Long build up of Sediments

The mississippi spews out a whole load of sediment, but I don't think its all from one mountain chain. Check out how much . All those miles of river bed sediment come with it.This message has been edited by Modulous, Sun, 12-March-2006 10:48 PM

Kilometers deep you say?And are the sediments deposited by the Mississippi different sediments of the sort seen in the geo column, say in the Southwest USA, only one kind for some great depth topped by another for another great depth and so on?

couldn't tell you to be honest, I'd google around and see what I can come up with, but I'm about to AFK (watch this space).I was just saying that it isn't just mountains that are getting eroded, in many cases, hundreds or even thousands of miles of land are being eroded by the river too.

Faith wrote:

quote:

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I see how the rising and falling of sea level might be an explanation, at least of a stack of different kinds of sediments, although I still have trouble with the abrupt change in sediments from one to another.

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For another (very good) explanation of these sharp boundaries, besides the one I gave you (erosion surfaces), see this link:
lordibelieve.org/time/age2.PDF
(Happened upon it from a Google search.) Most significantly, see part 9: Sedimentary Deposition and Lithification.

I'm going to answer your last reply to me here since it pretty much asks the same kinds of questions. For my own example I was trying to simplify everything down to a hypothetical example. Source material is igneous rock pushed up to the size of mountains. You can look at the Hawaiian islands, or Iceland as examples of how material is spewed up to form large volcanic land masses. How much sediment do you think you could get out of the rock of the Hawaiian island chain? That's for starters. Is it not possible to get a lot of sediment spread out into a layer?You want to jump from that simple example to answers about the Grand Canyon. Again, you will then have to understand a much more complex set of conditions before getting to the grand canyon. So far you have ignored what people are saying to you, saying it is irrelevent when it is not. I am not going to make that same mistake, and I am not going to try and explain the Grand Canyon based on the simplest hypothetical I created (because it is not enough).Okay so you have the mountain which can be eroded into sediments. Igneous rock is not some uniform mass. If you see it up close you will find crystals of different shapes and sizes and materials. It is made from a mass of different molecules which crystallize and fall out of solution based on conditions. Thus magma cooling inside the earth looks different than lava at the surface, same for ocean igneous rock.Okay so an igneous "mountain" is a huge collection of different types of crystals compacted together. That means as it erodes it erodes into various different components. Large boulders vs stones vs small grains. You can even have some minerals dissolve over time. Thus the sediment coming off the "mountain" creates a stream of different seds which will sort themselves based on what is transporting them. Just as you understand the flood will do to "everything", that happens with the mountains of material, though in a longer term and more directional flow.Do you see how this is beginning to work? A huge mass of varied material gets exposed to elements and erodes differentially, and transports differentially away from the mass, creating different types of strata. Same source, different seds, different strata.Again this is just a hypothetical beginning. Very simplified. Before we get to what could form the grand canyon, we need to move through this.

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holmes
"What you need is sustained outrage...there's far too much unthinking respect given to authority." (M.Ivins)

Are you actually claiming that volcanic action could create the stack of separate sediments that is the geo column? Is there actual evidence of this somewhere?This message has been edited by Faith, 03-12-2006 07:35 PM

I think there may have been a miscommunication here. Strata several kilometers thick is not a problem, especially adjacent to a young and high mountain range.Five kilometers of pure sandstone, or pure limestone, or pure shale, etc. is less likely, however. That's why I'm trying to clarify what exactly you think edge is stating, because reading back through the posts, I don't see him suggesting such a thing.The strata in the Grand Canyon area are not pure though. Yes, they are a thick package, but you still see marine and terrestrial strata fluctuating between each other, gradational contacts, unconformities, angular unconformities.In addition, are you also wondering about the lateral extent of formations? Marine formations can be quite extensive, but so can some terrestrial settings, such as deserts.

Perhaps I misread him, but then he must have misread me in the first place. Here is his original statement: I was talking about ONE layer of a million feet in my effort to pin down deposition rate. He answered in terms of "sequences" not one layer.I think I'll just give up on this thread for now. I appreciate that you have maintained an even tone but that can't be said for all the participants and then I get repetitive and the whole thing deteriorates. Best just let it rest.

It depends entirely on what the mountains are composed of, what is being exposed, and the rate of uplift. Mountains can be composed of ancient marine rocks, terrestrial sediment, and cored by igneous and/or metamorphic rocks. In addition, rocks are all affected differently by weathering and erosion.So if some mountain is composed entirely of granite, then the stuff (grains, cobbles, boulders, etc.) coming off of it is going to be composed of quartz, feldspar, biotite, magnetite, etc.But if the mountain is basalt on top, marine rocks underneath, and metamorphic rocks inside, then you can have a variable combination of sediment coming off the mountain range. The stuff up top will weather and erode first, followed by the stuff underneath. If they are different, they will result in different sediment.The resulting seds can be picked up, sorted, and transported by fluvial systems. Transport causes cobbles and boulders and even individual minerals to break down and become round.Of course it's much more complex than the above. That's why it's very difficult to answer general questions about geology. The answer to the questions depends on the geology.

Yeah, I think there is a miscommunication in there.Right now I am in the process of logging core that contains many layers of bentonite, which is altered volcanic ash. Using radiometric dating, I can choose two bentonite layers above and below a 10 foot package of shale, and date the bentonites.If the difference in the dates is 1 million years, then we can get an average depositional rate for that 10' of shale. However, it's not that easy, we much make several assumptions, such as how much compaction has occurred, so it's not perfect. But what is useful about these averaged rates is that it tells us how average depositional rates have changed over time and then attempt to see if the rocks and/or fossils point to such a conclusion. If they do, then we can wonder about climatic conditions or other things. If not, then we have to wonder why, maybe unconformities. We keep looking, though, for explanations.

I don't understand your first question. Could you elaborate?There are places around the world where you can drill 2000 feet and hit nothing but volcanic and intrusive rock, so in those areas, yes, the geologic record/column is composed entirely of igneous rock and completely barren of other sediment.

Again, you confuse erosion with deposition.Also, I don't think you understood. I was referring to basins which have accumulations of sediments that thick; not formations or beds or strata. No, it was not one kind of sediment. I don't believe that I said anything about one rock type. If that was your intent, it was not clear at the time. Well, not in detail. There are a number of formations that come and go in the Colorado Plateau. For instance the Navajo Sandstone practically pinches out before arriving in Colorado.Again, it is the details that trip you up. You have a completely wrong impression of geology and geological features.

Correct. In fact, what I had in mind was the Belt Supergroup. Not a bed, or a stratum, or a formation or even a group... There are numerous rock types within the Belt, even including some volcanics. It is, however a prodigious amount of sediment, and it is mostly fine grained, as I remember. I brought this up just because Faith seemed incredulous that large packages of sediments could be deposited anywhere.

OK I'm going to take a break from this thread because I don't like the way it is going but first I will answer this much. Here again you misread me and that is apparently a pattern I'm not going to be able to stop.I am not confusing erosion with deposition. The deposition is the ORIGINAL kilometers of accumulation you seemed to be saying could have occurred. The erosion is what reduced that putative accumulation to the present much smaller quantity. Got it?If so, great. If not, let's just not talk to each other any more.Bye for now.This message has been edited by Faith, 03-12-2006 11:08 PM

Very well, I stand corrected. However, it is not always clear what you are talking about. But your statement above is not completely true, either. Erosion is also the source of sediments for your huge thicknesses of sedimentary rocks. As such, is pertinent to your question about where all of these sediments came from.