My third day in Montana this summer, Lily and I took a hike in the Bridger Range, going up the west side of the range via Corbly Gulch to a cirque opposite the "usual" route up Sacagawea Peak, which starts at Fairy Lake on the east side, then goes up into Sacagawea Cirque* and south to the peak. Instead, we went up Corbly Gulch and got a whole new look at Bridger stratigraphy. First, orient yourself with this topographic map:



The Fairy Lake route brings you to the ridge crest from the upper right (northeast), wheras the Corbly Gulch route brings you to the same ridge crest from the lower left (southwest). Now take a look at some satellite imagery:



The green line at upper right is the ridge crest; Sacagawea Peak is just off-screen to the right. It will not surprise you to learn that stratigraphic contacts strike NW-SE in this area. The forested left-hand part of the screen is underlain by Mesoproterozoic LaHood Formation, a coarse-grained formation in the Belt Supergroup. Then there's a little gap of grassy area and a thin line of trees atop a light-brownish layer. This is the Cambrian Flathead Sandstone, which is chock-full of interesting sedimentary structures and trace fossils. The prominent light-colored ridge-forming layer traversing the screen from upper left towards lower right is the Cambrian Pilgrim Limestone, which shows "fossil hurricanes" in the form of limestone-chip conglomerates.

Here's some of the trace fossils in the Flathead Sandstone:


Here's a limestone-chip conglomerate from the Pilgrim Limestone, which I interpret as a paleo-hurricane deposit: rip-up clasts from a carbonate bank tumbled and re-deposited together in a big jumble:


We hiked up to the ridge, and peered down into Sacagawea Cirque (getting pummeled by the wind!), but didn't feel like we had sufficient time to attempt summiting Sacagawea, since I had to be back on MSU's campus for an evening session as part of "Bahama Montana" class. More on that tomorrow...

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* The following week, my Regional Field Geology students proposed to rename Sacagawea Cirque as "Death Cirque," for reasons I will explain in due course...

Labels: cambrian, fossils, limestone, montana, msse, primary structures, proterozoic, sediment

There's an article on the Burgess Shale and another on inventorying all the plants on Plummers Island (home of the Plummers Island Thrust Fault, between DC and the Billy Goat Trail).

Labels: cambrian, dc, faults, fossils, plants

Yesterday, I was fortunate enough to be able to tag along on the University of Maryland's petrology field trip, to five locations in Maryland showcasing a variety of igneous and metamorphic rocks. I'd like to thank Rich Walker and Roberta Rudnick for allowing me to come along on the excursion, and UMD graduate student Ryan Kerrigan for alerting me to the trip's interesting rocks in the first place. They have a crew of enthusiastic students, and some cool outcrops!

Our first stop was in northern Maryland's Cecil County. Along the banks of the Susquehanna River, just upstream from the I-95 bridge, is an abandoned quarry of the Port Deposit Tonalite.

Here's Rich and Roberta leading us into the quarry:


UMD students examine the semi-overgrown outcrops of the tonalite:


Tonalites are kind of like granites, except they have only very low amounts of potassium feldspar. This particular tonalite has a magmatic crystallization age of 515 Ma (U/Pb in zircon) and a metamorphic age of 490-480 Ma (Rb/Sr in biotite). Close-up of the rock's texture:


ADDITION: Kim notes in the comments that I didn't draw an explicit connection between the metamorphism and the metamorphic foliation that is so prominent in this photo. She's right: The wavy linear pattern you see in this photo is produced by minerals aligned by differential pressure. Squeeze the rock "top to bottom" and you produce a foliation that runs "left to right."

On the basis of isotopic evidence, the Port Deposit Tonalite is interpreted to have formed as an igneous pluton offshore of ancestral North America, underneath an island arc in the Iapetus Ocean. Later, subduction brought the island arc into contact with North America, triggering the Taconian phase of Appalachian mountain-building.

Here's a closer look at the texture and mineralogy. You can see some k-spar present here, though this was not a common mineral to see at the outcrop...


There were some nice xenoliths present, indicative of the host rock into which the PDT intruded:


Here's a quartz vein cutting through the tonalite. You'll notice that the vein is emplaced approximately perpendicular to foliation, suggesting the same maximum stress which imparted the foliation also extended the rock parallel to the foliation place, opening up fractures that when then fill with the most mobilizable minerals available (in this case, quartz):


If you look closely, you'll see that the fracture which opened up in the tonalite to allow this vein to be emplaced has a ragged edge (not a clean break):


Next up: the Setters Schist...

Labels: appalachians, cambrian, granite, igneous, maryland, metamorphism, ordovician, structure, teaching, xenoliths

When critters interact with their environment, sometimes they leave behind traces of that interaction. If we're lucky, these traces fossilize and can be preserved through time to tell us interesting things about the past. This past summer, when I rafted down the Grand Canyon with my father and two brothers, I saw some cool trace fossils. In chronostratigraphic order (earliest first), here they are:

The Bright Angel Shale can be found atop the Tapeats Sandstone, and below the Muav Limestone along the river in much of the canyon. The Bright Angel is middle Cambrian in age. For my money, it's one of the most spectacular sedimentary layers there, because it's so varied. The colors of the individual strata range from purple to green to brown to tan, and they are in many places chock full of horizontally-oriented feeding traces. Here's some of those wormy shapes along the trail to a waterfall we hiked to... (sorry, don't remember the name or exact location... I think it was day 4 or so of the overall trip... Hmmm, I guess I should have blogged this in early July when I photographed it...)





Nearby, we saw a spectacular trilobite crawling trace (Cruziana?):



Earlier in the trip (day 1, at lunchtime), and higher in the stratigraphic stack (the Permian Coconino Sandstone, which is a sand dune deposit), we saw these reptile (synapsid?) footprints:



This is a trackway left by an ancient reptile as it was walking up and down the dunes, preserved on the slip-face (which defines the feature we recognize from a side-view as "cross bedding"), and now, 260 million years later, I'm viewing those same tracks from underneath, as the older slip-faces of the dune have peeled off, and only the overlying (younger) ones are preserved in this particular alcove. Pretty spectacular stuff. And it offers some nice lunchtime shade, too... Can't complain about that. Here's another shot, with a sense of scale in it:



You can see the individual toes! Wild!

Labels: arizona, cambrian, fossils, grand canyon, permian, primary structures, travel

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, unconformities, wyoming