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Author Topic:   Exploring the Grand Canyon, from the bottom up.
roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 82 of 283 (296243)
03-17-2006 12:37 PM


Unkar Group
Here is an interesting figure that shows another package of rocks below the Bass Limestone and included in the Unkar Group - the Houtata Conglomerate. It does not appear to be a widespread formation and may pinch out against paleo topographic highs. Red lines are unconformities.


Click to enlarge

SOURCE of figure. The link also has the following:

Abstract

Unkar Group (1.25-1.10 Ga) shale geochemistry and petrography coupled with detrital zircon geochronology of interbedded sandstones, identify specific source terranes for Unkar Group sediments. The dominant sediment source for most of the Unkar group is the southern Grenville Orogenic Front (GOF). The Hakatai Formation is composed dominantly of GOF volcanic detritus with some material derived from the Southern Granite Rhyolite Terrane (SGRT) and the Yavapai - Mazatzal crustal provinces (YMP). Unconformably overlying the Hakatai is the Shinumo Sandstone, which is composed primarily of locally-derived YMP and SGRT detritus with a minor GOF component. The uppermost Dox Formation, which conformably overlies the Shinumo, is largely derived from the GOF. These data indicate that a protracted period of orogenesis along the southern GOF resulted in uplift of the interior Rodinian platform which evolved from an epicontinental sea or marine embayment to a foreland with well-developed fluvial drainage.

Variable provenance of the Unkar Group, as revealed by detrital zircon data and mineralogy, occurs despite relatively homogeneous bulk chemical trends. It is concluded that Unkar Group shales are a well-mixed recipe of variable ingredients that subtley reveal the distinct nature of the components. This is attributed to a number of factors that include; rapid rates of uplift and erosion along the GOF, the similarity in composition of a large number and volume of crustal domains (YMP and SGRT), a significant component of intraformational recycling, and the distal location of the Unkar Basin, ~800 km, from the dominant sediment source.

If you open up the PDF presentation in the above link, you can find the same figure on page 7 with interpreted depositional settings. The PDF document has some good pictures of shallow water sedimentary structures (ripples, dessication cracks, trace fossils, etc.). However, no text is available so it's impossible to know which formation we're looking at in each image. My guess is the images span the Unkar Group from the Bass Limeston to the Dox Sandstone.

OOPS!! It just occurred to me, Jar, this is generally discussing the setting for the lower portion of the Supergroup and ignores the Bass Limestone. Is this too much info? Should I perhaps remove the info until we get past the Bass Limestone?

I personally find it easier to understand the big picture first before diving into the details, but that may not work best for non-geos. Let me know what you think.

Changed img to thumbnail version to fix page width - The Queen

This message has been edited by roxrkool, 03-17-2006 12:49 PM

This message has been edited by AdminAsgara, 03-17-2006 12:32 PM


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 83 of 283 (296255)
03-17-2006 1:29 PM
Reply to: Message 79 by jar
03-16-2006 10:31 PM


Re: Moving on to the Bass Limestone layer.
Jar writes:

Question 2
What does the change from sandstone to limestone tell us?


First, what sandstone are you referring to, jar?

Second, THIS SITE appears to be pretty well researched and written. There really isn't a whole lot on the Bass Limestone, especially online, but I thought the following was an important observation (I noted the same thing in another thread about the term 'shale' being a misnomer):

"Bass Formation" has been proposed by some technical workers as a more accurate description who contend "Bass Limestone" is really somewhat of a misnomer due to the variety of rock types within this formation. The diversity of rock types composing the Bass is not atypical as virtually all of the strata in Grand Canyon popularly designated and labeled as "Sandstones", "Limestones" and "Shales" exhibit complex structures and members composed of differing lithologic types.

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.

It would be nice to see a strat column of the Bass Limestone and a picture or two of the stromatolites (algae) found in the Bass, but without digging through all the literature, that may not be possible.

More on the Bass:

Layer II – Algonkian Grand Canyon Supergroup (1.6 Ga – 850 Ma)

Post Mazatzal uplift and erosion set the stage for the deposition of Layer II (the Grand Canyon Supergroup which consists of the Unkar and Chuar Groups in Figure 24). Layer II consists of a vast sequence of intercalated sedimentary and volcanic rocks of predominately marine origin. 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. The Bass Limestone (and dolostone) varies in thickness from 120’ to 340’ and forms the basal unit of the 3,000’ thick Proterozoic Unkar Group.

[SOURCE]


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 88 of 283 (296392)
03-17-2006 9:33 PM
Reply to: Message 84 by Minnemooseus
03-17-2006 5:29 PM


Re: Moving on to the Bass Limestone layer.
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.

I agree. That's why it was interesting to see that the Bass Limestone is not purely a limestone-bearing unit. It's a bit hard to imagine how limestone would directly overlie metamorphic/igneous rocks. However, if the diagram is drawn accurately, it appears the conglomerate may pinch out in places so it's still possible we can find limestone directly on top of Vishnu.

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.

I wish we had a detailed strat section, but what I'm thinking is coarse material at the bottom, sand above that, maybe a calcareous sandstone, and then limestone. Marine carbonate generally does not form in the presence of too much clastic material. Based on the following and what was stated in the Grand Hikes link, it appears conglomerates are present throughout the entire Bass Limestone and they are at least 4 or 5 feet thick.

THIS SITE states the following about the conglomeratic unit:

quote:
The Hotauta Conglomerate member, which is the lowermost unit of the Bass Limestone, was deposited in low areas of the Vishnu terrane. The Hotauta conglomerate is composed of rounded gravel-sized clasts of chert, granite, quartz, plagioclase crystals and micropegmatites in a quartz sand matrix. This conglomerate is found in the eastern Grand Canyon. In the western Grand Canyon, intraformational breccias and small pebble conglomerate make up the base of the Bass Limestone, suggesting the source of these clasts was to the east. It is also important to note that the clasts found in the conglomerate come from the Vishnu Complex, suggesting an extended period of erosion of the Vishnu terrane.

The last bolded portion is particularly interesting. We have Vishnu-derived rounded material at the base of the Bass Limestone.

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.

They are referring to the depositional age of the proto-sediments, but because they don't have any hard evidence, such as an absolute date, they are speculating, hence the quotation marks.

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?

If a scale is present on a strat section/column, it implies true thickness. However, the 12,000 foot section may be a composite section where workers have compiled sections from different parts of the canyon to form one complete section, or they've constructed a section by taking the thickest or average thicknesses for each unit to form a generalized section of the Supergroup.

Or a better explanation might be that the scale in post 82 is in meters rather than feet.

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.

Thanks, moose. No more POTMs means less pressure in the future. :) Really, though, the entire thread is good because of all the participants.

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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 93 of 283 (296413)
03-17-2006 11:16 PM
Reply to: Message 87 by jar
03-17-2006 8:42 PM


Re: What sandstone???????
To change sandstone into schist I understood that it took time, higher temperatures and pressure.

Correct.

To change the magma into granite I understood that it took time, long slow cooling and pressure.

It takes time, but not pressure. Large plutonic bodies can take millions of years to cool, but dikes and smaller igneous bodies can cool quickly when intruding cold country rock. (Country rock is the rock being intruded.)

Since we are still at that point in time were the Vishnu layer has been laid down and the magma intruding into it, there has not been any time for the transformation. In addition, so far there are no layers on top of the Vishnu layer so no pressure to cause the metamorphose.

Is that correct?


Now that I've read a little more, I have a clearer picture of what happened, but I will have to use dates to illustrate the temporal progression.

During the latest part of the Archean, deposition of the Vishnu proto-sediments begins in a marine basin located just seaward of a young North American continent, and adjacent to a volcanic island chain similar to Indonesia. The island chain is slowly moving towards the continent. I'm not sure how thick the sediments were able to accumulate in the basin, possibly several thousand feet, but the deepest sediments begin the lithification process sooner than the shallower sediments due to temperature and pressure.

Later (by 1700 Ma), Vishnu proto-sediments of unknown thickness are fully lithified (I believe) and the volcanic island chain is in the process of colliding with North America. It's at this point in time the sediments, during collision and after lithification, are metamorphosed and deformed into schist due to intense pressures associated with collision. The collision results in mountain building (think Mt. Everest). It is also during this time (~1700 Ma) that granitic bodies are forming deep in the schist and migrating up into the shallower and distal portions of the schist. The resultant mountain range is composed of the Vishnu Schist and igneous intrusions.

Sometime after 1700 Ma, uplift stops and the mountain range is eroded down to small rolling hills. At around 1200 Ma, the erosive event is for the most part complete and deposition begins with the Bass Limestone as a result of a marine incursion.

Did that make better sense?


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 96 of 283 (296416)
03-17-2006 11:47 PM
Reply to: Message 95 by jar
03-17-2006 11:28 PM


Re: Need a short break to absorb...
That's perfectly understandable, Jar. :) I wish I was better at explaining this stuff in simpler terms, but it really is quite difficult.

Geology makes the most sense when we can visualize the progression of events through time and so to really understand geology, you need to think in 4-dimensions. Unfortunately, that is a skill not easily picked up by people, especially if you aren't a natural visual learner - thankfully I am or geology would have been absolute Hell.


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 100 of 283 (296507)
03-18-2006 4:11 PM
Reply to: Message 99 by Minnemooseus
03-18-2006 1:59 PM


Re: Okay, trying another summary
Good point about clay in the original sediments, moose. If no clay was present in the proto-sediments and it was just sand, metamorphism would result in quartzite, not a schist.

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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 111 of 283 (296682)
03-19-2006 10:57 PM
Reply to: Message 110 by edge
03-19-2006 7:19 PM


Re: Thanks and moving on.
This is something I keep trying to point out to Faith. Just because the pretty geologic images online or in books show straight lines and single-lithology formations doesn't mean it is anywhere near representative of reality. And it's not just in the Grand Canyon that this happens - it's everywhere, as you know.

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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 115 of 283 (296971)
03-20-2006 10:58 PM
Reply to: Message 112 by jar
03-20-2006 10:23 AM


Re: To the Bass Formation.
First, the article referenced says that there is a big chunk of material missing betwen the Vishnu Schist and the Bass Formation. Since it is not there, just what is the evdence that at one time something was there?

I'm not sure, but my guess is that it's simply an assumption based on the amount of time the disconformity represents - 500 million years.

Second, the article says that the Bass Formation was created by an intruding and then retreating sea. It even provides the directionality of the event.

How is that determined? What evidence led to that conclusions?


Good question!

The easiest way is to look at it a strat section of the Bass limestone. First of all, we know that the oldest layers are at the bottom and the youngest at the top. So, from the description, we can see that coarse clastics (e.g., conglomerates and sand, etc.) occur at the bottom of the formation; limestone, dolomite, and shale probably occur somewhere in the middle; and then more coarse clastics at the top.

Below is a simplified section not exactly what occurs in the Bass Limestone, but perhaps somewhat reflective of it:

5. clastics- TOP
4. limestone
3. deep marine shale (deepest water accumulation)
2. limestone
1. coarse clastics - BOTTOM

The best way to look at and think about sedimentary rocks is from the bottom up because this is the order of deposition.

1. Coarse clastics, such as conglomerates makes me think of a high-energy terrestrial environment such as a stream - maybe a braided stream dumping into the ocean. Or maybe a glacial stream that is carrying cobbles sourced from a retreating glacier. HERE is a great site that describes a cobble beach. Is it analagous to the Hotauta Conglomerate? I don't know. Maybe. I believe there is evidence of Proterozoic glaciation.

2. Above the clastics, you have limestone. The only way to get limestone on top of clastics is to raise sea level or for the basin to subside, either way, it's a relative rise in sea level.

3. On top of limestone is shale, which is a sediment generally deposited in deeper water than limestone, so that again tells you there must have been a relative rise in sea level from the time limestone was deposited to the time shale was deposited. This shale represents a period of maximum flooding (i.e., inundation, incursion, etc.).

4. Then from shale, you have limestone, which is generally deposited in shallower water than shale. This indicates a relative drop in sea level. In that one spot, water depth has decreased because limestone production can only occur above a certain depth.

5. On top of the limestone you have, clastics again. Water level has decreased to the point of again exposing that particular spot to the surficial environment - another drop in relative sea level.

So as you move up through the simplified section above, you have a transgressing sea up to the shales/mudstones, which represent sedimentation when the water was at its deepest and highest level. Then water levels started dropping until the cobbles were able to make an appearance again. It's possible the cobbles continued to be deposited while the sea was rising, but that beach environment was pushed backwards because of rising sea levels.

Figuring out from which direction the water was coming in from is generally a matter of mapping out the aerial distribution of the lithologies or cross-sections derived from drill holes in various locations. For example, a regional cross-section of an "easterly transgressing sea" will have beach sands moving east as you travel up the stratigraphic column.

The following is a simplified example of trangressive-regressive sequence:


Click to enlarge

For the aboveimage imagine you are looking north so that the x-axis represents west (seaward side towards the left) and east (landward side towards the right). The y-axis represents time - oldest at bottom, youngest at top.

As you move up through time, water levels are rising and so the sea is transgressing in an easterly (e.g., landward) direction (towards the right). The middle of the 'V' represents the period of time when water levels are at their highest, so the beaches have been pushed way back into the continent. Later, water levels start dropping and so all the lithologies are moving in a seaward direction (towards the left).

Did that help or are you even more confused?

Third, Roxrkool mentioned a layer pinching out. What does that mean and how are such things identified?

I made that assumption based on the cross-section below:

Click to enlarge

The person who drew that section made it a point to show the conglomerate is not a continuous layer at the bottom of the Bass Limestone - it's shown pinching out to the right. Now that doesn't necessarily mean it always pinches out to the east or west. In addition, in the link edge and I have both provided, the author states:

"An underlying layer of conglomerate known as the Hotauta Conglomerate Member deposited in low areas of the eroded and hilly Vishnu terrain composes the lowermost unit of the Bass Limestone."

This tells the reader the erosional surface developed on the Vishnu is irregular with high spots and low spots (in other words, topographic highs and lows) and the low spots are often filled with conglomeratic or other coarse clastic material. Since I don't think the low spots have vertical sides like a swimming pool, but rather sloped sides like a parabolic dish, I'm assuming the conglomerate is thickest in the middle and thins out - or pinches out - on the sides of the parabola.

This message has been edited by roxrkool, 03-20-2006 11:06 PM


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Replies to this message:
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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 117 of 283 (296980)
03-21-2006 12:40 AM
Reply to: Message 116 by jar
03-20-2006 11:17 PM


Re: To the Bass Formation.
What edge said is also true. The thing is, by the time people started studying the Grand Canyon, many geologic principles were already in place. The large gap in time was mainly the result of fossil assemblages and dating techniques.

I haven't studied the Grand Canyon much so I really don't know for sure, but in other places in the Southwest, it's likely that a more complete section is present. One that perhaps contains rocks that represent time or deposition prior to the Bass.

Okay, what is Ash and what does an Ash layer tell us?

Ash is very fine dust-like material ejected from a volcano during eruption. It is variably composed of glass shards, which is rapidly cooled very fine-grained molten material (like obsidian), and other minerals such as feldspar, quartz, sulfide, biotite, pyroxene, as well as rock fragments.

The chemistry of ash is very diagnostic and can be chemically characterized by looking at grain sizes, mineralogic compositions, and trace-element compositions. Each volcanic eruption, even if it's from the same volcano, will be slightly different, and so by studying these differences and mapping out the extents of the ash layers themselves, we can often trace them back to their source.

When deposited on land, these ash layers can remain relatively fresh retaining their primary mineralogy, though glass will devitrify (to clay) over time in the presence of water. When the ash falls on water, however, like the ocean or an inland sea, and is subsequently deposited in a subaqueous settings, the glass shards quickly devitrify into clay particles. (Note: devitrification means that the amorphous glass structure, which is unstable at surface conditions and free of water, will change into a more stable, orderly, and water-bearing form, and so in the case of volcanic glass, the glass converts to clay - typically bentonite.)

These ash layers, because of their volcanic origin, can also be dated using radiometric dating methods.

This message has been edited by roxrkool, 03-21-2006 12:42 AM


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 124 of 283 (297114)
03-21-2006 5:47 PM
Reply to: Message 118 by jar
03-21-2006 1:39 PM


Re: Okay, trying a summary and see if we are ready to move on.
A few things:

1. Conglomerate does not require increased pressure or temperature. Just the rounded material and some amount of finer grained matrix. If there is more cobbles than matrix, it is called a clast-supported conglomerate. If there is more matrix, it is called a matrix-supported conglomerate.

A conglomerate that has been subjected to increased pressure and temperature, but still retains its conglomeratic appearance, would be called a meta-conglomerate.

2. First, I should point out that there are absolute and relative fluctuations in sea level. Absolute fluctions are globally felt and recorded (in the rock record), while relative fluctutations can be locally felt and recorded... I think that's right.

Second, both jazz and moose are correct in that ice ages (growth and decay of ice sheets) and spreading rates (changes volume of ocean basins) affect relative sea levels, but so can tectonics (subsidence/uplift of land), increased rates of continental erosion (dumping a lot of seds into the ocean will displace water and raise sea levels), variations in how water is stored (groundwater, lakes, etc.), and thermal expansion and contraction of sea water (cold water causes seas to contract, warm water causes seas to expand).

One way to figure out what caused a transgressive-regressive cycle is by looking at how formations behave locally over lateral distances and then globally. If you have a transgressive cycle in a couple basins in New Mexico, but nowhere else, then you might have a localized relative drop in sea level due to basin subsidence - which may be the result of faulting. If you see this transgressive sequence all over the world for similar aged rocks, then this rise is sea level is likely a global event.

For the Bass Limestone, the following is technical, but discusses the depositional environment for the Grand Canyon Supergroup:


Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis

The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted "Grenville-age" NW-directed shortening. New U/Pb age determinations from an air-fall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and 40Ar/39Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 °C between ca. 1300 and 1250 Ma, refining exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal flexures indicate NW-directed synsedimentary contraction at ca. 1250 Ma. A large disconformity (~75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafic magmatism, and early Nankoweap deposition: (1) intraformational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nankoweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of southwestern Laurentia during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma.

SOURCE

So basically, the GC Supergroup transgressive-regressive sequences appear to be the result of tectonics.

This message has been edited by roxrkool, 03-21-2006 05:50 PM


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 129 of 283 (297154)
03-21-2006 10:22 PM
Reply to: Message 126 by jar
03-21-2006 9:10 PM


Re: Got yet another question for you.
That's a really nice website, jar. Good for beginners.

Horsts and grabens are the result of extension and the entire Basin and Range Province is the result of this type of tectonics - at a much larger scale than shown in that link of yours, however.

Is this what is refered to in the discussion of the Bass Formation?

What I picture when I read the description above is this: imagine placing a ball of dough between your hands, which are parallel to each other. Bring your hands together slowly - this is called the direction of compression or the shortening direction. Compressing the dough causes the ball of dough to squish up between your hands (similar to mountain building).

However, not only is the dough moving upwards, it is also moving in the orthogonal direction, which is 90 degrees from the direction of shortening. The dough is extending outwards toward your fingertips and wrists because it's just as easy or easier for the dough to spread out in that direction than to continue going straight up between your hands. The direction in which the dough is moving outwards, towards your fingertips, is the direction of extension. Extension can result in horsts and grabens.

So you are getting both compression and extension happening at the same time.

I should remind you that this compressional event is entirely separate from the one that formed the Vishnu Schist.

Under compression (increased pressure), rocks behave plastically, but under extension, rocks are brittle. So compression results in folding, deformation, metamorphosis, etc., but because it becomes easier for the crust to move laterally instead of straight up, they start extending in the orthogonal direction, forming horsts and grabens.

It's my understanding that the lower half of the Unkar Group was deposited early in the compressional history during regional folding and deformation (the monoclinal flextures) and the upper half was deposited when the crust started extending (horsts and grabens).

Abe: I've actually never heard of the term 'fold trains' - same thing as chevron folds? And older term or European?

This message has been edited by roxrkool, 03-21-2006 10:38 PM


This message is a reply to:
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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 131 of 283 (297295)
03-22-2006 12:01 PM
Reply to: Message 130 by jar
03-21-2006 10:40 PM


Re: Okay, more important points.
Yes, rock deformation labs all over the world have performed these sorts of experiments.

Actually, jar, I think that second image may be an angular unconformity, but the first one is nice.

Rock deformation affects the integrity of the rock and hence, the minerals that make up that rock. The minerals in unconsolidated sediment will not be affected by strain (or other metamorphic effects) because the minerals can freely move about.


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roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 134 of 283 (297318)
03-22-2006 1:18 PM
Reply to: Message 132 by jar
03-22-2006 12:15 PM


Re: Time to play stump the chump.
Another nice picture. Those do appear to be angular unconformities. The upper one shows a nice erosional channel, too (just above center).

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 Message 132 by jar, posted 03-22-2006 12:15 PM jar has responded

Replies to this message:
 Message 135 by jar, posted 03-22-2006 1:26 PM roxrkool has responded

  
roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 136 of 283 (297337)
03-22-2006 2:43 PM
Reply to: Message 135 by jar
03-22-2006 1:26 PM


Re: Time to play stump the chump.
Correct. If you go back to the link you provided and click forward through the images, you'll see it's believed the channel was stream-cut... I think.

It's a shame those images are not individually annotated.

This message has been edited by roxrkool, 03-22-2006 02:46 PM


This message is a reply to:
 Message 135 by jar, posted 03-22-2006 1:26 PM jar has responded

Replies to this message:
 Message 137 by jar, posted 03-22-2006 3:05 PM roxrkool has responded

  
roxrkool
Member
Posts: 1495
From: Nevada
Joined: 03-23-2003


Message 138 of 283 (297567)
03-23-2006 12:03 PM
Reply to: Message 137 by jar
03-22-2006 3:05 PM


Re: On Angular Unconformities and other wonders
The evidence for the unconformity is that layers simply stop, are cut off. That means that sometime after they were laid down, the surface was eroded away. Then there is another layer that is continuous above the parts that was worn away.

So we are looking at time and events here.


Basically yes, although the sediments were likely somewhat consolidated to fully lithified when they were eroded. However, without knowing more about that particular photo, I hesitate to state any sort of conclusion.

Abe: I now think the are probably turbidite deposits, which means the Bouma Sequences (see below) were deposited simultaneously.

In the picture we are discussing there are five main features, two angular unconformities where erosion happened and then a layer was laid down on top, the two layers, one separating the angular unconformities and the other above the higher unconformity and an erosional channel.

But there are also many, many layers in the picture.

What do those layers tell us? Is each one of those layers a change in the local environment or deposition?


Well, like I said above, it's a bit difficult for me to know what's going in that picture since I don't really know what the lithologic units are.

I think they are part of the Brushy Canyon Formation down somewhere in Texas because if you click through the other images in that slide 'presentation' it discusses the Brushy Canyon.

If so, those rocks are sandstones, siltstones, and limestones, and the the actual depositional setting appears to be the submarine environment. The channel, instead of being stream-cut, may instead be the result of underwater gravity-induced 'slides' - similar to a landslide. These sediments are not likely to have been lithified, but they were somewhat consolidated/compacted prior to being eroded.

From the photo alone, I really can't tell if those layers are turbidite-deposited, but they do have the look of it, and the other images appear to suggest they are.

That means some of the layers were deposited simultaneously. The heavy stuff (gravels, etc.) will settle out first and the finer muds last. The layers are graded and the sequence of layers is called a Bouma Sequence. In some settings, these sorts of deposits can be found one atop another for several hundred feet, possibly more.

Below are some images that might help.

How turbidites develop (SOURCE):


Click to enlarge

The resulting Bouma Sequence (SOURCE):


Click to enlarge

Turbidite images:

Folded turbidites

Cretaceous turbidites

Underwater turbidites

This message has been edited by roxrkool, 03-23-2006 12:04 PM


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