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Author | Topic: Evolution. We Have The Fossils. We Win. | |||||||||||||||||||||||||||||||||||||||||||
edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined:
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This is really mystifying edge. My eyes are bad but the picture shows an area with deep grooves> How is this any kind of example of a flat planar surface or a surface that could become a flat planar surface? I don't get it.
Unfortunately, I thought you understood how flat surfaces form in marine deposition and I was trying to show how a flat surface would form in transition from a terrestrial setting, to a marine setting. In this case the land gets planed off by waves and then the flat marine sediment would be deposited on top. As the sea moves across the continent, this process moves along also creating a regional unconformity that is essentially flat. The bonus was that the sedimentary bedding planes are standing on end so that when the new sediments are deposited on top of them you would get an angular unconformity. In the distant future. Sorry that, I placed too many ideas into one image. It was too good to pass up.Someday, this location will look like Siccar Point if the sea continues to rise. Well, maybe someone else is interested ....
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
I haven't read further downthread, but I think that a big part of the misunderstanding here is the lack of an obvious scale indicator in the photo. I think Faith is seeing that photo as a large area of substantial ridges and valleys. I see a relatively small area with a relief of maybe a couple of feet maximum.
Yah, I should have chosen a better image, but it was so interesting ... sorry about that. Faith also doesn't see the "ridges and valleys" as being differentially weathered near vertical sedimentary bedding. Looks to be a slightly plunging tightly folded syncline to me. Maybe a graywacke/slate sequence. That's the ocean in the background. But the image shows how Faith's 'landscapes' are demolished by transgressing seas and how angular unconformities form.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
But how do terrestrial landscapes become preserved as strata? I can only guess. I think transgression would have to be more rapid, either because of rapid sea level rise or rapid subsidence. For example, a subsidence of the land by 10 feet would cause coastal areas to be inundated, killing and preserving dinosaurs and their nests. A sudden land subsidence of 10 feet is nothing unusual - the Alaska earthquake of 1964 caused subsidence of as much as 7 feet and uplift of as much as 30 feet (not in the same locations of course).
First, remember our friend 'vertical exaggeration', so the valleys are usually not quite so pronounced as in the diagram. But basically, correct. Rapid subsidence could cause surface features to be preserved as irregular surfaces. IIRC, this is shown in the subsidence of a part of Kingston, Jamaica, after. an earthquake. Lot's of artifacts such as foundations and small hills and probably tree trunks, etc. will be forever preserved in the next layers of sediment. But also, when we have low relief and a fluctuating sea level we get a lot of inter layering of terrestrial streams and swamps, etc. with shoreline deposits. This is the case in places like the coal measures of the Appalachians or the Bone Beds of Florida. Changes in sea level move the land/sea boundary back and forth across an area burying swamps and river channels.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined:
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I still have absolutely no idea how my "landscapes" are "demolished by transgressing seas."
In a word, waves. Wave action is probably the most relentless and destructive and widespread form of erosion on the planet.
What transgressing sea for starters?
Any of those we see in the geological record. The Tapeats Sea, for instance.
And I see nothing getting "demolished" in the picture, ...
Yep, that shows you how effective wave action is.
... just a lot of lumpy ridges, ...
Yes, eroded bedding planes of a folded sequence of siltstones.
... and certainly nothing that could become a flat sedimentary rock of the sort seen in the geo/strat column. Don't see it, no idea what you mean.
Yes, it hasn't been deposited yet. That's what makes the image so instructive. It is geology in action.
And the idea you'd get angular unconformities from what, further deposits of sediment? makes no sense at all. Why wouldn't the sediment just fill in the "valleys."
Exactly all of those half-foot valleys would fill in and then become completely covered by sand and gravel, and then on up through the entire expected sequence. In fact, you could say that we see the future.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined:
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Starting with the last question first ...
Question for Edge/Moose: why the irregular left edge of the diagram?
That's a schematic device to show the effects of weathering. More resistant units would extend farther to the left.
Again I'll need confirmation from real geologists, but I think the salt layers lying between limestone layers may represent warm shallow seas that receded/evaporated, and that the salt layers between shale layers may represent coastal areas that receded/evaporated.
Good observation. In this case it looks like there is not much 'salt tectonics' going on. There is no deformation of the layers above the salt layers, so they caused no changes. Remember, the vertical scale is exaggerated. Salt deposits that rise through strata and can cause distortions and faulting typically involve massive amounts of salt, and these skinny salt layers don't appear to have migrated at all. The interpretation of such evaporite layers is that they formed in a restricted basin in which evaporation was more than replenishment of water. There are several lines of evidence for this.
Well, not really. While erosion is responsible for the unconformities (cessation of deposition is another possible cause of unconformities, but it's always seemed unlikely to me), the actual final surface that represents the contact between strata is usually going to be due to Walther's Law. Land surfaces are likely only preserved intact when rapidly inundated, which probably doesn't happen that often. Ours is an active but not frequently a catastrophic planet. In general sea levels slowly rise and fall, landforms slowly uplift and erode.
When you have such variation, it usually means that the relief was low so you had minor transgressions and regressions. If you look up the term 'cyclothem' you'll get a boatload of information on that. However, in those cases, the rise and fall of sea level is not fast enough to plane off the mountains and sea cliffs. It's always a race (relative) between uplift and erosion. When the deposition is terrestrial (lakes and rivers and swamps, etc.), you can fill in some of the lowlands and stream channels that way. One thing to remember is that in such a stratigraphic column, whenever you see a wavy or jagged line, it indicates an unconformity. In this case there are a lot of unconformities at times when Faith says that there is no erosion occurring. Well, that might be true in the GC, but there was plenty going on elsewhere. We also see cases where some terrestrial deposits occur in channels which cut entirely through older formations. Another thing to remember is that the unconformity lines are actually isochronous, so even though they cut across the time scales they are the age of the younger formation. Edited by edge, : No reason given.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined:
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I certainly have no problem with even extreme erosion of cliffs, but I can't regard some sand on top of tilted and apparently deeply buried siltstone layers as an angular unconformity, ...
I have always suspected that you have a hard time visualizing in three dimensions. Maybe you can visualize the paged of a book turned on end and then another book laid across those pages. If not, well, so be it. Everyone else can.
... just as there is no way that I can see how any current landscape could ever become a slab of rock such as we see in the geo/strat columns.
Other than the fact that we just explained it to you, sure.
I understand I'm probably not going to be able to persuade anyone of this, though I'll keep trying anyway.
Truth. And that is because your viewpoint contradicts reality. Edited by edge, : No reason given.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined:
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How can you have an angular unconformity unless the overlying sediment, whatever it is, forms a flat slab of rock across the tilted rocks? Are you saying it does, or that it's not necessary?
Technically, an unconformity is simply a discontinuity between two sedimentary layers. An angular unconformity occurs when the sedimentary layering in the two layers are different. There is nothing in the definition of unconformity that says one must be a flat slab of rock.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
Sigh. The Holocene "covers the entire Earth."
It does?
Sigh. And you expect the Holocene to end up as a flat slab of rock?
Parts of it. yes. More than you can imagine.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
[qs]Sometimes, you just gotta let things go, Herbie. As tempting as they are ...
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
It is very common to find time periods associated with their rocks, it's not considered a confusion and I'm certainly not making up the idea. You can find a map of "the Jurassic period" which obviously associates it with the rocks.
What rocks?
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
"Overlying horizontal layers."
And?
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
My "bolding". I think Edge didn't state that real well. I think he meant "An angular unconformity occurs when the sedimentary layering in the two layers are of different attitudes", or something like that. Better stated as "An angular unconformity occurs when the later sedimentary layer has a different attitude (strike and dip) than the earlier layer.
Well, my explanation had two parts. One was what an unconformity is and then what makes an angular unconformity. But you are correct that it might be a bit obtuse for the layperson anyway. Your point is exactly correct, though. There is no reason that any layers in an angular unconformity need to remain horizontal after deposition. In fact, the Old Red Sandstone, the upper layer at Siccar Point is also not horizontally bedded. I'm not sure that this addresses Faith's point completely or not, but I'm sure we'll find out.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
Here's a paragraph from Wikipedia, Geological Formations, that is about the connection between the strata and the time periods. "Usefulness of formations[edit] ...Formations were at first described as the essential geologic time markers, based on their relative ages and the law of superposition. The divisions of the geological time scale were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries." And what is meant by "Usefulness of formations"? Words have meaning, Faith.
Without the strata there would be no Geological Time Scale. How convenient that each time period has at least one such rock.
Since they are the original data, I would say, of course.
The following are descriptions and some photos of some of the formations in the Colorado Plateau, from top to bottom, to show, first, how extensive they are, covering far more area than any pseudo"flat" landscape you can come up with, and certainly making it impossible for the dinosaurs to live there at that time; second, how recognizably different they are from each other, which is really the point of my calling them "single sediment."
Okay so why is the Navajo not as extensive as the Tapeats? And if the strata are so flat and extensive why does its thickness range from zero to 270 feet in the Grand Canyon? Why is the Temple Butte Formation not found everywhere in the Grand Canyon? What makes you think that the depositional environment of the Navajo Formation was the same as, say, the Grand River Formation of Michigan? Why should marine deposits have a "landscape"? Why do the Navajo Sandstone and the underlying Kayenta 'interfinger' at their contact if the contacts are sharp and "single sediment"?
They don't have to be literally single sediments to be homogeneous enough to be recognizable, unlike your ordinary earth surface mixtures.
Most sedimentary rocks are mixtures of various sediment types.
And third, examples of very straight flat layers such as the Entrada formation with tight contact between Entrada and Curtis formations; and the Navajo as straight, flat and tight as I could find it with the Entrada above it etc. The clear differences between all of these formations in color and composition certainly don't suggest any normal earth surface landscape to me, but I know you'll pretend it is.
I'm not sure why this is a problem. This is an issue that does not exist. There is clearly some discontinuity of deposition reflected in source material, or other environment of deposition.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
This idea that "flat" land of the sort you are always showing photos could ever become a rock like those in the geo/strat columns needs to be answered but I don't think any answer will do it for you. I don't know how anyone could possibly be convinced of such an idea but of course you'll remind me that incredulity is not an argument. Too bad, it really should be in a case as obvious as this.
Again, to all appearances, you conflate terrestrial deposits with marine deposits. The eolian sandstones of the Jurassic do not have nearly the extent of formations such as the Tapeats or the great Paleozoic limetstones of North America. That is because they are terrestrial. There is no landscape of the sort you illustrate with photos that is anywhere near the extent of the rock formations you think could come from such land. I got descriptions of the Navajo formation and the formations above it, and they all cover from four to six or seven BIG current states of the USA. I also found one photo showing the straightness and flatness with knife-edge contact and was about to track down the others when I moved to another page without saving the post and lost it. None of your "flat" landscapes could ever form a knife-edge straight contact with another, let alone cover even a hundredth of the territory the rocks actually cover. If you can't see it I despair of ever getting anything across at all. Same thing with the Claron where I spent much of the last two days. It is not continental in scale. And neither are many of the formations that Percy has provided to you from the lower peninsula of Michigan. They are smaller in extent and do preserve landscapes that you are so hung up on. Some, such as the Hell Creek Formation contain abundant dinosaur fauna.
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edge Member (Idle past 1731 days) Posts: 4696 From: Colorado, USA Joined: |
"Preserve landscapes???
Topography.
You mean dead dinosaurs constitute a "landscape?" What?
Is that what I said? If dinosaurs lived on a landscape (your definition), would they not die there also?
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