Day #100 Outcrop: Baked Angular Unconformity

During the course of the last week I’ve been busy geotagging, describing, and uploading to Flickr field photos from my trip to the northwest USA last summer. In so doing, I think I’ve discovered a fitting photo for my hundredth daily Deskcrop/Outcrop post. (Kudos to Ian Stimpson for reaching the 100th daily rock post on his Posterous blog, too!)

Today’s outcrop is located near the northern entrance to Yellowstone National Park and just across the Gardiner River Valley from Mammoth Hot Springs. Geologically it features a textbook angular unconformity, with southward dipping 2.1 Ma Huckleberry Ridge Tuff unconformably overlying northward dipping Paleozoic sedimentary rocks. That much I realized when I shot the photos and GigaPan below last summer. What didn’t dawn on me in the field, but jumped right out at me after our recent geomeme of baked contacts, was that it’s also a great example of a baked contact in its own right. How’s that for a concrete example of how blogging advances one’s understanding and appreciation of geology?

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Huckleberry Ridge Tuff in Angular Unconformity Over Paleozoic Sedimentary Rocks



Baked Angular Unconformity – Huckleberry Ridge Tuff Unconformably Overlying Paleozoic Sedimentary Rocks
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Let’s take a closer look at two snapshots from this GigaPan. You can see that the Paleozoic rocks immediately below the contact have a reddish tint that fades with distance from the contact. I interpret this to be oxidation caused when the thick Huckleberry Ridge Tuff erupted 2.1 million years ago and blanketed and baked the rocks below it.

Baked Angular Unconformity
Baked Contact at the Angular Unconformity

Note that the baked red rocks are not merely talus coming from the Huckleberry Ridge Tuff. In the snapshot below you can clearly see talus from the HRT above the contact that is not colored red. Thus, the rusty color of the contact metamorphic aureole clearly originates within the Paleozoic rocks beneath the contact.

Distinction of Talus Origins
Clear proof that the rusty color is the contact aureole, not HRT talus.

Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!

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