8 May 2013
I hadn’t been able to get the next phase of imaging that site ready in time for the post, but here it is: an annotated view of the outcrop.
Annotation color code:
PINK = Granite contact
BLUE = Sericite after staurolite pseudomorphs
YELLOW = Outlines of stretched clasts within the metaconglomerate
GREEN = Edges of lichens growing on the surface of the outcrop
Comparative viewer screenshot and link, so you can explore it for yourself:
Pretty cool, eh?
7 May 2013
Yesterday we examined primary sedimentary structures (including trace fossils) at an outcrop of Devonian-aged Brallier Formation turbidites between Deerfield and West Augusta, Virginia. Today, we’ll zoom in on the tectonic structures at the site: folds, faults, and joints.
Remember, you don’t have to take my word for it. You can explore it for yourself in this M.A.G.I.C. GigaPan:
One thing that’s kind of cool about that GigaPan is the very subtle view of folding. Note the the prominent sand stratum (a big turbidite) dips away from us on the left side of the outcrop, but twists about halfway across, and dips moderately toward the viewer on the right. It’s subtle because we’re not viewing the fold in profile view (i.e., on an exposure orthogonal to the fold hinge). Instead, the outcrop face is oblique to, and almost parallel to, the hinge. Here is a different view, from a hundred yards north up the road, and here the fold is more obvious:
Folding here is Alleghanian in age: that’s when the Valley & Ridge province enjoyed the bulk of its deformation.
Folding was accomplished partially by flexural slip – that is, each relatively stiff bedding plane slid relative to its stratigraphic neighbors above and below. This means that the bedding planes are essentially being reactivated as a kind of fault plane. And, like fault planes, they are decorated with polish and slickensides:
Elsewhere, the bedding planes accomplished folding by internally fracturing. Check out this chunk of graywacke from one of the bigger turbidites:
You can see a series of small extensional fractures on the upper edge: this would have been the outer part of the fold.
Many of the joints (fractures) at this site exhibit lovely plumose structure, that feathery fine anatomy that shows how the fracture surface originated and grew over time.
These joints are easy to find in the GigaPan above. In places, two intersecting joint sets give a lovely “sawtooth” to the edge of the beds where they crop out:
Fractures, of course, are great “plumbing” for subterranean fluid flow. Here, fluids flowing through the two bed-cutting joint sets produce a beautiful cross-hatching pattern highlighted by rust:
Some of the fluids were silica-rich, and deposited perfect little euhedral quartz crystals on the walls of the joint surface, transforming them from mere joints into veins.
We spent a nice 45 minutes at the site, GigaPanning and looking around. There was a lot to see. I’d love to bring students back, but it’s a long way from NOVA.
Maybe some summer I’ll run a “Geology of Virginia” class that can travel around the state they way my summer Rockies field course travels around Montana. Then we could hit these outcrops that are further afield.
6 May 2013
Last week, I mentioned some geologizing with the family in the Staunton area. The furthest west we ventured was to the road connecting Deerfield, Virginia, with West Augusta. There, the Brallier Formation is well exposed in a dramatic roadcut.
Explore it for yourself in this M.A.G.I.C. GigaPan:
The Brallier is turbidites, shed off the Acadian Orogeny during the Devonian, kind of like Martinsburg Formation is turbidites shed off the Taconian Orogeny during the Ordovician.
I saw plenty of trace fossils there…
There were also some non-biogenic primary sedimentary structures, like graded beds:
Here are two mud chip rip-up clasts in a graywacke block:
A few blocks showed Bouma sequence details:
I’ll return to this same site tomorrow, for a look at the secondary (tectonic) structures…
3 May 2013
Last weekend, after we checked Lily in for her race, I spotted some boulders near the check-in site. The next morning, once the race had started but before we could cheer her on, my field assistant and I went back to the boulders to check them out.
My field assistant’s planners had forgotten to pack him a hat – so we improvised with a pair of fleece pants inverted on his noggin:
We were glad to have gotten back to take a closer look at these boulders, which showed limestone and dolostone layers that had been both cleaved and strongly folded.
There was one array of en echelon tension gashes, too.
Look at these lovely folds! They reminded me very much of some folds I saw last summer in Yoho National Park, British Columbia.
In some layers, my field assistant and I spotted limy rip-up clasts:
In some places, these rip-up clasts had been thoroughly distorted (and aligned) through compressional strain:
The differential weathering between the dolostone layers and the limestone was obvious and profound:
Another boulder showed this even more elegantly, and the dolostone was a rich, caramel color to boot!
My field assistant and I were very pleased with what we found. Though these boulders were not in situ, I suspect they are from the Conococheague Formation, which exhibits similar lime/dolostone interlayering. And that does crop out nearby.
My field assistant reminded me it was time to go cheer on mommy as she ran past mile marker 5, so we skeedaddled, right after I made him pose for one more shot…
2 May 2013
After our jaunt down in Staunton and Waynesboro, Lily and I headed north through the Shenandoah Valley, and tried a new winery. This has been a thing we’ve been doing lately – visiting various Virginia wineries and thinking about their geology while sampling their wares. This time, we went to Cave Ridge Vineyard, which is located just east of Little North Mountain, on Cave Ridge, a topographic high held up by the dolomitic Beekmantown Formation. When they were digging the foundations for the tasting room building, they uncovered lots of fossils, one of which stays in the tasting room for guests to view:
That’s a coiled nautiloid (note the smooth, gently curved septa that subdivide the shell into chambers). Unfortunately, VirginiaWine.org incorrectly identifies this as an ammonite, which shows they need a geological consultant familiar with the ranges of local fossil species. (Ammonites are limited to the Mesozoic, and these rocks are early Paleozoic in age.)
Here’s a second photo, sans flash, and with the scale pencil instead of fingers for scale:
Here is a geologic map of the area, modified from Young and Rader (1974):
Because the Beekmantown is dolomitic, it weathers more slowly than do the limestones which are stratigraphically above (southeast) and below it (northwest). There were big chunks of holey dolostone ringing the trees in the Cave Ridge Vineyard parking lot, and if you visit, you can search them for fossils. I didn’t find any myself.
This lovely beast graces the label of one of their red wines, the Fossil Hill reserve, a blend of Cabernet Franc, Chambourcin, and Petite Verdot. While we liked that wine, the one that really impressed us was Cave Ridge’s Traminette. We got a bottle of that, and enjoyed a glass in the sun, with our baby crawling around and being very inquisitive and cute. The traminette wasn’t named for a fossil, but that’s okay. It was delicious and had a lovely floral aroma.
My visit to Cave Ridge was “field work” for a new Smithsonian Associates class that I will be running this fall on the geology of Virginia wine country. While the geological tie-ins of Cave Ridge are superb, I think its location is probably a bit far for a one-day tour that originates (and must return to) Washington, DC.
Young, R.S., and Rader, E.K., 1974, Geology of the Woodstock, Wolf Gap, Conicville and Edinburg quadrangles, Virginia: Virginia Division of Mineral Resources, Report of Investigations 35, scale 1:24,000
1 May 2013
Following a tip from my colleague Pete Berquist (Thomas Nelson Community College), I did some graptolite collecting this weekend. Our family went down to Waynesboro, Virginia, so my wife could run a half-marathon, and on the way back home, we spent some time geolgizing.
One site we visited was Mint Spring, Virginia, in the parking lot of the Days Inn.
Here, at the contact between the Lincolnshire Formation and the overlying Edinburg Formation, there are some papery shales exposed:
They include bulky graptolite fossils preserved as carbon films:
Graptolites were (are?) colonial animals, and each of the little “sawteeth” you see on either side of these long, dark shapes (technically called “stipes”) is a “theca,” a little cup that held a small graptolite animal.
I love the contrast of the dark graptolites against the light-colored background of shale.
My experience with graptolites is limited, but these were easily the best I’ve ever seen.
30 April 2013
Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty to hold your attention in these rocks. Close looks will reveal sericite-after-staurolite pseudomorphs (evidence of retrograde metamorphism) and an intrusion of granite.
29 April 2013
Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under brittle-ductile conditions parallel to the foliation, which kinked it.
Here’s the site (map modified from a detail of Tony Fleming, Avery Drake, and Lucy McCartan’s 1994 Geologic Map of the Washington West quadrangle:
A few screenshots to whet your appetite:
26 April 2013
Near the end of the stellar pre-GSA-Minneapolis-meeting “Structural Geology of the Sub-Province Boundaries” field trip in the southern Superior Province, we visited a small but intriguing pavement exposure of biotite schist intruded by small dikelets of the Burntside trondhemite. These dikes were then deformed through boudinage and folding. Here, for example, was a lovely “S” fold:
So my gut reaction on seeing such a thing, is to interpret it thus:
In other words, my instinct was “S folds indicate sinistral (top to the left) shear.”
…But this outcrop was going to provoke a moment of insight. It turns out not to be so simple. Peter Hudleston suggested that asymmetric folds aren’t reliable indicators of simple shear. Audaciously, he suggested to me that in fact, this “S” fold could instead mean this:
What sacrilege was this?!?
Think about progressive simple shear, Peter suggested…
I sat down and thought about it for a minute, and sketched a series of folds in my notebook, and realized that if shear strain was high, Peter was right: the long limbs and short limbs would swap places!
This was an important realization for me. An insight like that is a moment of illumination that makes a person look at the rocks differently than they did before the realization. I see it as an important development in the evolution of my own understanding of rock structure: one insight like that is worth the cost in time and money for a trip like the Superior Province field excursion before GSA. Here, a few feet away from the “S” fold (with its “sinistral” implications) are plenty of indications of dextral kinematics:
We can zoom in there (at upper right) on a “big fish,” for instance, which is basically bounded top to bottom by C surfaces and left to right by S surfaces:
Kinematically, this is the standard interpretation:
This makes us re-examine the fold we started off with: instead of being a rock-solid (pun very much intended) indicator of sinistral simple shear, it’s in fact entirely consistent with what the rest of the outcrop was suggesting: dextral simple shear!
Son of a gun. I’ll never trust those S folds again…
Anyhow, I am prompted to blog this now, this week, because last week a new article came out in Geology that looks at a similar phenomenon: the unfolding of folds during simple shear.
The idea is this: if you have an original layer oriented at a certain angle relative to a simple-shear-dominated shear zone, you can first fold that layer, then rotate it backwards and unfold it!
As with my example from Minnesota, this phenomenon is a case of how a moderate amount of strain yields one structure, while a higher amount of strain produces another, totally non-intuitive structure. In Figure 1 of the paper (produced above, with color added), compare the unstrained left box with the very strained right box. They look like mirror images!
In particular, if the viscosity contrast between the folded layer (black) and the surrounding medium (green) is high (above ~50), the folded layer is capable of being entirely unfolded given a sufficiently high state of shear strain. Modeling results from Maria-Gema Llorens and her co-authors indicate that the presence of intrafolial folds and cusp-like folds in the matrix adjacent to otherwise straight layers are indicators that the “straight” layer may once have been folded.
Here’s a detail from their Figure 3, showing the relation of these apparently sinistral folds to the larger dextral shear zone that contains them:
This is an exciting new development in the study of folded layered rocks – and layered rocks that seem to be mostly non-folded.
Llorens, M., Bons, P., Griera, A., & Gomez-Rivas, E. (2013). When do folds unfold during progressive shear? Geology, 41 (5), 563-566 DOI: 10.1130/G33973.1
25 April 2013
Today, let’s go back to the Pike Dam, where we spent some lovely moments last week, agog at the lovely graded beds and flame structures visible there. In contrast, today we want to examine the deformational structures seen elsewhere at this same outcrop. There are folds and faults and joints and more exotic fare: tension gashes and Riedel shears.
The deformation here is the youngest to affect the Vermillion District of the Superior Craton. Their orientation is apparently conjugate (at a 60° angle to) a northeast-trending regional fault, according to the field guide to this trip (Bauer, et al., 2011).
You’ll notice some of these show a left-lateral sense of shear, while others show right-lateral kinematics. So… the deformation of these rocks is complicated.
There are also more subtle features to the deformation: joint sets and very small-scale-offset faults.
I love seeing examples of fractures weathering out in positive relief – a sure sign of fluid flow through these joints, and reinforcing their walls with whatever was dissolved in those fluids (silica, probably?). Here’s an example:
Finally, a year and a half later, I’m close to finishing off my blogging coverage of this excellent field trip. One more topic is a special one – I’ll save it for tomorrow…