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Author Topic:   Deposition and Erosion of Sediments
Parasomnium
Member
Posts: 2206
Joined: 07-15-2003
Member Rating: 4.8


Message 16 of 127 (191980)
03-16-2005 5:39 PM
Reply to: Message 7 by Faith
03-16-2005 4:37 PM


Faith,

There's no need for emotional outbursts. Let me paraphrase you: "If you can't control your emotions, get off this site."

Here's what I gleaned from a little booklet I bought at one of the bookshops near the Grand Canyon, quite a few years ago.

  • 2 BYA: sediments and volcanic material accumulated
  • 1.7 BYA: mountains are uplifted
  • 1.5 BYA: mountains eroded to a nearly level plain
  • 1.2 BYA: plain subsided; Grand Canyon Supergroup layers deposited
  • 800 MYA: fault block mountains formed
  • 700 MYA: mountains eroded to hilly topography
  • 600 MYA: area subsided; Paleozoic layers deposited
  • 230 MYA: Mezozoic sediments deposited
  • 65 MYA: uplift and erosion of mezozoic sediments
  • 4 MYA: Colorado river began to cut the Grand Canyon; volcanic activity within the last 1 million years in the western Canyon

So, you see, there has been erosion of the layers over time. There have even been layers which are now completely gone.

You also need to know that most of the layers were formed in marine circumstances. Of a total of 15 layers mentioned in my booklet, eight have a depositional environment labeled "sea", one is labeled "floodplain", one "swamp" and one "metamorphosed sea sediments". The other layers are labeled "desert", "disconformity", "the great unconformity", and "molten intrusion".

The bottom of a sea is a very tranquil place and can remain that way for millions of years, giving it time enough for sediment to build up undisturbed without being eroded faster than it builds.

This message has been edited by Parasomnium, 16-Mar-2005 10:49 PM


We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further. - Richard Dawkins

This message is a reply to:
 Message 7 by Faith, posted 03-16-2005 4:37 PM Faith has replied

Replies to this message:
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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.2


Message 17 of 127 (191981)
03-16-2005 5:43 PM
Reply to: Message 13 by Faith
03-16-2005 5:15 PM


Is the GC laid out in nice even layers?
I found a couple drawings that might help you visualize the Grand Canyon in another way. These take two slices through the canyon and the surrounding lands, one north-south and the other east-west. It only looks at the major sections but you will see similar signs of change and erosion within each major section.


Click to enlarge

You can find the images and write up here

Interestingly, IIRC, you'll find whole sections of rock missing in the Grand Canyon itself. For example, all of the rock from the time of the dinosaurs eroded away and simply isn't present. We know it must have been there at one time because if we wander over into Utah or Montana we can find it present. But erosion eliminated that whole massive section in the Grand Canyon itself. That's why the only dinosaur remains ever found in the Grand Canyon are those that washed in.

So what is seen in the Grand Cayon cannot be the result of one incident. There are pieces parts missing. It's not a neat, continuous act of creation, rather it's,like life itself, a random record ofconstruction and destruction carried out over billions of years.

This message has been edited by Admin, 03-17-2005 09:26 AM


Aslan is not a Tame Lion

This message is a reply to:
 Message 13 by Faith, posted 03-16-2005 5:15 PM Faith has replied

Replies to this message:
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PaulK
Member
Posts: 17178
Joined: 01-10-2003
Member Rating: 2.6


Message 18 of 127 (191984)
03-16-2005 5:50 PM
Reply to: Message 14 by Adminnemooseus
03-16-2005 5:25 PM


Re: Time out for me for a while
It's a bit late for that isn't it ?

Faith's argument is simple. Her assumptions about geology do not allow for the erosion observed by geologists. Therefore the erosion cannot exist. The idea that her uninformed assumptions might be less reliable than actual observations doesn't seem to occur to her.

Faith would have done much better to ask for an explanation of the problems she believes to exist - that would at least have shown a willingness to consider the issue.


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Jazzns
Member (Idle past 3222 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 19 of 127 (191986)
03-16-2005 5:57 PM
Reply to: Message 1 by Faith
03-16-2005 12:25 PM


Deposition on dry land
You seem to have the largest problem with sedimentary depostion on dry land. Is this correct?

I will go ahead and proceed assuming that it is.

The first thign to note is that most deposition does happen under water. The majority of the layers of the grand canyon were formed under water so there is no problem with those. When a particle of silicate falls to the bottom of the ocean there is not much that will necessarily move it and it will likely get buried. Just so we are clear, do you have any issues with this basic fact? Do you recognize that as material enters calm water it will settle out and pile up over time? Do you recognize that it is very difficult for a natural process to remove material from the bottom of a body of water?

No on to the dry land sediment which you claim cannot build up because it is doomed to be eroded. First of all, the one dry land layers of the GC that I can think of off the top of my head has ample evidence of erosion. The Coconino sandstone is cross-bedded showing that wind moved around the sand particles in a way that creates that feature like how we think of sand dunes in a desert. The reason this layer was preserved is that the wind did not erode the sand into other locations faster than sand was being put into the environment by other types of weathering. We can see this happening today in deserts that are near mountain ranges. Let me explain.

A high profile area like a mountain is subject to much more weathering than a flat coastline or plain. Because of this a the rate of erosion on a mountain is much greater than the rate of erosion on a plain. So if the source of material for the desert that created the Coconino sandstone was a more high profile area then the desert that produce the Coconino sandstone then it is likely that more sand was ending up in the desert then was being removed by wind, rain, etc. This is how depostion works. If more sand HAD been removed the Coconino sandstone would not have existed and we would be looking at an erosional disconformity in the GC rather than a layer of sandstone.

Enough sand was deposited over the life of the desert that, when the sea level rose again, the majority of it was shoved under water where erosion rates are MUCH slower then on land.

Key note here. No is is saying that the all the sand that ever got dumped into the desert that produced the Coconino sandstone stayed there. All the layers tell us is that, overall, deposition was greater than erosion at and between the layers of sediment at the GC when it was being deposited.

A basic understand of this does not have to be difficult. All it takes are a few key concepts.

1. Weathered material settles in low places.

2. Over a very wide area the tendency of settling material is to spread flat with respect to the area. This is easily observable today in modern lakes, oceans, deltas, and deserts.

3. The rate of weathering in these low areas affects how much weathered material will stick around. Material in calm bodies of water like the ocean or lakes experiences the least amount of weathering so most of the material that makes it here will stick around.

4. When the type of material changes all it does is begin to cover up the material already there. When this happens for a long enough period of time a differnt type of layer is made on top of the old one.

5. As long as the rate of deposition is greater than the rate of erosion, material will pile up and bury older material. We can watch this happening today in modern depositional environments.

6. If the rate of erosion is higher then the rate of deposition then layers are removed rather than formed. This is also happening today in high profile areas. At the top of Sandia peak where I live are old layers of limestone. It is very hard for material that falls on top of the limestone to stick around because the rate of weathering is extremely high. Eventually, the evidence that that limestone exists will be destroyed.

Faith's argument boils down to this. Faith believes that no where on dry land does there exist a place where the rate of erosion is less than the rate of deposition for any significant period of time.

Unfortunatly for Faith this is not true. I happen to live in an environment where more deposition is happening than erosion. The Rio Grande Valley is sitting on top of many layers of ancient alluvial fans. Alluvial fans are large fans of sediment of very coarse material straight from a mountain. You can see many alluvial fans when flying over a mountain range as steep hills of material that don't look solid like the granite that comprises the rest of the mountain. Over the course of the weathering of the Sandia mountains, more of this material stuck around than was carted off by weathering because the mountain happens to be near the Rio Grande Valley which is a very low place compared to the surrounding mountains and mesas. If you drive in Albuquerque from the vally to the foothills often your ears will pop like when they do when you take off in an air plane due to the abrubt altitude adjustment.

Basically, the only material leaving the valley is via the river. Because the mountain weathers MUCH faster than the river can carry away the results, the net result is that alluvial fan after alluvial fan have been buried over the course of time.

As always, I am sure any geologists by trade rather than by mere mild academia will probably correct anything I have said here.

Hope this helps,


FOX has a pretty good system they have cooked up. 10 mil people watch the show on the network, FOX. Then 5 mil, different people, tune into FOX News to get outraged by it. I just hope that those good, God fearing people at FOX continue to battle those morally bankrupt people at FOX.
-- Lewis Black, The Daily Show

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 Message 1 by Faith, posted 03-16-2005 12:25 PM Faith has replied

Replies to this message:
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roxrkool
Member (Idle past 300 days)
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 20 of 127 (192000)
03-16-2005 7:16 PM
Reply to: Message 11 by Faith
03-16-2005 5:06 PM


"Erosive events" that occurred over millions of years would have obliterated any layer in its formative stage. This is elementary logic.

and

Such minuscule observations of the strata are not required for the obvious effects real erosion would have caused during a period of twenty million years or so. That geologists take such evidences as in any way representing the actions of erosive processes during the building-up period of sediment-to-rock of a single layer over such long ranges of time is evidence only that geologists are in thrall to the false evolutionistic theory, and not thinking about the reality at all.

Faith, first of all, I am having a really hard time following your arguments. They don't make any sense geologically.

It seems to me you are saying that as soon as sediment is deposited, it is subjected to erosion. If that's what you're suggesting, then that is not necessarily true - especially if we are referring to deep marine sedimentation.

The beach area and shallow marine environment, where the waves are washing back and forth, IS someplace where sediment is continually being moved around. If the water level never changes and the basin never subsides, and tectonics are not active, then this process would likely never result in net sediment accumulation. What is washed in, is washed out.

However, the deeper into the sea you go, the more stable the environment. Out there, depostional processes dominate over erosional processes, and there IS a NET accumulation of sediments (thanks for the word, Jar! :)). Sediment is building up and weighing down the basin. The basin subsides and sediment continues accumulating.

So NO, all erosion does not occur immediately preceding deposition, except in localized settings.

There would not be merely minuscule evidences of "erosive events" you'd have to examine the rocks to discover over such a period of ages, there would simply be not much of a layer left at all. As I've repeatedly said, what actual real erosion obviously does in the Grand Canyon area, is create the Grand Canyon itself.

Again, your statements don't make much sense. What do you mean by layer? The entire unit (e.g., Muav, Redwall) or the individual layers (i.e., beds) that make up the units?

Because if you attempted to erode 5,000 feet of limeSTONE, millions of years would likely only erode a portion of that, leaving some behind. If instead you are eroding 2 feet over millions of years, then yeah, that stuff would likely be completely eroded.

Take a look at the following images and note all the unconformities:

There are at least 14 unconformities that have been identified. Those represent gaps in the rock record - that means MISSING rock. The amount of time represented is in the millions of years, perhaps some of that time is accounted for by non-depostion, but for the most part, it represents eroded rock. What you see in the GC is what was left behind AFTER erosion. It's the rock that survived due to changes in climate, eustasy, tectonics, etc.

So Faith, the erosive evidence is hardly "miniscule." The channels carved into the top of the Muav Limestone are large and often deep. You can't see them in pictures of the GC unless the pictures are taken close enough. Unfortunately, I didn't find any online. And the other unconformities, except for the Great Unconformity don't pop out in the photos. Just because YOU can't see them, does not mean they aren't there.

Additionally, these channels tell you that at one time, the Muav Limestone was exposed to the surface. If the Muav was carved by channels, then logically, the Muav must have been thicker than what is currently seen at the GC. And perhaps there were all sorts of other rock layers on top of that. But erosion worked it all down to its current thickness and all those other rocks are gone from the GC.

Not only that, but freshwater limestone, the Temple Butte Limestone now fills those channels. Originally, how thick was that? Were there other layers on top of that, too? But erosion worked all those rocks down, too.

So at that one contact, there are AT LEAST TWO erosive events and who knows how much was eroded. I suspect that more research would find the answer or simply looking through the literature as there are likely Muav- and Temple Butte-equivalent limestones in other parts of the region.

If you require posters to take geology field trips and speak geologese here, it should be posted up front at this site so that we can stay away and you won't have to deal with mere logical thinkers without a degree in Geology. Or just don't post to us.

I don't require anything of you. It was a suggestion that, instead of relying on photographs to do your geology, perhaps you should actually see the rocks for yourself before making judgements and calling into question the work of thousands of professional scientists.

No one likes an armchair geologist - no matter how 'logical' they think they are.

What is your point? Which theory are you giving for the formation of a single layer of the Grand Canyon? Only it's lowest layers were tilted so why mention that at all?

PLEASE explain what you mean by "a single layer." Is that a lithologic unit or a single bed IN the rock? Because your question again makes no sense.

This message is a reply to:
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Arkansas Banana Boy
Inactive Member


Message 21 of 127 (192009)
03-16-2005 8:31 PM
Reply to: Message 11 by Faith
03-16-2005 5:06 PM


When in Rome...
To think logically about geology you have to know the language, some terms and concepts.

The concepts of net sedimentation and protection of sediment by subsidence seem key to your understanding.

This site http://www.pitt.edu/AFShome/c/e/cejones/public/html/Geology0040/8_RiverSystems.pdf discusses sedimentation via streams and rivers. I will find some info on alluvial fans and deep sea sedimentation next as they have been mentioned here as net depositional events.

p.s. The pdf file on that site takes a few seconds to load.

ABB


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Faith 
Suspended Member (Idle past 755 days)
Posts: 35298
From: Nevada, USA
Joined: 10-06-2001


Message 22 of 127 (192067)
03-17-2005 3:05 AM
Reply to: Message 16 by Parasomnium
03-16-2005 5:39 PM


There's no need for emotional outbursts. Let me paraphrase you: "If you can't control your emotions, get off this site."

Such as for instance Crashfrog's outburst "Grand F-ing Canyon" etc., which is the post to which I was responding although apparently you didn't catch that one.


This message is a reply to:
 Message 16 by Parasomnium, posted 03-16-2005 5:39 PM Parasomnium has replied

Replies to this message:
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Parasomnium
Member
Posts: 2206
Joined: 07-15-2003
Member Rating: 4.8


Message 23 of 127 (192069)
03-17-2005 3:27 AM
Reply to: Message 22 by Faith
03-17-2005 3:05 AM


You are right, Crashfrog's outburst was uncalled for too. But I was answering your post with on-topic material and I wanted to shift your attention from outrage to the points I made. How about adressing them?


We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further. - Richard Dawkins

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Faith 
Suspended Member (Idle past 755 days)
Posts: 35298
From: Nevada, USA
Joined: 10-06-2001


Message 24 of 127 (192072)
03-17-2005 4:08 AM
Reply to: Message 5 by Minnemooseus
03-16-2005 2:07 PM


I fear you have a very simplified view of what are sediment and sedimentary processes.

Dunno if I have in any way that affects what I'm trying to say which to me seems pretty straightforward and obvious. I was in fact reading a site on the subject a earlier this evening and it didn't contradict anything I've been thinking though it was really quite fascinating to read about all the different kinds. I saved a bunch of sites on sedimentation, rock formation, the mineral content of everything whatever, and the chemical contents of each type of mineral to read when I have more time. Side question for a topic that doesn't exist here, Ask The Geologist: How do you KNOW what is at the center of the earth pray tell? (For this question at least I'm sure there is a clear trustworthy answer).

Indeed, a long history of deposition and erosion, repeated over and over, may well be the route to what is seen in the end. The marks of erosion and/or non-deposition may be large (Grand Canyon) or they may be very subtle.

My problem is with the enormous time frame. The idea that ANYTHING could sit still for 50 million years is simply preposterous to my mind. How can ANYTHING "subtle" happen in a 50-million year period? Hurricanes alter seacoasts and beaches, tornados move tons of stuff from here to there, one good rain causes mudslides all over California that rearrange local landscapes drastically, not subtly, and destroy houses; all in one year; but the redwall limestone stays in place for 50 million years even in the phase where it's quietly sedimenting away and not yet lithifying?

How long did it take for the sediment to accumulate during that 50 million years before the next layer of completely different sediment started accumulating? You guys are all talking about geological processes that you see occurring in human time, such as sedimentation in rivers, and extrapolating to millions of years of time as if it made sense to do that. How deep is the sedimentation in a river? can it possibly compare to the thicknesses of the strata? And since you say below that those same sediments are re-eroded and re-deposited, how can THAT be extrapolated to the neat thick layers of the canyon walls? (Yes I looked ahead and saw the diagrams of the whole area -- VERY much appreciate those pictures. Those strata are nice and parallel, even where they have sunk down at some distance from the canyon, but I'll get to that when I get to it). And yet you ignore other processes such as big changes in the landscape within a year or certainly a human lifetime, and apparently don't extrapolate THOSE things to the millions of years. Such processes would certainly obliterate a layer of sediment over millions of years (if not underwater). Somebody says oh but it was covered up (underwater or not underwater?). But with what? And how soon (how long did it take the limestone to accumulate, as obviously it had to be all there before the covering accumulated)? And where's the evidence of the covering? And how come when the covering got so conveniently removed the strata is so (relatively) nice and neat and horizontal over such a huge distance?

If the idea is that the Grand Canyon formed in water, that helps deal with the questions about erosion to a great extent anyway as presumably the sediment just falls to the bottom and stays there. But even in that case FIFTY MILLION YEARS? Again, these strata are different from each other, made up of different hardened sediments, a limestone here, a different limestone on top of it, a shale, etc etc. Are you guys REALLY thinking about FIFTY MILLION YEARS when you are trying to explain to me how Oh well a bunch of stuff WAS on top of it but it got eroded away?

Were these layers formed under water or not? You have to make up your mind. If you're talking about erosion apparently you aren't talking about an underwater environment where you all seem to be saying that sediments settle to the bottom and stay put, and harden as the bottom drops or something along those lines. If it got eroded it must have NOT been under water. But then you all agree the canyon layers were formed under water. This does get hard to follow.

One example of a sedimentary environment is that of a river (referred to as the fluvial environment). In a modern river environment, the sediment of the river banks and flood plain continue to be re-eroded and re-deposited. What you see is the current (no pun intended) result, subject to further change. In either a modern example or an old now rock example, evidence of much of the process can be seen.

I answered some of this above. I'm getting very tired and maybe shouldn't even be trying to post right now.

Certainly there are many processes that have been studied and understood, but

Concerning limestones: The geo-cliché is that limestones mean that nothing was happening. They are there because the was no tectonic activity in the area, and thus no detrital (fragmental) sediment was being brought in and deposited. They are the result of a very placid environment. Much of the limestone is directly or indirectly of biological origin - it is a graveyard of past life.

The Geo Column idea assumes that each layer is a "landscape" taht endured for a very long period of time. I keep coming to the Mississippian period because it seems simplest: Only one kind of sediment, redwall limestone, and the period is said to have lasted some 50 million years. ONLY redwall limestone made up that "environment," that "landscape" for fifty million years? How long was it in the sedimentary form? Was this underwater? Did it lithify during those 50 million years before the next layer / era began?

Oh, and over what extent of planet earth did this Mississippian era stretch, and is that "era" all characterized by limestone everywhere on earth? I gather there were some different limestones in different areas, but it was all limestones? Great swaths of the whole earth covered in sedimentary limestone for how long? before it became lithified limestone, and for how long? before the next stratum began to accumulate?

I am too sleepy to continue this. Maybe I'll have some time in the morning before the day gets busy.

This message has been edited by Faith, 03-17-2005 04:15 AM

This message has been edited by Faith, 03-17-2005 04:16 AM

This message has been edited by Faith, 03-17-2005 04:22 AM


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PaulK
Member
Posts: 17178
Joined: 01-10-2003
Member Rating: 2.6


Message 25 of 127 (192074)
03-17-2005 5:16 AM
Reply to: Message 24 by Faith
03-17-2005 4:08 AM


quote:

The Geo Column idea assumes that each layer is a "landscape" taht endured for a very long period of time. I keep coming to the Mississippian period because it seems simplest: Only one kind of sediment, redwall limestone, and the period is said to have lasted some 50 million years. ONLY redwall limestone made up that "environment," that "landscape" for fifty million years? How long was it in the sedimentary form? Was this underwater? Did it lithify during those 50 million years before the next layer / era began?

The problem here is that you need to actually appreciate the data that is to be explained.

This webpage makes a good start:
http://www.geocities.com/earthhistory/grandb.htm

Some relevant quotes are:


The Redwall is divided into four members: the Whitmore Wash, Thunder Springs, Mooney Falls, and Horseshoe Mesa members. The Whitmore Wash is nearly pure calcium carbonate (98% pure). The Thunder Springs member consists of alternating layers of chert and carbonate. The Mooney Falls member is once again almost totally pure calcium carbonate (99.5%). The Horseshoe Mesa member consists of thinly-bedded carbonate with occasional chert lenses.

So much for "one type of sediment".

The Surprise Canyon formation is also Mississipian, and so that must also be included in any discussion of the "types" of sediment.


Though only a few million years are thought to seperate the end of Redwall deposition and the begginning of Suprise Canyon deposition, the surface of Redwall Limestone was altered considerably during this time. During this time, a series of westwardly-deepening channels were incised into the surface of the Redwall, up to 400ft thick in some places. Blocky knolls and small erosional "mesas," up to 40ft high, are present on the upper surface of the Redwall, buried by basal Supai sediments

So much for there being no erosion.

I would also suggest that the question "over what extent of planet earth did this Mississippian era stretch" is rephrased. As written it makes about as much sense as asking what portion of the Earth is currently in the year 2005AD by the Gregorian calendar.


This message is a reply to:
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Arkansas Banana Boy
Inactive Member


Message 26 of 127 (192079)
03-17-2005 6:09 AM
Reply to: Message 24 by Faith
03-17-2005 4:08 AM


Your argument from incredulity is that millions of years is way too long. Many here find it hard to believe that the world is 10000yrs old or less, but refuse to use it as the basis of an argument.

The bottoms of some oceans and the deltic basins appear to have existed for a long time. These areas accumulate much more sediment than they lose and are not sitting still. If the ocean recedes then deposition stops and a portion may even erode away, but over the long haul any area that is covered by a sea for most of the time accumulates sediments.

Tectonic movement, over millions of years, may change an area that was for millions of years an ocean into, for instance, a brackish swamp. If this swamp persisted for a fairly long period then it would be later represented as a different layer on top of the previous marine sediment.

The alteration and fossil composition of these layers posed a problem for geologists of the 18th and 19th centuries, who noticed comonalities of strata in different areas when strata maps were compared. These common rock layers compise the geologic column. This concept was accepted by most scientists almost 200 years ago. Most of these scientists were originally creationist or were still subsequently religious. The 20th century brought radiometeric dating which furthur correlated these layers.

Yes, most geologists extrapolate from present day processes...this is uniformitatarian view as opposed to your catastrophist view that things must have been quite different on earth just a few thousand years ago.

And if 50 mil yrs for the Redwall bothers you then the 600+ mil yrs for the whole formation and 4.6 bil yrs for the age of earth must make you very incredulous.

So your objections are the deposition problem and extreme age.

Deposition occurs in some areas at such a rate as to exceed erosion, such as sea beds and deltic fans. That some areas have more deposition than erosion (and therefore sedimentation) has been shown to you in many forms.

It has also been shown that young earth creationists have more inconsistencies to deal with than old earthers, with fossil stratigraphy, radiometric dating, and lack of sufficient time for rock lithification or metamorphasis as just the beginning.

What particular scientific objections do you have to an old earth may be the next question to furthur plumb the depths of your geologic doubts.

ABB


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Replies to this message:
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Arkansas Banana Boy
Inactive Member


Message 27 of 127 (192103)
03-17-2005 9:39 AM


re; last message
I retract my request about the age question unless Faith wishes to pursue it; these threads can get off track soon. I'll just listen to others about the Grand Canyon as I'm still learning about it.

Till then its our old friends erosion,deposition,and sedimentation. Do they occur? Are they important in the rock cycle of an old earth or the unimportant remnants of a young degenerative world? Tune in next time for the thrilling conclusion!

Also here http://geology.about.com/library/bl/images/blalluvfan.htm is some info on alluvial fans. I'll work on finding some seafloor info next.


  
Percy
Member
Posts: 20948
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.1


Message 28 of 127 (192111)
03-17-2005 10:18 AM
Reply to: Message 24 by Faith
03-17-2005 4:08 AM


Hi Faith,

There was a lot in your post, but I'm just going to focus on one small point:

Faith writes:

My problem is with the enormous time frame. The idea that ANYTHING could sit still for 50 million years is simply preposterous to my mind. How can ANYTHING "subtle" happen in a 50-million year period? Hurricanes alter seacoasts and beaches, tornados move tons of stuff from here to there, one good rain causes mudslides all over California that rearrange local landscapes drastically, not subtly, and destroy houses; all in one year; but the redwall limestone stays in place for 50 million years even in the phase where it's quietly sedimenting away and not yet lithifying?

You use the example of the Redwall Limestone layer of the Grand Canyon, so I'm going to focus on that. This layer represents what was once a shallow sea. We can tell by the makeup of the sediments that it was a shallow sea, and it persisted for a long time, perhaps longer than 10 million years. We know that it existed about 340 million years ago by radiometric dating.

The catastrophes that are so devastating on land often go unnoticed under water. The hurricanes, tornados, rains and mudslides that you mention have impacts that are felt primarily on land. Hurricane Andrew devastated southern Florida in 1992, causing billions of dollars worth of property damage, yet if you had visited the ocean floor some miles off into the Atlantic after that hurricane you would have found no sign of it at all.

The redwall limestone layer of the grand canyon does not happen to include the shoreline area of this shallow sea. The massive amounts of sediments that would have been recorded near shorelines after major storm and flood events have no effect far away from shore. Out in the middle of this quiet sea sediments slowly accumulated undisturbed eon after eon.

There are many examples of the accumulation of sediments over time - every ocean is another example. The floor of the Atlantic ocean forms at the mid-oceanic ridge at the rate of about 10 cm per year. The ridge goes pretty much north/south through the center of the ocean - it goes right through Iceland, accounting for its volcanic activity. Sea floor near the ridge is very young, while sea floor near the American, European and African continents is very old. Sea floor cores taken of sediment near the ridge are very shallow before hitting rock, while sedimentary cores taken near continents are deepest. This is because sediments gradually accumulate over time, and in general the oldest sea floor will have the greatest amount of deposited sediment (it also depends upon sediment sources, of course).

The ultimate fate of all sea floor is subduction beneath continents (continental crust is lighter than sea floor crust, so sea floor always subducts beneath continents). While there are continental rocks in some places that are billions of years old, there is no sea floor anywhere in the world older than 200 million years. The only record we have of seas and oceans older than that is those that were pushed up by tectonic forces and became part of continents, such as large areas of Arizona. The Grand Canyon is unique in that it provides visibility to the layers that underlie Arizona, but it is important to keep in mind that these layers extend for miles and miles in all directions, you just can't see them.

--Percy


This message is a reply to:
 Message 24 by Faith, posted 03-17-2005 4:08 AM Faith has not replied

Replies to this message:
 Message 31 by Minnemooseus, posted 03-17-2005 1:22 PM Percy has replied

  
Jazzns
Member (Idle past 3222 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 29 of 127 (192138)
03-17-2005 1:11 PM
Reply to: Message 26 by Arkansas Banana Boy
03-17-2005 6:09 AM


Hopefully Helpful Info About Sedimentation
Side question for a topic that doesn't exist here, Ask The Geologist: How do you KNOW what is at the center of the earth pray tell? (For this question at least I'm sure there is a clear trustworthy answer).

There was a topic very recently about this. I believe it was this one:

A Critique of the "Evolution Essay" A GREAT DEBATE S1WC and anglagard ONLY

The topic is closed but much good information was presented about how we know that the center of the earth is like. In a nutshell, we know the earth is liquid at certain depths and solid at certain depths. We also know the density of substances at these depths. There are only a few elements that exist that have the right density and other properties to fit this data.

My problem is with the enormous time frame. The idea that ANYTHING could sit still for 50 million years is simply preposterous to my mind. How can ANYTHING "subtle" happen in a 50-million year period? Hurricanes alter seacoasts and beaches, tornados move tons of stuff from here to there, one good rain causes mudslides all over California that rearrange local landscapes drastically, not subtly, and destroy houses; all in one year; but the redwall limestone stays in place for 50 million years even in the phase where it's quietly sedimenting away and not yet lithifying?

That is part of the problem. Over 50 million years, things like a hurricane ARE subtle events. Over the span of your life have any of the hurricanes removed the Florida everglades or the beaches in the southeast coast? Did El Nino destroy the Mojave Desert? Sure it might mix stuff up but it is still there. These "events" do not stop deposition and over the course of time the average will be positive deposition.

No one is claiming that everything must sit still for 50 million years. All it has to do is sit sill long enough for more stuff to pile on top of it on average. Once it is buried then either it gets further buried or it gets eroded. The stuff that gets eroded is carted off to some other location where it may or may not get buried there. It has to go somewhere and it WILL and up someplace where it is less and less likely to be eroded even if it has to travel all the way to the bottom of the ocean.

How long did it take for the sediment to accumulate during that 50 million years before the next layer of completely different sediment started accumulating? You guys are all talking about geological processes that you see occurring in human time, such as sedimentation in rivers, and extrapolating to millions of years of time as if it made sense to do that.

It does make sense because we are not just looking at the stuff that gets dropped out of the Mississippi river while we are watching it. We know that the Mississippi river delta is composed of material that the river has weathered from upstream. We can look at the sedimentation rate today and we can drill and see all the layers that have been deposited over the lifetime of the delta. To say that these layers in particular are from a different source other than the delta is rediculous because they are the same as the ones we can watch being deposited today. Therefore we can look at the whole Mississippi delta and all the layers it has produced ever.

When we practice geology on these layers we are looking at something that is "current" with respect to geologic time. The Mississippi delta system is "live" so we know a lot about the characteristics of the sediment in the delta. Then when we find layers buried somewhere else where there is not a river but has the same characteristics as the layers of the Mississippi delta we can hypothesize that there was once a delta there. Then we search the area to try to find evidence of an ancient river and when we do it confirms our ability to take our current knowledge that we gathered from the current "live" formation and apply it to something different.

This applies to many other aspects of geology. We can watch deserts today and how they work. We can watch sand dunes and cut into them to see what they look like on the inside. We can see that they are one big heap of sand with internal cross-bedding. A desert is a "live" geologic system even if we never saw the sand that is at the bottom of dune be deposited. Then when we find the Coconino sandstone that just looks like a lithified version of a sand dune we have strong evidence that the Coconino sandstone came from a desert.

How deep is the sedimentation in a river? can it possibly compare to the thicknesses of the strata? And since you say below that those same sediments are re-eroded and re-deposited, how can THAT be extrapolated to the neat thick layers of the canyon walls?

Well, first off the canyon sediments were not deposited by a river so there can be no extrapolation to the same kinds of things we see in the GC. River deposits are very different from ocean deposits so your request for a correlation between the sediment in the GC and river sediments is unwarrented. In a river, the re-erosion and re-deposition happens as the river carries material down stream. Rivers tend to erode things but they also leave evidence of their erosion and when they flood. The process of re-erosion and re-deposition creates ripple marks in some rivers that get preserved if that sediment lithifies. This is similar to the cross-bedding in desert sandstone. It is an indicator that the sediment was being moved around by some process (wind or water) before it was lithified. It directly shows that sediment in question was exposed to weathering yet not enough to overcome the amount of sediment that entered the system which allowed it to be buried.

And yet you ignore other processes such as big changes in the landscape within a year or certainly a human lifetime, and apparently don't extrapolate THOSE things to the millions of years.

Big changes like what?

Mud slides? There are tons of preserved examples of ancient mud slides. Sometimes that is the best way to bury some dry land geologic features. Polystrate trees and such are often the result of some local catastrophic event such as a mud slide that quickly buries something. These things are preserved in the geologic record. IN the case of the GC, mud slides are rare at the bottom of the ocean.

Volcanoes? These are also preserved in the geologic record and even in the GC there is a disruption of sedimentation due to a volcano erupting and leaving behind its ash and igneous rock. But it buried the stuff that was already there. It did not remove it.

Tornadoes/Hurricanes? Sure these things cause weathering but are actually minor in comparison to the overall time that sediment is being deposited. Also, consider that the material removed by these events still ends up somewhere and often it is probably nearby. SO when you look at a formation that spans hundreds upon hundreds of square miles, the fact that this was in one spot but is now a hundred miles away as a part of the same feature is moot.

Such processes would certainly obliterate a layer of sediment over millions of years (if not underwater).

No actually either they would help preserve the layers or at worst are a minor disruption in the net sedimentation which probably only serve to move stuff around a bit.

Somebody says oh but it was covered up (underwater or not underwater?). But with what?

Sediment on the surface is subject to weathering. If the weathering tends to dump more stuff out then take stuff away then material will pile up and be buried. This is observable both in real time and as a part of "live" geologic system.

And how soon (how long did it take the limestone to accumulate, as obviously it had to be all there before the covering accumulated)? And where's the evidence of the covering? And how come when the covering got so conveniently removed the strata is so (relatively) nice and neat and horizontal over such a huge distance?

Limestone is a neat geologic feature. YECs usually dodge my questions about limestone because it is very hard to fit limestone formation in a young earth and espeically a flood. Limestone is created from the fossils of dead sea creatures. A little sea creature will die and its body will fall to the bottom of the ocean just like a particle of silicate does for "regular" sedimentation. In order for limestone to accumulate in any thickness it requires a relativly calm warm environment like the bottom of the ocean or a lake. Near the shore where the tides are always mixing things up, limestone does not form so it it hard to imagine how a trillion trillion of dead sea creatures decided to settle out together during a worldwide flood.

Since the environment that allows limestone to form must be calm it is easy to see why we would expect limestone formations in particular to be pretty neat and flat. The bottom of the ocean and the bottom of lakes are pretty flat so there is no mystery there. Limestone does not form on land at all so if you were thinking that then I hope this clears that up.

Some limestone is also due to reef systems, (also a problem for how a 1000ft reef of delicate sea creatures could form during a flood). These like reefs today are large habitats for little sea cretures and like most all limestone are packed full of marine fossils.

If the idea is that the Grand Canyon formed in water, that helps deal with the questions about erosion to a great extent anyway as presumably the sediment just falls to the bottom and stays there. But even in that case FIFTY MILLION YEARS? Again, these strata are different from each other, made up of different hardened sediments, a limestone here, a different limestone on top of it, a shale, etc etc. Are you guys REALLY thinking about FIFTY MILLION YEARS when you are trying to explain to me how Oh well a bunch of stuff WAS on top of it but it got eroded away?

We can tell very easily when something was eroded away. It leaves behind what is called and erosional disconformity. Basically when you look at the rocks in detail you can tell that there was an abrupt break in the deposition only to be resumed by some drastically different sediment. Sometimes the deposition will even be at a different angle which means that the feature was tilted before sedimentation resumed. Sometimes there is not an abrupt change between the layers because all that happend was that the environment changed. If sandstone is being deposited near the shore and sea level rises then it will probably start to deposit silt instead without there ever being a break. All that happens is that the sand stops being buried by more sand and starts being buried by silt.

How we know something "WAs on top of it" is by diagnostic means. It requires a very detailed look at the rocks that you just won't get from looking at a big picture of the GC.

Were these layers formed under water or not? You have to make up your mind.

Some were formed on land and some were not. That is part of how we know it took a long time to deposit the layers of the GC. Sea level rose and fell often during many many years. Sometimes it fell enough to expose that area to the air and either eroded some of the layers or deposited more stuff like the Coconino sandstone. This happened more than once during the geologic history of the GC. We would actually expect this and it would be kind of strange to see hundreds of millions of years of NO evidence that sea level has changed. That would be better evidence of a flood. Unfortunatly we do not see that evidence.

So to make up our mind the answer is yes. Sometimes the deposition of the layers of the GC happened on land and sometimes it happend on water. Sometimes the layers were eroded and we see an erosional disconformity. Either way we know that sometimes the area was under water and sometimes it was not.

If you're talking about erosion apparently you aren't talking about an underwater environment where you all seem to be saying that sediments settle to the bottom and stay put, and harden as the bottom drops or something along those lines. If it got eroded it must have NOT been under water. But then you all agree the canyon layers were formed under water. This does get hard to follow.

I can see how it is hard to follow if you have not been exposed to the concept before. Ocean levels rise and fall so sometimes land will be covered by water and other times it may not be. This is the case for the GC and why we see limestone which is very much a marine feature covered by the Coconino sandstone which is very much a desert feature.

The Geo Column idea assumes that each layer is a "landscape" taht endured for a very long period of time. I keep coming to the Mississippian period because it seems simplest: Only one kind of sediment, redwall limestone, and the period is said to have lasted some 50 million years. ONLY redwall limestone made up that
"environment," that "landscape" for fifty million years?

As was mentioned in another post. The one layer of limestone is actually very complex. It would be very strange maybe to see 50 million years worth of sediment without some kind of dynamics. Luckily, we do see these dynamics and the redwall limestone is a good example of this.

How long was it in the sedimentary form?

I am not sure what this question is asking. I assume you are asking how long was it unlithified sediment? Well lithification does not happen all at once. As something gets buried it is subject to pressure and heat. The more it gets buried the more it gets compacted until it finally becomes stone. There are even varying degress of this when talking about sedimentary rocks. You can find sandstone that is very loose and where you can still see the individual grains of sand very well. Then you can find different sandstone that is extremely hard and still harder to see the individual featuers of the sediment. For limestone, you can range from these very messy chunks of shells and material to very fine compressed rocks that do not even look like limestone until you pour some acid on it and watch it fizz. Basically, sediment stays sediment until it gets buried enough to compress it into rock. So a few layers down from the surface something might still be just loose sediment.

Was this underwater? Did it lithify during those 50 million years before the next layer / era began?

The bottom part of the layer probably started to lithify as soon as it was buried enough. The other important thing to think about is that the layers is MUCH thinner than it was when it was loose sediment. As things get buried they get compacted sometimes very drastically. A layer that is dozens of feet now may have been many hundreds of feet when it was still being deposited.

Oh, and over what extent of planet earth did this Mississippian era stretch, and is that "era" all characterized by limestone everywhere on earth?[

The Mississippian was a time and no limestone was not being deposited everywhere during the Mississippian. There is Mississippian sandstone, igneous rocks, etc depending on what was being deposited where. There are also places where the geologic history is missing any Mississippian rocks at all. These either represent areas where the erosion was geater than sedimentation during the Mississippian or where the Mississippian rocks have been eroded since that time.

I gather there were some different limestones in different areas, but it was all limestones?

A single layer of global limestone would mean that the whole world was under water. This would be great evidence for some kind of global flooding if anyone could find such a layers that was global. No one has so far.

Great swaths of the whole earth covered in sedimentary limestone for how long?

Nowhere has anyone found a global layer of anything except maybe the K-T Boundary which is a very thin layer of iridium probably from a stellar impact.

Please let me know if any of this helps,


FOX has a pretty good system they have cooked up. 10 mil people watch the show on the network, FOX. Then 5 mil, different people, tune into FOX News to get outraged by it. I just hope that those good, God fearing people at FOX continue to battle those morally bankrupt people at FOX.
-- Lewis Black, The Daily Show

This message is a reply to:
 Message 26 by Arkansas Banana Boy, posted 03-17-2005 6:09 AM Arkansas Banana Boy has replied

Replies to this message:
 Message 42 by Arkansas Banana Boy, posted 03-17-2005 10:18 PM Jazzns has not replied
 Message 93 by Faith, posted 04-02-2005 4:49 PM Jazzns has not replied
 Message 94 by Faith, posted 04-02-2005 8:03 PM Jazzns has not replied

  
Jazzns
Member (Idle past 3222 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 30 of 127 (192140)
03-17-2005 1:14 PM
Reply to: Message 24 by Faith
03-17-2005 4:08 AM


Previous Post
My post immediatly prior to this was a respose to you but I think I accidently click the reply button to ABB. Just want to give you a heads up and the last message I wrote was actually supposed to be a rely to you.

Thanks,


This message is a reply to:
 Message 24 by Faith, posted 03-17-2005 4:08 AM Faith has not replied

  
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