This summer, I saw "the Great Unconformity" in a couple of locations.

An unconformity is a break in the local geologic record -- a period of time which elapsed without being recorded by the deposition of rock units. Often unconformities mark places where erosion has erased part of the local rock record, but sometimes they just mark periods of non-deposition. (Analogy: You can get a blank page in your diary two ways. You can either take a day off from writing, or you can write that day's entry and then later go back and erase it. Either way, you end up with a day going by and no journal entry.) People call the major break between metamorphic and igneous "basement" rocks and overlying sedimentary layers the "Great" Unconformity, though it's not the same age everywhere. It's just shorthand, really.

Anyhow, here it is in the Grand Canyon (photos provided below are both unadorned and annotated versions):





Give or take, there's about 1.2 billion years missing along this ancient erosional surface. Intuitively, this probably makes sense, since metamorphic rocks like schist and 'distilled' intrusive rocks like granite are characteristics of mountain belts, where they form at depth. In order to get those interior-mountain-belt rocks to the surface takes lots of erosion over lots of time (though not necessarily that long -- in DC, for instance, we have interior-mountain-belt rocks exposed that 'only' took 360 million years to make it to the surface). In the above photos, the metamorphic rocks and granites below the unconformity formed about 1.7 billion years ago, during the Mazatzal Orogeny, and the sedimentary layers on top (both quartz sandstones) were deposited in the Cambrian period, about 543-488 million years ago. They represent passive margin sedimentation along an ancient transgressive seashore, something like modern day beach sands along the east coast of North America. So, to get something like the Great Unconformity, take something like coastal Maine (Acadia National Park, say), and bury it beneath something like Virginia Beach.

And here "it" is again, in Wyoming's Wind River Canyon (between Thermopolis and Shoshoni):





A zoomed-in look at this same outcrop:





This time, however, the rocks below the unconformity are much older* metamorphics (schist & amphibolite) and granite. According to Maughan (1987), these are the oldest rocks exposed in Wyoming, having formed about 2.9 billion years ago. They were then metamorphosed at 2.75 billion years ago. These truely ancient rocks (Archean) were then eroded and exposed at the surface, where quartz-rich sand was laid down atop their burnished roots. Aside from the difference in the age of the underlying basement rocks, the story is very similar to the one at the Grand Canyon.

* Thanks very much to Kim, who pointed out my error in under-stating their age in an earlier, more-poorly-researched version of this post.

Reference:
Maughan, E.K. (1987) "Wind River Canyon, Wyoming." In: Geological Society of America Centennial Field Guide - Rocky Mountain Section. S.S. Buess, ed. p. 191196.

Labels: archean, cambrian, dc, grand canyon, primary structures, proterozoic, wyoming

A quick note here, just for the sake of completeness, on my final MSSE class of the summer: "Examining Life in Extreme Environments." This was a cool class, but structured in a different way from my other MSSE courses: it was set up more like a conference, with a variety of different speakers on different topics, interspersed with activities. The organizers, Susan Kelly and Monica Brelsford used a grant from NASA to help fund the course, which meant they had the money to fly speakers in from NASA Ames, the University of New Mexico, and the Wrigley Marine Science Center on Catalina Island, California. We also had a presentation piped in from Woods Hole. The goal of the class was to look at living organisms that manage to survive in 'extreme' environments, like really salty, really hot, really cold, really acidic, and so forth. Why study these bacteria and archaea? We're hoping they will give us insights into (a) the origins of life on Earth, and (b) the possibility of life on other planets or moons elsewhere in the universe. We had a field trip to Yellowstone National Park to look at microbial mats; we looked at cultures of hyperthermophiles; we listened to excellent talks by Mark Young (viruses as a source of genetic diversity), Ed Adams (new subzero lab tour), and Robert Szilagyi (thermodynamics of the origin of life). As you can see, it was pretty diverse -- all week long, always something new and interesting. I really enjoyed it!

Labels: archean, astronomy, cold, hot springs, montana, msse, wyoming, yellowstone