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Author Topic:   reply to Faith: Deposition and erosion
roxrkool
Member (Idle past 989 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 1 of 5 (191857)
03-16-2005 2:39 AM


From the first go round:
Faith writes:
If the column was built up from the bottom to the top, then each layer was the surface of the column for some period of time, so whatever it happened to be composed of at the time was at the surface.
That depends on whether we're speaking of water-lain sediments or subaerial sediments. "Surface" to me implies subaerial deposition or erosion. But yes, essentially your statement is correct - each layer was at one time at the top of the heap.
Faith writes:
rox writes:
A column represents landscape evolution through time. At the 'bottom' of a colum you may have limestone (this represents a very specific depositional system and environment). Above that you may have a marine shale, which represents a deeper marine environment. Many times, you will see a gradation between limestone and shale, something that could be called shaley limestone or calcareous shale, sometimes you do not.
It won't help for you to give a hypothetical abstract scenario here. You should address the example of the Grand Canyon so we can know we are talking about the same thing.
Well, Faith, the problem with us discussing the Grand Canyon in such detail requires quite a bit of research on both of our parts. Time neither one of us has, I suspect. However, I'll do what I can.
There being or not being a gradation doesn't matter here. What you have to explain is how that limestone at the bottom, if it represents millions of years of time, stayed put for those millions of years, how it hardened if it did, how it remained neatly horizontal as the column in the Grand Canyon certainly does for those millions of years, and how everything abruptly suddenly became marine shale instead of limestone.
Lithologic contacts are EXTREMELY important to geologists because they too are part of the story - sometimes the most important part. So they do, in fact, matter very much.
Additionally, large thickness of sediment can accumulate in basins by a process called basin subsidence. The weight of accumulating sediment causes the crust beneath the basin to deform and sink, thus providing amply accomodation space for continued basinal deposition. As long as the basin remains stable, it will continue to subside.
Now, what do you mean I have to explain how limestone stayed put for millions of years? Where exactly would it go? It would simply stay in the basin, the bottom of which would get deeper and deeper as long as the basin in which it's being deposited subsides. The deeper the limestone gets, the more it's subjected to increasing temperatures and pressures, both of which are responsible for lithification. Cement, in the form of either calcite or dolomite (for carbonate rocks), is also responsible for lithification, and it is sourced from the same rocks (usually).
As for how it remained horizontal. Well, that's what you would expect for deposition in a marine setting (or any other body of water, for that matter).
Tectonics are usually responsible for tilting and/or deforming strata. The presence of a low angle unconformity between the underlying Thunder Springs Mbr. and the overlying Mooney Falls Mbr. suggests tectonic movement and erosion preceding deposition of the Mooney Falls Mbr., which disproves your assumption that all the layers are perfectly horizontal and neat.
A gradational contact implies a transgressive sea. Meaning sea level at that point is rising, causing the sea to be deeper and deposit marine muds. A sudden, knife-sharp contact between shale and limestone is likely an erosional contact.
Faith writes:
rox writes:
A gradational contact implies a gradual sea level rise so that at that particular spot, limestone was deposited and then gradually overlain by more and more shaley constituents (shale = clay), until eventually the water was too deep, too poor in O2, too dark/murky, for deposition of carbonate to occur. If the sea level keeps rising, you may eventually get siliceous ooze (e.g., chert), etc. to deposit over the shale. If sea levels slowly drop over time, you will again get carbonate deposition, and if the sea level drop even further, sandstone (beach and eolian deposition), estuarine deposits, conglomerates (e.g., fluvial), etc. This is what Sequence Stratigraphy is all about.
The more sediment you dump in one spot, the more the ground and crust underneath deforms due to the weight. You can get thousands upon thousands of feet of sedimentary deposition as long as the basin is stable. It will simply continue to downwarp amidst the weight of the overlying material. However, tectonics will not allow the landscape to go too long before it takes effect, except in the stablest portions of the continents - the cratons.
You are not addressing my example of the Grand Canyon nor explaining the homogeneous content of each layer over the millions of years of each layer, nor explaining the horizontal disposition of the layers, over miles and miles of landscape. MILLIONS of years, horizontal, level, straight, ONE content.
Um... yes, I am, Faith. You just don't know enough of the units to see that. In fact, I am pretty much describing the Redwall Limestone - albeit I did it unknowingly at the time.
The Redwall Limestone is composed of four members:
Horseshoe Member (youngest) - a fine-grained, thin-bedded, gray limestone with occasional chert beds in the lower part of the member. 45 - 125' thick.
Mooney Falls Member - a course-grained, light olive-green nearly pure (99.5%) limestone containing abundant crinoid segments, the upper portion of which is interbedded with thin, white chert beds. Locally dolomitized. This unit pinches out laterally from a max of 3'. 200 - 400' thick.
Thunder Springs Member - limestone interbedded with chert and 100 - 150' thick. Low angle unconformity with the overlying Mooney Falls Mbr., possible the result of minor tectonic activity and erosion.
Whitmore Wash Member (oldest) - a fine-grained, thick-bedded, gray, nearly pure (~98%) limestone that locally grades into dolomite and contains interbedded chert beds at the top. Thins to the east from ~200' to ~70-80'. 100 - 200' thick.
The Redwall Limestone is far from being homogeneous. As you can see, each member represents a distinct depositional setting, which in turn results in differing lithologies, textures, grain-sizes, colors, etc. Additionally, all the units pinch out to the east and thicken to the west. This represents the deep sea (west) and land (east). So if you are assuming uniform thickness, you are wrong. Sure, laterally, the units persist for many miles (hundreds), but how large is the ocean basin? It's certainly not impossible.
Faith writes:
rox writes:
If instead the contacts are sharp and abrupt between limestone and sandstone, that tells you deposition was punctuated by either erosional periods or possibly periods of non-deposition (those are generally contentious interpretations).
The problem with this idea is that the horizontal configuration of the layers defies the idea of erosion of any given layer (wish I could draw you a picture) and periods of non-deposition could be millions of years long depending on how long you think the deposition took.
You are under the mistaken impression that the strata are PERFECTLY horizontal - meaning 0 dip - for the entire stratigraphic package at the Grand Canyon. This is not true if you look at the lateral extensions of each formation. My guess is that they initially dipped slightly to the west - towards the deepest portion of the paleosea.
In pictures (2-dimensional representations), the strata appear horizontal - this is called apparent dip - but only the deepest marine settings and lakes, would result in almost perfectly horizontal deposition. In truth, most strata, have some dip, either incipient from deposition or due to tectonics. It's just not especially noticeable at the GC.
And for the record, I did not suggest millions of years of non-depostion. In fact, I did mention that interpretations of non-deposition are rather contentious, and in truth, I am not a fan of
them, for the most part.
The idea of millions of years of non-deposition is just surreal, echoes nothing that happens normally on planet earth, and actually so does the idea of millions of years of erosion seem surreal. Erosion would leave a humongous hole in the layer at many points and in fact down through lower layers in many cases. Erosion doesn't occur by nearly invisible centimeters, it creates gullies and mudslides and leaves gashes in a landscape, the Grand Canyon itself being an example, not these (relatively) nice neat layers.
Again, I do not suggest there were millions of years of non-deposition; however, millions of years of erosion are certainly quite possible. As I've already stated before, the Rockies have been eroded down to almost peneplains at least twice. How long would that take?
I am not following your impression of what erosion entails. What do you mean by "humongous hole?" What does that look like exactly? Because your explanation that follows is pretty accurate. However, the effects of erosion can be vastly different depending on where and how it occurs.
Highlands (mountains, etc.) will erode much differently than lowlands (beaches, etc.). In the highlands, wind, rain, and chemical erosion all dominate (e.g., landslides, gullies, rivers, etc.), while in the lowlands, the beach can be slowly eroded along vast horizontal distances by a rising sea. The beach environment is highly erosive. Imagine what a slowing rising beach environment can do over several thousand years! You would also get erosion of channels and depostion in on flood plains/banks in estuarine settings.
Erosion cuts across all lithologies, eroding some rocks faster or slower depending on their composition. If you do some research into the Muav Limestone, which is overlain by the Redwall Limestone, you will see that it's surface is eroded. It is characterized by well-formed channels, formed either fluvially or through marine scour (possible in an estuarine environment). These channels are in turn filled with a carbonate known as the Temple Butte Limestone, and this limestone ONLY occurs in these channels. This channeling, as well as the infilling limestone, are strong evidence of erosion - and those channels are far more than centimeters deep.
Faith writes:
rox writes:
And the affected rocks may either be semi-consolidated or lithified. Meaning you are either eroding sediment or rock. If you bury limestone, it lithifies, and then bring it back to the surface, you will erode the limestone until it either erodes completely away, or depositional (instead of erosional) processes dominate. In which case, limestone (the lithified rock) will then be covered by whatever is being deposited at the time, be it alluvium, colluvium, conglomerate, sands, silts, etc.
But looking at the Grand Canyon layers we are supposed to imagine millions of years during which ONE limestone was laid down neatly and presumably lithified. These layers do not show signs of real erosion. They are neat and straight.
Most signs of erosion are not visible in a photograph on a webpage. You can see the large, angular unconformities easily, but the ones parallel or even sub-parallel to bedding are difficult to pick out in a photo. Additionally, you cannot see the erosion at the top of the Muav in a photo either. When you get down on the rocks, looking at them with your own two eyes, the evidence is there.
And again, the strata are not neat and straight except in the scale in which you are looking at them. The strata pinch out laterally, they thin and thicken laterally, they intercalate with other rock types laterally, etc. They are not as you imagine them to be.
And the huge ages that are supposed mean nothing covered it for millions of years. There is no evidence of covering. You have to read it into the scenario but there is no evidence of it. Why would a covering appear and then disappear anyway, leaving no trace in all those many many layers?
Do you mean erosional periods? Those are pretty much the only times when rocks were exposed for "huge ages." Because I agree with you, it's highly unlikely that there would be millions of years of non-deposition - except perhaps on a mountain. The interpretaion of erosion is usually (not always!) based on evidence for erosion or vast periods of surface exposure, such as the presence of channels, scour marks, paleosols, karst, or accumulation of eroded material (conglomerates, for example.). When evidence of erosion is lacking, that's when some like to turn to 'non-deposition.'
Faith writes:
Rox writes:
It is only in this scenario where you can have 20 million year old gaps between one depositional process and a preceeding (i.e., overlying) one.
You are reading in a gap that has no evidence for it whatever. The actual situation is a LEVEL surface of rock upon which another entirely different LEVEL HORIZONTAL rock appears. Erosion doesn't do that!
YES, it can and does!
A million years of non-deposition would not let the rock just sit there level and all. These are all jerryrigged explanations to explain what cannot be explained. They do not describe anything that could possibly happen on this real planet in real time.[/qs] No! Erosion, for the most part, WILL leave evidence - as stated above. And a "level surface upon which another level horizontal rock appears" can indeed be the result of erosion. Erosion is responsible for the CONTACT between the two rock. And if the sediment was originally deposited and later eroded and then deposited again by marine trangressions/regression cycles, then 'level' erosional surfaces CAN occur.
Faith writes:
rox writes:
Not only that, but 1,000 years later, all that stuff that originally covered the limestone, may be completely eroded itself.
There is NO evidence of such erosion in the layers of the Grand Canyon. You would have lumpy lopsided layers, not straight layers ANYWHERE if it happened. The actual appearance of the real world canyon defies your explanation.
YES THERE IS! Evidence is everywhere, but you cannot see it in a photograph taken hundreds or thousands of feet away. Go to your local library and do some research on EACH stratigraphic formation of the GC. The erosive episodes are documented, right along with all the different lithologies, fossils, etc.
Faith writes:
rox writes:
The only records we have of some erosional processes are remnants of that eroded material. Therefore, the time implied in ONE geologic column is not only apparent in the amount of time needed to deposit the sections, but in the amount of time to deposit and erode sediments and rocks that we will never see represented in situ.
This is apparently how geology tries to account for the obvious irrationality of the geologic column idea, the clear demarcations and the sharply different kinds of rocks /sediments, but it doesn't work. You are postulating something that does not happen according to any known natural laws.
Come on. Open your eyes and your mind. Look around you, the mountains are being eroded - what used to be there? Are all mountains homogeneous granite? Or are they all different, some formed from igneous rocks, other from differing layers of rocks.
The tops of some mountains are limestone. Isn't it possible there was a shale on top of that limestone? To check this, you would look for other occurrences of the same limestone, perhaps in lower lying areas. Perhaps on the same mountain where the limestone was downdropped by a fault. And on top of that limestone - shale. Shale overlain by sandstone, overlain by coal. Hmm... what would that mean???
Sometimes geologists resort to using their imagination when it comes to recreating paleolandscapes, for the most part, however, there is ample evidence.
You are postulating that erosion acting upon a perfectly flat surface over millions of years removed material in such a way as to leave perfectly even horizontal strata. Wouldn't happen that way in a month let alone a million years. It would disrupt layer upon layer of material. And why would ALL the additional material erode away anyway, in ALL those many many layers? This is fantasyland. This is not science.
If the rock was lithified, or even semi-consolidated, the layers would not necessarily be disrupted. Disruption of unconsolidated material occurs via fossils, compaction, fluid movement, earthquakes, landslides, etc. If erosion was caused by trangressing/regressing seas, then no, layers may not be disrupted. Go to your local beach and dig a hole. Are all the layers disrupted? Horizontal erosive surfaces will happen during marine transgressions.
You really are not addressing the actual visible situation of the Grand Canyon. There is NO evidence of erosion that amounts to a hill of beans in the layers themselves. The erosion all occurred after the entire column was built.
Well, in fact, I was not referring to the GC in the post you replied to. It was more of a general post, but as it turns out, it addressed the GC just fine. You simply do not know enough about the formations comprising the Grand Canyon to know this, because the actual geology is far different that what you have imagined.
Faith writes:
rox writes:
How do we know time/rock is missing? Because if you travel a few miles away to another section/column, that missing rock might not be missing.
I'm sure there are different deposits in different places even over a few miles, but there is no REAL reason to assume that rock that appears in one place was "missing" anywhere else.
Of course there are REAL reasons. If you find an outcrop that contains 10 different types of rocks. You describe each unit in detail. One mile down the road, you find the same 10 units, in the same order, same color, same fossils, same EVERYTHING, except that this new location has an extra unit smack dab in the middle. How would you interpret this?
sources:
http://www.geocities.com/earthhistory/grandb.htm
Classzone.com has been retired
Angelfire - error 404
This message has been edited by roxrkool, 03-16-2005 12:27 PM
This message has been edited by roxrkool, 03-16-2005 12:29 PM
This message has been edited by roxrkool, 03-16-2005 12:35 PM

AdminSylas
Inactive Member


Message 2 of 5 (191860)
03-16-2005 3:28 AM


Thread moved here from the Proposed New Topics forum.

AdminSylas
Inactive Member


Message 3 of 5 (191865)
03-16-2005 3:35 AM


Thread moved here from the Geology and the Great Flood forum.
Oops. I'm having a very bad day as admin.
I have moved this back into Proposed New Topics. Sorry about that! This original post here is given as a reply to Message 298, posted by Faith.
On reflection, this post is not yet the kind of continuation requested. The previous thread was getting a bit out of hand, with lots of she said, he said, I said exchanges, and not always very polite. This post is continuing in the same old vein. The substance is very strong, but still think we can start with a bit less focus on persons.
We do want to start up the topic again, but please take into account the requests of Adminjar in Message 310.
jar writes:
If there is any point in continuing let's start a new PNT. Also, if that step is taken please try to narrow the focus to one small part such as how an initial layer is created, eroded, modified and preserved.
Also from Admin in Message 303.
Admin writes:
To everyone: please stay focused on the topic and avoid the "did not, did too" and "you don't realize how wrong you are" digressions.
Past experience teaches that by 300 posts most threads have drifted way off topic, but that's not the case here. This thread is a bit repetitive, but this seems natural. I don't know if one of the moderators will close this thread, but if they do I hope someone proposes a continuation thread. I think the questions being raised on both sides will find better answers if the discussion continues.
To that end, we want to be pretty strict about the start of the new thread. Thanks for this but I think we can get off to a more focused start. Perhaps I should have let Adminjar handle this proposal.
This message has been edited by AdminSylas, 03-16-2005 03:46 AM

Replies to this message:
 Message 4 by roxrkool, posted 03-16-2005 11:37 AM AdminSylas has not replied

roxrkool
Member (Idle past 989 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 4 of 5 (191921)
03-16-2005 11:37 AM
Reply to: Message 3 by AdminSylas
03-16-2005 3:35 AM


I shall re-read the post, Sylas. I wrote it late last night and was getting rather tired by the end. I just wanted it over with, to be honest, and posted it rather quickly without a second glance. lol
I also just wanted to respond to Faith since the other thread was closed.
What exactly do you think I need to do? Was I rude in some spots? I was trying to be more illustrative, than anything.
Any suggestions would be helpful.

This message is a reply to:
 Message 3 by AdminSylas, posted 03-16-2005 3:35 AM AdminSylas has not replied

Replies to this message:
 Message 5 by Adminnemooseus, posted 03-16-2005 12:36 PM roxrkool has not replied

Adminnemooseus
Administrator
Posts: 3974
Joined: 09-26-2002


Message 5 of 5 (191927)
03-16-2005 12:36 PM
Reply to: Message 4 by roxrkool
03-16-2005 11:37 AM


Faith's version of new topic has been approved, this topic rejected
Going to reject this "PNT".
Please take the discussion to Deposition and Erosion of Sediments (I did like this topics title, and used it for Faith's topic).
Please bring any discussions of topic promotions to "Considerations of topic promotions from the "Proposed New Topics"", link below.
Adminnemooseus

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This message is a reply to:
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