You've mentioned coal seams several times. How would a coal seam get formed?
In between eruptions, in a swamp-type of environment. Remember there are sandstone and shale interbedded with the ash as well. Some of the sand is pretty clearly fluvial and it's only a short step from there to a swamp or bog.
And here is a picture of fluviatile sediments associated with coal in a New Mexico coal mine.
Coal occurs at the bottom of the mine, with sandstone and mudstone layers above it. You can see a sandstone channel at the top of the highwall on the left edge of the image.
Now, just imagine this sequence being deposited during a volcanic episode of rather large proportion, derived from one or more of the larger volcanic fields in the region.
Perhaps I'm not explaining very well here, but I know that as we drilled for deep oil fields in the region, we encountered a lot of coal on the way down through the clay beds that you've seen in the these photos. I've also seen layers of 'clinker', the term for baked clay related to coal seam fires, interbedded with the ash; certainly ancient in timing.
You have said that there are layers of coal found between layers of volcanic ash and also layers of sandstone.
Yes, let's just say that they are interbedded.
We can also see layers within the ash package, probably due to a number of factors such as multiple eruptions, amount of water present at deposition, degree of oxidation, contamination by silt, etc., etc.
We saw pictures that show a layer of ash with sandstone above and below.
In the picture of the Lee Ranch Mine that I showed you, there is a sequence of coal, sandstone and mudstone, without apparent ash layers. The idea is to imagine the same sequence expanded by a number of very thick ash deposits.
Note that I say 'deposits'. I'm pretty sure that some are 'ash fall', some are 'ash flow' and others are 'water lain' ash, or they are otherwise contaminated by silt and sand. Because the beds are so thick and continuous, I'd say that most of them were deposited in large basins, at least partly containing water. This would facilitate turning the glass to clay.
In this picture, you can see what they call 'popcorn' texture caused by dessication of bentonite on the land surface. If you look closely, it isn't very pure, but there is enough clay to make a distinctive surface. In rain storms this terrain is almost impossible to traverse.
Here is a clue as to what the environment looked like. In among the the clay beds there are petrified trees. While the ash altered to clay, the trees were replaced by silica and now weather out as petrified logs in the clay beds.
You can see some coal beds in the background.
The next picture shows interbedded sandstone, ash and coal in the Bisti Badlands. The basal clay is quite organic (dark gray) suggesting that it was a paleosoil of some kind, probably derived from the bentonite deposits.
The last picture is an image, again in the Bisti Badlands, of clinker where a coal seam has burned and fired the clay around it. The rock is now fragmental because the overlying rock has collapsed into the burning coal.
The topic is the story in the rock. Tell us the story (hopefully with actual pictures not drawings) that those rocks tell you?
The geology of volcanic rocks is very complex and rife with jargon suggesting all of the different combinations of deposition, alteration, age, fragment size, etc., etc. However, the types of rock we see in these places are pretty suggestive of large intermontane basins such as the Green River Basin which we have also discussed with respect to fossils. The accumulated a large amount of transported volcanic ash with successive forests and swamps between volcanic events.
You mention a coal seam burned. Is there evidence to show that the coal seam was buried when burned and not simply a wild fire?
The best evidence is that the roof rocks are also baked and that they collapsed into the burning seam. These fires are extremely hot and virtually impossible to extinguish. They burn today in many coal fields.
Then there a layer of volcanic ash that is many meters deep.
Then a layer of clay or mudstone.
Then a layer that was once a marsh or bog over the volcanic ask that was later covered by another volcanic ash flow and the bog or marsh material compressed and turned into coal.
In a word, yes. The order and timing and thickness vary.
Clay that was above and below the coal layer was later baked by a coal seam fire.
Well, not everywhere, but yes, that is a possibility. Remember you were asking how we know about layers in the ash. The burnt coal beds are often a bright red, creating very distinct layers.
As petrophysics has mentioned oxidizing pyrite or other iron sulfide can oxidize rapidly enough to actually ignite a coal bed. In other cases, when there is uplift of the basin followed by erosion, the coal can be exposed to lightning or wildfires. Some of the modern fires are man-made.
The result is an odd looking formation that we call 'clinker'. And as petrophysics mentioned it does look very like a crushed coffee cup glued into an unrecognizable mass.
Along the North Fork of the Gunnison River east of Paonia you can see a normal looking sedimentary sequence, but notice very extensive, thin layers of red rock that just don't make sense. Then you realize that you are in coal country.
And finally there is sandstone at the current surface. If that is correct, how did the sandstone instead of soil or marsh or bog or woodland like we see when we walk around today get to be the current surface?
Well, not necessarily, but sandstone, being more resistant to erosion would tend to protect whatever is beneath it.
Here is a hoodoo showing how a cap rock of sandstone can resist erosion from above and protect a column of mudstone or ash below it.
Just as an aside to petrophysics, once I drilled an exploratory hole for coal and intercepted a vertical marcasite vein about the width of the core. You can imagine picking up that box after drilling a bunch of coal...
I understand that but how can sandstone be the top layer instead of the soil and dirt and loam and sand and pebbles be a top layer?
Because sandstone, being composed largely of quartz grains does not alter to weaker material like clay. And being cemented by either silica or carbonate, it is more resistant to weathering.
Now, if you developed a modern soil, of course, that would be at the surface. I don't think that any of the places we are talking about have soil.
Can sandstone form at a topmost layer or the surface?
Not sure. I doubt it. In some places they turn compacted sand into building material, but I know of no true rock unless it was somehow cemented by silica from a hot springs deposit or something like that.
Can sandstone form at a topmost layer or the surface?
As I mentioned earlier, I'm not sure that it happens, but here is a landslide debris deposit that has been cemented by travertine (a carbonate) that has permeated the debris, and can now be considered a new rock, and it is certainly at the surface.
It would not be hard to imagine a sandstone forming this way, but I haven't found any references.
Yes, this is a little off-topic, but I thought I'd relate some reading that I've done lately.
This summer is the 200th anniversary of the writing of Frankenstein by Mary Shelley. That year (1816) was known as the is known as the 'Year without Summer' because of the eruption of Tambora in April of the year before. It turns out that she was cavorting around a lake in Switzerland with Percy Bysshe Shelley and Lord Byron, among others, but the weather was so disagreeable that they couldn't enjoy the weather so they sat around campfires and told ghost stories. Well, they decided to see who could write the best horror story as a kind of game. At the age of 18, she composed the story of Frankenstein and his monster. The rest is history.
I post this simply to show that the earth has not only dictated the evolution of our species, but also has affected our culture in more recent times.
Just because you see tilted rocks and (apparently) flat lying rocks in the same view doesn't automatically mean angular unconformity. You might, for example, be seeing a tilted limb of an anticline in the foreground and the flat lying crest of the same anticline in the distance. I think you need to see more detail to make a judgement.
Well, it's always good to see the two geometries in contact.