Encore - The Great Unconformity

May 23, 2020

The Great Unconformity

Today and every Saturday Earth Science Picture of the Day invites you to rediscover favorites from the past. Saturday posts feature an EPOD that was chosen by viewers like you in our monthly Viewers' Choice polls. Join us as we look back at these intriguing and captivating images.

Photographer: Stan Celestian  
Summary Author: Stan Celestian

July 2014 Viewer's Choice One of the most geologically interesting views displayed on the walls of the Grand Canyon in Arizona, is the Great Unconformity. This view was captured from Desert View, in the eastern portion of Grand Canyon National Park. Its greatness lies in the vast amount of missing time it represents as well as its large lateral extent. Specifically, the Great Unconformity is an angular unconformity. The younger (550 million years ago) Cambrian Period Tapeats Sandstone rests unconformably on the much older sedimentary Grand Canyon Supergroup (about 740 to 1,200 million years ago). At about 730 million years ago, these rocks were tilted 15 degrees to the northeast and faulted during the Grand Canyon orogeny. This tilting created a mountain range in the area that was subsequently eroded to a nearly flat horizontal surface. The deposition of the coarse sands and gravels on this erosional surface was later to become the Tapeats Sandstone that defines the upper limit of the angular unconformity. Thus, this unconformity represents a gap in the geologic history of the Canyon that varies along its lateral extent and spans about 200 to 1200 million years -- enough time for an entire mountain range to be eroded flat! Photo taken on June 5, 2014.

Photo Details: Camera: NIKON D800; Lens: 70.0-200.0 mm f/2.8; Focal Length: 70mm (35mm equivalent: 70mm); Focus Distance: Infinite; Aperture: f/6.3; Exposure Time: 0.0025 s (1/400); ISO equiv: 100; Software: Adobe Photoshop CS5.1 Windows.