Exploring the Grand Canyon, from the bottom up.

"hand sample" means?"very thin laminae which are parallel to bedding (bedding is very roughly analogous to the amount of sediment laid down during one sedimentation event--one event more or less equals one bed)." means?"So where as mudrocks are bedded sedimentary rocks, shales are bedded and laminated sedimentary rocks. " means?Now I do remember what wedded and bedded meant, least wise I think I do, but now you tossing in lots of new stuff.Are you saying that if I found a layer of mudstone that I would be looking at one event? Say what? Remember old and not to bright? What different conditions?

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Aslan is not a Tame Lion

Okay, this could get ugly really quick, which is why I threw out generalities (to avoid a detailed bogged down discussion of shale).A hand sample is what we would normally collect in the field. A sample that is large enough to display all of the important characteristics of the rock in question but still small enough to be manageable. Maybe 3-5 inches on a side. Okay, let's see...if we're standing on a creek during a major rain event and the creek floods and that flood deposits a layer of sand a foot thick on a basically flat floodplain, that layer will be basically planar. It will have a bottom surface and a top surface, both of which will be more or less parallel (called subparallel). These surfaces are referred to as bedding planes. If the bedding planes are really really thin (I forget the # right now), they're laminae rather than beds. In a stacked sequence of multiple layers of sand, the bedding planes serve as zones of weakness along which a rock will break (but formationally, they often designate event boundaries). Shales tend to break along these very thin laminae but there can also be bedding planes (larger planes of weakness enclosing packages of laminae). So shales can be both bedded and laminated rocks. Again, this isn't really that important here I don't think. Often yes. If you can identify a layer of mudstone between two bedding planes, you're often dealing with one sedimentary event. Now, sometimes these planes are so thin that you cannot discern the boundaries and lots of things destroy the boundaries after deposition, so you won't always be able to find these beds in a given outcrop of mudrock, but yeah, if you can find two bedding planes, then the rock between them often represents one sedimentary event. Now, this in and of itself says nothing about the length of time it took to deposit that bed (duration of the event) nor how much time passed between that event and the one which produced the bed overlying it. Other factors go into figuring that stuff out. Like in our flooding creek example above, that foot of sediment might have been deposited in an afternoon. I've seen this happen...a single small channel flood depositing a foot of sand. If that happens to get preserved, it is recording one interesting afternoon in Pennsylvania. Beds of mud often take much longer to deposit and the bedding planes record larger-scale environmental or other changes than a single storm. There are usually a lot of clues preserved in any given package of sediment telling us how they came to be, but it isn't always trivial to tease the answer out.

Actually both useful and important.One of the claims often made is that folk are just making assumptions when they talk about how some rock was made. But you say: For the benefit of those of us who are stumbling in the dark, what are examples of such clues and can you link to pictures of samples we could see that would help us understand such differences?AbE:Just for my benefit, look at the following pictures. I know I'm asking alot but if I came across something like these, what should they tell me? Image 1Image 2Image 3Image 4Edited by jar, : add images and questions.

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Aslan is not a Tame Lion

Yes...and statements like that really reflect their ignorance. People (on both sides) go on and on about how geology and paleontology are simply historical sciences and that all the evidence is observation-based and that no experiments are ever done in either discipline. This is simply untrue. Sedimentology in particular is great. We do lots of actual experiments on things like how sand grains settle or calcite precipitates out of water columns and carcasses degrade and break down and get introduced into the local environment, etc. Also, because many many sedimentary processes operate at temperatures and pressures we live in (unlike, say, magma cooling into granite 20km below the surface), they can be directly observed (e.g., you can go to a beach and watch the beds form). People who say things like "we don't really know how rocks form" and that "no one was there when it happened" and "all the evidence is indirect and based on guesses from the past" are making statements that are as untrue as "there is no such thing as a car." There is a lot left to learn of course (tons!) and we absolutely infer present processes to have happened in the past in the same ways, but unless some deity decided to entirely change physics just when geologists started looking at sediments in a rigorous way, then we know a great f-ing deal about how sedimentary rocks form and in what paleoenvironments. Statements to the contrary (like "all sediments are laid down in water" or "the Grand Canyon records an epic global flood") are just...sorry if this is insensitive...stupid. Yes, absolutely. But let's do this case by case based on the rocks in question, unless we want to turn this thread into just a discussion of sedimentology. It might be more interesting if threaded in to the actual stuff being discussed rather than some long winded discourse on how sand grains accumulate on river beds, say. Does that make sense?IMAGE 1: Difficult to say much about this rock from the picture. It looks to be coarse siltstone or sand and the feature important in the picture (called a sedimentary structure) is what we call graded bedding. The lines cutting diagonal across the picture are the bedding planes--the coin is sitting on a bedding surface. It is probably the top surface. If so, the bottom surface of that bed is the next most distinct line to the right (so the bed is that package of lines that is more or less the same color and is about 1/3 of the photograph width in thickness). It is called a graded bed because...it is graded. The sizes of the particles in the bed grade from bottom to top. Note that the sand/silt grains where the coin is are much finer than those at the base of that same bed, where you can see lots of shiny individual grains right at the ?bottom surface. Each bed here is a depositional event and what has happened is that the water was flowing with more energy at the beginning of the event (and thus moving around heavier grains) than later in the event. What you're seeing is the water actually losing energy over time during the event. First the water loses enough energy that the heaviest grains can no longer be moved along and so they start to collect, forming the bottom of the bed deposit. Then flow drops off so that the next largest grains can no longer be carried and so they are left behind and accumulate and so on until the flow is low enough that silt-sized particles begin to settle out of suspension and accumulate at the top of the bed. The coin is sitting where the event ends...where the water basically stops moving sediment of that size range. These rocks could be recording floods (each bed is an event...there is a time gap right where the coin is sitting between that event and the next one). Floods often deposit graded beds because of course over time flood waters wane (a flood has more energy right when, say, the river floods, than a week later when the waters recede). Other processes also create graded bedding. Not much more can be said just looking at this photo. This is the kind of image used to teach what graded bedding is...not about what processes produced that graded bedding. Probably, the rocks get younger to the up-left. But these rocks could be reverse-graded. I don't think so, but I cannot be sure just from the photo. If so, the oldest rocks are to the top-left. Opposite process--flow increases over the duration of the event rather than decreases. I would have to be at the outcrop to figure that one out...I cannot do much from a photo.IMAGE 2: These are sandstones that were deposited by wind in an arid environment. They're called aeolian or eolian and these particular ones are interpreted as being dune deposits. These are coarse-grained rocks and what gives them away as being aeolian are the high-angle large-scale cross-beds. These are the fine beds that are subparallel to the line being made by the person's arms. If you were to draw a line along the trend being made by the person's arms, this is basically parallel to these very thin beds (that kind of look like striations). This is cross-bedding. The actual beds in the rock are the deeper cuts in the rock that are subparallel with the path upon which the person is standing. The cross-beds are made by sand grands avalanching down the dune face. The size and angle of these are what gives them away as being deposited by wind rather than water. Wind direction was basically right to left. What you're seeing recorded here is a stacked sequence of desert sand dunes with lots of erosion surfaces in the sequence.IMAGE 3: Same thing. The person in the phone is walking directly on an erosion surface--note that the cross-bedding below her feet and above her feet are at different angles to the camera. She is walking right on a surface that was scoured after the deposition of the lower cross-beds and before the deposition of ones at about her knee level.IMAGE 4: I cannot tell much from this picture except that it looks to be sandstone or mudstone. Almost certainly water-lain. Just from this photo, I wouldn't necessarily call those polygonal cracks in that top bedding plane mudcracks, but I presume the person who took the photo could see better what was going on (sometimes photos are very difficult to interpret). If those are mudcracks, they are a crap example of them. If they're mudcracks, then this is recording a snapshot in time where the sand/mud was exposed out of water after deposition and dried for a while before the next layer of sediment was deposited. If those are mudcracks, there is a gap in time between the event that laid down that top bedding surface that we see and the one which sat on top of it but has now been removed by erosion. The pen looks to be sitting subparallel to bedding--it is essentially on a bedding surface. Note that the pen is sitting on a different (probably older unless the beds are overturned) bedding surface than the one with the green staining. There is also a time gap between these two bedding surfaces.Okay...did all of that make sense? If not, I'll try to clarify what was cloudy.

No you did great.That was very helpful.The next formation is the Chuar Group. I gather that there are two main formations in this group and many layers.What can you good folk tell us about this group?

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Aslan is not a Tame Lion

Chuar Group: Again, the Proterozoic is way not my forte, so here is the abstract from a good article in Geology (Karlstrom et al., 2000 28(7):619-622). The second author on the paper is Sam Bowring of MIT, who is one of the heavy hitters of Proterozoic geology. So basically, the Chuar Group consists of about 1600 meters of clastic sediments (these are sedimentary rocks made of up of fragments of pre-existing rocks (e.g., sandstone)) that records some evidence of the breakup of Rodinia (I presume you've been talking about Rodinia--this is a supercontinent like the much later Pangea, where all major world landmasses were crushed together at low latitudes) and provides evidence of late Neoproterozoic life (apparently mostly things like stromatolites...not Ediacaran stuff). It records nearshore environments (so beaches, tidal flats and the like) and the life living in those environments. There is also evidence of major tectonic unrest (I'm presuming (I don't have this article on hand, but can dig it up quickly if needbe) that this evidence is interbedded deposits of more coarse clastics than one would expect to find in nearshore environments (maybe coarse sands and conglomerates with fragments that can be traced to paleohighlands) as well as the ash layers, which not only provide nice age control for the unit but also indicate that this was an tectonically active area at the time. Does this synopsis make sense to all?

That's fine since we are not dealing with any terms such as Proterozoic in this thread. We are just looking at the rocks themselves to see what they tell us. Well, we are not touching on stuff like Pangea or Rodina or Ediacaran or Neoproterozoic so those kinda fall out.BUTclastic sediments is important.We covered this earlier in the thread but I would like to try explaining it in my words just to see if I understand such things.You recognize clastic sediments because they are made up of once existing rocks that have been further weathered, broken down into smaller pieces, transported somewhere else and reformed into another piece of rock.The important thing to me in all that is that first it was necessary to weather and even earlier formation, transport, compact and raise the secondary rock formation, then weather, transport, compact and raise the next formation.Basically clastic sedimentary rocks are second or more generations of reprocessed rocks. Is that correct?

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Aslan is not a Tame Lion

From what I have seen in studying the past 50+ years, yes.

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There is no better love between 2 people than mutual respect for each other

Great.Now JB said: It appears from that that we are looking at a succession of events, nearshore but also times when it was deeper water. Also the mention of ash layers.Does anyone of you good folk know the order of such things in the Chuar Group from bottom to top?

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Aslan is not a Tame Lion

Yes. You have it exactly. Reformed as used by you above means deposited. Well, mostly yes. Lava coming out of a volcano on Hawaii will weather and erode. Fragments of basalt will be deposited at some distance (might be very close if the wind is doing the moving of tiny little pieces of basalt) from the parent rock. This is pile of material is now a clastic sedimentary body. The compaction and raising step isn't necessary. Yes. Exactly. And of course the source rock can be any kind of rock, including a pre-existing clastic sedimentary rock...

This is a Geological Society of America abstract of a talk presented in 2001. The Chuar Group contains two formations. The rock is mostly shale all the way through. The older unit is the underlying Galeros Formation. The younger unit is the Kwagunt Formation. The significant stuff from this abstract are that the rocks in the older Galeros are coarser than those of the Kwagunt (slightly higher energy) and are sourced to igneous and metamorphic rocks. The overlying stuff is more highly weathered and comes from different sources. There is ash all the way through. So, it is more the older stuff that is interpreted as being evidence of Rodinia's breakup. Sounds like there is inferred to be a lot of time separating the deposition of the two units...seems as though a lot of the older stuff weathered and redeposited into the Kwagunt Formation.

Great and I almost understand such stuff.Now one more basics question.Sorry if so much of this seems silly or self evident, but I want us to present the evidence in pretty much the same way that those seeing it for the first time might have approached it.Generally transport is from a higher to lower location. So to weather a surface it has to be higher than where it deposits the weathered material. Is that generally true?

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Aslan is not a Tame Lion

Yeah...generally...the source area which is weathered is going to be "higher" than the sink area where the resulting material gets deposited (not uniformly true of wind deposition of course...). But in this case yeah, we can presume that the material in these units eroded from sources that were substantially elevated relative to the shallow marine environment in which they came to rest (we might be missing rivers here that moved the material to the coast...).

No worries. This stuff bores the hell out of some folks, but I find it very interesting.

Great.Next, the info you supplied said:

quote:

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The significant stuff from this abstract are that the rocks in the older Galeros are coarser than those of the Kwagunt (slightly higher energy) and are sourced to igneous and metamorphic rocks.

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So let me try to see if I understand some of what is seen.We saw the Cardenas Lava Flows which shows an active environment where there are direct lava flows.Later they get overladen by a formation first of fairly fine sedimentary materials then somewhat coarser ones, and now we see still another change.One key thing is that this current formation, Galeros and the next higher, Kwagunt , require two things.First instead of the somewhat slow sea or shore line environment of the Nankoweap, there needed to be some pretty active volcanism to create the igneous rocks that get weathered to make the Chuar group.Second we seem to see two different transport mechanisms, a more active higher energy one that deposits only the larger material then followed by a slower less energetic period that deposits a smaller finer material.At various times though throughout the time we see yet another event, volcanic activity but one that is mostly ash flow and not lava flow.Is that reasonable so far?Does that also mean that the lower, larger material likely came from a closer, steeper source and as that source was weathered down, away and back from the point of deposition, we see small material that has been transported further and thus more weathered? Also if we were seeing something being worn down, would we see the slope decrease with weathering and so slower, lower energy transport?

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Aslan is not a Tame Lion