The Great Unconformity in the Grand CanyonWe’re finally here. I’ve literally spent a couple weeks in

The Great Unconformity in the Grand CanyonWe’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 eroded and washed out to the ocean, where it finally comes to rest. Once the rocks of the Grand Canyon Supergroup were exposed at the surface, 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 deposition in those basins stopped once sea levels dropped and this area was exposed at the surface. This created an erosional boundary that cuts across much of the continent - the Great Unconformity.In the Grand Canyon, only a small slice of those sediments are preserved. 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 dropped down on these normal faults.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. See the person putting their finger on the unconformity? At that point, her finger is on about 1.2 billion years of time.A recent paper proposed that the Great Unconformity is the end result of a major, global process - the “Snowball Earth” events where glaciers advanced to the equator and the planet froze over for tens of millions of years. During this time, many areas of the planet were eroded, and just after that event there was a change in the isotopic composition of rocks being erupted by volcanoes. To account for the amount of mass needed, the scientist estimated that at least 3 kilometers of sediment were scraped off by the glaciers - around the entire world. This single line, running around the world, at this spot might be the entire record of one of the largest climate shifts in Earth’s history.-JBBImage credits: https://www.flickr.com/photos/brewbooks/5216562085https://www.flickr.com/photos/ceedave/4647303077https://www.flickr.com/photos/ceedave/4647304033/Previous articles:https://www.facebook.com/photo.php?fbid=71718732167564https://www.facebook.com/photo.php?fbid=717596974968016https://www.facebook.com/photo.php?fbid=718487278212319https://www.facebook.com/TheEarthStory/posts/718917208169326https://www.facebook.com/TheEarthStory/posts/719035941490786https://www.facebook.com/TheEarthStory/posts/719534524774261https://www.facebook.com/photo.php?fbid=720485404679173https://www.facebook.com/photo.php?fbid=720916891302691https://www.facebook.com/TheEarthStory/posts/721282287932818https://www.facebook.com/TheEarthStory/posts/721455997915447https://www.facebook.com/TheEarthStory/posts/722212221173158https://www.facebook.com/TheEarthStory/posts/722332104494503https://www.facebook.com/TheEarthStory/posts/723288294398884https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/723925267668520/?type=1https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/724756080918772/?type=1https://www.facebook.com/TheEarthStory/posts/724792024248511https://www.facebook.com/352857924775258/posts/2081922038535496/https://www.facebook.com/352857924775258/posts/2083451545049212/Sources: http://www.lpi.usra.edu/science/treiman/greatdesert/workshop/greatunconf/http://geoscience.unlv.edu/pub/rowland/Virtual/geology.htmlhttp://www2.nature.nps.gov/geology/education/foos/grand.pdfhttps://www.pnas.org/content/early/2019/01/07/1804350116 -- source link

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