Jointed Virgelle

One of the stops my Rockies students and I made this summer was a dinosaur paleontology tour through the Two Medicine Dinosaur Center in Bynum, Montana. The folks there are very accommodating, and at my request gave the class a bit of stratigraphic context for the dinosaur fossils. For instance, we visited the geologic formation which underlies the dinosaur-bearing Two Medicine Formation: it’s a beach sandstone called the Virgelle Formation. The Virgelle was deposited along the shore of the Western Interior Seaway, a Cretaceous-aged transgression of seawater onto the North American continent.

While our guide Corey discussed the primary structures that showed the unit to be “beachy” to my students, I got distracted by this outcrop:

virgelle_crackedField notebook for scale (long side 18cm).

So what’s so great about this? It struck me as a nice little demonstration of the relationship between stress directions and joint orientations. σ1 is our maximum principal stress direction (i.e., the direction of greatest stress), in this case caused by acceleration due to the force of gravity. σ2 is perpendicular to the screen of your computer (and the plane of the photograph): that is the intermediate principal stress direction. σ3 is our minimum principal stress direction (weakest stress), in this case pushing in from the sides (atmospheric pressure only, no overlying rock weight):

virgelle_cracked_2

By definition, σ1 is greater than σ3.

So we have a low-level confining stress paired up with the differential stress imparted by the heavy rock pushing down on the slab of sandstone beneath it. As long as that difference in stresses is greater than the strength of this weakly lithified Virgelle sandstone, then the rock will break, and the orientation of those breaks will be ~parallel to σ1, and ~perpendicular to the extension direction, σ3:

virgelle_cracked_3

You’ll also note that the bedding planes in the Virgelle sandstone are planes of weakness, accommodating the extension by allowing blocks of sandstone to slip sideways over what amount to small-scale “detachment faults” (low-angle, upper block sliding downward relative to lower block).

So does an understanding of these stress directions and the resulting structures’ orientation do us any good beyond this one lone slab of fractured sandstone?

Indeed it does. Keeping in mind that we are rotating our perspective from horizontal (“side view”) to vertical (“bird’s eye view”), consider the following map of central Asia:

baikal_ext_sigma

As the Indian subcontinent impacts the Eurasian continent, it moves towards the northeast. This results not only in the northwest-southeast-trending Himalayan mountain front at the site of impact, but also in extensional faulting further into the heart of the continent. Down-dropped blocks of crust in desert areas show up as northeast-southwest-striking rift valleys, but in wetter areas, those low-lying cracks fill with water, and show up to us as linear lakes.

baikal_ext_lakes

Lake Baikal in Russia is a famous example of this, but Mongolia’s Lake Hovsgol is a smaller version of the same thing. The lakes are oriented with their long axis ~parallel to the σ1 direction, as they have been opened up due to stretching in the σ3 direction.

Caveat blog-reader: The kinematics and dynamics of central Asia are actually a lot more complicated than this simplistic picture I’ve painted. My main point in drawing the parallel between the two examples is that outcrop-scale structures can serve as analogues that can help us understand regional-scale processes.

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The LaHood Conglomerate

The Belt Supergroup is a series of sedimentary strata laid down in the Belt Sea, an inland sea (like modern Hudson Bay) that existed in the northwestern (by present coordinates) part of ancestral North America during the Mesoproterozoic era of geologic time. Estimates of the absolute age of these rocks range from 1470 to 1400 Ma. Mostly, it’s siltstones (argillites) and limestones, including the multicolored strata so gloriously displayed at Glacier National Park in Montana. But there are some coarser units, too. My favorite is the LaHood Formation, which is a beautiful diamictite well exposed in the canyon of the Jefferson River just east of Cardwell, Montana.

Check out this gorgeous rock (sawn, polished, lacquered, and scanned):

Here it is again, rotated by 90°:

You can click through (twice) for both of these images to get big versions if you want more details. The prominent pinkish clast in the upper scan is a myrmekitic granite, and the prominent grayish clast in the lower scan is marble. These are pieces of the Archean basement complex (Wyoming Terrane), such as we see exposed in the Gallatin Range or the Beartooth Plateau. Note their well-rounded shapes: these cobbles traveled some distance before reaching their depositional location. You can also see pebbles of potassium feldspar, milky quartz, and rock fragments, all set in a dirty sandstone matrix.

Because of the coarse grain size, especially relative to other Belt units, the LaHood conglomerate is interpreted to have been deposited along the southeastern shore of the Belt Sea. Here’s a sketch from my field notebook (from 2008) showing this basic depositional interpretation:

belt

While some of the sediment has been sourced to Montana rocks, I am told that some of it comes from some other landmass, maybe Antarctica or Siberia, indicating their possible paleo-proximity to what is today the northern Rocky Mountains, USA.

Rockies course applications open

For those of you who are potential NOVA students (really, that’s pretty much anyone on the planet), I wanted to let you know that applications are now open for the July 2010 Regional Field Geology of the Northern Rockies course that I co-teach with Pete Berquist of Thomas Nelson Community College. A more detailed description is available on my website.

Contact me via e-mail if you want more information or download an application here.

To whet your appetite, here’s Rockies 2009 student Jason Von-Kundra mapping Mississippian-aged carbonates in the Bridger Range of Montana:

Jason_maps

Fossil crinoid stem

Today, you get a photo of a fossilized crinoid stem, from the Mississippian-aged Lodgepole Limestone of the Bridger Range, north of Bozeman, Montana. A pencil is provided for scale:

crinoid_stem

Zoomed-in a bit, and cropped. The segments (“columnals”) show up nicely:

crinoid_stem_2

Crinoids are echinoderms, the invertebrate phylum which includes sea urchins and sea stars. However, at first glance you might think they were plants, as they are sessile (mainly sessile, anyhow) and have an overall form much like a kindergartner’s sketch of a flower. This morphology is where their common name, sea lilies, comes from.