My post #26 from the Grand Canyon thread, followed by others. This one starts by responding to jar's OP:
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jar writes:
I'd like to see explanations for each layer, it's composition, the environment when it was created,
Its composition is physical fact, but "the environment when it was created" is all interpretation, explanatory conjecture.
Then roxrkool on the Grand Canyon thread,
message #6
The simple.
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.
This is a typical description of the sort one gets both about ancient geological events and ancient biological events (the ToE). It is a mixture of factural description with interpretive conjecture presented as fact.
Conjecture presented as absolute fact: This layer averages about 1,700 to 2,000 million years old
Physical fact (actual science): 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. (since this also includes some speculation about how it was formed it's not pure fact)
Conjecture presented as absolute fact: 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.
Then she goes on to The technical description:
This appears to be physical fact : "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). "
This, however, is conjecture : 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).
Back to scientific fact : "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.SOURCE (PDF document)The link above has a nice picture of the relict bedding on page 1153, which clearly indicates a sedimentary origin for the Vishnu."
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I hope the above will help make it clear what I mean about interpretation as opposed to scientific fact, which I'm always referring to. There is a lot that geologists can know about the sheer physical presentation of the rocks, but when they go on with their interpretation of how it was formed in such and such an environment in such and such a time period they have stepped outside science into sheer speculation, yet they often don't make that distinction.
In the above "technical" quote, the interpretation is at least referenced and not stated dogmatically, but in the "simple" quote above that, it IS stated dogmatically: "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."
This cannot be known, and yet it was presented with just as much certainty as the physical facts were.
This is what one runs into all the time in discussions of both the geo timetable and the ToE, the treatment of what is nothing but fanciful notions about what happened in the ancient past as if they were fact.
I understand that these interpretations are based on some facts, but often the particular facts they are based on don't even get described.
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{The following is in answer to roxrkool on the Grand Canyon thread,
message #8
I am simply continuing my discussion of how interpretation tends to get interwoven with and confused with fact:
All of the following is pure interpretation :
quote:
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).
Anyway later, the sea eventually retreated and exposed the Precambrian sediments to erosion. ...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.
At least some of the physical facts involved are appealed to, but basically all the above is nothing but imaginative speculation presented as if it were fact.
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{The following is in answer to roxrkool on the Grand Canyon thread,
message #11
The following statements are about physical facts (whether correct or not is another issue)
quote:
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).
All the above is free of interpretive fancy. Whether the facts are correct or not is for the scientists to determine, but it is all in the category of scientific fact.
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{The following is in answer to roxrkool on the Grand Canyon thread,
message #18
I've bolded the interpretive parts:
quote:
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.
Evidence is given, but still the interpretations have that aura of fact. No contrary interpretations are suggested. There is no way to verify or falsify such an interpretation. This is what I find so frustrating about both OE and TOE discussions.
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{The following is in answer to roxrkool on the Grand Canyon thread,
message #21
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.
ALL the above is interpretive but it is not treated as interpretive or as if there could be any other thoughts about it. In the process of describing this scenario, various physical facts (deposition of volcanid and marine sedimentary rocks; igneous intrusion) are alluded to, but they are not in themselves discussed, only the supposed way they contributed to the scenario is focused on.
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.
All the above is speculative but at least the reasoning is given.
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This message has been edited by Faith, 03-15-2006 10:33 AM
This message has been edited by Faith, 03-15-2006 10:36 AM
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