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
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.