As I understand it, Silica Cement is the product of diatoms, the carcasses of microscopic shelled critters, that have disolved in water and then precipitated out to form the cement. Is that correct?
Well... sorta... I think. It's been a while since I've done much reading about clastic cements, but I can't say I remember hearing that diatoms directly contribute silica for cementing purposes - though it's entirely possible they do in some settings. Siliceous oozes are generally deposited in areas where high Si-content prevails and these are the result of either diatoms or radiolaria in the deep marine environment.
In the case of the Shinumo Quartzite, which appears to have been deposited in a shallow marine environment, it's really difficult to hazard a guess without knowing more about it. It's possible the silica cementation is the result of pressure solution where grain-grain contacts result in partial dissolution of the quartz grain (at the contacts) and that dissolved silica then re-precipitates in the pore spaces as coatings on the sand grains. However, pressure solution requires, well, pressure. Possible during burial, sure, but I'm not aware of a whole lot of low grade metamorphism affecting the other formations in the Super Group. But then, maybe that's not always necessary, either.
Also, it's possible something in the depositional environment contributed silica during diagenesis or perhaps a siliceous fluid was introduced via structure and this fluid was able to move through the porous sandstone.
THIS LINK has a nice SEM (Scanning Electron Microscope) image of a rounded quartz grain (near center) with a siliceous overgrowth that is also filling the pore space. The link also briefly discusses pressure solution.
Okay, one more question then. I thought radiolaria were another living critter, like diatoms, common in the marine environments from earliest times on. So regardless of whether it was radiolaria or diatoms, we are looking at a source that began life as something living in the oceans.
Okay, trying another summary through the Shinumo Quartzite layer..
We began with the Vishnu Schist because it is the lowest exposed layer. It is schist, so originally it was sandstone. To get sandstone there first had to be some higher rock source which was weathered, accumulated in a basin, was later compressed and metamorphed into schist. There was clay, which is also a product of weathering mixed in with the sand.
So to this point, rock was created, weathered into smaller and smaller particles until we had sand and clays, washed or blown downhill (eroded) to collect in a basin, settled, compressed and over time, under pressure and heat became schist.
At sometime during this process, magama pushed through the sandstone or schist and over long periods of time cooled to become granite.
The whole structure, Vishnu Schist and Zoroaster Granite was pushed upward or the whole area was eroded down to where everything was at about the same level. We know this happened because accumulations of the sandstone stopped and the area began to erode. We know that because there is a non-conformity between the Vishnu Schist/Zoroaster Granite and the layers above them. Another indicator that that was what happened is that the Zoroaster Granite does not intrude into the layers above.
Then we began to look at the next level.
The Bass Formation is actually a a composite of several types of material including limestone, conglomerates, ash and other materials. At the very bottom of the Bass Formation is a layer of Ash. This would indicate a period of volcanic activity somewhere and the ash was brought in, likely by wind, and layered over the eroded surface of the Vishnu Schist.
One of the layers is the Houtata Conglomerate which again layers over the Vishnu Schist and likely simply filled eroded valleys in the Vishnu Schist and so is seen to be pinched out, or gradually disappear, in some areas. Conglomerates, as we learned earlier, are made up of rocks that have been worn down and weathered, have rounded edges, that are then cemented into some matrix under pressure, temperature and over time. We can see an example of a conglomerate here.
The important issues are that the Vishnu Schist being eroded at the top shows that biulding up had ceased. It was no longer accumulating things but was higher than the surrounding areas and so was wearing away.
The Bass Formation shows an intruding, and then a retreating sea. We can tell that by looking at the composition of the formation as we move from top to bottom as outlined in Re: To the Bass Formation. (Message 115).
This means we see the land lowering to allow the sea in, and then later in the process gradually rising again to make the sea retreat or that sea level rose and later fell.
The next layer, the Hakatai Shale, points to another change in the environment. Shale is much finer grained than sandstone or schist. This tells us that the environment was one of deposits in fairly calm water. From the colors of the shale we can get some idea of the depth of the water where it was deposited, the darker colors being laid down in deeper, oxygen lacking water while the redder ones show greater reaction with iron and a higher oxygen content, most likely from shallower water.
The fact that we see these in bands shows us that the sea level rose and fell cyclicly over and extended period. The overlying sandstone shows a return to a more energetic flowing water.
Now we look at the Shnumo Quartzite layer. As I understand it, it is one of the thicker remaining layers in the Unkar Group of the Grand Canyon Supergroup, with well over a 1000 feet remaining. There is evidence that the Underlying layer was partially eroded away before the Shinumo layer was laid down.
That is important because it tells us that the underlying Hakatai Shale group must have stopped forming, been pushed up above the surface of the water to become dry land, weathered and part of it was eroded away. In the same way, the evidence is that the Shinumo Quartzite was also produced in a shallow marine environment, meaning that the land was either eroded back down to where it was once again under water, that sea levels once again rose, that the land itself once again was lowered or some combination of all of those. There are remains of streams and river borne sediment and it looks like it was once a near coastal delta like the mouth of the Mississippi.
So, here are the steps so far.
Intially rock is produced somewere. It is weathered and then eroded to sand mixed with the smaller, finer clay, compressed, heated and over time turned into schist.
At some point during that process, magma intrudes into the sandstone or schist.
The whole body is later weathered and eroded which tells us it was exposed for sometime and higher than the surrounding areas. Accumulation stopped and erosion began.
At some time it was once again lower than the surrounding areas and we find a layer of conglomerate, larger weathered rocks cemented together in a matrix. We also find a layer of ash which indicates volcanic activity.
Above that we find a layer of limestone and conglomerate, which from the record left show a sea intruding and then regressing. The sea intrusion could be because sea levels rose and later fell, or becaise the land lowered and later was raised, or a combination of both.
Finally (so far) we find a layer that was laid down in what must have been a coastal swamp with slow moving water that allowed the finest particles to settle out forming shale. The changes in coloring of the shale indicated that the water level rose and fell over time. And capping that we find more sandstone and conglomerate indicating a return of more active water flow.
Either water levels dropped or the land rises because part of the layer gets weathered and worn away.
Next we find another return to a marine environment. The Shinumo Quartze layer is laid down in a shallow sea. It is made of cemented sand. The land continues to change and in the Sinumo layer we find the remains of streams and a river delta environment.
Through the Shinumo Quartzite layer including suggested changes from Roxrkool.
This message has been edited by jar, 03-28-2006 12:15 PM
Re: Okay, trying another summary through the Shinumo Quartzite layer..
A fair summary except for one thing, the Shinumo was not eroded prior to deposition of the overlying Dox Formation. It was eroded as part of the Great Unconformity much later in the Cambrian, prior to deposition of the Tapeats Sandstone. So we still need to deposit the rest of the Unkar and Chuar Groups before erosion begins.
The upper contact with the Dox Formation is conformable, meaning it's an unbroken depositional sequence with no apparent interruption of sedimentation.
I still reallly like the Grand Hikes site, so I'm going to post a few important points from there:
The only complete sequence of members in the Dox Formation occurs in eastern Grand Canyon where its overall thickness has been variously reported between about 3000 and 3200 feet. Its contact with the underlying Shinumo Quartzite is regarded to be generally conformable and marked by a relatively rapid subsidence and marine incursion. Geologists have divided the Dox Formation into four members: the lowermost Escalante Creek Member, Solomon Temple Member, Comanche Point Member and uppermost Ochoa Point Member. Their respective horizons are gradational and they are distinguished on the basis of color changes, topographic features and depositional environments.
Escalante Creek time was marked at its onset by relatively rapid subsidence of the area followed by gradual basin filling. By the close of Escalante Creek time the basin had filled and the region was at or near sea level, where it remained throughout the rest of Dox time. The overall lithologic sequence of the Dox members suggest the environment of deposition evolved from an underwater delta into a floodplain which was followed a tidal flat environment.
According to the Grand Hikes site (above link), members of the Dox Formation include (number indicates depositional order):
4. Ochoa Point Member: At least 300 feet thick of micaceous mudstone, reddish-colored silty sandstone. Sedimentary structures include salt casts, ripple marks, and cross beds. Inferred depositional environment: tidal flat or shallow marine.
3. Comanche Point Member: At least 620 feet thick of white (leached) to reddish (oxidized sulfide?) shaley siltstones, mudstones, and sandstone, intercalated with stromatolitic dolomite. Sedimentary structures include mud cracks, salt casts, ripple marks, and irregular, wavy bedding. Inferred depostional environment: tidal flat(?).
2. Solomon Temple Member: At least 920 feet thick of reddish shaley siltstones, mudstones, and sandstone. Sedimentary structures include channels and cross-beds. Inferred depostional environment: floodplain.
1. Escalante Creek Member: At least 1,280+ feet thick; ~800 feet of siliceous (probably the cement and similar to Shinumo) sandstone possibly gradational from underlying Shinumo Qzte, overlain by ~400 feet of shale and mudstone. I can't exactly remember what all can impart a green color (it's usually a result of deposition in a reducing environment - no oxygen), but I think it can either contain chlorite, clay, glauconite, or fine-grained sulfide such as pyrite or pyrrhotite. My guess is sulfide because the overlying Solomon Temple Member contains reddish colored rocks, suggesting an oxidizing environment. However, the original sediments need to contain something that is capable of oxidizing and so I think that may be sulfide. Inferred depostional environment: delta(?).
Okay, I'm going to try this again after losing it TWICE!!! :mad:
ESCALANTE CREEK MEMBER
The Escalante Creek Mbr. (~390 meters thick) is broken into 4 units based on lithologic and depositional differences and appears to record the transgressional and regressional sequence of a deltaic system following rapid subsidence of a tectonic(?) basin. Number represents depositional order: . . . . (SOLOMON TEMPLE Member) ----------------------- Unit 4 (140 m thick): Composed primarily of less-resistant brown to grayish siltstone with lesser thin, greenish brown sandstone interbeds. Shale fragments and normal grading is observed in the sandstone beds.
Inferred sedimentary environment: pro-delta slope deposits. These sediments represent a continuation of the basin in-filling and are located somewhere near the apex where older basin shales meet and are being covered by younger deltaic sediments in a [much?] shallower marine environment than unit 3.
----------------------- GRADUAL FILLING OF BASIN CONTINUES
Unit 3 (96 m thick): Composed of resistant lithic (containing rock fragments) and feldspathic (containing feldspar) calcareous (carbonate) sandstone. The quartz grains are subangular to subrounded and poorly to moderately sorted. Microcline, orthoclase, plagioclase, mica, clay, and micaceous shale clasts are present in the sandstone. Also present are thin to discontinuous shale beds or lenses. Sedimentary structures present are ripple cross-laminations, normally graded sandy beds.
Inferred sedimentary environment: proximal turbidites. Unit 2 represents distal turbidites and these represent proximal turbidites indicating a shallowing (hence, filling) of the basin. The 'proximal' interpretation is derived from the morphology, composition, and sorting of the sandstone beds. The quartz grains are only somewhat rounded and sorted, suggesting the did not travel far before being deposited. In addition, the presence of feldspar and lithic fragments also suggests a nearby source.
----------------------- GRADUAL FILLING OF THE BASIN
Unit 2 (80 m thick): Composed primarily of green to greenish-brown shale and mudstone with lesser sandstone interbeds. Cyclic sequences of sandstone grading upwards into siltstone are common. Sedimentary structures present are normally graded detrital layers, scour marks and load casts.
Inferred sedimentary environment: distal turbidites. Following basin subsidence, which resulted in deeper water shale setting, terriginous sediment was occasionally cascading down the delta front as the delta was growing (in a basin-ward direction) and some of the sand and silt made it far enough to settle out atop the shale.
----------------------- RAPID SUBSIDENCE and DEEPENING OF BASIN
Unit 1 (73 m thick): Composed primarily of white to gray-tan, friable, medium-grained quartz sandstone, the basal 12' of which is composed of dark green to black, fissil shale interbedded with thin sandstone lenses. Sedimentary structures present in this lowest unit include soft sediment deformation (resulting from slurry slumps) and cross-bedding.
Inferred sedimentary environment: Lagoonal shales atop Shinumo Quartzite and an encroaching distributary mouth-bar system which later buried the lagoon with delta-front sands.
----------------------- SHINUMO QUARTZITE - Considered a shallow marine deposit, it seems that the basin had filled enough to allow the formation of a lagoonal system, which marks the beginning of Escalante Creek time. . . . .
The lowest layer is the Escalante Creek Member. At the bottom of it we find a material similar to the Sinumo Quartzite.
How do they tell the two apart? What is it that makes this a different layer?
This is a perfect example of why it's exceedingly difficult to discuss geology without knowing the details of the various formations.
From reading a few online descriptions of the Dox, it seems that we have sand of the Escalante Mbr. overlying sand of the Shinumo, but upon reading a more detailed paper on the Dox we find that in fact we have a 12 m thick shale sequence sitting directly on top of the Shinumo. And that's how we can delineate the two formations.
Then on top of that we see shale and mudstone. We've talked about shale, but not mudstone. What is mudstone and what does mud stone tell us about the environment?
Mudstone is basically the same thing as shale only it lacks the visible stratification (fissility) we see in shale.
Before the availability and capabilities of radiometric dating, was there anyway to estimate timespans? Or where things simply established as sequences?
We could guesstimate, but that's about it. Everything was relative prior to radiometric dating.
Yes. After Shinumo sediment was deposited, a lagoon formed on top. It's possible that a stream/delta was near the lagoon originally and as it started weaving back and forth, deltaic deposits eventually covered the lagoon. The data show the delta was prograding out into the ocean (i.e., the deltaic coastline was migrating in a seaward direction), which may indicate an increase in stream run-off and sediment load.
Here we seem to see more filling of the basin, and I get the feeling, form the description, of a series of slopes being created.
Is this a continuous process? Or are these a real series of discreet steps? From the description in Message 161:
Inferred sedimentary environment: distal turbidites. Following basin subsidence, which resulted in deeper water shale setting, terriginous sediment was occasionally cascading down the delta front as the delta was growing (in a basin-ward direction) and some of the sand and silt made it far enough to settle out atop the shale
it looks to me like we are now seeing larger particles settling out which implies that the flow is slower.
it looks to me like we are now seeing larger particles settling out which implies that the flow is slower.
Is that correct?
What you have is a sort of underwater landslide with sediment cascading down the steeper front (see post 138) and these are known as turbidity currents.
These underwater slides are composed of various sized sediment with the heaviest stuff settling out first, either ON the slope or near the toe of the slope and forming thicker sequences, and the finer stuff traveling out further into the basin before being deposited once the current loses energy. The turbidite deposits closer (i.e., proximal) to the source will be coarser grained and thicker, while the turbidite deposits further out (i.e., distal) in the water will be thinner and finer grained.
In the image below, you can get an idea of what the depositional environments may have looked like, as well as where they were located with respect to the beach.
What I'm not sure about is exactly where the distributary mouth-bar system was located (I'm not up on all the terminology). I originally pictured it similar to the upper deltaic plain (upper right in image), but the location according to the image below suggests it was much deeper. I placed the timing of subsidence nearer the middle or end of Unit 1, but from the image, it appears subsidence must have been very fast and it's beginning represented by the lagoon. I need to read the paper again, it seems.
Source of image is the previously referenced paper by Stevenson and Beus, 1982.
According to the image below, the distributary mouth-bar system IS located in the submarine/subaqueous environment. (Note the depth in meters.)