To all our geologists, please help. So far we are still in the sub basement of the Grand Canyon and have miles to go before we sleep, miles to go. Our last glimpse was of the Dox formation and if someone could pick this up by explaining what happens between the Dox and the next higher layer I would appreciate it.Aslan is not a Tame Lion
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. :)
This is a fascinating thread. I'm not that familiar with GC geology. When I hiked it years ago the section that the trail crosses has the Tapeats Sandstone resting directly on inner gorge rocks. I was familiar with the GC Supergroup but it isn't exposed in that part of the canyon. I'm not familiar with the Dox Formation, but looking at the Wikipedia article that was linked, it looks like the next formation upward is the Cardenas Lava which would seem to do it for the Unkar Group. Once again I'm not too familiar with the Cardenas. But just being an unmetamorphosed Precambrian basalt makes it interesting. Let the discussions begin!Brent
Oh I'm sorry. I havn't read the whole thread yet. In my limited understanding of Grand Canyon geology there are the horizontally layered deposits overlying the Vishnu Schist of the inner gorge, and in the eastern part of the canyon there are the gently dipping deposits of the Grand Canyon Supergroup (to which the Dox and Cardenas belong). I could be wrong but I believe these rocks were determined to be Precambrian from their stratigraphic position and complete lack of fossils. That makes them some of the oldest sedimentary rocks in the world. It's unusual to find supracrustal rocks (rocks that are deposited on the earth's surface like sediments or lava flows) this old that havn't been metamorphosed (deeply burried and altered mineralogically). As for how it formed, that may be less interesting from a flood geology perspective than the layer above it, which according to the Wikipedia article was deposited on the eroded surface of the Cardenas. No matter how you slice that requires the passage of substantial amounts of time between the two units.Brent
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We are holding off on the layer above until we determine all we can about the layer we are on.
You mention "supracrustal". That's yet another new term in this thread. Can you explain what that means?
And how does a geologist tell whether rock has been "metamorphosed"? How is this different than the intrusions found in the Vishnu Schist?
How thick is the Cardenas layer? Can we tell things like directionality? Are there signs of weathering or erosion and if so, what do they tell us? Is it all one lava flow or is it a series of events? If a series, how is that determined?
What do we know about the intersection of the lower Dox Sandstone and the Cardenas Lava Flow(s)?
Thanks for the welcome AdminPD! I will certainly put my responses in the right place now. I just didn't want to needlessly quote a lot of material,but I see now that's not how it works. I belong to several forums like this and they're all a little different.Brent
Great. I wasn't trying to move on from the Cardenas, just pointing out the interesting relationship with the overlying unit.
Supracrustal really means above the crust or on top of the crust. Any rock deposited on the surface of the earth is supracrustal, in other words sedimentary and volcanic rocks. The word is used when refering to rocks that make up the stratigraphic column, as opposed to igneous intrusions or metamorphic rocks.
Sometimes it is very difficult to know if a rock has been metamorphosed. When different types of rocks are buried and subjected to progressive increases in temperature and pressure, they undergo characteristic changes in mineralogy that range from no noticeable change at all to melting. The Vishnu is a fairly high grade of metamorphism. Rocks like that have been almost totally recrystallized. Microscopic examination would show the growth of new minerals and recrystallization of existing minerals. Almost nothing from the original rock is recognizable. OTH some rocks that undergo low grade metamorphism only exhibit mild alteration and growth of characteristic low grade minerals like chlorite, sometimes giving the rock a green appearance.
You ask some really good questions about the Cardenas. Those are the types of things a geologist would want to know. I can't wait for someone smarter than me to answer them! But most likely it is more than one flow. Often individual flows can be seen, or mapping might indicate multiple flows. It's easy to think of it as a 2d slice through a layer cake, but the rocks exist in 3 dimensions, forming a landscape with some rocks in some hills, some in others. I have personal fondness for basalt geochemistry. I know that's sick and I need help. But I studied it in grad school, and there is so much information that can be pulled out of basalts by chemistry. Also the fact that these rocks are so old could make their chemistry very interesting, perhaps providing information on changes in the mantle through time.
quote: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). 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.
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.
Edited by Minnemooseus, : Mention the later erosion as being Paleozoic.
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