#chuar

Supergroup!

Wikipedia tells me that in music, a supergroup is a group composed of artists who have been successful on their own or in other bands. For the last week+, we have seen a series of geologic formations that make up the Grand Canyon Supergroup. Aside from making beautiful rock music (ba-dum tss), what is a supergroup to a geologist, or for that matter, a formation?

A formation is the building block of the geologic history of a spot. Geologic formations are sections of related rocks, deposited together, where the rocks have similar properties. We started at the bottom of the Grand Canyon in the Vishnu Schist – a formation defined by having been heavily metamorphosed. The Grand Canyon supergroup includes a number of different units – the Dox Formation, the Hakatai Formation, the Cardenas Formation, etc. Every one of these has something in common – the Cardenas formation is a series of igneous rocks, the Hakatai formation is a shale that has mostly turned reddish.

When you look at the Grand Canyon, you see a stairstep sequence on the side. That stairstep pattern is defined by formations. Some formations erode easily and they step backwards, some formations are strong against erosion and they stand up as cliffs.

Formations are named based on some property of where they are first described, often at an example or “type” location. The Shunimo formation, for example, is named for a site in the Grand Canyon known as Shunimo wash.

A “Group” to a geologist is made up of several formations that are somehow related. The Grand Canyon Supergroup contains 2 major “Groups” – the Unkar Group and the Chuar Group. The Unkar Group is made of the Bass Formation, the Hakatai Shale, the Dox Formation, the Shunimo Formation, and the Cardenas Formation. The first 4 formations are sedimentary rocks that formed over a billion years ago – they record sea level rising and flooding the area that is today the Grand Canyon at that time. The Cardenas formation is an igneous rock that sits on top of all those units; these rocks likely represent what was happening in the Canyon Area as a Precambrian Supercontinent formed – the predecessor to Pangaea, a supercontinent that formed about a billion years ago is known as Rodinia.

The Chuar Group contains sedimentary units including the Nankoweap Formation, the Galeros Formation, and the Kwagunt formation. These rocks represent sediments deposited in this area about 800 million to 700 million years ago – they were deposited as Rodinia rifted apart. Sediment stopped being deposited in this area at the start of the Cryogenian, a time when the full Earth’s surface was covered by glaciers.

The Chuar Group, the Unkar Group, and one more formation – the Sixtymile Formation – all of these rocks are considered part of the Grand Canyon Supergroup. The Supergroup is a package of tilted rocks found beneath the flat-lying sediments deposited in the Paleozoic. The rocks in the Grand Canyon Supergroup were once deposited as flat-lying sediments and a basaltic lava flow sequence, but a series of faults allowed those rocks to tilt early in the Cambrian. Today, the Grand Canyon Supergroup tilts off to the East by about 10 to 30°. In this photo, all of the rocks that tilt off to the right side - those are the rocks of the Supergroup.

-JBB

Image credit: https://commons.wikimedia.org/wiki/File:Precambrian_-_Cambrian_Unconformity_in_Grand_Canyon.jpg

Street food: Chuar, Shanghai, China.