Exploring the Grand Canyon, from the bottom up.

Conglomerates are conspicuously absent in the Vishnu metasedimentary rocks That's nice. But what does that mean?

Metasedimentary rock = Metamorphic rock that was formed from the metamorphism of sedimentary rock.

Conglomerate - Sedimentary rock made up of gravel or larger sized clasts. There are also commonly found "hybrid" sediments - Conglomeratic sandstones. The conclomeratic compontent are lag deposits, with the sand being deposited around and above the larger clasts.

The pre-existing sedimentary rocks are called the protoliths. An earlier form that became a later form.

In the case of the Vishnu protoliths, the sediments show no evidence anywhere of being from the processes that form conglomerates.

Bottom line - For whatever reason, the protolith sediments did not contain gravel or larger sized clasts.

Moose

Per the diagram of message 8, the granite is shown to be intruding only the Visnu Shist (metasediment).

... is there any way that these intrusions could have happened while the strata was still uh, muddy?

I'm going to ask a (sort of) variation on that question. I am going to presume that the sediments were at least already lithified (not "muddy").

What was the order of the events?

1) Sediments were metamorphosed and then later intruded by the granite?

or

2) Sediments were intruded by the granite and then later metamorphosed?

or

3) Sediments were intruded by the granite and metamorphosed at the same time?

Now, at least superficially, the answer would seem to be #1, at least as opposed to possibility #2. If the metamorphic event were last, then the granites would have also been metamorphosed.

The kicker is (as I recall), because of their mineralogy, granitic rocks are highly resistant to metamorphic effects. Pressures and temperatures that would substantially change a mudstone or a basalt might not change a granite at all.

Perhaps the detailed study of the schist/granite contact would shed light on this question. Offhand, my guess is that #3 is the case. Actually, #3 may be a variation of #1, in that the granitic intrusion may be a later phase of the same event that caused the metamorphism.

Comments?

Moose

First of all, I'm going to repost the strat column originally posted in message 8.

Click to enlarge

SOURCE of the section.

The first question, "What is limestone?", has and is being discussed in the currently active Limestone Layers and the Flood. I don't see any point in rehashing that material here. The esential point there, is that limestone is mostly a direct or indirect product of biological activity.

The interesting point is that the Bass Limestone is a preCambrian limestone; Per the diagram, it has an age of 1250 million years. preCambrian limestones are relatively uncommon.

Per the second question, the sandstone/limestone transition - The Bass Limestone unconformibly overlies the Vishnu Schist, dated at 1700 million years. As such, there was no real transition from sandstone to limestone. The Bass was some sort of (marine?) deposit upon a schist/granite surface.

The interesting point, as I see it, is how did the Bass Limestone form, being that it is seemingly of an age before much any biologically produced calcium carbonate. In other words, the research topic is preCambrian limestones, or more specificly, Proterozoic limestones.

By the way, I have just noticed that the "preCambrian" in the diagram is designated as being a geologic period. That is wrong.

The geologic time scale is divided into the preCambrian and Phanerozoic. The preCambrian is in turn divided into the older Archean era (aka Early preCambrian) and the younger Proterozoic era (Proterozoic = proto life) (aka Late preCambrian). The Phanerozoic consists of the Paleozoic era (Paleozoic = early life), the Mesozoic era (Mesozoic = middle life), and the Cenozoic era (Cenozoic = recent life).

Moose

Added by edit: Possibly relevant article (abstract only, full paper access requires membership)

Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution

Abstract The temperature and salinity histories of the oceans are major environmental variables relevant to the course of microbial evolution in the Precambrian, the “age of microbes”. Oxygen isotope data for early diagenetic cherts indicate surface temperatures on the order of 55–85 °C throughout the Archean, so early thermophilic microbes (as deduced from the rRNA tree) could have been global and not just huddled around hydrothermal vents as often assumed. Initial salinity of the oceans was 1.5–2× the modern value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean. Because O2 solubility decreases strongly with increasing temperature and salinity, the Archean ocean was anoxic and dominated by anaerobic microbes even if atmospheric O2 were somehow as high as 70% of the modern level. Temperatures declined dramatically in the Paleoproterozoic as long-lived continental cratons developed. Values similar to those of the Phanerozoic were reached by 1.2 Ga. The first great lowering of oceanic salinity probably occurred in latest Precambrian when enormous amounts of salt and brine were sequestered in giant Neoproterozoic evaporite basins. The lowering of salinity at this time, together with major cooling associated with the Neoproterozoic glaciations, allowed dissolved O2 in the ocean for the first time. This terminated a vast habitat for anaerobes and produced threshold levels of O2 required for metazoan respiration. Non-marine environments could have been oxygenated earlier, so the possibility arises that metazoans developed in such environments and moved into a calcite and silica saturated sea to produce the Cambrian explosion of shelled organisms that ended exclusive microbial occupation of the ocean. Inasmuch as chlorine is a common element throughout the galaxy and follows the water during atmospheric outgassing, it is likely that early oceans on other worlds are also probably so saline that evolution beyond the microbial stage is inhibited unless long-lived continental cratons develop.

The above was found via a scirus.com seach for "Bass Limestone". Apparently it is relevant to the Bass Limestone, even though such is not mentioned in the abstract. Seemingly, the Bass Limestone is a result of microbial processes. Note also the "Cambrian explosion" mention.

This message has been edited by minnemooseus, 03-17-2006 05:59 AM

The Hotauta Conglomerate (apparently mispelled in the other figure) occurs in the basal portion of the Bass Limestone and this suggests the possibility of a more gradational transition into limestone/dolomite than previously thought.

I would think that some sort of residual conglomerate would be expected at the unconfomity (nonconformity in this case). Such is said in the following quote box.

The sequence starts with deposition of the Hotauta Conglomerate which grades upward into the Bass Limestone. The Hotauta contains fragments of igneous and metamorphic rocks of Layer I, preserved remnants of the older “Archean” sequence.

I didn't look at the source article, as it takes a long time to download on my slow dial-up connection (the earlier mentioned PDF was even worse). I wonder how thick this gradation is? Are we talking along the lines of at most just a few feet? My image is that the gradation is limestone infilling of the conglomerates voids. In all, I suspect the conglomerate is a pretty insignificant feature.

Side note: The Vishnu is dated at 1700 million years. I presume this is the age of the intruding granites and of the metamorphic event. By saying "Archean" above, I presume they are talking about the age of the Vishnu protolith.

Side note 2: I note that in the diagram of message 8 (repeated in 80), the total thickness of the Grand Canyon Supergroup is along the lines of 12,000 feet. In the geological section of message 82, that thickness is 4000 feet. My guess is that the message 8 "thickness" is not a true stratigraphic thickness. Maybe it's exposure distance along the river?

Mucho kudos to roxrkool, for doing all the work to dig up the information, diagrams, and references. Many more POTM's should be coming your way, but that would mean we're again getting into the "roxrkool posts again, gets POTMed again" situation. Maybe you should get a GMOAPOTM (grand mother of all POTM) when this topic is concluded.

Moose

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Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.

"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham

"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith

"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

Seems to me that you are bringing up enough details that we could have several seperate other topics.

Basicly agree, but a few comments (warning - may contain oversimplifications):

The process of making many small things out of fewer large things is erosion.

That is weathering. It may be chemical, mechanical, or both. Erosion is the moving of the smaller particles from their location.

Next, schist is sandstone that was changed due to heat, pressure and time.

In the case of the Vishnu Shist, yes. I think it was because the protolith was a clay rich variety of sandstone. It is the clay minerals that are metamorphosed into the micas that give the schist the property of schistosity. Not all sandstones can metamorphose into schists, and not all schists are metamorphosed sandstones.

Granite is the result of magma which changed form as it slowly cooled over time.

Granite is a variety of rock that is formed when a very hot solution (magma) cools, and different chemical composition units (crystals) are formed. In general, slower cooling rates result in larger crystals. A very fast cooling rate results in glass.

We can see an example of a granite intrusion...

Your second example is a pegmatite, which is a special late stage, high volatile (water etc.) variety of igneous intrusion. Probably best not discussed further in the context of this topic.

Crossbedding - Formed from the movement of sediments to form ripples of various scales. The crossbeddings are the successive down flow direction faces of the ripples. Dunes (from either wind or water sediment movement) are essentially just large ripples.

Conglomerates - Rock formed from coarse grained sediments with the "grains" being rounded because of abrasion from movement.

Breccia - Angular broken up rock, not rounded from movement. Offhand, your photo example appears to be a fault zone breccia.

Moose

Edits: Fixed a misspelling, tweeked a couple of phrases.

This message has been edited by minnemooseus, 03-18-2006 03:06 PM

This is again getting into being something that could well be a topic in itself.

Most sea level changes (correct me rox or IHR or edge) that you see in transgression/regression sequences occur due to the cyclic nature of ice ages on earth.

Such can be the causes, but it is a relatively minor effect.

The cause of major sea transgressions onto land is thought to be because of major increases in seafloor spreading rates. I'm not going to get into the details of why and how the effect happens, but essentially, increased spreading rates cause the sea floor to rise to higher levels. This in turn causes sea level to rise, and displaces water onto the continents.

Of course, since the volume of the Earth remains constant, this rise in sea floor levels strongly implies a fall in the levels of the continental crust.

To visualize what I mean by level, think distances of the oceanic and continental crust surfaces from the center of the earth.

I think this has previously been covered elsewhere, with Joe Meert being the primary contributor of information. Probably the discussion has been mostly in topics relating to Baumgardner's (sp?) catastrophic sea floor spreading ideas.

Moose

Your photo is from http://strata.geol.sc.edu/PermianBasinTxNMx/pages/049-West-Face.html

Clicking on that photo gets you to http://strata.geol.sc.edu/PermianBasinTxNMx/pages/050-Guadalupe%20Mts-Delaware-Group.html, which seems to be an attempt to explain the geology shown by the photo.

The process shown is that of a dropping sea level. My impression is that the non-horizontalness of some of the strata relative to other strata may be a result of some of the strata actually being deposited on a slope. In other words, an exception to the general rule that sediments deposition results in horizontal layers.

There is also the possibility that there was some soft sediment deformation, such as from the slumping of whole multiple layers.

Perhaps what looks to be the major angular unconformity in the photo is actually just a larger version of a channel scouring and filling.

It is hard to do a great analysis from a photo. But, of course in the context of this forum, that is what we have to work with.

Again, getting pretty remote from things Grand Canyon.

Moose

Source:

quote:

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Studies of the sequence of rocks show that the Vishnu Group underwent at least two periods of orogeny mountain-building. These orogenies created the 5 to 6 mile (8 to 10 km) high Mazatzal Mountains (Yavapai-Mazatzal orogeny).[3] This was a very high mountain range, possibly as high as or higher than the modern Himalaya. Then, for over 500 million years, erosion stripped much of the exposed sediments and the mountains away. This reduced this very high range to small hills a few tens to hundreds of feet (tens of meters) high, leaving a major angular unconformity. The once deeply buried mountain roots were all that remained of the Mazatzal Mountains as the sea reinvaded.

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The point I want to make here is that metamorphism of the Vishnu sort requires substantial pressure. In other words, deep burial. I was unable to come up with much about the metamorphic grade, but the above cited did include "...garnet-studded layer the Vishnu Schist". Garnets are characteristic of medium grade metamorphism. My wild ass guess (WAG) is that somewhere in the neighborhood of 10 kilometers (30,000 feet) of burial was required.

So, the original rock (protolith) of the Vishnu were deposited and then buried to a (WAG) 30,000 foot depth. This is approximately 6 times the current depth of the Grand Canyon. Then this 30,000 feet of rock was eroded off during the preCambrian, resulting in an unconformity. Then the preCambrian Grand Canyon group of sediments were deposited, followed by another major erosion event resulting in another unconformity.

Events:
1) Deposit of Vishnu protolith.
2) Much more sediments deposited, resulting in Vishnu deep burial and metamorphism.
3) Much erosion resulting in unconformity.
4) More sedimentation.
5) Erosion again (a Paleozoic event) - another unconformity.

That gets us to the top of the preCambrian. A LOT of sedimentation and erosion happened.

Moose

Edited by Minnemooseus, : Mention the later erosion as being Paleozoic.

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Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.

"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham

"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith

"Nixon was a professional politician, and I despised everything he stood for — but if he were running for president this year against the evil Bush-Cheney gang, I would happily vote for him." - Hunter S. Thompson

"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose