I'm coming in to this late and for that I apologize. This might not be critical to the point at hand, but it does help to understand the difference between various sedimentary materials From the perspective of a geologist, there is no real appreciable difference between rock and sediment. Beware the wikis of the world. A rock is an aggregate of minerals. No lithification component is needed. I have seen rocks containing dinosaur fossils that are 67 million years old but which are hardly cemented at all and can be pulled apart with your fingers. In reality a pile of sand is a rock as it is an aggregate of minerals (sand grains are usually grains of individual minerals). The important thing to think about when contemplating sedimentary rocks is the process of formation. This is what it's all about. Whether or not the rock is lithified is of tertiary importance at best.
It appears that mudstone can be the same as siltstone & shale just different mineral content
Yes for siltstone versus mudstone (mud and silt are grain-size classifcations)...no for these two versus shale. Mudrocks (e.g., mudstone and siltstone) are distinguished from shale by virtue of shale being fissile. Fissile means that a hand sample of shale tends to be comprised of many 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). You can think of laminae as just very very fine beds (again rough analogy). So where as mudrocks are bedded sedimentary rocks, shales are bedded and laminated sedimentary rocks. For most of the discussion we're likely to have here, we don't really need to worry about the differences between mudrocks and shale and we can think of them as roughly equivalent (although in reality they form under slightly different conditions and thus are different rocks).
Did that make sense or was it more confusing than helpful?
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.
"So where as mudrocks are bedded sedimentary rocks, shales are bedded and laminated sedimentary rocks. " means?
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.
Are you saying that if I found a layer of mudstone that I would be looking at one event?
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.
One of the claims often made is that folk are just making assumptions when they talk about how some rock was made.
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.
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?
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.
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.
The Chuar Group (~1600 m thick) preserves a record of extensional tectonism, ocean-chemistry fluctuations, and biological diversification during the late Neoproterozoic Era. An ash layer from the top of the section has a U-Pb zircon age of 742 Â± 6 Ma. The Chuar Group was deposited at low latitudes during extension on the north-trending Butte fault system and is inferred to record rifting during the breakup of Rodinia. Shallow-marine deposition is documented by tide- and wave-generated sedimentary structures, facies associations, and fossils. C isotopes in organic carbon show large stratigraphic variations, apparently recording incipient stages of the marked C isotopic fluctuations that characterize later Neoproterozoic time. Upper Chuar rocks preserve a rich biota that includes not only cyanobacteria and algae, but also heterotrophic protists that document increased food web complexity in Neoproterozoic ecosystems. The Chuar Group thus provides a well-dated, high-resolution record of early events in the sequence of linked tectonic, biogeochemical, environmental, and biological changes that collectively ushered in the Phanerozoic Eon.
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?
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.
Yes. You have it exactly. Reformed as used by you above means deposited.
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.
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.
Basically clastic sedimentary rocks are second or more generations of reprocessed rocks. Is that correct?
Yes. Exactly. And of course the source rock can be any kind of rock, including a pre-existing clastic sedimentary rock...
LITHOSTRATIGRAPHIC VARIATIONS IN THE NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON- INSIGHTS INTO PROVENANCE FROM GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF SHALES BLOCH, John D.1, CROSSEY, Laura J.1, and DEHLER, Carol M.2, (1) Earth and Planetary Science, Univ of New Mexico, Albuquerque, NM 87131, email@example.com, (2) Utah State Univ, UT
The Neoproterozoic (c.a. 800-742 Ma) Chuar Group in the eastern Grand Canyon is a shale-dominated succession approximately 1600 m in thickness comprising seven members in two formations. Preliminary geochemical and petrographic analyses of Chuar shale samples show significant stratigraphic bulk-rock and mineralogical trends that suggest changes in provenance, basinal sediment distribution and/or weathering intensity in the source region.
Enriched Si, Al, Ti and depleted Fe abundances in the Awatubi and Walcott members indicate that the Kwagunt Formation is composed of sediment either of differing provenance or more highly weathered than the underlying Galeros Formation. BSEM data confirm textural (grain size and sorting) and mineralogical (abundant kaolinite, rutile and quartz) characteristics consistent with variation in weathering or provenance for shales of the Kwagunt Formation. In contrast, the Carbon Canyon, Jupiter and Tanner members of the underlying Galeros Formation are chemically (enriched Fe, Mg) and texturally (coarser and more poorly sorted) less mature. In addition, the Galeros Formation generally contains more detrital mica, chlorite and feldspar. These detrital mineral components suggest a significant plutonic or metamorphic provenance. If our interpretations are correct, the upper Chuar may record more intense silicate weathering, consistent with models for drawdown of atmospheric carbon dioxide and cooler conditions perhaps tens of millions of years prior to the first recognized "snowball" event (Sturtian glaciation).
Petrographic analysis further indicates a significant volcanic ash component in the Tanner, Carbon Canyon and Awatubi members. Both detrital monazite (<10 micrometers) and zircon (<10 micrometers) are present suggesting the possibility of dating of source region or (optimistically) ashfall components. These results, in conjunction with high-resolution lithostratigraphy, C-isotope stratigraphy, and the distribution of Chuaria and Melanocyrillium in the Chuar Group, provide a means of dating and better correlation with other Neoproterozoic shale-bearing sequences in the southwestern US. Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29â€“May 2, 2001) Session No. 11 Meso to Neoproterozoic of the Western US: Record of Supercontinent Assembly and Breakup, and a Snowball Earth? Sheraton Old Town Hotel: Alvarado AB 8:00 AM-12:00 PM, Tuesday, May 1, 2001
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.
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?
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...).
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.
This is probably true, but doesn't have to be (e.g., if there were preexisting igneous rocks in the Nankoweap or something, that could eliminate the need for active volcanism bewteen Nankoweap and Chuar time.
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?
What about the ash layers though? Is it common for a volcanic event to produce both ash and lava?
No, you're totally right. Volcanoes tend to do one or the other and those require active (duh...) volcanism. I was saying (poorly) that it is possible that you could have an older igneous source weathering to produce the sediment in the lower Chuar that might NOT be related to the activity producing the periodic ash layers. It is a more complicated explanation than necessary but nature does tell Occam to piss off on occasion. I wasn't talking about the ash layers right there but that probably wasn't clear.
Would each ash layer indicate a separate event?
If so, I assume that there would be some time between such events to account for the intervening material?
Yep...exactly. You're a quick study.
Any chance you can help me over on Thread Salt of the Earth (on salt domes and beds)
Hah...you caught me. I've been ignoring that one for the time being because whereas I know a bit about salt deposition, I don't remember all that much detail about salt domes and salt tectonics. I've got an old book that has some stuff on salt tectonics but I haven't been able to put my hand on it. So, the answer to your question is probably...but give me a bit to see what I remember/can find.