This is potentially a huge and complex thread, so it's a great idea to start from the bottom and work your way up gradually.
This layer averages about 1,700 to 2,000 million years old and consists of mica schist. These were originally sediments of sandstone, limestone and shale that were metamorphosed and combined with metamorphosed lava flows to form the schist. This layer along with the Zoroaster Granite were once the roots of an ancient mountain range that could have been as high as todays Rocky Mountains. The mountains were eroded away over a long period of time and new sediments were they deposited over them by advancing and retreating seas. The color of this layer is dark grey or black.
Vishnu Schist. The Vishnu Schist consists of pelitic schist and quartz + biotite +muscovite schists interpreted as metaâ€“lithic-arenites, metagraywackes,and calc-silicate lenses and pods. Metaâ€“lithic-arenite and metagraywacke sequences show thick sections (kilometre-scale) of rhythmically banded (centimetre- to metre-scale) coarser and finer layers, with locally well-preserved bedding and graded bedding (Walcott, 1894; Clark, 1976; Fig. 4c). Locally, the Vishnu Schist contains pelitic and semipelitic schists that variably contain andalusite, sillimanite, staurolite, chloritoid, cordierite, and garnet. Original grain size in the Vishnu Schist metasedimentary rocks probably ranged from medium-grained sand to silt and clay.
Conglomerates are conspicuously absent in the Vishnu metasedimentary rocks (Campbell and Maxson, 1933). Relict graded bedding (Fig. 4c), association with metavolcanic rocks containing pillow structures (Fig. 4b), lack of coarse sediments, and geochemical data (Babcock, 1990) indicate that the metasedimentary units accumulated in an oceanic island-arc environment, as suggested for the Yavapai Supergroup rocks of central Arizona (Anderson and Silver, 1976; Bowring and Karlstrom, 1990).
The contact between the Vishnu Schist and the Brahma Schist is generally concordant, and the rocks are interlayered as in Clear Creek,Â»1 km from the Colorado River. A reference section for the Vishnu Schist is in Vishnu Canyon. Other easily accessible exposures of Vishnu Schist metasedimentary rocks occur (1), for highest-grade Vishnu rocks, along the river between mile 78 and Hance Canyon; (2), for lowest-grade Vishnu rocks, between Vishnu Canyon and Clear Creek, near 91-Mile Canyon, from Monument Canyon to 96-Mile Canyon, and from 96-Mile Canyon to Crystal Creek; and (3), in isolated septa, from Crystal Creek to mile 102.6 and from Lower Bass Camp to Waltenberg Canyon (Fig. 1). In the latter area, metasedimentary rocks with graded bedding clearly overlie metavolcanic rocks. As with the metavolcanic rocks, we expect that the Vishnu Schist may be further divisible in the future into different units and/or originally disparate tectonic packages juxtaposed across shear zones. However, present understanding of the stratigraphy and structure does not suggest obvious further subdivision.
I haven't really done a lot of research into the history of the GC, so this will be a really interesting topic.
I'm not sure where to start, but I posted a couple of things to introduce the Vishnu. I think probably the most interesting thing about the schist is how it started out - as simple sedimentary rocks. What I don't know is if the Vishnu has been divided into 3 units, which are discussed in the technical paper I linked to above, or if the Vishnu IS the 3rd unit.
I also think it's important to show the relationship between the various rock units. Based on the column below, we can see the sequence of events prior to the Cambrian.
Deposition of the proto-sediments and subsequent lithification. Later, the Vishnu protolith (original rock) was folded and metamorphosed during a mountain building event into schist (i.e., the Vishnu Schist). After the main phase of mountain building, the Vishnu was intruded by at least two generations of magma - mafic and granitic phases. The mountain range was then eroded (and we know this because the Zoroaster Granite does not intrude any of the overlying sedimentary rocks) down to small rolling hills (I think), and soon after covered by a transgressing sea which deposited limestone, shale, sandstone, and even volcanic lava (somewhat hard to explain during a flood). I don't remember if this package of PC sedimentary rocks outcrops anywhere else in the region or if this is it. The reason I mention this is because it's possible we are only looking at a very small package of precambrian rocks of what was once a thick sequence.
Anyway later, the sea eventually retreated and exposed the Precambrian sediments to erosion. The only reason these rocks are visible in the GC is because of normal faulting. Later another series of marine transgressions and regressions during the Paleozoic occurs and these we see clearly in the GC.
Just in the Precambrian rocks do we see lithologic relationships that point to deep time.
Sorry, Jar, I did miss this post. This can be a pretty difficult topic to convey, so I'll try to keep it pretty non-technical. And if I make any mistakes (which is highly likely since I'm no mineralogist), I hope someone will jump in to correct me.
The mica group is comprised of several minerals of which biotite and muscovite are probably the most commonly known and recognized varieties. Biotite is dark brown and muscovite is clear to yellowish in color and the color is pretty much as result of their elemental composition. Biotite is a ferromagnesian (or mafic) mineral which means it contains appreciable amount of iron and magnesium. When an igneous rock is composed primarily of these Fe- and Mg-rich minerals, it is called 'mafic.' Muscovite is a felsic mineral and light colored because it generally contains very little if any Fe or Mg in it's mineral structure.
They are part of a group of minerals called sheet silicates because the atomic structure (composed of linked (Si,Al)O4 tetrahedra with a hydroxyl ion) resembles a sheet. These sheets are stacked one upon the other, but the bond between sheets is somewhat weak so they part (or 'cleave') perfectly between the sheets, forming what is called perfect basal cleavage. And of course that is one of the most fascinating things about mica, how they can be separated into thin elastic sheets. This and other properties (such as their high melting temperature and some good electrical properties) is exploited by various industries such as the electrical, lighting (in lamp shades), ovens (the glass window), etc.
Mica can range in size from minute particles impossible to see with the unaided eye to large massive sheets sometimes several meters in diamter. The finer stuff is found in all three rock types, but the massive stuff I believe is restricted to pegmatites where they are the result of primary (as opposed to secondary in metamorphic/diagenetic settings) mineralization if a residual fluid rich in volatiles.
In my own experience, hotter rocks, such as ultramafics don't typically have much mica (in the form of biotite) because the high temperatures tend to drive off the water, which is required to form the hydroxyl ion. However, is many rocks, particularly layered intrusives, we do find biotite in pegmatitic lenses or horizons, which tells us (or maybe just me lol) that even in the hottest magmas, water is either able to be retained in 'pockets' of residual fluids, or it is introduced from elsewhere (perhaps the country rock - which is the rock being intruded).
Darn! I'll be back Jar. Forgot about an appointment. oops haha
Let me know if you have anymore questions and I'll come back to answer the other one about the conglomerate (if no one else has).
This message has been edited by roxrkool, 03-14-2006 03:20 PM
Re: Vishnu Schist - No conglomerate in the protoliths
From reading the technical paper I linked to earlier, it seems the lack of a coarser grained component (i.e., the conglomeratic sequence), points to a marine depositional setting rather than a terrestrial setting for the original proto-sediments; which is consistent with previous theories regarding the geologic history of that particular terrane.
In another thread, I briefly touched on how the U.S. grew via island arc accretion south from about the Wyoming/Montana area. Wyoming is located on the Archean Wyoming craton and everything south and basically west are progressively younger accreted terranes - generally considered to be island arc terranes (volcanic island chains similar to Japan and Indonesia that develop along subduction zones).
According to the linked paper (Ilg et al., 1996), the protosediments which later became the Vishnu Schist, are remants of one of those island arc terranes that collided with the continent back in the Precambrian (early Proterozoic time?). Evidence for this is in the relict (i.e., original unmetamorphosed rock with primary texture) volcanic and fine-grained sedimentary textures/rocks still visible in some portions of the entire Vishnu schist sequence, which Ilg et al. (1996) renames to Granite Gorge Metamorphic Suite.
Re: Vishnu Schist - No conglomerate in the protoliths
That's basically correct, Jar.
What metamorphism also does, because of increased heat and/or pressure, is cause recrystallization and secondary mineral growth. Subjecting slate to higher temps and pressures can result in a schist.
So with increased temperature and/or pressure, you will get new [secondary] minerals forming from the destruction of the primary minerals due to mineral instability. The new minerals form because they are stable within the new pressure/temperature regime and the original ones were not (because they were formed and stable under other temp/pressure conditions).
What metamorphic petrologists can do is look at the minerals comprising metamorphic rocks and pretty much tell you what the original protolith was and often how much water it contained. Clays, such as in shale, when metamorphosed become chlorite during low-grade or green schist metamorphism and will alter to mica under higher temps (and pressure?).
Each metamorphic grade from low to high is represented by specific suites of minerals, which are indicative of ranges of temperature and/or pressure.
Re: Vishnu Schist - No conglomerate in the protoliths
The general sequence of events as I understand them from reading around the internet:
1. Formation of island arc at subduction zone located off the coast of the U.S. (in Utah or Arizona at the time??) which is slowly migrating toward the continent;
2. Deposition of volcanic and marine sedimentary rock (protosediments) at the island arc;
3. Collision (accretion)of island arc onto North American continent, metamorphism, and possibly igneous intrusions soon after;
4. Some time later 'mountain building'/uplift (more metamorphism?) began in the vicinity of the arc accretion (possibly due to deep seated structural sutures developed during accretion);
5. Erosion of mountain range which cuts across both metaseds and igneous intrusions;
6. Deposition of Grand Canyon supergroup.
Ilg et al. (1996) appear to suggest that metamorphism occurred while the island arc was still in the marine environment and possibly due to collision. Uplift of the island arc system to sea level exposed the surficial volcanic environment to marine (wave?) erosion and continued uplift eventually exposed the metamorphosed core. They don't mention any mountain-building episode. When the island arc system was eroded far enough, eventually marine sediments were able to be deposited atop the erosional surface.
Re: Vishnu Schist - No conglomerate in the protoliths
Well, I did mention it, but the Supergroup does not make an appearance until #6, after the major erosive event.
The Vishnu Schist, apparently now known collectively as the Granite Gorge Metamorphic Suite, is formed from the metamorphism of the volcanic rocks and marine sediments deposited by the island arc system. This package, which we've been calling the Vishnu Schist, was later uplifted and eroded prior to deposition of the Grand Canyon Supergroup.
Is that more clear? It's a bit difficult to keep all this stuff straight and I am probably doing a terrible job of explaining it.
Unfortunately, I won't have time to get to your comments today as I will be in a class all day today, but I think they are good ones.
Your comments illustrate the different mindset that exists between what Creationists and some laity consider 'testable science' and what scientists consider testable science.
One thing that popped out at me immediately is your implication that I present everything I have stated (in the interpretive portions of my posts) as 100% correct or absolute fact. That is simply not true. Everything I've stated beyond the facts is interpretive AND tentative AND subject to change. Some of it is a bit conjectural, but it all is based on the factual observation.
I can explain why graded bedding or pillow structures are significant observations in metamorphic rocks, while you haven't yet been able to adequately support your positions or arguments with any definitive evidence whatsoever.
This message has been edited by AdminAsgara, 03-15-2006 08:49 AM
Boy, would it be wonderful if there was a geologist here for each geologic topic that came up. There have been a lot lately.
The degree of metamorphism seen in the Vishnu and other schists is quite pervasive over long distances and very little of it is the result of igneous intrusions as far as I can tell. The reason for this conclusion is that the degree of metamorphism, which includes both deformation fabric/structure and mineralogy, is present throughout the entire body of schist regardless of the presence of intrusive bodies.
Contact metamorphism is a point source for the heat and so heat is radiated out from the dike/intrusion through the rock. This sort of thermal conductivity results in minralogic and textural changes in the country rock (rock being intruded), which, depending on it's depth in the crust, can be cold, warm, or hot. The higher in the crust, the colder the country rock, the more water it typically has, and (I think) the more pronounced the reaction of the country rock to the intrusion. The presence of water or other fluids is easily detected by the resultant metamorphic minerals and this water can come from the country rock or the intrusion. We can tell the difference using isotopic signatures.
So in contact metamorphic zones, the highest degree of alteration is typically located immediately adjacent to the intrusion (again especially if the rock is cold) and the alteration becomes progressively less pronounced the further away from the heat source we move.
Because I don't see such a pronounced zoning (an alteration halo) around each intrusion, and the degree to which the Vishnu Schist is deformed, the metamorphism event that formed the schists in the Grand Canyon area are not primarily the result of contact-metamorphism.
That doesn't mean there is not any contact metamorphism present in the schists, there very likely is, and this would probably result in increasing the grade of metamorphism in the vicinity of the intrusions. But again, it would be like a halo radiating away from the intrusive bodies unless there was a large concentration of intrusive bodies in one location. The resulting minerals found in a contact metamorphic halo would be different that what is found in the rest of the schist simply because by intruding the schist with an igneous body, you are introducing new fluids and minerals (in other words, new elements/chemistry) into the system.
From what I've read about these intrusions in the paper I linked to on the first page (Ilg et al, 1996), there are two separate types of intrusions that occurred at different times and are not genetically related to each other.
One is an earlier and more mafic group of gabbroic to granoditoritic intrusions probably related to arc magmatism, which may have been the source of the volcanic material (pillow basalts, volcanic seds, flows, etc.) erupted by the island arc. The mafic bodies are highly deformed, though not too alteres as to obliterate primary mineralogy and structure apparently. But the fact that they are deformed suggests they preceded the major deformation event and were deformed during the same event that created the schist.
The other group of intrusives is a later granitic phase that may have been the result of partial melting of the schist or sourced by other processes, but these appear to have intruded the schist at the same time the schist was forming. This is based on various things such as lack of deformation and how dikes tend to follow folds and fraacture/jointing patterns as the schist later cooled and hardened. Basically, they took advantage of incipient weaknesses in the schist while it was forming and after it started cooling.
This message has been edited by roxrkool, 03-15-2006 03:00 PM
Re: Unlithified sediment vs. sedimentary rock vs. metasediments
As far as the Vishnu Schist goes, the granite was formed at about the same time as the schist was forming. In other words, they were forming contemporaneously - at least at deeper levels in the crust.
The granite may represent either a completely separate igneous body, or it is a partial melt of the Vishnu Schist itself. When rocks are subjected to high temperatures and pressures, as the Vishnu was, they can partially melt to form a magma (aka partial melt). It is called a partial melt because only a portion of the minerals present in the parent rock, in this case the Vishnu Schist, will melt. This is because temperatures during a metamorphic event generally do not get as hot as temperatures lower in the crust where ALL the minerals melt.
Ferromagnesian minerals, such as pyroxene and olivine, have higher melting temperatures, and it takes very high temperatures to get those to melt. So in a [shallow?] metamorphic setting, only the lower temperature felsic minerals will melt, forming a felsic magma - granite.
This granitic magma, because it was more buoyant than the surrounding rock, worked its way up into the schist, specifically exploiting and traveling up areas of weakness, such as along faults, joints, and fold axes. This suggests the schist was already somewhat brittle, though not completely cooled, when the granite intruded some distal portions of the schist.
Sorry, I don't think I'm doing a very good job at describing the temporal relationship between the schist and granite.
I haven't read all the pertinent data, so I don't know whether any sediments exist below the Supergroup that may have been intruded by the granite. That is certainly possible. But what we do know is that the granite intruded the Vishnu long before the Supergroup was deposited atop it. The granite did NOT intrude any of the Supergroup rocks - as far as I know.
Re: A few more questions before moving on to the Bass Limestone.
Now Question 1.
Does the accumulated sand need to be under pressure of an overlying layer to change from sand to sandstone, or is it simply that the lowest layers of sand are compressed by the weight of overlying sand?
Would it simply be a layer that is sand at the top gradually turning to sandstone as the weight and pressure increase with depth.
1. Generally, all you really need for lithification and/or induration of marine rocks is compression (gets all the water out) and it helps to have some sort of cement or maybe even a fine-grained component such as clay. So more deeply buried sediments are more likely to harden than shallow sediments. Diagenesis, which is basically very mild form of metamorphism, happens soon after deposition sometimes and this too will help 'set' the sediments. Your last sentence is possible.
Is it likely that the magma intrusion extended through the layer of sand--->sandstone and that what happened is the the softer protosandstone layers that also contained magma were what was eroded away before the Bass Limestone and other layers were laid down?
Jazz answered this question well, but yes, what you stated is possible as far as I know.
I haven't read many papers about the Precambrian strata in the area, so I don't know if a 'complete' section exists (between the Vishnu and the Supergroup) elsewhere in the area.
When did the sandstone become schist and the magma become granite? Wouldn't both have to be buried under lots of material to create the temperatures and pressures need for sandstone ---> schist and magma ---> granite?
Jazz also answered this one well. Based on other papers, it seems that most workers agree the schist and the granite formed at about the same time, especially at depth. There is some question whether the granite formed from the schist or whether the granite is a completely separate body of magma. The paper I linked to earlier (Ilg et al., 1996) state that, based on metamorphic mineralogy, peak temps in the study area reached as high as about 725 degrees Celsius and depths of about 22 km.
By the way, I have just noticed that the "preCambrian" in the diagram is designated as being a geologic period. That is wrong.
They also spelled "Vishnu" wrong. heh
Abe: Good post, by the way. I'll see if I can find some more on the Bass Limestone. I'm really interested in this portion of the thread. I don't remember ever reading any literature about the Supergroup, much less individual formations.
This message has been edited by roxrkool, 03-17-2006 12:16 PM