Angular Unconformity in the Grand Canyon

July 24, 2018

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MarliM_180518-27 (3)

Photographer: Marli Miller
Summary Author: Marli Miller 

July 2018 Viewer's ChoiceThe photos above, taken in the Grand Canyon of Arizona, show a classic angular unconformity. The tilted red-colored rocks near the bottom of the upper photo belong to the 1.1 billion-year-old Hakatai Shale; the overlying light-colored horizontal rocks were deposited during the Cambrian Period, which began about 540 million years ago. The surface of contact between the two is called an unconformity because there's a significant amount of rock record missing --about 600 million years at this locality! This particular example is an angular unconformity because of the angular relationship between the two. There must have been a period of tilting and erosion after the older rock but before the younger rock was deposited.

If you look towards the right side of the photo, you can see a thick ledge of rock, called the Shinumo Quartzite, on top of the Hakatai Shale. It protrudes into the overlying Cambrian rock. The lower photo shows the relationship more clearly. This block of Shinumo Quartzite persisted as an island while the Cambrian rock was being deposited around it. Photos taken on May 18, 2018.

Photo Details: Upper photo - iphone 6 panorama. Lower photo - Sony a6000 camera; 18-105G; 40 mm; f8; 1/320 sec. exposure.