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The Dox formation: another red shale

The last 2 units in the Grand Canyon were, in order, the Hakatai Shale and the Shunimo Quartzite. These rocks reflected a period of up to 50 million years where the land was extending and faulting. This tectonic motion created low-lying areas that were first filled with quiet water, tidal flats, followed by the overrunning of the area by river sediments.

In other words, the land first subsided slightly below sea level, then moved slightly back above sea level due to high sediment loads delivered by rivers.

The next step? Back down.

Over a period of about 10 meters at the top of the Shunimo Quartzite, the sandstones grade back into reddish shales that are thinly bedded and easily eroded. This sequence is very reminiscent of the Hakatai shale seen previously and represents a similar depositional environment. After the land moved above sea level to deposit the quartzite, the land moved back below sea level and resumed depositing shales in tidal-flat settings.

Just like the Hakatai and the Shunimo, there are tectonic features recorded within the Dox formation, indicating that the land was experiencing earthquakes and faulting which probably helped create the space where the Dox formation was deposited.

An interesting feature of the Dox formation is found in its detrital zircon population. The population of zircons found in this formation comes from a source that didn’t exist when the previous units were deposited. These zircons derive from the Grenville orogeny; an enormous mountain building event clear on the other side of the country; in the area that is today eastern North America and eastern Canada. Sediments from that mountain range, thousands of kilometers away, represent an important new source of material that appears in the Dox formation.

The zircon population also tightly constrains the age of this formation, deposited between 1.14 and 1.10 billion years ago.

Previously we saw that the Hakatai formation erodes easily and the Shunimo Quartzite stood strong and hard to erode. The Dox formation is another fine-grained, reddish shale, and just like the Hakatai, it erodes easily and does not outcrop well.

One other feature is illustrated here; the red color of formations like this is due to the presence of a lot of oxidized, reddish (rusted) iron. In the Dox formation, there are all sorts of white spots like this, created by the presence of some other compound such as organic carbon in the rocks that could instead reduce the iron, changing it from Iron (III) to Iron (II) and removing the red color.

-JBB

Image credits: Wayne Ranney (with permission) www.wayneranney.com http://earthly-musings.blogspot.com/

And http://en.wikipedia.org/wiki/Unkar_Group#mediaviewer/File:Grand_Canyon_Supergroup_showing_Cardenas_Lava.JPG

Details: http://bulletin.geoscienceworld.org/content/117/11-12/1573.full

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