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

Cleaning up my hard drive today, before switching over to the laptop for my summer travels. Thought I would share a few annotated photos from my "Geology of Glacier National Park and surrounding areas" class that I took last summer.

Here's Chief Mountain:


On the trail to Firebrand Pass, here's the contact between the Altyn Formation (lowest of the Belt Supergroup exposed at Glacier) and the overlying Appekunny Formation:


The Purcell Sill is a readily recognizable feature high on the glacially-carved walls of Glacier National Park. This shot is from the trail on the way up to Grinnell Glacier:


Here's a shot from Sun River Canyon, showing one of the many imbricate thrust faults there, with some glacial till thrown in as a bonus feature:


Just outside of Sun River Canyon, we saw some nice recumbent drag folds on some thrust faults in the Cretaceous rocks:


This one was from early in the trip, on the road from Helena up north towards Glacier. Specifically, we stopped in Little Prickly Pear Canyon, near Wolf Creek, and saw these chevron folds in the Cretaceous rocks there:


Along those same lines (folded Cretaceous strata), here's a gorgeous fold just outside the park's boundary, on the road leading north from Two Medicine towards Many Glacier:


No annotations on this one, but I wanted to share it anyhow: a blind thrust / drag fold complex, in the Grinnell Formation (exposed on the trail up to Grinnell Glacier):


Lastly, some snow photos. I took this shot on my way up the trail to Grinnell Glacier, because the holes in the snow reminded me of the scary mask face from the Scream movies. But then on the way down, I realized I had the opportunity to document how much snowmelt occurs in six hours of Glacier NP summer weather. Hence, the bottom "after" shot:


That's it for today... Enjoy!

Labels: field trips, montana, msse, plutons, proterozoic, sediment, structure

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

Ahhhh.... the semester's just about over. Yesterday, I gave my last lecture and delivered two lab practical exams, and now all that's left to do is give the final exams on Tuesday. Not a moment too soon! It's been a very busy time over the past couple of months. What with my regular teaching duties, my Audubon class, my online MSSE class, GSW, various talks (like Wednesday's "Geology along the C&O Canal" at NSF), supervising homeschoolers visiting the NOVA chemistry lab, grant finagling, writing projects, and just life, I'm dog tired. I'm seriously ready for a nice break.

This ought to mean I'll have more time for posting on this blog, and hopefully that the posts will be richer and more thoughtfully composed.

Anyhow, let's share some pictures today. These are photos I took last summer on Dave Lageson's "Geology of Glacier National Park and Surrounding Areas" course at Montana State University - Bozeman. Dave is a great field trip leader, and I'm looking forward to another of his courses this summer: "Northern Rocky Mountain Geology."

For the Glacier course, we loaded up the vans in Bozeman and drove northwest through Helena and up to Sun River Canyon, one of the best areas in the world to look at multiple imbricated thrust sheets. Dave's been taking students here for a long time, and in fact "wrote the book" on it as a field trip location. In the photo below, the prominent cliff is Paleozoic limestone. The gently-sloping hill in the foreground, however, is Cretaceous shale. As is often the case, tectonics trumps superposition. Compressional tectonic forces have shoved the older rocks up on top of the younger rocks. (An analogous situation in the east is the Blue Ridge's Grenvillian rocks thrust up and to the west over Cambrian and Ordovician carbonates of the Shenandoah Valley.)


Here's a map showing how the Canyon trends east-west across the north-south strike of these mutliple thrust sheets:

Next up: Waterton Lakes Park, Alberta. We slipped over the border and spent an evening drinking beer in the southernmost of the Canadian Rockies. ...Purty.


Here's us looking at the next day's field stops.


Still life with fun stuff:


The next day, crossing back into the U.S., we stopped to get a good look at Chief Mountain, another scene of thrusting older rocks on top of younger rocks. Again, the lower unit is Cretaceous, but this time the upper rocks are older, much older. They're Mesoproterozoic rocks of the Belt Supergroup, thrust eastward along the Lewis Thrust, which underlies the base of this mountain. Chief Mountain is an erosional remnant of the Lewis Thrust sheet: that is to say, erosion has cut into the thrust sheet and left behind this one isolated outpost of what was once a continuous sheet of allochthonous rock. (It's a klippe!) The thrust sheet picks up again in the mountains of Glacier National Park.


Next day: a hike up to Grinnell Glacier, a classic glacier in a park named for classic glaciers. Like all of Glacier's glaciers, however, Grinnell is melting. It's receded quite a lot, as repeat photography shows:


Here's a view looking down the Grinnell Valley at a string of pater noster lakes blue with "glacial flour."


Here's what's left of Grinnell Glacier:


Where the glacier once stood, there's now a new lake. Several of my classmates decided that they would go for a dip. Note: all these guys are from Montana...


As for myself, I stayed out of the water, amusing myself with the amazing sedimentary structures displayed by the Belt rocks. Here's an outcrop of the Grinnell Formation, showing amazing Mesoproterozoic mudcracks. (As David Byrne said, "Same as it ever was, same as it ever was...")


Glacier's Belt Supergroup rocks are reknowned for their stromatolites, fossilized cyanobacterial mats. Here, a stromatolitic layer in the Helena Formation was exposed in cross-section by glacial erosion. Penny for scale (atop middle stromatolite).


And here's another view of the same stromatolitic layer, exposed in map-view section (a horizontal slice, as opposed to the vertical outcrop above). Enthusiastic geologist for scale, imagining doing the backstroke through the Proterozoic Belt Sea.


And... that's it for today. I'm off to the Blue Ridge this weekend, so I won't be posting again until Monday or so. But hopefully I'll have some cool new images from Virginia's oldest rocks to share at that time. Be good.

Labels: canada, montana, msse, primary structures, proterozoic, structure