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Author Topic:   Exploring the Grand Canyon, from the bottom up.
roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 142 of 283 (297786)
03-24-2006 11:11 AM
Reply to: Message 141 by jar
03-24-2006 12:09 AM


Re: But what is shale?

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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 144 of 283 (297902)
03-24-2006 6:26 PM
Reply to: Message 143 by jar
03-24-2006 12:22 PM


Re: Summary up to the second layer of the Supergroup.
That was a pretty good summary. The only thing I guess I would mention or suggest is to start thinking or practicing thinking of the rocks from bottom to top - the order of deposition. It takes some getting used to, but once you get the hang of it, the processes might start making more sense.

Now for the big question. It seems that this last layer is the first to actually show signs of live, but only of cyanobacteria. Is this correct?

I believe the first signs of life in the Grand Canyon are visible in the Bass Limestone. I recall reading about stromatolites in the Bass.

This message has been edited by roxrkool, 03-24-2006 06:26 PM


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 146 of 283 (297955)
03-24-2006 9:47 PM
Reply to: Message 145 by jar
03-24-2006 9:04 PM


Re: Great, glad it passed. Now we can move on.
I'll have to read a little about this one first. I think this one is a pretty interesting unit. Generally, 'quartzite' is a metamorphic term, but the Shinumo is underlain by a shale (not a slate, which is metamorphosed shale) and the Dox Formation, which is also unmetamorphosed (as far as I know) sedimentary rocks. It's pretty hard to sandwich a metamorphic layer between two unmetamorphosed units.

My guess is that the Shinumo is not the result of metamorphic processes, but simply a well to strongly cemented sandstone. I'm going to go read now. :)


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 149 of 283 (298718)
03-27-2006 2:11 PM
Reply to: Message 148 by jar
03-27-2006 11:29 AM


Re: Where we are?
Jar, I can't seem to find anything (easily accessible) that discusses the Shinumo in more detail; however, in the Grand Hikes link it states:

Geologists have recognized four or five poorly defined members based on varying lithologies of the sandstones within the Shinumo. These consist of a bottommost member of conglomeratic subarkose and subminiature quartz sandstone, overlain by a mature quartz sandstone, followed by a brown quartz sandstone containing clay galls and fossilized mud cracks, followed by a hard, fine grained sandstone with rounded and well sorted quartz grains lithified with a siliceous cement.

The lithology and structure of sediments making up the Shinumo sandstones suggest a shallow marine environment with stream and river born sediments deposited in near coastal delta environments. The contact between the Shinumo and overlying Dox Formation is regarded as generally conformable, with the end of Shinumo time marked by a rapid transgression of the Dox sea and ensuing deposition of the lower shaley and mudstone members of the Dox.


It's pretty clear that in this case, the term 'quarzite' refers to a rock of sedimentary origin that has been strongly cemented - in this case with silica cement. (Apparently I did not read this link very well since I missed the reference to silica cement.)

Without reading a more detailed description of the Shinumo, I can't offer more on the presence of the 'silica cement.'


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 151 of 283 (298881)
03-28-2006 12:26 AM
Reply to: Message 150 by jar
03-27-2006 2:19 PM


Re: Silica Cement
As I understand it, Silica Cement is the product of diatoms, the carcasses of microscopic shelled critters, that have disolved in water and then precipitated out to form the cement. Is that correct?

Well... sorta... I think. It's been a while since I've done much reading about clastic cements, but I can't say I remember hearing that diatoms directly contribute silica for cementing purposes - though it's entirely possible they do in some settings. Siliceous oozes are generally deposited in areas where high Si-content prevails and these are the result of either diatoms or radiolaria in the deep marine environment.

In the case of the Shinumo Quartzite, which appears to have been deposited in a shallow marine environment, it's really difficult to hazard a guess without knowing more about it. It's possible the silica cementation is the result of pressure solution where grain-grain contacts result in partial dissolution of the quartz grain (at the contacts) and that dissolved silica then re-precipitates in the pore spaces as coatings on the sand grains. However, pressure solution requires, well, pressure. Possible during burial, sure, but I'm not aware of a whole lot of low grade metamorphism affecting the other formations in the Super Group. But then, maybe that's not always necessary, either.

Also, it's possible something in the depositional environment contributed silica during diagenesis or perhaps a siliceous fluid was introduced via structure and this fluid was able to move through the porous sandstone.

THIS LINK has a nice SEM (Scanning Electron Microscope) image of a rounded quartz grain (near center) with a siliceous overgrowth that is also filling the pore space. The link also briefly discusses pressure solution.


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 153 of 283 (298887)
03-28-2006 1:54 AM
Reply to: Message 152 by jar
03-28-2006 12:40 AM


Re: radiolaria??????
Yes, radiolaria are similar to diatoms in that they extract silica from marine water and form skeletons from it.

As for whether they contributed to the silica cement in the Shinumo (is that what you're asking?), I'm really not sure, but my guess is no.


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 155 of 283 (298979)
03-28-2006 11:54 AM
Reply to: Message 154 by jar
03-28-2006 10:15 AM


Re: Okay, trying another summary through the Shinumo Quartzite layer..
A fair summary except for one thing, the Shinumo was not eroded prior to deposition of the overlying Dox Formation. It was eroded as part of the Great Unconformity much later in the Cambrian, prior to deposition of the Tapeats Sandstone. So we still need to deposit the rest of the Unkar and Chuar Groups before erosion begins.

The upper contact with the Dox Formation is conformable, meaning it's an unbroken depositional sequence with no apparent interruption of sedimentation.


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 157 of 283 (299502)
03-29-2006 10:39 PM
Reply to: Message 156 by jar
03-28-2006 12:58 PM


Re: Moving on.
Sorry, I've been a bit busy lately.

I still reallly like the Grand Hikes site, so I'm going to post a few important points from there:

The only complete sequence of members in the Dox Formation occurs in eastern Grand Canyon where its overall thickness has been variously reported between about 3000 and 3200 feet. Its contact with the underlying Shinumo Quartzite is regarded to be generally conformable and marked by a relatively rapid subsidence and marine incursion. Geologists have divided the Dox Formation into four members: the lowermost Escalante Creek Member, Solomon Temple Member, Comanche Point Member and uppermost Ochoa Point Member. Their respective horizons are gradational and they are distinguished on the basis of color changes, topographic features and depositional environments.

Escalante Creek time was marked at its onset by relatively rapid subsidence of the area followed by gradual basin filling. By the close of Escalante Creek time the basin had filled and the region was at or near sea level, where it remained throughout the rest of Dox time. The overall lithologic sequence of the Dox members suggest the environment of deposition evolved from an underwater delta into a floodplain which was followed a tidal flat environment.

Grand Hikes: Dox Formation

According to the Grand Hikes site (above link), members of the Dox Formation include (number indicates depositional order):

4. Ochoa Point Member: At least 300 feet thick of micaceous mudstone, reddish-colored silty sandstone. Sedimentary structures include salt casts, ripple marks, and cross beds. Inferred depositional environment: tidal flat or shallow marine.

3. Comanche Point Member: At least 620 feet thick of white (leached) to reddish (oxidized sulfide?) shaley siltstones, mudstones, and sandstone, intercalated with stromatolitic dolomite. Sedimentary structures include mud cracks, salt casts, ripple marks, and irregular, wavy bedding. Inferred depostional environment: tidal flat(?).

2. Solomon Temple Member: At least 920 feet thick of reddish shaley siltstones, mudstones, and sandstone. Sedimentary structures include channels and cross-beds. Inferred depostional environment: floodplain.

1. Escalante Creek Member: At least 1,280+ feet thick; ~800 feet of siliceous (probably the cement and similar to Shinumo) sandstone possibly gradational from underlying Shinumo Qzte, overlain by ~400 feet of shale and mudstone. I can't exactly remember what all can impart a green color (it's usually a result of deposition in a reducing environment - no oxygen), but I think it can either contain chlorite, clay, glauconite, or fine-grained sulfide such as pyrite or pyrrhotite. My guess is sulfide because the overlying Solomon Temple Member contains reddish colored rocks, suggesting an oxidizing environment. However, the original sediments need to contain something that is capable of oxidizing and so I think that may be sulfide. Inferred depostional environment: delta(?).


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 159 of 283 (299794)
03-31-2006 11:28 AM
Reply to: Message 158 by jar
03-29-2006 10:55 PM


Re: Exploring the Dox Formation.
I found a paper on the Dox Formation. Give me a bit and I will summarize what it says.

Paper:
STEVENSON, GENE M., BEUS, STANLEY S., 1982, Stratigraphy and depositional setting of the upper Precambrian Dox Formation in Grand Canyon, Geological Society of America Bulletin, 93: 163-173.


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 161 of 283 (300627)
04-03-2006 2:39 PM
Reply to: Message 160 by jar
04-02-2006 4:58 PM


Re: Exploring the Dox Formation.
Okay, I'm going to try this again after losing it TWICE!!! :mad:

ESCALANTE CREEK MEMBER

The Escalante Creek Mbr. (~390 meters thick) is broken into 4 units based on lithologic and depositional differences and appears to record the transgressional and regressional sequence of a deltaic system following rapid subsidence of a tectonic(?) basin. Number represents depositional order:
.
.
.
.
(SOLOMON TEMPLE Member)
-----------------------
Unit 4 (140 m thick): Composed primarily of less-resistant brown to grayish siltstone with lesser thin, greenish brown sandstone interbeds. Shale fragments and normal grading is observed in the sandstone beds.

Inferred sedimentary environment: pro-delta slope deposits. These sediments represent a continuation of the basin in-filling and are located somewhere near the apex where older basin shales meet and are being covered by younger deltaic sediments in a [much?] shallower marine environment than unit 3.

----------------------- GRADUAL FILLING OF BASIN CONTINUES

Unit 3 (96 m thick): Composed of resistant lithic (containing rock fragments) and feldspathic (containing feldspar) calcareous (carbonate) sandstone. The quartz grains are subangular to subrounded and poorly to moderately sorted. Microcline, orthoclase, plagioclase, mica, clay, and micaceous shale clasts are present in the sandstone. Also present are thin to discontinuous shale beds or lenses. Sedimentary structures present are ripple cross-laminations, normally graded sandy beds.

Inferred sedimentary environment: proximal turbidites. Unit 2 represents distal turbidites and these represent proximal turbidites indicating a shallowing (hence, filling) of the basin. The 'proximal' interpretation is derived from the morphology, composition, and sorting of the sandstone beds. The quartz grains are only somewhat rounded and sorted, suggesting the did not travel far before being deposited. In addition, the presence of feldspar and lithic fragments also suggests a nearby source.

----------------------- GRADUAL FILLING OF THE BASIN

Unit 2 (80 m thick): Composed primarily of green to greenish-brown shale and mudstone with lesser sandstone interbeds. Cyclic sequences of sandstone grading upwards into siltstone are common. Sedimentary structures present are normally graded detrital layers, scour marks and load casts.

Inferred sedimentary environment: distal turbidites. Following basin subsidence, which resulted in deeper water shale setting, terriginous sediment was occasionally cascading down the delta front as the delta was growing (in a basin-ward direction) and some of the sand and silt made it far enough to settle out atop the shale.

----------------------- RAPID SUBSIDENCE and DEEPENING OF BASIN

Unit 1 (73 m thick): Composed primarily of white to gray-tan, friable, medium-grained quartz sandstone, the basal 12' of which is composed of dark green to black, fissil shale interbedded with thin sandstone lenses. Sedimentary structures present in this lowest unit include soft sediment deformation (resulting from slurry slumps) and cross-bedding.

Inferred sedimentary environment: Lagoonal shales atop Shinumo Quartzite and an encroaching distributary mouth-bar system which later buried the lagoon with delta-front sands.

-----------------------
SHINUMO QUARTZITE - Considered a shallow marine deposit, it seems that the basin had filled enough to allow the formation of a lagoonal system, which marks the beginning of Escalante Creek time.
.
.
.
.

jar writes:

The lowest layer is the Escalante Creek Member. At the bottom of it we find a material similar to the Sinumo Quartzite.

How do they tell the two apart? What is it that makes this a different layer?


This is a perfect example of why it's exceedingly difficult to discuss geology without knowing the details of the various formations.

From reading a few online descriptions of the Dox, it seems that we have sand of the Escalante Mbr. overlying sand of the Shinumo, but upon reading a more detailed paper on the Dox we find that in fact we have a 12 m thick shale sequence sitting directly on top of the Shinumo. And that's how we can delineate the two formations.

Then on top of that we see shale and mudstone. We've talked about shale, but not mudstone. What is mudstone and what does mud stone tell us about the environment?

Mudstone is basically the same thing as shale only it lacks the visible stratification (fissility) we see in shale.

Before the availability and capabilities of radiometric dating, was there anyway to estimate timespans? Or where things simply established as sequences?

We could guesstimate, but that's about it. Everything was relative prior to radiometric dating.

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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 163 of 283 (301554)
04-06-2006 12:18 PM
Reply to: Message 162 by jar
04-04-2006 8:37 PM


Re: Exploring the Dox Formation.
Yes. After Shinumo sediment was deposited, a lagoon formed on top. It's possible that a stream/delta was near the lagoon originally and as it started weaving back and forth, deltaic deposits eventually covered the lagoon. The data show the delta was prograding out into the ocean (i.e., the deltaic coastline was migrating in a seaward direction), which may indicate an increase in stream run-off and sediment load.

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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 165 of 283 (301651)
04-06-2006 3:17 PM
Reply to: Message 164 by jar
04-06-2006 12:30 PM


Re: Exploring the Dox Formation.
it looks to me like we are now seeing larger particles settling out which implies that the flow is slower.

Is that correct?


What you have is a sort of underwater landslide with sediment cascading down the steeper front (see post 138) and these are known as turbidity currents.

These underwater slides are composed of various sized sediment with the heaviest stuff settling out first, either ON the slope or near the toe of the slope and forming thicker sequences, and the finer stuff traveling out further into the basin before being deposited once the current loses energy. The turbidite deposits closer (i.e., proximal) to the source will be coarser grained and thicker, while the turbidite deposits further out (i.e., distal) in the water will be thinner and finer grained.

In the image below, you can get an idea of what the depositional environments may have looked like, as well as where they were located with respect to the beach.

What I'm not sure about is exactly where the distributary mouth-bar system was located (I'm not up on all the terminology). I originally pictured it similar to the upper deltaic plain (upper right in image), but the location according to the image below suggests it was much deeper. I placed the timing of subsidence nearer the middle or end of Unit 1, but from the image, it appears subsidence must have been very fast and it's beginning represented by the lagoon. I need to read the paper again, it seems.


Source of image is the previously referenced paper by Stevenson and Beus, 1982.

According to the image below, the distributary mouth-bar system IS located in the submarine/subaqueous environment. (Note the depth in meters.)


(SOURCE of above image)


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 169 of 283 (301968)
04-07-2006 11:45 AM
Reply to: Message 166 by petrophysics1
04-07-2006 9:16 AM


Re: Exploring the Dox Formation.
Great visual. I often attempt to build those same sorts of images for people because I think it's easier for the layperson to understand. I'm not sure how well they work, however. No one ever says, "AHA!!! I get it now!!" lol

Hope you stick around and add to this discussion. I suspect these depositional settings are right in your ballpark (several friends are petrophsycists in Houston and New Orl... errr Houston :)).


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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 170 of 283 (303007)
04-10-2006 6:11 PM
Reply to: Message 168 by AdminJar
04-07-2006 10:24 AM


Re: Welcome to EvC
Should we move onto the next member or do you still have questions, jar?

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roxrkool
Member
Posts: 1497
From: Nevada
Joined: 03-23-2003


Message 185 of 283 (369060)
12-11-2006 3:16 PM
Reply to: Message 184 by jar
10-13-2006 11:54 AM


Re: bump for petrophysics
Hi Jar,

I'd like to come back to this thread. I had taken a "leave of absence" to finish my thesis. My thesis is finished (I'll be defending next Monday) - WOO HOO - so I'd like to continue this discussion. Hope you're still interesed.

Hmmm... I read that you're no longer around. If you come back, we'll continue. :)

Edited by roxrkool, : No reason given.


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