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Where's a geologist when you need one?

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Where's a geologist when you need one?

Postby mtnfiend » Mon Feb 25, 2013 6:15 pm

I was looking at some photos from a trip to Lake Powell a couple years ago and came across the following pictures of what looked to be quartz. It seemed odd to me that a vein of quartz was sandwiched within the sandstone. The only explanation I could come up with was a volcanic eruption or meteor impact large enough to melt dirt at some point in the past. I don't remember seeing this in other areas of the lake, but on this particular trip we were hiking near the Rincon if that matters?

I know there are a few geologists on the site, so I figured I'd put the question to the geological contingent: how did this quartz form?

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Re: Where's a geologist when you need one?

Postby Teeboy » Mon Feb 25, 2013 6:58 pm

http://www.quartzpage.de/gen_occ.html

Geologist here....this seems like a nice, simple explanation of the process. Has a cool pic associated. Geology rocks!

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Re: Where's a geologist when you need one?

Postby Monster5 » Mon Feb 25, 2013 7:31 pm

I've attempted to dumb-ify the process below for the casual reader. Somebody else can throw in complicated stuff even fewer people understand.

Basically, water flowing along fractures. I suspect the white mineral is actually calcite and not quartz, but it'd be a simple test.

The process is diagenetic (occurs after the sediments become a rock in this case, not while the sediment is being deposited or the rock forming).

During a period with active tectonics (shifting plates over millions of years, not necessarily full blown volcano mode), the rocks are fractured - jointed, faulted, etc. Notice in your third pic how some of the bands crossing the vein are offset? The white band on left is lower than the right band on right? This is an example of a normal fault caused by tension, which is common in the region. As an example, think about grabbing a semi-dry length of Play Doh and pulling it apart. You'll get a good number of cracks and the middle section will droop down.

After the rock is fractured, water flows easier along the open fractures than within the sandstone itself. That's pretty much the whole ideal behind the ever-controversial "frac"ing: create fractures in the rock that'll allow fluids/gases to flow easier. Taking the HOV lane rather than weaving through traffic.

The water picks up minerals as it flows through the rock. The minerals are re-deposited and re-crystallize/precipitate along the fractures. Similar, but not quite, to the way gunk builds up on faucets and in the shower, though with a bit more complexity and considerably more time. Keep in mind that it doesn't all occur along the surface. The rock could be buried beneath thousands of feet of other sediments which have subsequently been eroded away.

Edit - the above link explains it better.
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Re: Where's a geologist when you need one?

Postby Matt Lemke » Mon Feb 25, 2013 11:57 pm

I understood every word Ryan...coming from a fellow geologist I now officially declare your post peer reviewed! :lol:

Any other geologists here know where I could get a job?
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Re: Where's a geologist when you need one?

Postby siop » Tue Feb 26, 2013 12:15 am

I've done quite a bit of research documenting meteorite craters and can say with near certainty that these veins were not caused by a meteorite impact. Based on the photos alone you can't really differentiate if the veins are quartz, calcite or halite. Lake Powell has a history of volcanism, although it pre-dates this sandstone. Tectonic activity is a common cause of similar veins. To get a better idea of the cause you'd want to know which sandstone formation this is and a preferred orientation of the veins. Just looking at these photos there are numerous possibilities, but as a structural geologist I'd have to say tectonic activity.
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Re: Where's a geologist when you need one?

Postby geojed » Tue Feb 26, 2013 6:17 am

Another Geologist here, and I second Matt's approval of Ryan's description.

The gunk & play-doh references were a nice touch! :D


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Re: Where's a geologist when you need one?

Postby Mark A Steiner » Tue Feb 26, 2013 9:52 am

Another geologist here - 41-year degreed.

This is an expression of a low-temperature geochemical condition, resulting in the precipitation of a water-transported, silica rich solution as quartz into fractures. These are often called "fracture fillings". Another common solution for fracture fillings precipitates calcium carbonate (calcite). Examination of a thin-section of adjacent sand grains using a polarized-light microscope may reveal silica overgrowths of quartz grains, contributing to the sandstone's cementing matrix and resultant porosity.

IP's photos also reveal fine examples of cross-bedding (stratification) in the sandstone.

Looks like the Navajo Formation - any thoughts, geoscientists/stratigraphers out there?
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Re: Where's a geologist when you need one?

Postby rijaca » Tue Feb 26, 2013 10:31 am

Navajo Mountain, which is south of the Rincon, is a laccolith (igneous intrusion), and could be the source of the hydrothermal fluids.
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Re: Where's a geologist when you need one?

Postby MonGoose » Tue Feb 26, 2013 11:16 am

I agree with the assessment of the fellow geologists. To paraphrase in the simplest of terms: A long time after the sandstone was formed, a fracture occurred in the rock. Water flowing through this fracture deposited minerals which crystallized and formed the vein.

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Re: Where's a geologist when you need one?

Postby geojed » Tue Feb 26, 2013 11:22 am

Most likely Navajo Sandstone with cross-beds that big. There's a lot of Kayenta and Wingate in the area too.

Here's a Geologic map of the Rincon with some fun perspectives of it. The light/faded green is Navajo Sandstone, medium green is Kayenta Formation, and the darker green is the Wingate Sandstone. A fun fact to know is that the Rincon itself is an abandoned meander (aka: gooseneck) of the Colorado River. When the river cut through the meander it shortened the river by ~6 miles.

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Geologic Map of Rincon
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Aerial View
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Oblique View with Navajo Mtn in the background.
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Re: Where's a geologist when you need one?

Postby boudreaux » Tue Feb 26, 2013 12:24 pm

The Little Rockies/Henrys are laccoliths as well. Anyone wanting to see how those rocks pushed up through the strata over there should take a trip to Utah. You talk about a maze of Flatirons. Texas A&M does a big field trip down there to study those formations in June just about every year, with their basecamp at Starr Springs CG. One of the coolest views from there is the sunset on Navajo Mountain, every fin and flatiron is hi-lited in pink, simply amazing.

There is a nice abandoned meander on the San Juan River, way east of the Colorado River. I think another one is Jackson's Hole in Moab, right off Amassaback/Kane Creek Rd area.
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