I'd like for us to discuss the Grand Canyon, beginning with the lowest, oldest layers and then working up to the top, layer by layer. I'd like to see explanations for each layer, it's composition, the environment when it was created, and get questions about that layer answered before we move up to the next layer.
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 meta-lithic-arenites, metagraywackes,and calc-silicate lenses and pods. Meta-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,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.
One of the things you mentioned was mica. I remember as a child exploring some of the mountains in western Maryland. There I sometimes found mica. It always fascinated me since it's unlike almost any other rock I'd find (except maybe asbestos). It was almost clear and would separate into very flat thin sheets.
Can you tell me a little about Mica and how it is formed?
Conglomerates are conspicuously absent in the Vishnu metasedimentary rocks
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
Maybe a dumb question, but where did the upper layers come from? Common sense tells me that if the surface layers were 1 mile lower 100 million years ago than today, the earth must have been smaller in diameter then. If so, where did the top 1+ mile of dirt/sediment come from?
The rocks were deposited when the area was at a much lower elevation. The area has undergone a lot of uplift (and that is why we have a very deep canyon there).
I would add that as I understand it, the topographical variations we see on the Earth's surface don't really make a big difference to the size of the planet. The diameter of the Earth is 12,756 km, so a 100 km increase would be less than 1% of that..
Paul, thanks for the post and I hope you'll continue to contribute, but I really want to keep this one very narrow and organized so that it doesn't get pulled off into the "Big Picture". Can you help by sticking to the Vishnu Schist until we move on to the next layer?