The Great Unconformity in the Grand Canyon
We’re finally here. I’ve literally spent a couple weeks in a small slice of the Grand Canyon. The basement rocks and the Grand Canyon Supergroup rocks make up a small piece of the Western U.S. They tell hugely important stories of how the continent was assembled, but compared to the units we’re about to see they appear as but a sliver.
Everything we’ve seen so far shows up in small areas in the inner gorge. Suddenly in literally 1 plane, everything is going to change and we’re going to look at rocks that outcrop across the entire Western United States.
Each of these photos shows the same boundary; generally it is called the Great Unconformity (although that term is also used for Hutton’s unconformity in Scotland). It is so obvious that it was recognized as a major boundary during John Wesley Powell’s first trip through the Canyon in the late 1800’s.
The Great Unconformity is an erosional boundary key to the history of the Western U.S. When we last left the rocks of the Grand Canyon Supergroup, they were being folded and pushed upwards. Sediments deposited below the waters were giving way to conglomerates deposited by rivers.
Most of the sedimentary rocks in the world are deposited when the rocks move below the water line. In the ocean it’s easy for sediments to accumulate and stay there. Above the water line, sediment tends to be washed out to sea and eroded. Once the rocks of the Grand Canyon Supergroup were uplifted during a pulse of mountain building at the end of the Precambrian, they started to erode.
This process was repeated throughout the Western U.S. After Rodinia broke apart, there were large sedimentary units deposited in basins, but those basins were uplifted and spent several hundred million years eroding.
In the Grand Canyon, only a small slice of those sediments are preserved at all. Along one normal fault there is a single block of the Grand Canyon Supergroup rocks and that’s it. In most areas of the Canyon, whatever sediments were deposited have washed away and the rocks above sit directly on the Vishnu Schist basement.
The overlying unit is a sandstone we’ll talk about next. It even contains clasts of the Vishnu Schist and the Grand Canyon Supergroup at its base, showing how the Great Unconformity is an erosional boundary.
Western North America was pushed up above sea level for several hundred million years. It eroded and then finally found the ocean again as sea levels rose and the rocks subsided.
This boundary and this story repeat throughout the Western U.S. Personally I’ve put my finger on this layer in Wyoming, Montana, and South Dakota (no that’s not me in this photo, photo taken from Flickr account). This boundary occurs over thousands of kilometers. At this slice, we step from the Precambrian into the Cambrian and we find the layered rocks that define the upper part of the Grand Canyon. See the person putting their finger on the unconformity? At that point, her finger is on about 1.2 billion years of time.
Image credits: https://www.flickr.com/photos/brewbooks/5216562085
The United States Geological Survey recorded a Magnitude 3.2 Earthquake at a depth of 4.5 km in northeast Payne County, Oklahoma, slightly before 5.20 pm local time (Slightly before 10.20 pm GMT) on Sunday 27 July 2014. There are no reports of any damage or injuries associated with this event, though it was felt locally.
Gold // Fimiston Open Pit Mine (Super Pit), Golden Mile Mines, Kalgoorlie-Boulder, Kalgoorlie-Boulder Shire, Western Australia, Australia
Spinal column of mosasaur dinosaur period marine reptile, mosasaurus beaugei, around 70 million years old www.thefossilstore.com #mosasaur #marine #reptile #beaugei #fossilsforsale #geology #archaic #archaeology #palaeontology
I spent a night and half a day in Badlands National Park in South Dakota while on my way to Michigan. A truly stunning landscape that reminded me vividly of Bryce Canyon National Park in Utah. More photos from there as I make my merry way across the Midwest.