19 September 2014

Friday fold: Miette Group anticline in Banff National Park, Alberta

Here’s an outcrop of Miette Group slate, seen at the intersection of the Icefields Parkway with the Trans-Canada Highway, just north of Lake Louise, Alberta:

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There’s a lovely anticline just to the right of Zack, who obligingly provided a sense of scale. Also note how cleavage which is subparallel to bedding on the far left side of the outcrop, becomes perpendicular to bedding along the crest of the anticline…

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Happy Friday!

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12 September 2014

Friday fold: Quartz vein in Catoctin Formation, Point of Rocks, Maryland

I took this image in 2005, when I was working up a geologic history of the C&O Canal National Historical Park. It’s a vein of quartz, gracefully folded within the Catoctin Formation. The exposure is along the railroad tracks at Point of Rocks, Maryland, easternmost extent of the Blue Ridge province on the north shore of the Potomac River. The Culpeper Basin begins about 100 meters to the east of this site.

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Note that the Catoctin (a former basalt) has been metamorphosed to greenschist here, with a pronounced foliation. Here’s a Photoshop-highlighted and annotated view of the same fold:

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It is typical when folding and foliation form at the same time due to the same consistent tectonic differential stress that the foliation should be parallel to the axial plane of the folds. We saw the same parallelism in completely different rocks in the Canadian Rockies a few weeks ago. You know what they call that? Uniformitarianism!

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8 September 2014

Grizzly pig

Archaeotherium skull, on display at the Royal Tyrrell Museum in Drumheller, Alberta:

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I love these beasts since I first encountered mention of them at Badlands National Park, and reading them dubbed “grizzly pigs” in the excellent book Cruising the Fossil Freeway really stuck with me – these were pigs filling a predatory ecological niche we don’t really see them in today.

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5 September 2014

Friday fold: Kink banding in the Arran Islands

This one is in my folder of ‘structure’ images on my computer, but it’s not one of mine. I’m not sure where it came from. A TinEye search turns up nothing. Perhaps one of  you can tell me?

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Lovely subparallel kink bands… such gorgeous structure.

Happy Friday!

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3 September 2014

Rugose corals in the Clearville member of the Mahantango Formation

Here are some rugose coral fossils (along with some cross-sectioned articulate brachiopod shells) to be seen in the Clearville member (~80 feet thick) of the Mahantango Formation, exposed on the north side of route 55, just west of the West Virginia / Virginia border.

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These fossils are cool in their own right (what fossils aren’t?) but here they’re serving another purpose – they’re letting us know where we are in the stratigraphic stack. This is really useful when you’re out in the Valley & Ridge, and all the strata are folded and faulted and “reshuffled” in a way that makes it hard to keep track of where/when you are. They serve, in other words, as a marker horizon. I’m grateful to Dan Doctor (USGS) for cluing me into them.

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1 September 2014

Field work on the Grinnell Formation at Red Rock Canyon, Waterton Lakes National Park, Alberta

While in Waterton Lakes National Park, Alberta, my field class visited beautiful Red Rock Canyon to practice our field observation skills. This is a deep gorge cut into red and green argillite of the Grinnell Formation. Here is the view across the middle of Red Rock Canyon, at Tony, Kaatje, and Tom on the opposite side:

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We gave the students 45 minutes to explore the place and make observations before we met up to knit those observations together into an interpretation, making sure we didn’t miss any salient details. Here’s a collection of shots showing Team Rockies at work:

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So what did they see there besides a bunch of very red rock?

There’s some structure to be discerned as well… noticed readily thanks to the few green argillite layers amid all the vermillion.

Here’s a view from the bridge at the upstream end of the pedestrian walkway:

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At first glance, that looks like a bunch of parallel faults showing a small amount of offset, but that’s just a trick of the perspective. Really, this is nothing more than a joint set, progressively weathered out to deeper and deeper levels. (So I guess those would be “fauxlts?”)

…But fear not; there are genuine faults to be seen here, too. Sean has noticed one obviously faulted outcrop:

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Here’s another example of these apparently normal faults:

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Note here the parallel joint set to the fault, highlighted with light green reduced chemistry, surrounded by so much oxidized rock:

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These joints probably pre-existed when the stresses that triggered faulting went to work on these rocks. Select joints, ideally positioned, were then exploited as faults.

Here’s another example showing the parallelism of the dominant joint set and the normal faults:

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The reduction of the walls of the joints raises a question in my mind: was the reduction of the “strata” primary or diagenetic? The color is pretty much the same between the two, and in places, such as just left of the fault above, you can see the reducing front wicking upward from the bedding parallel reduced zones into the joints and faults.

Hmmmmmm.

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29 August 2014

Friday fold: Pleistocene glacial folding of Cretaceous sedimentary rock?

It’s Friday! Time for a fold. Here’s one in the Horseshoe Canyon Formation of eastern Alberta, seen on the bluff east of the Red Deer River near Willow Creek (“The Hoodoos”).

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This is anomalous – it’s unusual to see deformed strata out here, so very far from the mountain front to the west. One possibility is this representative of soft sediment deformation in the sediments; slumping, say, shortly after deposition.

Another possibility is more astonishing — the idea that Pleistocene ice sheets may have plowed into these pre-existing strata of Cretaceous age and crumpled them up as they moved along. (Subsequently, the crumpled uppermost Cretaceous rocks were buried in post-glacial sediments (loess and outwash). Such a thing has in fact been documented in the Great Plains north of here, at about the same distance from the Rockies. And previously on this blog, we’ve examined a similar case of glacially-induced deformation just east of Glacier National Park, Montana.

What do you think? Happy Friday!

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27 August 2014

Dinosaur bone

In July, I found a dinosaur bone in Dinosaur Provincial Park!

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It was lying in a wash coming off a small mesa, and sure enough, when the students and I walked up the little draw, we came to in situ bones poking out of the cliff above.

After showing it to the students, I put it back down exactly where I had found it, of course.

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23 August 2014

An examination of the Horseshoe Canyon Formation in eastern Alberta

One of the delights of this year’s Canadian Rockies regional geology field course was the serendipitous acquisition of an expert in the stratigraphy and sedimentology of the strata of the Great Plains. We started our trip in Drumheller and Dinosaur Provincial Park, spots where Cretaceous strata of the Judith River Group, Bearpaw Shale, and Horseshoe Canyon Formation are exposed through the downcutting action of the Red Deer River. My friend Astrid Arts, who I met through Twitter, joined us in the field for a few days and brought along her friend and colleague Jason Lavigne, who had studied the Horseshoe Canyon as the basis of his master’s thesis at the University of Alberta in Edmonton. Both Astrid and Jason work in the “oil patch” in Calgary. Jason agreed to take over the field instruction for half a day, showing us some key sites that were foundational to his thinking about the depositional setting of the Horseshoe Canyon Formation.

The first stop that Jason took us to was south of Rosedale, along Willow Creek, just south of a spot called “the hoodoos.” There, we looked north across a small valley to see this:

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Note that at the base of the outcrop, the strata are inclined about 5° to 12° relative to horizontal (and the overlying strata). These are point bar deposits, prograding (laterally accreting) out from left to right. They’re older at the left (west), and younger toward the right (east). These strata were deposited by a meandering river.

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This river was draining the newly-rising Rocky Mountains to the west, and flowing downhill into the Western Interior Seaway. Portions of it are clearly terrestrial, and portions are clearly marine. It’s a transitional unit.

Here’s a detail from one of Ron Blakey’s excellent paleogeographic maps (Northern Arizona University / Colorado Plateau Geosystems) of the setting:

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Directly overlying the inclined point bar deposits is “coal 0,” the oldest (and stratigraphically deepest) coal layer in these strata. As  you go up in the stratigraphic sequence, each successive coal is numbered 1, 2, 3, 4, etc. Here’s another look at coal 0, exposed east of the road where we parked on the way in:

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The other thing to notice here is the disconformity up top: the yellowish massive layers overlying the Cretaceous strata are Pleistocene glacial sediments – loess and silty outwash.

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We then stopped in the town of East Coulee, a few short kilometers to the south, to look east at a fine mud-filled channel.

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This mud layer overlies coal 0. There are clam borings in the top of the coal, and dinoflagellate fossils in the mud-filled channel – indicating a marine incursion. The river probably avulsed to a different course, and this channel became a tidal channel instead of a fluvial channel. The distinctive seismic signature of this mud channel is very easy to image geophysically. It’s about 10 km long and 2 km wide.

So here’s a summary of what we saw at these two sites:

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Jason favors a deltaic interpretation to make sense of these outcrops, but the area shows signatures of all three classic end-members of the Galloway ternary diagram for delta classification: we see evidence of fluvially-dominated portions (point bars), akin to the modern Mississippi “Bird’s foot” delta, and elsewhere there are tidal-dominated portions (our marine mud channel), like the modern Sao Francisco delta in Brazil, and elsewhere (we didn’t see this on our trip) there is hummocky cross-stratification, which is cited as being consistent with storms that periodically rework wave-dominated deltas (like that of the modern Ganges). The Ebro delta in Spain (Flash Earth link) was cited by Jason as the best modern analogue for the depositional system that he thinks best explains the characteristics of the Horseshoe Canyon Formation, because different areas of that single delta are wave, river, and tide dominated.

I was so grateful to have Jason there to explain all this to us – not only is it geographically out of my range of expertise, but the sedimentological details are also unfamiliar to me. The students really appreciated him sharing his time and experience with us on the trip. Thanks, Jason, and thanks for bringing him along, Astrid!

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12 August 2014

More Canadian Rockies student projects

In addition to the projects I linked to last week, here are a few more:

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