22 June 2011

The Rockfish Conglomerate

Posted by Callan Bentley

After visiting the folded granite dike that served as last Friday’s featured fold, my Tuesday field trip (a week ago yesterday) with Chuck Bailey and Jay Kaufman took us to “Ayres Rock, Virginia,” where underneath a small bridge spanning the Rockfish River, you can find the type locality for the Rockfish Conglomerate. We investigated it there over lunch, deducing from graded bedding and the dip of the beds that the facing direction had been flipped upside down, which makes for a more complicated story when you try and figure out the contact with the underlying basement complex, exposed just a few tens of meters to the west.

The Rockfish Conglomerate is a Neoproterozoic (probably) clastic sedimentary unit which nonconformably overlies the basement complex in the Blue Ridge province. It occupies the same place in the stratigraphic sequence as the Fauquier Formation of the eastern Blue Ridge (i.e. near Aldie, VA), or the Swift Run Formation of the area around Shenandoah National Park. Hebert et al. (2010) interpret the outsized, faceted clasts in the Fauquier Formation as Snowball Earth glacial outwash, and we wanted to investigate the Rockfish Conglomerate in the same light. Chuck had been out canoeing with his family earlier this spring and rediscovered a large quarry of the Rockfish near his Uncle Joe’s property. So after lunch, we went back and checked in with Uncle Joe and his goat herd, and then hiked out to the quarry. The exposures there were a lot better than the sub-bridge Ayres Rock location, though I’ll mix them the two locations up here in this blog post to demonstrate key features that we observed.

The quarry featured both “pavement” perspectives and significant vertical exposures:

On the lip of the quarry, Chuck and Molly engage in conversation (right) while everyone else searches their legs for ticks (left):

Here’s a gigapan of the quarry (You can go to a full screen version of it by clicking the word “Gigapan” in the lower right):

And, while I’m at it, here’s another gigapan (much less detail, since it was hand-shot and stitched with ICE) showing the empty space where the quarry was excavated, and the mountains in the distance, just to provide a sense of place:

Some examples of the conglomerate’s texture:

This one looks like it’s got some aligned feldspars in it, like we saw at the Lawhorne Mill High Strain Zone:

Here’s an animated GIF to give a big of three-dimensional perspective on the shape of one particular clast that was exposed on the sides as well as cross-sectioned by the outcrop:

We were able to see some primary structures in places, like these examples of graded bedding:

Clasts with long axes oriented perpendicular to bedding: makes the most hydrodynamic sense if the were dropped in from above, rather than washed in from the side:

Closer in:

(This was one of the beneath-the-bridge exposures, which is why it looks like it has a patina of rust on it and a spatter of tar droplets: it does.)

Here’s some other bedding:

Bedding like this was relatively rare; most of the exposure was poorly sorted and massive.

A complicating factor here was that we also saw a lot of evidence of significant strain, which makes it harder to interpret the rocks. Are those layers bedding or are they foliation? The Rockfish Conglomerate has clearly been metamorphosed, as this muscovite sheen shows:

Chuck poses monkey-style (note the monkey Band-aid on this forearm) in front of a ~vertical splitting surface that appears to be bedding:

The rock that Chuck’s hand is on in the previous image shows clear bedding:

Note the grading there, which youngs towards the top of the photograph (white arrows pointing up). Also, this same exposure shows another one of those putative dropstones, with the long axis perpendicular to bedding:

So we’re pretty confident that there are some real primary structures here, and that we can deduce geopetal information from them. But the Paleozoic brought an Appalachian structural overprint to these rocks, and we want to be careful not to dupe ourselves by mistaking metamorphic foliation for bedding. This is really important when you want to interpret a particular clast as a dropstone.

I saw several features that make me feel very cautious about attributing too much Snowball Earth glacial significance to this package. For instance, the augen-like shape of the dark clast here suggests it has been remolded by ductile flow, and therefore this may not be the  most reliable place in the world to use the sedimentary structures as support for a proglacial depositional setting:

Ditto for this one: looks like a metamorphic fabric to me:

And here are two clasts that resemble asymmetric porphyroclasts with a top-to-the-right sense of shear:

There was also a lot of evidence of pervasive pressure solution (as we once saw in another conglomerate). Here are some examples that caught my eye of what Chuck likes to call “P sol“:

This pressure solution evidence is important, especially when considering the question of whether some of these outsized clasts were iceberg-rafted to their site of deposition. Consider the following two images:

Both show a large clast with a prominent asperity pointing downwards. These asperities have dark layers wrapped around them. But are these dark layers bedding deformed by the impact of a dropstone? Or are they later metamorphic foliation wrapping around a resistant porphyroclast as the volume of the rock is reduced by pressure solution? The interpretation may be in the eye of the beholder. A Snowball Earth expert like Jay is primed to see primary sedimentary structures here, while as a structure geologist who worked on shear zones, I’m more likely to see this as a deformational fabric.

Here are a few examples (white arrows) showing what looks to me like clear signs of clasts impinging on neighboring clasts, with sutured boundaries between them:

Or check out how cosy these two clasts are to one another, separated only by a thick seam of (insoluble) mica:

Here’s another line of textural evidence that gives me pause:

The boundaries of the clast in the previous photo are very sharply defined at top and bottom, where the quartz has apparently gone into solution and concentrated the mica. But the sides are far less well defined, as they lie in the pressure shadow of the main clast, and thus the quartz there is less susceptible to dissolution.

Then there were a bunch of clasts that showed odd shapes that suggested they had been forced to flow during recrystallization of the Rockfish:

The next two display tapered tips at the right end, which suggests to me a “pinching out” sort of phenomenon:

There are also some more pronounced zones of strain localization in these outcrops. For instance, check out this small fault offsetting two halves of a clast:

We also spotted several mylonitic shear bands that cross-cut the conglomerate:

Take home lessons (for me) from the Rockfish Conglomerate:

1) It’s a really interesting rock.

2) Its diamictic grain size distribution, as well as the mix of rounded and faceted clasts, could be plausibly interpreted as glacial outwash from the Neoproterozoic, and thus is a putative “Snowball Earth” sedimentary deposit. However, no striated clasts were identified.

3) It is deformed, perhaps significantly so, and thus I do not have high confidence that we can identify dropstones in it (which would “seal the deal” on the pro-glacial interpretation). This deformation has changed the orientation of the beds (overturned at Ayres Rock, and in multiple orientations at the quarry) and has recrystallized the unit through metamorphism and pressure solution, which have overprinted original (pre-Appalachian) sedimentary structures with an Appalachian structural fabric.

4) It’s a really interesting rock, worth more than thirty photos and two gigapans and half a dozen chigger bites and a six hour round-trip drive (thanks for chauffeuring us down there, Jay!).

I’m grateful to Chuck for taking time to show us around down there.