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.

Crystal ghosts

The first time I went to the Billy Goat Trail (Potomac, Maryland) with geology as the goal (as opposed to mere recreation), it was 2002. The trip was led by a professor at the University of Maryland. I was a graduate T.A. then, and didn’t know anything about the local geology. I remember at the end of the trip, the professor sent us out to search for “kyanite ghosts” (pseudomorphs of sericite after kyanite, produced during retrograde metamorphism). We didn’t find them on that trip, but the evocative phrase “kyanite ghost” stuck in my head.

Several years later, after I had cultivated a deeper understanding of the story told by Billy Goat Trail rocks, I was poking around in the area near the trail’s “emergency exit,” and found something that fit the “kyanite ghost” bill. I took a photograph of it:

ghosts_3

My next step was to confirm what I found with my mentor and local rock guru, the geologist E-an Zen. E-an had been training me to take over leading geology hikes as a volunteer for C&O Canal National Historical Park. I e-mailed him the photo above. E-an wrote back to congratulate me on finding and photographing the exact same outcrop that was used in Cliff Hopson’s 1964 book The Crystalline Rocks of Howard and Montgomery Counties to illustrate the pseudomorphs. Hopson used a pencil for scale, and I used a Swiss Army knife, but otherwise the photos are identically composed:

hopsonImage: Plate 20, Figure 2; Hopson (1964)

That’s pretty uncanny, eh? Two photos taken just over half a century apart, of the exact same square foot of clue-bearing rock.

So, we have here large, bladed crystals that formed as porphyroblasts of metamorphic minerals during prograde (↑P,↑T)  metamorphism, then those same porphyroblasts found themselves unstable as temperatures and pressures dropped (retrograde metamorphism; ↓P,↓T). Their elemental constituents found themselves in disequilibrium, re-reacted, and formed new minerals which occupied the same space and shape as the large, bladed porphyroblasts. Today, you’ll finded that these “large, bladed crystals” are really aggregates of sericite (super-fine-grained muscovite).

So the question is, what were the metamorphic porphyroblasts that formed at peak P/T (and were subsequently replaced)? I mentioned kyanite as one possibility, right? However, Hopson noted these ‘ghostly’ shapes as “sillimanite (?).” Kyanite and sillimanite have a lot in common, but they aren’t the same thing. Like their polymorph andalusite, both kyanite and sillimanite have the chemical formula Al2SiO5. Both also grow in long bladed crystals. Check out these examples to prove this to yourself: kyanite | sillimanite

But in spite of these similarities, there’s a big difference between kyanite and sillimanite: they are stable at different combinations of temperature and pressure. Consider this classic P/T diagram:

Al2SiO5 triple point

If the sericitized pseudomorphs on the Billy Goat Trail were once sillimanite, then it implies higher temperatures. If they were once kyanite instead, then the temperatures were potentially lower. These rocks have plenty of un-retrograded sillimanite, but George Fisher (1971) was the one to invoke kyanite as the peak-P/T-porphyroblasts. He uses petrologic evidence to make the case that they were once close to ky/and/sil triple point. He says:

…the pelitic rocks contain many stubby crystals of andalusite, partially altered to sillimanite, and now largely pseudomorphed by fine aggregates of sericite. Andalusite partially altered to sillimanite is common at this end [south] of the island*, while at the north end of the island only bladed crystals of kyanite altered to sillimanite have been found. It appears as if the rocks at this end of the island must have entered the sillimanite field from the andalusite field, while the rocks farther north entered the sillimanite field from the kyanite field. If so, the rocks in the center of the island must have passed close to the triple point in the system Al2SiO5., about 5000 bars pressure [0.5 Gpa], and 650° C. The presence of muscovite and quartz in the sillimanite-bearing rocks reinforce this conclusion…

I assume he’s basing those statements on detailed petrologic evidence, but I haven’t seen his thin sections myself.

Tangentially, we’ve only been discussing the metasedimentary rocks so far, but E-an Zen and Phillip Candela point out in a 1998 guide to the area (for the University of Maryland geology department’s 25th anniversary hike) that the amphibolite units (meta-igneous, presumably) also contain kyanite or sillimanite but have not melted, which suggests temperatures in the range of 540° to 680° C, and pressures between 4.2 and 7 kbar (0.7 GPa).

So which is it? Kyanite or sillimanite? I can’t claim to know the answer: perhaps someone with more metamorphic petrology experience than me can shed some light on which mineral they they think they see in these ghostly pseudomorphs.

When I was out on the Billy Goat Trail last Friday with my GMU Structural Geology students, we ended up in that same general area. I challenged them to find the pseudomorphs, and it wasn’t five minutes before several of the students found excellent (though small) outcrops. Not the one that Cliff Hopson and I found, but other ones! Here are some shots to show their discoveries:

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IMG_2709

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I have two questions for you: (1) What’s your favorite example of retrograde metamorphism? and (2) Have you had a similar photographing-the-same-spot-someone-else-did-many-years-before-you experience?

______________________________________________

Bierman, Paul, Zen, E-an, Pavich, Milan, and Reusser, Luke (2004). The Incision History of a Passive Margin River, the Potomac Near Great Falls, in USGS Circular 1264: Geology of the National Capital Region. Field trip guidebook.

Fisher, George  W. (1971). The Piedmont crystalline rocks at Bear Island, Potomac River, Maryland. Maryland Geological Survey Guidebook No. 4, prepared for the 1971 annual meeting of the Geological Society of America, Field Trip No. 4.

Hopson, Clifford A. (1964). The Crystalline Rocks of Howard and Montgomery Counties. Maryland Geological Survey, Baltimore.

Zen, E-an, and Candela, Philip (1998). Department of Geology, University of Maryland: 25th anniversary geology hike to Great Falls, and the Chesapeake and Ohio Canal National Historical Park. Field trip guidebook: September 19, 1998.

* The “island” in question is Bear Island, which is not really an island (except during times of highest flooding). It’s just the land between the C&O Canal and the Potomac River in the vicinity of the Billy Goat Trail.

Easter egg

Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped?

This is as close as I got:

owens6_01

This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a faulted xenolith. I love outcrops like this, with a combination of primary structures (like the xenolith) and secondary structures (like the fault). And the fault surface appeared to have hosted some fluid flow, encouraging epidotization (hydrous metamorphism) along its surface. How appropriate, considering both the “cracked egg” implication of the round xenolith and the pastel tones of the green epidote.

I’ll annotate it up for you, because I know you love it when I do that:

owens6_015

Happy Easter, folks. Focus on the bunnies and candy, and not the zombies.

Here, ptyggie ptyggie ptyggie!

Yesterday, I took my GMU structural geology class to the Billy Goat Trail, my favorite local spot for intriguing geology. Unlike last year, we managed our time well enough that we got to clamber around on the rocks downstream of the amphibolite contact. Here’s Sarah, Lara, Kristen, and Alan, negotiating a steep section:

IMG_2700

Justin, Joe, Nik, Aaron, Jeremy, and Danny find a chunky amphibolite boudin in metagraywacke. Notice how Jeremy is gesturing about the orientation of the metagraywacke foliation wrapping around the boudin.

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The thing that we found that really made me happy were these ptygmatic folds. Most of my readers will doubtless already be familiar with ptygmatic folding, but in case you’re new to this, check out this photo (ballpoint pen for scale):

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Ptygmatic folding is “intestine-like” in appearance. It results where there is a particularly high viscosity contrast (viscosity is resistance to flow) between the folded layer and the surrounding matrix. The higher viscosity material makes broad lobes, while the lower viscosity material may be found in the pointy cusps between those lobes. If ptygmatic folding is well developed, the limbs become parallel to one another (isoclinal), and the visual similarity to guts is disconcerting. Here’s a smaller version, a few feet away from the first one:

IMG_2706

I’m headed back to the Billy Goat Trail today to discuss the trail’s geology with a crew from Sigma Xi‘s D.C. chapter. I wonder what we will discover today?

Sugarloaf

Sunday morning, NOVA adjunct geology instructor Chris Khourey and I went out to Sugarloaf Mountain, near Comus, Maryland, to poke around and assess the geology. Sugarloaf is so named because it’s “held up” by erosion-resistant quartzite. It’s often dubbed “the only mountain in the Piedmont,” which refers to the Piedmont physiographic province. Here’s a map, made with GeoMapApp and annotated by me, showing the general area:

A larger version of the map can be viewed by clicking here.

On the far west, you can see the Valley & Ridge province, which ends at the Blue Ridge Thrust Fault. Then the Blue Ridge province runs east from the Blue Ridge itself to Catoctin Mountain. From there, you enter the Piedmont, including both the “crystalline” Piedmont (Paleozoic metamorphism of various ocean basin protoliths, plus infusions of granite) and the Culpeper Basin, a Triassic/Jurassic rift valley. The Potomac River cuts a series of three spectacular water gaps across the Blue Ridge province just west of Sugarloaf. Harpers Ferry, West Virginia, is located at the confluence of the Potomac and the Shenandoah Rivers by the westernmost of these water gaps, and the name for the easternmost one is “Point of Rocks.”

Here’s a look at a detail from the southeastern corner of the geologic map of the Buckeystown, MD quadrangle, by Scott Southworth and David Brezinski:
sugarloaf_geol
sugarloaf_geol_key

The map pattern shows a that the area around Sugarloaf Mountain is a doubly-plunging anticlinorium of Sugarloaf Mountain Quartzite [SMQ] and overlying (younger) Urbana Formation. Overall, it’s got that typical “Appalachian” northeast-southwest trend. Notice the thrust fault on the west side: a typical hanging wall anticline? The ridges, including the summit of Sugarloaf Mountain itself, are held up by the toughest quartzite. This overall “squashed donut” shape shows up pretty well in the physiographic map up at the top of this post.

Sugarloaf is quartzite (metamorphic), but you can clearly see the sand grains that composed its protolith (sedimentary). There’s also reports of cross-bedding, and so Chris asked me to take a look at a few structures to assess them with my point of view. I found a pervasive cleavage in the rock, far more than I would have suspected would be there. We did find bedding exposed as compositional/grain size layers in several locations, including on the summit. I also paid a lot of attention to the many quartz veins which cut the metasedimentary quartzite. These veins of “milky quartz” are often arranged in lovely en echelon series, like these tension gashes:

tension_gash_array_sugarloaf_web

I took the above photo several years ago on a visit there, but it’s typical of the sorts of stuff we saw Sunday. The kinematic sense of this outcrop would be “top to the right.” Interestingly, none of the Sugarloaf outcrops show really deformed tension gashes (i.e., they’re not folded into Z or S shapes like those I showed you a few days ago).

What we really wanted to get a sense of, though, was which way was up in these rocks. We were in search of geopetal structures: primary sedimentary structures that indicate the “younging direction” of the beds. Graded beds can do this, though I didn’t see any unambiguous graded beds in the SMQ on Sunday’s trip. We wanted some cross-beds. We found some hummocky / swaley examples, looking approximately like this USGS photograph (black & white; hammer for scale) of an outcrop somewhere “north of the summit”:

crossbedding_USGS_sugarloafImage source: USGS

Ours wasn’t as beautiful as the one pictured above, but it was clearly hummocky cross-bedding, and it was right-side-up (in beds tilted at ~30°). Interestingly, the SMQ has been correlated by Southworth and Brezinski (2003) with the Weverton Formation of the Chilhowee Group, a rock unit exposed in the Blue Ridge. Just as the Weverton is overlain by the finer-grained Harpers Formation, so too is the SMQ overlain by a finer-grained unit, the Urbana Formation. Both are interpreted as metamorphosed continental margin deposits. The Urbana is mostly phyllite in the areas I’ve seen it (including phyllite that’s full of quartz grains, a first for me). The Urbana is well exposed in a creek-side outcrop north of Sugarloaf Mountain, and I took Chris there to show him the lovely intersection of bedding and cleavage.

Here is a weathered piece of the Urbana Formation that Chris collected there, looking at the plane of cleavage (ruler in background for scale):

urbana Image source: Christopher Khourey

You can see the bedding running ~horizontally across it, though the photo cannot convey the lovely phyllitic sheen that results from waggling these samples back and forth in good light. It’s pretty cool. In places, the transition from sandy to phyllitic is gradational, probably relict graded bedding.

So, what does it mean if Southworth and Brezinski (2003) are correct in their correlation, and the Weverton and the SMQ are really the same rock layer, but in different provinces and at different metamorphic grades? Recall that the Blue Ridge province to the west is also a thrust-faulted anticlinorium, launched up and to the west by the Alleghanian Orogeny from an original position deeper in the crust and further towards the east. It’s a shard of the craton, snapped off and shoved bodily up and to the northwest. (In class, I often liken it to Joe Theismann’s leg: a compound fracture of the continental crust.) Might the Sugarloaf Mountain Anticlinorium [SMA] be a smaller version of the Blue Ridge pulling the same trick? It too is arched up and snapped off …but it would be a “Mini-Me” that’s only just surfacing, like a baby whale swimming above momma whale’s back…

whales_analogy

We know that deeper down in the Blue Ridge stratigraphy, we find the Catoctin Formation, the Swift Run Formation, and the basement complex. If we drilled down through the crest of the SMA, would we find the same units (or more metamorphosed equivalents thereof)? It’s an intriguing thought…

Transect debrief 8: late brittle deformation

The final chapter in our Transect saga is now here. In some ways, it’s the least thrilling of the bunch. On the other hand, when I see a nice example of this structure, it makes me squeal like a little girl.

I refer, of course, to plumose structure, the small-scale architecture of a joint surface. We saw multiple great examples on the trip, but my favorites came with the first post-lunch stop on Transect Trip day #1, at an outcrop of the Weverton Formation showing a fine-grained deposit of siltstone.

I posted versions of both these photos previously via iPhone, but here I’ll give you the crisper Canon Elph version coupled with reposting of the iPhone shots for comparison purposes.

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tt_10

Plumose structure branches out in the joint propagation direction, the curvilinear “plumes” are thought to be perpendicular to the leading edge of the joint as it cracks through the rock. These late-stage brittle features may be related to the very latest part of Alleghanian deformation, or they may be related to recent uplift of these rocks.

All righty, then. I think that brings us up to the present day. Those of you who requested more details about the live-geoblogged photos, has this series answered your questions? If not, what do you need more details on?

Transect debrief 7: Brittle-ductile deformation

On the transect trip, I also saw some nice meso-scale “minor” structures that probably formed during Alleghanian deformation. Prominent among the ones that really impressed me were these en echelon tension gash arrays, deforming the Antietam Formation quartz sandstone and well exposed in blocks used to construct the wall along Skyline Drive and the Sandy Bottom Overlook in Shenandoah National Park:

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Good Lord! Ain’t those things beautiful? They also give us a lovely sense of the kinematics (relative motions) of the blocks of Antietam sandstone on either side of this sheared zone. In the case of the image above, the left side of the photo has moved “down” relative to the right side. The rock in between has torn and stretched, with the gashes opening up at right angles to the maximum stretching direction. As deformation proceeds, of course, the gashes rotate and deform, folding into “S” shapes.

Here’s one that’s more subtle:

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What you’re looking at in the image immediately above is a tension gash array that was a zone of weakness, exploited by later brittle deformation. The fracture which defines the edge of this block cracked through those old brittle-ductile tension gashes and split them clean in half.

Neat, eh? …Now check this out:

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Remember the Skolithos trace fossils? Here, you’re looking at a sideways cross section through some cylindrical Skolithos as they are disrupted by this zone of shearing. Note that the burrows tend to be highlighted by rust (hematite) staining: the brown lines that run roughly from the top left of the photo towards the bottom right. But look what happens to the orientation of those tubes where they are cut by the tension gash arrays: they are deflected into a new orientation, rotated from their original orientation!

If that’s a bunch of gobbledygook to you, consider this annotation:

tt_9_anno

I’ve drawn white lines to show the orientation of the Skolithos tubes in their undeformed and deformed states, colored the tension gashes yellow, and drawn on a set of blue arrows to show my kinematic interpretation (top to the left).

Here’s another block, showing the same phenomenon:

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Go ahead. Tell me you’re not impressed with that. I dare you. That is frakking AWESOME.

You are now dismissed.

Transect debrief 5: sedimentation continues

We just looked at the Chilhowee Group, a package of sediments that records the transition for the North American mid-Atlantic from Iapetan rifting through to passive margin sedimentation associated with the Sauk Sea transgression. Well, if we journey a bit further west, we see the sedimentary stack isn’t done telling its story. The saga continues through another two pulses of mountain building. Consider this “unfolded, unfaulted” east-west cross-section cartoon:

vr_stack

Part A of the image above shows the overall stratigraphic sequence for the Blue Ridge and the Valley & Ridge provinces in Virginia and West Virginia. You’ll notice that the small, detailed stratigraphic column I used to start the last two posts covers just the bottom 6 layers in this stack. Zoomed out to the bigger picture, we see ~40 layers overall. Lynn Fichter of James Madison University, one of the leaders of the Transect Trip, has published an excellent information-dense guide to the mid-Atlantic column. It’s a terrific reference for anyone looking to learn more about these rocks and the story they tell.

Part B of the image above shows the tectonic interpretation of these different packages of rock — some represent rifting, some represent passive margin sedimentation, some represent clastic influence from various orogenies occurring to the east (Taconian and Acadian).

The cartoon cross-section below, modified from an original by Steve Marshak in his excellent introductory textbook Earth: Portrait of a Planet, shows the tectonic evolution of the east coast over the past ~1 billion years of geologic time. It is reprinted here with Steve’s permission.

full_eastUS

The story begins with the Grenville Orogeny, an episode of mountain building that completes the assembly of the Rodinian supercontinent. This is followed by Iapetan rifting, followed by three pulses of Appalachian mountain-building: the Taconian (“Taconic“) Orogeny, the Acadian Orogeny, and the culminating event of Pangean supercontinental assembly, the Alleghanian (“Alleghenian”) Orogeny. Finally, Pangea breaks up in the Mesozoic, an event also known as Atlantic rifting. Two complete Wilson Cycles are preserved by the Appalachian mountain belt!

The Valley & Ridge province received sediment courtesy of the Taconian and Acadian Orogenies, but wasn’t directly involved with the tectonic collision in any deformational way. Notice how west of both those orogenies in the Marshak diagram you see a fresh layer of sediment being deposited atop the North American craton.

During the field trip, I posted some iPhone photos of the sedimentary strata that accumulated in the Valley & Ridge during the mid-Paleozoic, shed off from the orogenic activity to the east. For example, the Brallier Formation’s turbidites record a time when sea was west and mountains were east. Or the Juniata Formation’s red beds speak of a time in the late Ordovician when an advancing clastic wedge had piled sediment up above sea level. This shot of some of those red beds preserves some beautiful depositional relationships from ~440 million year old river systems.

Let’s annotate that, shall we?

juniata_anno

Even in the Ordovician, rivers did what they do today, spilling over their bansk and building up natural levees. Same as it ever was, people.

That “sediment only; no deformation” regime for the Valley & Ridge changed with the Alleghanian Orogeny. That’s when deformation propagated to the west, encompassing the flat-lying Valley & Ridge strata into a proper fold-&-thrust belt. Later, differential erosion of these folded and faulted layers would etch the landscape into a series of valleys and ridges… hence the province name. More on that deformation in the next post.

Transect debrief 4: transgression, passive margin

…So where were we? Ahh, yes: an orogeny, and then some rifting. What happened next to Virginia and West Virginia? Let’s consult the column…

shenandoah_column

After the rifting event opened up the Iapetus Ocean, seafloor spreading took place and tacked fresh oceanic crust onto the margin of the ancestral North American continent. As North America (“Laurentia”) moved away from other continental fragments (Congo craton, Amazonia craton), it got a little bit calmer ’round these parts. From the continent’s perspective, the spreading center moving further and further offshore.

This shift of the tectonic locus out to the middle of an ocean basin means that the edge of the ancestral North American continent could finally relax a bit. The magmatic intrusions became a distant memory, and the crust cooled, contracted a bit, and sank. This subsidence allowed seawater to lap up onto the edge of the continent, and with the seawater came sediments. Rivers draining the exposed North American continent brought sediments to the sea, and dumped them. We geologists call this “passive margin sedimentation,” and it results in relatively “mature” sediments: those that have been well-worked over, typically rich in quartz and well sorted and with more rounded component grains.

As time went by, the edge of the continent subsided more and more, and any given spot in the modern-day Blue Ridge transitioned from streams to beach to continental shelf. The sedimentary stack reflects this increasing distance from the shoreline: a transgressive sequence.

It starts at the bottom with Weverton Formation: conglomerates and sandstones (and as I discovered on the Transect Trip, siltstones too). Here’s a piece of the Weverton from a GSW trip several springs ago:

The Weverton is overlain by muddy deposits of the Harpers Formation, which can also include coarse sandy units, as I learned on the Transect Trip. Here’s a shot of the Harpers Formation at Harpers Ferry, West Virginia, the type locality. This was taken five years ago, back when I had just gotten out of grad school, and spent a year teaching at George Mason University (pre-NOVA). [The student pictured is Steve Elmore, who just earned his master's from GMU, working with Bob Hazen. Congratulations, Steve!]

steve_with_kink_bands_harpers_ferry

The Harpers is really important, because it contains some Olenellus trilobite fossils, which constrain its age to be Cambrian.

The Harpers is overlain by another sandstone: a clean, pure quartz package named the Antietam Formation. For me, the Antietam is a favorite local rock, because it is studded with Cambrian-aged Skolithos trace fossils. On the trip, I used the iPhone to upload a few photos of these, but here’s a higher-resolution image to savor:

You’re looking at the bedding plane of the Antietam in the above image, with your sight-line parallel to the paleo-vertical orientation of the tubes. Wow. Beyond all reason or deeper interest, I just love Skolithos tubes. I look at this outcrop, and I wonder: is this a palimpsest? or a small wormy Manhattan? In other words: was this multitude of burrows generated by a small population that dug in the same area over a long period of time, or by a huge population living cheek-to-jowl over a relatively brief moment?

Regardless, the sand-then-mud-then-sand-again picture painted by the succession of Weverton-Harpers-Antietam isn’t a “textbook” transgressive sequence, but it might make more sense if you consider the Antietam sands as barrier island deposits, with the Harpers being deposited in a Pamlico-Sound-type setting.

Finally, the transgression is complete when we get to the top of the Blue Ridge sequence and see the Tomstown Formation, a carbonate unit:

IMAGE CREDIT: USGS

The Tomstown tells of a time when sea level had gotten so high locally that the shoreline was way, way, way far away. There were no clastic sediments making it out to this location, and all that was available to precipitate were the ions dissolved in the seawater. No sand, no pebbles, no mud: only carbonate.

The sequence of sedimentary strata continues, but to follow its succession upwards, you’ll have to travel across the Blue Ridge Thrust Fault to the west, into the Valley & Ridge province. More on that in the next post. For the moment, let me share a cartoon sequence of images by Tom Gathright (1976), showing the overall stratigraphic evolution of the Blue Ridge province*:

*Note that Gathright used the outmoded names “Hampton” instead of Harpers, and “Erwin” instead of Antietam. Please forgive him and move on.

That last panel, showing Alleghanian deformation, is something we will attack in a future post. For now, I’m satisfied to have finally climbed to the top of the Blue Ridge stratigraphic stack.

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Gathright, Thomas M., 1976. Geology of the Shenandoah National Park, Virginia. Virginia Division of Mineral Resources Bulletin 86, Charlottesville, VA. [buy it from SNPA Bookstore]

Transect debrief 3: Rodinian rifting

The Grenville Orogeny, responsible for Virginia’s basement complex, was one mountain-building event among many that helped put together a Mesoproterozoic supercontinent called Rodinia. But Rodinia didn’t last: it broke apart during the Neoproterozoic to form the Iapetus Ocean basin. This rifting event is recorded in Virginia’s Blue Ridge province in the Swift Run Formation and the Catoctin lava flows.

It’s probably about time to start putting some of these rock units in stratigraphic context. Here’s my redrawing (and updating) of a cartoon Shenandoah National Park stratigraphic column based on an original by Tom Gathright (1976):
shenandoah_column

You’ll notice here that the Swift Run Formation is interbedded with the Catoctin Formation, a Neoproterozoic series of lava flows fed by fissure eruptions (kind of like what’s happening this week in Iceland).

Trickling downhill away from these fissure eruptions would have been flows of basaltic lava (tholeitic, indicating a mantle source chemistry). If you want a warmer modern analogue than Iceland, look to the Afar Triangle region of Ethiopia:

As with Neoproterozoic Virginia, the continental crust of modern Ethiopia is stretching, opening up topographic grabens which are being filled with clastic influx from the surrounding highlands and mafic lava which is formed from decompression melting in the underlying mantle, and funneled to the surface via feeder dikes. In places you will see streambed conglomerates interlayered with the mafic lava flows, and in places there are tuffs and rhyolites that are a (volumetrically-small) part of the package. Elsewhere there are lake sediments. The two bear a common geologic signature, despite being separated by thousands of miles and half a billion years. There’s that refrain again: Same as it ever was, same as it ever was.

Once on the surface, the lava cooled, and in some places, columnar jointing formed:

The cooling age on some of the rhyolitic upper units in the Catoctin Formation is 570-565 Ma (Rb/Sr on pyroxene by Badger and Sinha, 1988). Some mafic and felsic dikes (could be feeders) associated with the unit yield the same age via U/Pb (Aleinikoff and others, 1995).

At some point, ancestral North America (“Laurentia”) drifted away from the spreading center, and volcanism ceased. The crust cooled, subsided, and then a sequence of sedimentary rocks began to accumulate atop the cooled lava flows. This transgressive sequence of sediments (the Chilhowee Group) is the next thing up in the stack. More on that later.

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