7:19 PM
Reply to: Message 109 by jar
03-19-2006 6:59 PM


Re: Thanks and moving on.
Try this for background. The Hotauta Conglomerate is considered to be a member of the Bass Limestone.

http://www.rockhounds.com/grand_hikes/geology/bass_limestone.shtml

In reading this website, you will notice reference to the fact that the Bass includes several rock types. This kind of refutes Faith's statements on another thread about continuous deposition of a rock type and simplification of the section for mapping and descriptive purposes.


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roxrkool
Member (Idle past 872 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 111 of 282 (296682)
03-19-2006 10:57 PM
Reply to: Message 110 by edge
03-19-2006 7:19 PM


Re: Thanks and moving on.
This is something I keep trying to point out to Faith. Just because the pretty geologic images online or in books show straight lines and single-lithology formations doesn't mean it is anywhere near representative of reality. And it's not just in the Grand Canyon that this happens - it's everywhere, as you know.
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 112 of 282 (296769)
03-20-2006 10:23 AM


To the Bass Formation.
As we move on to the next layer, I have a few questions.

First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?

Second, the article says that the Bass Formation was created by an intruding and then retreating sea. It even provides the directionality of the event.

How is that determined? What evidence led to that conclusions?

Third, Roxrkool mentioned a layer pinching out. What does that mean and how are such things identified?


Aslan is not a Tame Lion
Replies to this message:
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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.6


Message 113 of 282 (296940)
03-20-2006 8:38 PM
Reply to: Message 111 by roxrkool
03-19-2006 10:57 PM


Re: Thanks and moving on.
This is something I keep trying to point out to Faith. Just because the pretty geologic images online or in books show straight lines and single-lithology formations doesn't mean it is anywhere near representative of reality. And it's not just in the Grand Canyon that this happens - it's everywhere, as you know.

Yah, probably wasting your time. Faith is impervious to facts. I would add that much of the simplification we see is due to scaling. We simply cannot show all of the detail unless we go to a one-to-one scale, which would kind of defeat the purpose of creating a map. But Faith has decided what is correct, no reason to listen to anyone who actually does the work.


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edge
Member
Posts: 4450
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 2.6


Message 114 of 282 (296945)
03-20-2006 8:52 PM
Reply to: Message 112 by jar
03-20-2006 10:23 AM


Re: To the Bass Formation.
First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?

Since there is deposit of eroded Vishu material on the unconformity, we can only say that there was time to weather and erode some of the Vishnu. We don't know how much, or what else might have been there. However, by judging the metamorphic grade of the Vishnu (in other words, these rocks probably formed deep in the core of a mountain range), we can assume that there was a considerable thickness that has been eroded.

Second, the article says that the Bass Formation was created by an intruding and then retreating sea. It even provides the directionality of the event.

How is that determined? What evidence led to that conclusions?

This is called a transgressive-regressive sequence. It is determined by mapping the paleoenvironments of the different formations. When a rock sequence fines upward (sand to silt to mud to limestone, for instance) it is consdered to be caused by deepening water. THe opposite would be shallowing water, or a regressive sequence. It is determined by mapping the rocks.

Third, Roxrkool mentioned a layer pinching out. What does that mean and how are such things identified?

It is the characteristic of a layer of rock thinning laterally so that it eventually disappears. This is very common in the geologic record. It can be caused by several different processes.


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roxrkool
Member (Idle past 872 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 115 of 282 (296971)
03-20-2006 10:58 PM
Reply to: Message 112 by jar
03-20-2006 10:23 AM


Re: To the Bass Formation.
First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?

I'm not sure, but my guess is that it's simply an assumption based on the amount of time the disconformity represents - 500 million years.

Second, the article says that the Bass Formation was created by an intruding and then retreating sea. It even provides the directionality of the event.

How is that determined? What evidence led to that conclusions?


Good question!

The easiest way is to look at it a strat section of the Bass limestone. First of all, we know that the oldest layers are at the bottom and the youngest at the top. So, from the description, we can see that coarse clastics (e.g., conglomerates and sand, etc.) occur at the bottom of the formation; limestone, dolomite, and shale probably occur somewhere in the middle; and then more coarse clastics at the top.

Below is a simplified section not exactly what occurs in the Bass Limestone, but perhaps somewhat reflective of it:

5. clastics- TOP
4. limestone
3. deep marine shale (deepest water accumulation)
2. limestone
1. coarse clastics - BOTTOM

The best way to look at and think about sedimentary rocks is from the bottom up because this is the order of deposition.

1. Coarse clastics, such as conglomerates makes me think of a high-energy terrestrial environment such as a stream - maybe a braided stream dumping into the ocean. Or maybe a glacial stream that is carrying cobbles sourced from a retreating glacier. HERE is a great site that describes a cobble beach. Is it analagous to the Hotauta Conglomerate? I don't know. Maybe. I believe there is evidence of Proterozoic glaciation.

2. Above the clastics, you have limestone. The only way to get limestone on top of clastics is to raise sea level or for the basin to subside, either way, it's a relative rise in sea level.

3. On top of limestone is shale, which is a sediment generally deposited in deeper water than limestone, so that again tells you there must have been a relative rise in sea level from the time limestone was deposited to the time shale was deposited. This shale represents a period of maximum flooding (i.e., inundation, incursion, etc.).

4. Then from shale, you have limestone, which is generally deposited in shallower water than shale. This indicates a relative drop in sea level. In that one spot, water depth has decreased because limestone production can only occur above a certain depth.

5. On top of the limestone you have, clastics again. Water level has decreased to the point of again exposing that particular spot to the surficial environment - another drop in relative sea level.

So as you move up through the simplified section above, you have a transgressing sea up to the shales/mudstones, which represent sedimentation when the water was at its deepest and highest level. Then water levels started dropping until the cobbles were able to make an appearance again. It's possible the cobbles continued to be deposited while the sea was rising, but that beach environment was pushed backwards because of rising sea levels.

Figuring out from which direction the water was coming in from is generally a matter of mapping out the aerial distribution of the lithologies or cross-sections derived from drill holes in various locations. For example, a regional cross-section of an "easterly transgressing sea" will have beach sands moving east as you travel up the stratigraphic column.

The following is a simplified example of trangressive-regressive sequence:


Click to enlarge

For the aboveimage imagine you are looking north so that the x-axis represents west (seaward side towards the left) and east (landward side towards the right). The y-axis represents time - oldest at bottom, youngest at top.

As you move up through time, water levels are rising and so the sea is transgressing in an easterly (e.g., landward) direction (towards the right). The middle of the 'V' represents the period of time when water levels are at their highest, so the beaches have been pushed way back into the continent. Later, water levels start dropping and so all the lithologies are moving in a seaward direction (towards the left).

Did that help or are you even more confused?

Third, Roxrkool mentioned a layer pinching out. What does that mean and how are such things identified?

I made that assumption based on the cross-section below:

Click to enlarge

The person who drew that section made it a point to show the conglomerate is not a continuous layer at the bottom of the Bass Limestone - it's shown pinching out to the right. Now that doesn't necessarily mean it always pinches out to the east or west. In addition, in the link edge and I have both provided, the author states:

"An underlying layer of conglomerate known as the Hotauta Conglomerate Member deposited in low areas of the eroded and hilly Vishnu terrain composes the lowermost unit of the Bass Limestone."

This tells the reader the erosional surface developed on the Vishnu is irregular with high spots and low spots (in other words, topographic highs and lows) and the low spots are often filled with conglomeratic or other coarse clastic material. Since I don't think the low spots have vertical sides like a swimming pool, but rather sloped sides like a parabolic dish, I'm assuming the conglomerate is thickest in the middle and thins out - or pinches out - on the sides of the parabola.

This message has been edited by roxrkool, 03-20-2006 11:06 PM


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 116 of 282 (296972)
03-20-2006 11:17 PM
Reply to: Message 115 by roxrkool
03-20-2006 10:58 PM


Re: To the Bass Formation.
jar writes:

First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?

to which Roxrkool replied:

I'm not sure, but my guess is that it's simply an assumption based on the amount of time the disconformity represents - 500 million years.

Well, I'd sure like to get more than that.

First, the time period is something we're not dealing with yet and it certainly wasn't something that the people that decided that what was seen couldn't be explained under a Young Earth Concept would have known about.

So is the missing material a new concept that was added after the physical layering was determined and based on dating techniques? If so, then we can just move on for now.

Or is there, as edge implied, evidence that there should be something there? Are there other places where there is stuff between the Vishnu Scist and the Bass Layer?

AbE:

One more thing. Your drawing points to one more thing we haven't discussed yet. Between the Vishnu Schist and the Bass Formation it shows som Ash layers.

Okay, what is Ash and what does an Ash layer tell us?

This message has been edited by jar, 03-20-2006 10:25 PM


Aslan is not a Tame Lion
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roxrkool
Member (Idle past 872 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 117 of 282 (296980)
03-21-2006 12:40 AM
Reply to: Message 116 by jar
03-20-2006 11:17 PM


Re: To the Bass Formation.
What edge said is also true. The thing is, by the time people started studying the Grand Canyon, many geologic principles were already in place. The large gap in time was mainly the result of fossil assemblages and dating techniques.

I haven't studied the Grand Canyon much so I really don't know for sure, but in other places in the Southwest, it's likely that a more complete section is present. One that perhaps contains rocks that represent time or deposition prior to the Bass.

Okay, what is Ash and what does an Ash layer tell us?

Ash is very fine dust-like material ejected from a volcano during eruption. It is variably composed of glass shards, which is rapidly cooled very fine-grained molten material (like obsidian), and other minerals such as feldspar, quartz, sulfide, biotite, pyroxene, as well as rock fragments.

The chemistry of ash is very diagnostic and can be chemically characterized by looking at grain sizes, mineralogic compositions, and trace-element compositions. Each volcanic eruption, even if it's from the same volcano, will be slightly different, and so by studying these differences and mapping out the extents of the ash layers themselves, we can often trace them back to their source.

When deposited on land, these ash layers can remain relatively fresh retaining their primary mineralogy, though glass will devitrify (to clay) over time in the presence of water. When the ash falls on water, however, like the ocean or an inland sea, and is subsequently deposited in a subaqueous settings, the glass shards quickly devitrify into clay particles. (Note: devitrification means that the amorphous glass structure, which is unstable at surface conditions and free of water, will change into a more stable, orderly, and water-bearing form, and so in the case of volcanic glass, the glass converts to clay - typically bentonite.)

These ash layers, because of their volcanic origin, can also be dated using radiometric dating methods.

This message has been edited by roxrkool, 03-21-2006 12:42 AM


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 118 of 282 (297079)
03-21-2006 1:39 PM


Okay, trying a summary and see if we are ready to move on.
So let me summarize where we are at this point.

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.

Is this a reasonable summary of what we see so far?

This message has been edited by jar, 03-21-2006 03:33 PM


Aslan is not a Tame Lion
Replies to this message:
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Jazzns
Member (Idle past 1886 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 119 of 282 (297096)
03-21-2006 4:09 PM
Reply to: Message 118 by jar
03-21-2006 1:39 PM


Re: Okay, trying a summary and see if we are ready to move on.
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.

I don't think this statement can be made definitivly. Sea level can rise by both the land dropping and more water being put into the oceans. Vice versa sea level can drop by the land raising or water being removed from the oceans. Most sea level changes (correct me rox or IHR or edge) that you see in transgression/regression sequences occur due to the cyclic nature of ice ages on earth. During an ice age more water is trapped in ice and sea level lowers. Of course this is just the mainstream view so if we are trying to be 'controversy' correct all we can say is that the water rose and fell in comparison to the land via EITHER the land or water raising or lowering.


Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.3


Message 120 of 282 (297099)
03-21-2006 4:20 PM
Reply to: Message 119 by Jazzns
03-21-2006 4:09 PM


Reasonable point
I agree that it's certainly a reasonable variation on the summary.

So...

for out geologists.

Is there something that could tell us whether what is seen in the Bass Formation is the result of changing land levels, changing sea levels, or some combination of both?


Aslan is not a Tame Lion
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