If you didn't already catch it elsewhere, there's a new fossil from the Chengjiang Fauna that suggests a bunch of arthropods following one another in a line. Matt at the HMNH reports on it here.

Labels: arthropods, blogs, cambrian, fossils

As a follow-up to yesterday's post on the "Great Unconformity," today I offer a few more shots of unconformities in the Grand Canyon, including (at the end), an angular unconformity...

First, here's a close-up of the contact between the Vishnu Schist and the Tapeats Sandstone:


Slightly blown-out because I was shooting into the sun, and the outcrop was in shadow, but that's why God invented Photoshop:


Same thing, but with the direct light, it's texture (rather than color) that allows you to discern the difference between the two rock units:


The Great Unconformity is visible here, with a boatload of river rafters for scale:


Same thing:


Same thing again...


Okay, here's something different. A waterfall shot. People apparently love waterfalls. Every place I went this summer with a waterfall, there were oodles of folks gathered around, and much flapping of camera shutters. I must be dim, because I kind of don't get it. Water flows downhill... What's the big deal? Anyhow, here the waterfall actually shows us something interesting: note where it emerges from:

That's right -- from the unconformity. Apparently, this is due to the stubborn resistance of the crystalline basement rocks, which are tougher to erode into than the overlying sandstone. The creek cut through the sandstone, but hasn't yet cut through the Vishnu Schist and Zoroaster Granite. However, the Colorado River has, and as the creek flows into the river, there's a difference in the elevation of the two bodies of water. Hence, the waterfall.

I went for a pretty amazing swim in the pool at the base of this fall: the water was cool and bracing, and the wind created by the waterfall was amazingly powerful, actually blowing swimmers downstream! Just the thing after a hot hike.

Lastly, a different aspect of the same unconformity, also seen in the Grand Canyon. Don't look in the foreground, but high up on the distant ridge. This one is an angular unconformity, with sedimentary rocks below the ancient erosional surface as well as above.

In this case, the angular unconformity separates the Grand Canyon Supergroup from the Tapeats. The Tapeats, as we've seen, is Cambrian (~543-488 million years old). The Grand Canyon Supergroup (1.25 billion to 825 million years old) was laid down on the basement rocks first, then faulted and tilted 15 degrees. These tilted blocks were then eroded. On many, the Grand Canyon Supergroup was totally burnished away, re-revealing the underlying basement rocks. In the more down-dropped blocks, however, little protected packages of the Supergroup were preserved. When sea level rose anew in the Cambrian, it deposited the Tapeats Sandstone. In some places, the Tapeats sand was laid down on granite and schist, and in other places on these tilted layers of the Grand Canyon Supergroup. Same erosional surface; different rocks below it in different locations.

Here's a Flash animation showing the various steps it took to put the Grand Canyon together, including the erosion that gave rise to these various unconformities.

Labels: cambrian, grand canyon, primary structures, proterozoic, sediment

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