Jay Kaufman of the University of Maryland and I met in the field this weekend to do some outcrop preparation and structural documentation with the GigaPan.

This is a tricky bit of business, as it turns out – or, at least, it’s trickier than the typical GigaPan shoot.

To start with, we had to clean the exposure, a block that shows the contact between the Neoproterozoic Fauquier Formation and the overlying Catoctin Formation.To clean it, we needed a pressure washer. But there was a good sized stream between the car/road and the rock. So here’s Jay hauling the pressure washer across Goose Creek with the help of his graduate student Huan Cui.

All in the name of science.

Next, we had to haul water from a flooded quarry pond up a steep, muddy slope to the site where the intriguing rocks live. There were several slips and spills, and my cell phone took a swim in the pond. All in the name of science. Jay’s daughter Colleen was helping, too, and she hauled tools like a saw and branch loppers to the site.

We power-washed the rock, a block of float with some intriguing structure. Pond water misted surrounded us: we breathed in its microdroplets and their microbial cargo. All in the name of science.

Then it was time to GigaPan: Jay hauled a ladder over, which was key for getting the GigaPan up to a proper height for photography orthogonal to the plane of the exposure.

_{Photo by Huan Cui, courtesy of Jay Kaufman}

_{Photo by Huan Cui, courtesy of Jay Kaufman}

As I supervised the GigaPan (it was on auto-focus, and needs supervision in that condition; otherwise it sometimes skips a shot if it can’t decide what to focus on), Jay cast shadow on the block, so as to have a consistent level of lighting across the final image. Braving muscle fatigue as he perched on a branch for 15 minutes, between the Sun and the rock, Jay persevered – all in the name of science!

_{Photo by Huan Cui, courtesy of Jay Kaufman}

_{Photo by Huan Cui, courtesy of Jay Kaufman}

Here are the four GigaPans that resulted from our labors:

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The happy team at the end of the expedition:

_{Photo by Jay Kaufman}

So what do we see in these images? I think our background strongly colors our interpretation.

Jay is a sedimentologist and geochemist, one of the four authors of the bombshell 1998 “Snowball Earth” paper that ignited the modern explosion in examining and re-interpreting Neoproterozoic sedimentary rocks. He interprets this contact as showing pillow basalts, and carbonate diapirs between the pillows. This is very important, because if indeed the block records primary soft-sediment deformation (hot submarine lava flowing onto submerged carbonate mud), it allows us to use the radiometric date for the Catoctin Formation (meta-basalt / greenstone) as a constraint on the age of the underlying Fauquier Formation (meta-limestone / marble). It would mean that the relationship is conformable (and the metamorphism later overprinted the primary configuration with a light enough touch that it wasn’t fundamentally reorganized), and that helps us say when the “Ediacaran” phase of Snowball Earth glaciation took place, since the lower Fauquier shows features that are consistent with glacial outwash. Here’s my rough interpretation of this reading of the rocks:

Photo by Jay Kaufman; Click to enlarge

Caveat: Jay and his colleagues may draw those lines differently; I refer you to their paper for a more complete discussion of their interpretation. Jay has also added snapshots to one of the GigaPans to help tell the story.

However, I’m trained as a structural geologist, and so I see something different when I look at this block. I see boudinage, tension gashes, and metamorphic reaction rims (chlorite + garnet most prominent minerals in these “skarn”-like veins), with enormous crystals of vein calcite filling most of the volume of the boudin necks. The fact is that we see the reaction rims and the multi-cm-scale calcite crystals only in the spaces between the blocks of greenstone (and not along the bottom/top contacts). The inverse is seen with the dark-colored, fine-grained “chill zone” that one might say is where the quenched obsidian formerly was located: you don’t see that in the spaces between the greenstone pods. This suggests to me that interpreting them as Alleghanian-age boudins and boudin necks is the more parsimonious interpretation. A fine way to test my interpretation would be to compare the orientation of these structures to Alleghanian-aged structures from elsewhere in the region. Unfortunately, this is a block that is displaced from its original position (float, not outcrop), so the geometry is divorced from its in situ orientation. At any rate, the pervasive Alleghanian deformation of Blue Ridge rocks gives me reason to suspect the that this contact isn’t pristine enough that I’d feel comfortable deeming it as definitely conformable.

Photo by Jay Kaufman; Click to enlarge

Jay sees primary structures, and I see tectonic structures. Maybe there’s a bit of both?

I love this site, and the rest of the Fauquier Formation, for this reason – it’s not 100% clear what it means. That means it’s a great place to train students to think for themselves about the paths we follow from the observations we make on site (or virtually “on-site”) to the stories we tell about the past based on those observations.

Regardless of the final interpretation, these GigaPans offer plenty of information to stimulate discussion and inquiry. And the great thing about our efforts last weekend is that now everyone can “visit” these rocks and use their observations to power their own independently-formulated hypotheses.

Enjoy exploring them, and viewing them through the lens of your own training and background. Let us know what you see!