Wednesday, December 9, 2009: 1437th Meeting


Refreshments start at 7:30 p.m. The formal program starts at 8:00 p.m.
Meetings are held at the John Wesley Powell Auditorium
2170 Florida Avenue NW, Washington, D.C. (directions)

Labels: appalachians, dc, gsw, meetings, plate tectonics

That's a slab of the Shawangunk Formation conglomerate, from eastern Pennsylvania. I collected it a couple of years ago when I drove up to go fossil hunting at the Whaleback, but it wasn't until last year that I slabbed and polished it. (The slab measures 10 cm wide by 27 cm in length.) Then a couple of months to get around to scanning it, and finally a few months more before posting it. Sheesh.

It's a lovely quartz-rich clast-supported conglomerate, a ridge former in the Valley & Ridge province of the Appalachians. Like the Massanutten Formation, it's Silurian in age, and thought to be part of the "molasse" sequence shed off the Taconian mountain belt, first raised during the late Ordovician. It is interpreted as a relatively-high-energy fluvial system deposit; sediments laid down by rivers as the mountains next door were weathered and eroded.

Labels: appalachians, pennsylvania, quartz, sediment, silurian, valley and ridge, weathering

First off, I'd like to say a big "Thank you!" to everyone who joined in the basins discussion yesterday after my post comparing depositional basins and structural basins. I haven't had a post generate that level of chewy discussion in a while, and it pleases me to see folks chiming in.

So here's some additional thoughts: yes, structural basins are big synforms wherein the bedding dips in all directions towards the center of the structure. They are the opposite of structural domes. It seemed that this was a sticking point with several readers, who weren't familiar with "structural basin" used in this way. Chris indicated that the term "structural basin" isn't part of structural geology vocabulary in the U.K., and in many ways I agree with him when he says, "calling a structure which was never a site of sediment deposition a 'basin' seems rather silly to me." But that is what our textbooks and lab manuals refer to them as... That's why students get confused, and that was my motivation to draw the graphic delineating the differences. (I didn't invent this term! Ed appears to back me up on this.)

Suvrat called attention to the erosion that I included as part of my structural basin "model," and while that's not necessary for a structural basin to be called a structural basin, I included it to show that there was no basin-like topography necessarily involved. And that word, topography, is likely critical to the discussion. Shame on me for not mentioning it yesterday. (Ed mentioned that's how he distinguishes the two.) Here's the way structural domes and basins are expressed in the second edition of Steve Marshak's textbook Earth: Portrait of a Planet (reproduced here with his permission):


In the uppermost part of the image, you have both topographic and stuctural domes and basins. In the central part of the image, you see erosion-gutted (and differentially eroded) structural domes and basins that are not topographically basinal or domal. Brian asked an excellent question after yesterday's post, which was "where's a good example of a structural basin?" I didn't know of any great ones offhand, so I Googled it, and as it turns out, Wikipedia has a list on their page about "structural basins." (Tragically, the fourth hit on that same search turned up yesterday's blog post! I hate it when that happens.)

And this brings us to the most interesting part of the discussions: Lockwood was the first to say it: "Basins can be both, can't they? i.e., a structural basin can become a locus of deposition." Ah, yes! As my friend John Weidner likes to say about simple geological explanations, "Actually, it's more complicated than that." Are there depositional and structural basins? "Yes...."

"...but actually, it's more complicated than that."

The reality is that many basins are both structural and depositional. I hinted at this yesterday, when I said "[Depositional basins] can also self-perpetuate, as the heavy sediment keeps the crust sagging downward at that location." But I didn't launch into a full-blown discussion then because I was mainly interested in generating crisp thinking in my students: understanding that the term "basin" gets used (at least in our textbooks) to mean two different things, which have similar patterns but independent means of generation. Yes, the reality is that crustal sagging creating a lowspot is itself a structural phenomenon, which then has sediment accumulate atop it, which can encourage through its weight additional sagging, and additional sediment accumulation, and so on. Howard pointed this out in yesterday's comments. The layers at the bottom of such a "hybrid basin" will be structurally deformed at the same time sediment is being deposited at the top of the stack in the resulting topographic low.

So, really, what I outlined yesterday are end-members of a spectrum:


Reality has shades of gray! Yesterday's post was about the "black and white." Today, we discuss the spectrum in between.

How can we tell them apart? The classic test of whether a basin represents a sag in the crust and a hence a paleo-crustal downward flexure is to look at the thickness of the sedimentary layers. If they thin towards the edge and thicken towards the middle, then you've likely got some topographical low, and hence elements of a depositional basin. In contrast, a purely structural downwarp in the strata will not necessarily show any such changes in bedding thickness across the structural basin; so you'll see uniform thickness across (so much as such a thing exists):



Many basins have aspects of both of these -- sometimes they look structural further down and depositional higher up. The lower half of the Marshak illustration above is a map that shows the various basins and domes of the Midwest U.S. (Sometimes the domes are called 'arches' in they're more elliptical in outcrop than circular.) So are these regional-scale basins depositional or structural? Or both? Both, pretty much. These basins do show bedding thickness changes over time, and as I understand it, those times of increasing crustal flexure have been tied to the various episodes of Paleozoic mountain-building on the east coast. The Cincinnati Arch, for example, appears to have developed by the Devonian, since the layers older than the Devonian appear to be uniform in thickness across Ohio, but the Devonian sequence is thinner atop the arch and thickens to the southeast. (I'm no expert on Midwest geology; if someone cares to clarify and/or enlighten, please do!)

Eric made another excellent point: that sometimes we refer to the volume of sedimentary rock that was deposited in a depositional basin as a sedimentary basin. Hence the volume of sedimentary rock comprising the tortured strata of the Valley & Ridge province is sometimes referred to as the Appalachian Basin: not because it's either a depositional or structural basin today, but because it was a depositional basin in the past, before it got folded and faulted. Interestingly, the Marshak map also shows a non-folded, non-faulted Appalachian Basin northwest of the Valley & Ridge province. Hmm. You mean there's one term that geologists apply to two different things?

"No! Say it ain't so!"

Howard asked about the basins of the Basin & Range province. In my parlance, those would be strictly depositional basins -- structurally controlled, yes, but by brittle faults rather than crustal downwarping. They are sites of sedimentary accumulation, but do not show any kind of synformal structure. Thus, they don't qualify as "structural basins." Tricky business! ...Yes, they're basins; yes, they're structurally controlled. But they don't meet the definition for "structural basin."

And lastly, both Eric and Howard noted that there's yet another kind of basin: a drainage basin, a topographical feature through which runoff is collected, essentially synonymous with "watershed." To summarize the difference between a drainage basin and a depositional basin, consider this: a topographical basin which is primarily the site of erosion would be a drainage basin. A topographical basin which is primarily the site of deposition would be a depositional basin. Can a single topographical basin host both erosion and deposition? Definitely! Consider the Mississippi River drainage: eroding in the high country headwaters, depositing in the lowlands nearer the mouth of the river.

Thanks again for all the thoughtful comments, folks.

Labels: appalachian plateaus, appalachians, devonian, sediment, structure, valley and ridge, words

I've been meaning to go check out the Soapstone Valley for years, but finally got around to it on Memorial Day. The park is a valley that shoots off to the east from Rock Creek Park, with an eastern terminus at Connecticut Avenue:



I didn't have far to walk before I found my first cobble of soapstone. It felt soapy in my hand, and was easily scratched by my fingernail. (Fingernail = 2.5 on the Mohs scale of hardness; talc = 1) I found it interesting that the soapstone cobbles had less algae growing on them than the other cobbles in the stream... Hmm. Because they slough off their outer layers more easily? Or because there's something chemical going on that prevents algae growth?


Why does anyone care about soapstone? Well, people who care about prehistory are interested in soapstone because it was easily carved to make various artifacts. As a geologist, I'm more interested in it because it's a metamorphic rock that implies an ultramafic protolith. In other words, as the various rocks that would become DC's bedrock were squished and squeezed and heated during Taconian mountain-building, one of the ingredients in the mix may have been a peridotite. As the graywacke around it metamorphosed to metagraywacke, the putative peridotite metamorphosed into soapstone.

The stuff I found in Soapstone Valley is a talc schist with porphyroblasts or relict phenocrysts of something dark and chunky in it:


Here's a close-up. The big crystals were dark green, like augite, but they had a texture that looked more like hornblende. Not sure as to their identity. I'll put one under the microscope later to try and suss out the relationship between the cleavage planes.


They're definitely mafic though! Here's an example where the large crystals are rusted out:


So there was plenty of soapstone float, but no bedrock outcrops. At first, I was in the highly foliated metagraywacke schist of the Rock Creek Shear Zone...


...but as I headed upstream I found boulders of the Kensington Tonalite, implying exposures of the KT further up the valley...


... and sure enough, that's what I found. This is the Kensington Tonalite, a late Ordovician granitoid.


Where I first crossed the contact, I thought it looked a little odd, and then a later look at the geologic map of the Washington West quadrangle (Fleming, et al., 1995):

Fleming, et al., list it as a sheared biotite tonalite of the Georgetown Intrusive Suite, which I guess explains its appearance as distinct from the Kensington Tonalite.

When I got up to the eastern edge of the park, I saw the source of the stream:


The valley widens out here, almost as if the rock is weaker... And where concrete has been poured (to stabilize the slope??) the underlying rock is etched away: it's the super-soft soapstone...


Here's a boulder of soapstone (my fingernail scratches it to demonstrate that it's soft):


Here's the geologic map of the area. You can see Soapstone Valley cutting an east-west swath across the strike of the structures. ("ss" means "soapstone"...)

My annotations on Tony Fleming's map (reference below).

Reference:
Geologic map of the Washington west quadrangle, District of Columbia, Montgomery and Prince Georges Counties, Maryland, and Arlington and Fairfax Counties, Virginia. Anthony H. Fleming, Lucy McCartan, and Avery Ala Drake. U.S. Geological Survey (Reston, VA), 1995.
_________________________________________________________________

A quick tangent to note a milestone: this is my 700^th post on NOVA Geoblog. Thanks to everyone for reading. Looking forward to 700 more...

Labels: appalachians, dc, granite, igneous, metamorphism, minerals, ordovician, piedmont

Done with the Cockeysville Marble and fortified with chocolate malts from the Twin Kiss, we ventured on to "Mineral Hill," interpreted as a paleo-black-smoker site from the deep Iapetus. This is a zone of mafic and ultramafic rocks that have been metamorphosed and also mineralized with a suite of sulfide minerals, including pyrite, chalcopyrite, bornite, covellite, and carrollite (in fact, this is the type locality for carrollite). Presumably it was a SedEx-type deposit in the Iapetus Ocean basin. It is geographically associated with the Baltimore Mafic Complex, which is most readily interpreted as a dismembered slice of the Iapetus oceanic lithosphere (that is, an ophiolite). As the Iapetus closed during the Taconian Orogeny, it was accreted to North America and metamorphosed.

The petrology students start picking up pieces from the massive pile of tailings in search of treasures:


Talc shist (soapstone) with malachite:


More of the same:


I forget what this one was, but I loved the "spray" pattern of its bladed crystals:


Chrysotile asbestos:


Pyrite:




And lots and lots of magnetite! These are some of my refrigerator magnets stuck to it:


One more stop to go: the Ellicott City Granodiorite...

Labels: appalachians, maryland, metamorphism, minerals, ophiolites, ordovician, ore

I just found out about the Appalachian Tectonics Study Group. They run a fun-looking weekender field trip each spring on topics of current research in Appalachian tectonics. I was not able to attend this year's event in the central Blue Ridge (due to the UMD petrology trip), but maybe I'll get to next year's event.

Labels: appalachians, field trips, mountains, plate tectonics

Yesterday, I showed you the Port Deposit Tonalite, stop #1 on the University of Maryland's annual ig/met pet trip. Today I'll share pictures of the next stop. We voyaged to the Hunt Valley Shopping Mall, where a lovely exposure of the Setters Schist can be found.

It's a lovely example of a classic-looking muscovite schist:


It is also chock-full of garnets! Millions and millions of them....

Some are small:


Some are medium:


Some are large:


Some are fresh:


Some are weathered:


Some are weathered-out:


There's also staurolite present:




Here's a nice big chunky staurolite:


In one localized zone, we also see some very big, rather lovely kyanite:




...Awesome! I love this suite of metamorphic minerals!

The Setters Schist is a highly metamorphosed pelitic rock (meaning that its protolith was clay-rich). It was presumably metamorphosed in the late-Ordovician-aged Taconian Orogeny, like everything else in the Mid-Atlantic Piedmont.

Next up, another member of the Glenarm Series, the Cockeysville Marble...

Labels: appalachians, maryland, metamorphism, minerals, ordovician, piedmont

Yesterday, I was fortunate enough to be able to tag along on the University of Maryland's petrology field trip, to five locations in Maryland showcasing a variety of igneous and metamorphic rocks. I'd like to thank Rich Walker and Roberta Rudnick for allowing me to come along on the excursion, and UMD graduate student Ryan Kerrigan for alerting me to the trip's interesting rocks in the first place. They have a crew of enthusiastic students, and some cool outcrops!

Our first stop was in northern Maryland's Cecil County. Along the banks of the Susquehanna River, just upstream from the I-95 bridge, is an abandoned quarry of the Port Deposit Tonalite.

Here's Rich and Roberta leading us into the quarry:


UMD students examine the semi-overgrown outcrops of the tonalite:


Tonalites are kind of like granites, except they have only very low amounts of potassium feldspar. This particular tonalite has a magmatic crystallization age of 515 Ma (U/Pb in zircon) and a metamorphic age of 490-480 Ma (Rb/Sr in biotite). Close-up of the rock's texture:


ADDITION: Kim notes in the comments that I didn't draw an explicit connection between the metamorphism and the metamorphic foliation that is so prominent in this photo. She's right: The wavy linear pattern you see in this photo is produced by minerals aligned by differential pressure. Squeeze the rock "top to bottom" and you produce a foliation that runs "left to right."

On the basis of isotopic evidence, the Port Deposit Tonalite is interpreted to have formed as an igneous pluton offshore of ancestral North America, underneath an island arc in the Iapetus Ocean. Later, subduction brought the island arc into contact with North America, triggering the Taconian phase of Appalachian mountain-building.

Here's a closer look at the texture and mineralogy. You can see some k-spar present here, though this was not a common mineral to see at the outcrop...


There were some nice xenoliths present, indicative of the host rock into which the PDT intruded:


Here's a quartz vein cutting through the tonalite. You'll notice that the vein is emplaced approximately perpendicular to foliation, suggesting the same maximum stress which imparted the foliation also extended the rock parallel to the foliation place, opening up fractures that when then fill with the most mobilizable minerals available (in this case, quartz):


If you look closely, you'll see that the fracture which opened up in the tonalite to allow this vein to be emplaced has a ragged edge (not a clean break):


Next up: the Setters Schist...

Labels: appalachians, cambrian, granite, igneous, maryland, metamorphism, ordovician, structure, teaching, xenoliths

After the Garth Run high-strain zone and a night hanging out by the campfire at Heavenly Acres with the William and Mary Structural Geology class, the second stop on our Structural Geology trip was in Shenandoah National Park, looking at the deformed meta-basalt columns on the Limberlost Trail. Longtime readers of the blog have seen these unique (in my experience) columns before, in a post from last May.

This is an outcrop of the Catoctin Formation, a series of (mainly) basaltic lava flows that erupted sometime older than 565 Ma (only the youngest, rhyolitic layers have been dated, and evidence suggested that significant amounts of time may have passed between the eruption of each stratum of basalt deeper down in the stratigraphic stack). As the lava cooled, it developed cooling fractures that formed perpendicular to the isotherms. These fractures likely initiated at the top and the bottom of the flow, and propagated towards the middle over time.

Later, during Alleghenian mountain-building (~300 Ma to ~250 Ma, roughly), the rocks were subjected to greenschist-facies metamorphism, and were deformed. The basalt's consituent minerals re-equilibrated and reacted to become other minerals, most notably chlorite and epidote (both of which are green).

Here's John and Joe checking out the columns:


Exquisite! Even arrest lines on the side of each column are preserved. In an undeformed basalt column, these arrest lines would be perpendicular to the column edge. Here, they have a pronounced angular relationship, indicating the shearing of the overall column:


Bobby measures the angular shear along the length of the column:




Goofball professor poses with column:


Jay plays the column like an electric guitar:


We found some nice plumose structure too:


Finally, we evaluated the concentric rings of minerals filling amygdules (vesicles that had been infilled with mineral deposits after lithification) in an attempt to determine whether they could be used as strain markers, or whether they may have attained their ellipsoidal shapes due to stretching of the bubbles in the originial lava (i.e. like this) and then been infilled with minerals:




...and then we were off to Field Study Area #3...

Labels: appalachians, basalt, field trips, igneous, metamorphism, national parks, primary structures, proterozoic, shenandoah, structure, teaching

A little follow-up to this morning's post of pictures from the Garth Run high-strain zone. A short distance away is the Quaker Run high-strain zone, which I visited in spring of 2003 with my geology graduate advisor, Dazhi Jiang, and two other structure students from the University of Maryland, College Park. Here's a beautiful sample of the mylonite I collected then:

Labels: appalachians, blue ridge, metamorphism, structure

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering

Last weekend, I took a group of students, mostly from NOVA but also 3 from GMU, up to hike Old Rag Mountain in Shenandoah National Park.

Here's a Google Map showing the terrain (and trails, which is a cool new addition to the already cool Google Maps):


The crew discusses debris flow deposits in the forest on the way up the mountain:

photograph by Charlie Corrick

The first spot where we get a nice view out over the valleys below:

photograph by Charlie Corrick

Spheroidal weathering in Catoctin Formation greenstone:

photograph by Jared Fortner

Spheroidal weathering in granite (the Old Rag Granite, 1.0 Ga):

photograph by Charlie Corrick


photograph by Charlie Corrick

Student Jared atop a spheroidally-weathered boulder of the Old Rag Granite:

photograph by me

Grain-size differences in the Old Rag Granite (balanced atop my leg):

photograph by me

Non-foliated Old Rag Granite (showing lovely "blue quartz"):

photograph by me

And the foliated version of the Old Rag Granite:

photograph by me

Labels: appalachians, basalt, blue ridge, field trips, geology, granite, igneous, mountains, national parks, nova, structure, weathering

Yesterday, four Honors students and I went out to West Virginia's route 55 (between Wardensville and Moorefield), to look at some sedimentary strata and associated tectonic structures. Our guide was my friend David Dantzler, an enthusiastic amateur geologist. Here's a map of the terrain we traversed:



As you can see, this is part of the Valley & Ridge province, an area of the country defined by Paleozoic rocks that were folded and thrust-faulted during the Alleghenian phase of Appalachian mountain-building. Recently, a new road has been constructed traversing these valleys and ridges. It's a bit of a boondoggle, a pet project of West Virginia senator Robert Byrd which funneled federal dollars into the Mountain State, ostensibly to make it easier for the chicken farmers of Moorefield to get their birdie bits to market on the east coast.

This image ought to give you a sense of the project's scale (big bridge), and how much use it gets (no one on the bridge):


But the U.S. taxpayer's loss is the geologist's gain... There are some pretty spectacular new exposures of Valley & Ridge rocks along the new route 55. Here's the NOVA van parked at an outcrop of Tuscarora Sandstone that is arched up into a broad anticline. Again, notice how few people are driving on route 55 here:


Ooh, look: heavy traffic!


Contact between the lower Tuscarora Sandstone (a Silurian-aged extremely pure quartz sandstone, variably fused to quartzite), and the overlying (darker-colored) formation, which is either the Rose Hill Formation or the Mackenzie Formation at this location:


We found oodles of cool trace fossils:







But it wasn't just sedimentary layers. There were also some cool tectonic structures, like this joint in the Tuscarora, showing a beautifully developed hackle fringe:



Here's some "pencil cleavage" where fine-grained shale develops cleavage that intersects the planes of fissility, causing it to fracture in long slivers:



I slammed on the brakes for this one: an awesome anticline...


I forced David and the students to act out the orientation of the bedding planes at this anticline:


Honors student Jason points out a small thrust fault in the outcrop above him: You can see the offset in a greenish/gray shale layer:


In case it wasn't obvious above, here's a zoomed-in shot, with the offset layer highlighted (the miracles of Photoshop!) and the fault labeled:


We all had a grand day outside, and the rain held off until our return trip, which was pretty great. Thanks to David for showing us these rocks, and thanks to my students for being smart and inquisitive and into field trips.

Labels: appalachians, devonian, field trips, fossils, mountains, nova, politics, primary structures, sediment, silurian, structure, teaching, valley and ridge, west virginia

Yesterday I mentioned finding a new (to me) outcrop of the Martinsburg Formation's graded beds (turbidite sequences shed off the late-Ordovician Taconian Orogeny here on the east coast of North America). Today, I'd like to share a few images of where John Graves and I went next: up into the heart of the Massanutten Synclinorium, the Fort Valley. To remind you of the relationship between the Shenandoah and Fort Valleys, here's a Google Map I've posted before:



There, defining the ridges of Massanutten Mountain (and thereby separating the lower Shenandoah Valley from the upper Fort Valley) is the Massanutten Sandstone, a Silurian-aged quartz sandstone (in some places it's a quartz-pebble conglomerate) that is correlated to the Tuscarora Sandstone further west in the Appalachian Mountains' Valley & Ridge province.

The Massanutten can show some nice primary structures, including some of the oldest known terrestrial plant fossils (preserved as fragmentary carbon films) and cross-bedding like this:



With regard to the cross-bedding, note that this is "reverse" cross-bedding, which records shifts in current direction over time. At the bottom of the sample, the current was flowing from left to right, and at the middle and top of the sample, it was flowing in the opposite direction, right to left. This sample shows well the distinctive shape of cross-beds: they are tangential to the main bed at the bottom, but are often truncated on top, making them superb geopetal indicators. (They tell you whether your rock is right-side-up or up-side-down.)

I took John on a hike up the Veatch Gap trail, because I wanted to show him the awesome anticline in the Massanutten Sandstone that NOVA adjunct geology instructor Chris Khourey and I had found on a reconnaissance trip out there in May of last year. John and I took a "group shot" with the fold:



And here's John showing those Montanans that we do actually have some cool geology out on the east coast:



So, what's going on here? Well... the Valley & Ridge province of the mid-Atlantic region is defined by folded (and thrust-faulted) sedimentary strata. These folds were produced about 300 to 250 million years ago, during the Alleghenian phase of Appalachian mountain-building. The tectonic cause of this deformation is interpreted to be North America's collision with Africa, closing the Iapetus Ocean and completing the assembly of the supercontinent Pangea.

More locally, the Shenandoah Valley and Massanutten Mountain are structurally underlain by a great fold, the Massanutten Synclinorium. Synclinoria are different from mere synclines because they are more complicated: the overall synclinal shape is "decorated" with numerous smaller anticlines and synclines. It's a big trough-like shape, but wrinkles are "parasitic" on the main fold. So, even within the big "canoe" shape of the Massanutten Synclinorium, there are little bulges and wrinkles that go the opposite direction. This anticline is one of them.

At that point, having seen the anticline, we weighed whether to keep hiking or not.

We opted to press on... and I'm so glad we did. ... Twenty feet further down the trail, we saw another two anticlines!



At its base, this one had a small cave I could crawl into:



And: a short distance further we found a hiker's shelter with an apt name:



Ha! I love it.

More tomorrow, when I'll revisit the issue of plumose structure and hackle fringes.

Labels: appalachians, geology, mountains, msse, ordovician, primary structures, sediment, silurian, structure, valley and ridge, virginia

Yesterday, I mentioned what my MSSE advisor John Graves and I saw along the Billy Goat Trail on Saturday afternoon. Today, I'd like to share some images and insights from our Sunday field trip, out to the Shenandoah Valley and the Massanutten Synclinorium which underlies it.

I would like to thank Rick Diecchio of George Mason University for sharing some key outcrop knowledge with me. I've found that information about good outcrops can be very difficult to obtain unless you know somebody who knows. The information is primarily passed on through the oral tradition, rather than written in sufficient detail in peer-reviewed literature or in field guides (...or posted on geoblogs?).

Anyhow, back in December, on our drive down to the Blue Ridge / Valley & Ridge Symposium in Charlottesville, I told Rick I was organizing a new Massanutten Synclinorium field course. It's a place he's very familar with. He recommended a good outcrop to see the turbidite sequences of the Martinsburg Formation, a late Ordovician clastic unit made of debris shed off the rising Taconian Mountains to the east. Rick drew me a map in my field notebook, and on Sunday I was finally able to schedule a visit. Since John is unfamiliar with the stratigraphy and structure of the Shenandoah Valley (or the east coast in general), we also stopped at a lot of the other stops I'll be taking students to, including the classic "Tumbling Run" section.

Today I'd like to share a sets of photos with you from this new (to me) outcrop of the Martinsburg Formation. Tomorrow I will share another set from the next layer up in the stratigraphic stack, the Massanutten Sandstone. Both outcrops a pleasing combination of sedimentary stratification and structural geology.

Here's the Martinsburg Formation outcrop, just west of the Shenandoah River's North Fork:


This, like the "Pet Store Anticline" that I have previously blogged about, is an excellent place to look at bedding/cleavage relationships. The beds are dipping east, but the cleavage dips steeply to the west, implying the outcrop's position within a much larger (kilometers-wide) cleavage fan.

Here's a eye-catching outcrop that shows the beds weathered out differentially, while pervasively cut by ~vertical metamorphic cleavage:


More beds, of alternating sand and mud, steeply dipping in the Massanutten Synclinorium:

Note how the muddier portions show cleavage development better than the sandier strata.

More pervasively-cleaved muddy layers:


Here's one that confused me. In this predominantly-sandstone layer, you can see that the cleavage is better developed on the right, lower side of the bed. Does this mean that the right, lower-side of the bed is more mud-rich? (and sand-poor?) It did appear to be finer grained. If so, does this imply this bed is upside-down? Ordinarily, I would have thought to only look for the primary sedimentary structure as a geopetal (right-side-up) indicator, but this is the first time it has occurred to me that structural susceptibility based on mineralogy (in this case, susceptibility to cleavage development) could be used as an indicator of younging direction. I should note that this particular photo was taken downhill of the main outcrop, and may well be overturned. It's a synclinorium, after all, not a smooth syncline!


In this photo, the turbidite sequences of the Martinsburg Formation show a cool feature, a primary sedimentary structure known as cross-bedding:

Note that this photo is taken with the photo's long axis ~parallel to bedding, but the reality of the outcrop is that this is all steeply dipping, rotated 90 degrees clockwise (see the inset for "true" outcrop orientation).

...But wait! There's stuff dipping to the left, and stuff dipping to the right! Which one is this purported cross-bedding? Try this labelled version to sort it all out:

Note how at the bottom, the cross-beds curve tangentially to subparallelism with the main bed. They are truncated at top by the overlying layers. This is a good geopetal indicator, and the photo is oriented in depositional position, with the top at the top. Furthermore, if you reconstruct the current direction from these cross-beds (after the strata have been "unfolded" and restored to their original horizontal orientation, it would have come from the east... that is, from the orogen itself (the roots of which are exposed along the Billy Goat Trail.)

The intersection of rock weaknesses along the planes of bedding and planes of cleavage can result in the rock fracturing into long pencil-like bits, a phenomenon known as "pencil cleavage." This is my Freddy Krueger impersonation using the Martinsburg's cleaved "pencils."


John puts his hand up to give a sense of scale to the axis of this small fold in the steeply-dipping strata:


I was all agog over this outcrop, really digging the relationship between the structure and sedimentological elements in the rock. Best of all, it's a very short drive from Tumbling Run, and will replace the hike to the Buzzard Rock outcrop in my Massanutten field trip in April. (For NOVA-area readers, there are still four spaces open in that class...)

Labels: appalachians, mountains, msse, ordovician, primary structures, sediment, structure, valley and ridge

The Virginia Community College System (VCCS) organizes conferences occasionally where faculty in different disciplines can get together. This weekend was the "peer conference" for the natural and physical sciences. It was held at the lovely mountain resort called Wintergreen, in central Virginia's Blue Ridge Mountains.

Here's a map of the area:

That's the Shenandoah Valley on the left (part of the Valley & Ridge province), the Blue Ridge in the middle (running from NE to SW), and the Piedmont province on the far right. Wintergreen is a bit SW of Charlottesville.

The conference was fruitful and interesting. I enjoyed getting to meet a bunch of the other VCCS geology faculty and discussing what we want to do in the future in terms of supporting one another and professional development. I gave a talk about new technologies in geology instruction, which included information about the geoblogosphere and other sundry web resources I use. My colleague Erik Burtis at NOVA-Woodbridge led us on a cool "field trip" to Glacial Lake Missoula, via Google Earth.

I spent a lot of time talking with Pete Berquist, from Thomas Nelson Community College, discussing next summer's Regional Field Geology of the Northern Rocky Mountains course. We laid out a series of goals for the students, and created a tentative itinerary. Pete and I took a great hike at the end of the first day, poking around in the rocks and watching the sun set over those gorgeous mountains. Friday evening, there was a cool astronomy session, where Ed Murphy from UVA showed us the Ring Nebula, the Andromeda Galaxy, and assorted other stuff in outer space. He had a great laser pointer that extended a green laser line up about 80 feet into the sky... Very useful for pointing things out. Low light levels in the forested mountains meant excellent stargazing. Saturday morning, Bill Warren of Lord Fairfax Community College gave a good talk about the global energy crisis, and potential solutions. I picked up a few good resources there that I'll use next semester in teaching Environmental Geology. And then when the conference concluded, there was a geology "hike" out to look over the landscape. By driving us to a couple of different overlooks, Doug Coleman of the Wintergreen Nature Foundation showed us spots where we were able to look east into the Piedmont, and west into the Valley & Ridge. Pretty cool, though we didn't look too closely at the actual rocks exposed there. Fortunately, I have an inclination to do that on my own... as you'll see below:

Catoctin Formation greenstone (meta-basalt), showing chlorite-rich portions (left) and epidote-rich portions (right). Quarter for scale.


More Catoctin, the volcanic breccia layer. Lots o' epidote. Quarter for scale.


Is this a quartz vein or a granite dike?
At first glance, it appears to be your standard hydrothermal quartz vein full of milky quartz, but then you'll notice that it's not just quartz. There are also two crystals of orthoclase feldspar in there. (The dark shapes are just empty holes & shadow, not mafic minerals.) I pointed this phenomenon out before, but I'll state it again: I think that hydrothermal quartz veins and granite dikes are not separate phenomena, but points along a spectrum of composition. Quarter for scale.

Looking southeast towards the Piedmont:


Looking northwest towards the Valley & Ridge:

Labels: appalachians, blue ridge, conferences, igneous, metamorphism, piedmont, valley and ridge

Labels: appalachians, climate change, global warming, new york, rain, sediment, snow

Labels: appalachians, geology, piedmont, rivers

Here's a quick recap of my cross-country journey, for those who are interested in such things.

I left Bozeman on Saturday morning, July 26, and drove east on the Interstate to Billings, then diverged southeast towards Little Bighorn. There, I verified a comment from a Lakota friend at MSU that with my new bushy mustache (see change in icon above), I look a wee big like George Armstrong Custer (Custer & his men were killed by Lakota and/or Cheyenne warriors). After a short picnic there, I kept driving across southeast Montana, and into northeast Wyoming. My goal for the night was Devils Tower, where I have positive memories from my "North by Northwesty" roadtrip two years ago. I got to Devils Tower in mid-afternoon, just in time for a wicked-looking thunderstorm to roll in. Pendulous looking mammatus clouds were hanging down, and the skies turned a darker grey than Lola. Rain and wind came through, and a big dead branch from one of the cottonwoods in the campground came crashing down, but not on anyone's car or tent. When the skies cleared up, I drove up to the visitor's center and took a walk around the tower. It's awesome: massive columns, some of them twenty feet across. The rock is a porphyritic phonolite, and it's quite pretty to look at: big feldspars (5mm) set in a fine-grained grey matrix. Lovely.

The next morning (Sunday), I headed for Red Bird, Wyoming (along Wyoming's eastern border), where Cruisin' the Fossil Freeway suggested there would be oodles of ammonites in concretions in the Pierre Shale, some a foot across. When I visited the Denver Museum of Nature and Science earlier this summer, Kirk Johnson reiterated to me that Red Bird was the place to go for ammonites. But once I got to where Red Bird should be (according to my road atlas), there were no highway signs indicating that the town existed. Worse, there were no outcrops, and no sign of public land. (And one thing that an amateur fossil collector does not want to do in Wyoming is trespass on a rancher's land.) So, no Red Bird ammonites for me. Oh well, no worries: I had collected ammonites from a tongue of the Pierre Shale (the Bearpaw Shale) earlier in the summer on BLM land near Glendive, Montana, and scored some good specimens there. I cruised south, stopping at the Sierra Trading Post outlet in Cheyenne, Wyoming, and dropping some cash on some new duds (STP is mainly a catalog business, famous ten years ago for their amazing deals, but the company seems to be shifting to more mainstream business nowadays, including multiple brick-and-mortar locations). Then another hour on the road brought me to Fort Collins, to the house of Larry Wiseman, where I stayed earlier in my trip. He and I got some pizza and 90-Shilling Ale (Odell's) and traded tales about our summers.


The next morning, we had coffee on Larry's front porch and watch the sun rise. I packed up and hit the road, heading for Kansas. In my rear-view mirror, the Rockies shrank and vanished from sight, a melancholy fade. Out into the plains... In mid-afternoon, I rolled into Oakley, Kansas, where I headed for the Fick Museum. The Fick Museum is interesting on multiple levels: it's got some stellar fossils from Kansas's Smoky Hill Chalk (member of the Niobrara Formation), like a Xiphactinus (massive fish) and a Tylosaurus skull (even more massive mosasaur). But it's also got some whacked-out art: the founder, Vi Fick, was into making art with local "art supplies," and so the walls show his portraits of eagles rendered entirely in rattlesnake tails (see image at right, from this online gallery), or his geometric arrangements of thousands of fossil shark teeth. There's even an oil painting Fick did of "God making the Cretaceous seas," which shows a bearded diety surrounded by flames (it kind of reminded me of Hindu art) making pleisiosaurs and pterosaurs. Not the usual way you see fossils displayed, or paleontology depicted!

At the Fick Museum, I met up with Ron Schott, doyen of the geoblogosphere, who graciously agreed to show me some cool Kansas geology. Ron and I headed south from Oakley towards Monument Rocks, an outcrop of the Smoky Hill Chalk. Ron was eager to gigapan the outcrop, and he set up the little device: essentially a robot that directs his camera to take high-resolution photos in a systematic grid. Pretty cool, really -- I guess I hadn't realized what a Gigapan really was before seeing it in action. I got to meet Ron's two little plastic elves that he uses for scale, and personally placed them on a ledge of chalk for the photograph. The grid of pictures eventually gets digitally stitched together by software, and available for sharing online.

From there, Ron and I headed back up to Oakley, stopping en route so I could collect a couple samples of the aquiferiferous Ogallala Formation, and then headed east, then south again, towards Castle Rock, another chalk outcrop. Here, we tested out my Prius' shocks on the dirt tracks, and checked out the largest cliff in Kansas (nearly getting blown off it by the intense wind), and then prospected for fossils below. I found some fish scales, and a shark tooth! Also inoceramid clam fragments, encrusted with oysters (apparently a common feature of the bottom of the Western Interior Seaway). No mosasaurs, though... Back to the road, and into Hays, Kansas, where Ron put me up in his guest room. We had dinner and a few beers at the Lb. Brewing Company, and thought about recording a PodClast, but then it slipped our minds. We discussed field trips, tenure, publications, and related topics. A good time! Thanks again to Ron for being such an excellent host.

The next two days (Tuesday and Wednesday) were essentially just driving. On Tuesday, I made it to Indianapolis, Indiana, and spent the night in a hotel there. On Wednesday, I turned north, and drove up into Michigan, and crossed into Ontario at Port Huron / Sarnia. Why go to Canada on my way from Montana to DC? Well, I'm teaching my Snowball Earth class this week at NOVA, and some of the rock samples I needed were stuck at Brock University in St. Catherines, Ontario. Usually they get shipped to educators who want to use them, but because of alleged border complications, I had to go get them myself; a five hundred mile detour! Fortunately, I have good friends who leave in Waterloo, Ontario, so I went and stayed with them. Mike and Natalie Leuty have been friends since 1996, and we had a good evening catching up. They have a sweet house in a suburb full of professorial types who teach at one of the several universities in town.

On Thursday morning, Mike and I had coffee on his front porch while his kids played in the yard, and then I packed up my kit and got rolling. I made it to Brock by 11am, and got the Snowball Suite. Because it's in a giant black case that looks suspiciously like a rifle case, I packed it under a pile of other gear in my car. At any rate, I crossed back into the United States without any static from customs officials, and rolled through Buffalo, New York (twice in one year!) I made my destination for the night Ithaca, New York, where I have a friend who's going to grad school at Cornell. I've never been to Ithaca, but I hear that it's "gorges" from many people. So I called my friend, Kathryn Werntz, and she was indeed around and accepting visitors, so I drove through the finger lakes region (five subparallel glacial troughs now filled with water), and found my way to her bungalow. Kathryn and I took a walk through Cornell's campus (two amazing gorges cutting through it), had some Indian food, and went to get dessert at Purity Ice Cream.

In the morning (Friday), I got up and we went to Gimme! Coffee for some caffeine. Thus fortified, I hit the road for my final day of driving. East to I-81, then south through Pennsylvania. At Harrisburg, I turned onto I-83, which took me to Baltimore, and from there it was a familiar zoom down the B-W Parkway into northeast DC. The dome of the Captiol was visible to my left, and then the comfortable sights of Florida Avenue and U Street. Up the hill, and a left on Harvard Street, and I was back in Adams-Morgan. Home! Finally!

Labels: appalachians, canada, chalk, colorado, field trips, glacial landforms, kansas, montana, national parks, new york, snowball earth, structure, travel, wyoming

Here's a few more photos from the recent field trip to the Massanutten Synclinorium in the northern Shenandoah Valley, Virginia.

Some more Arthrophycus (?) trace fossils in the Massanutten Formation:

Outcrop of the Massanutten Formation on Route 678, south of Waterlick, VA. Note that the bedding is dipping to the south (reflecting the overall "canoe"-shape to the structure of the Massanutten Synclinorium... this is the "bow" of the canoe...):

Shelly horizon in the Mahantango Formation. Mainly brachiopod debris, but also crinoid columnals:

Cross-bedding in the Martinsburg Formation's Bouma sequences. This is a sample I collected on Saturday. I sawed it open on Monday, then polished it and gave it a coat of clear acrylic. Sample length is about 5 cm:

Ditto. As above, we can see clear cross-bedding here, reflecting current flow in these ancient turbidites:

Bedding / cleavage relationships expressed at an instructive outcrop in the parking lot of a pet store north of Front Royal, Virginia. Bedding is clearly visible running subhorizontally across the picture, but the rock breaks vertically: a tectonically-induced cleavage:

You could hardly ask for a better outcrop to teach bedding / cleavage relationships. Here's a medium-sized anticline in the same outcrop (note quarter, center, for scale). It clearly displays a fan of cleavage orientations. Lovely!

Lastly, on that same note, here's a sample I collected fromthat locality, with bedding planes and cleavage planes highlighted through the magic of CorelDraw. The stripes you see on the face of the sample are formed by the intersection of bedding and cleavage planes, shown schematically in red:

Labels: appalachians, field trips, fossils, primary structures, valley and ridge, virginia

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

Snowball Earth. The Pleistocene Ice Age was the proving ground for our species. But an earlier episode of glaciation, dubbed Snowball Earth, stretches our conception of what the limits of climate change are: the ice reached from the Earth's poles to its equator! Scientists infer that the freezing event was ended due to volcano-induced global warming. The course examines the geologic, chemical, and biologic evidence for Snowball Earth. This course meets 8/4 to 8/10: three evenings (MWF, 6-9pm) and one Saturday field trip to local Snowball glacial deposits. GOL 299, section 071N: 2 credits. More details

Labels: appalachians, geology, nova, teaching

On Tuesday afternoon, four students and I drove from Annandale, VA, up to Buffalo, NY, for the NE section meeting of the Geological Society of America. On the way, we crossed the Pennsylvanian Appalachians, and pulled over to examine some beautiful redbed exposures on the Pennsylvania Turnpike. I think these are in the Hampshire Formation, but I could easily be wrong about that, considering I've never been here before. Here's a few photos. First, some beautifully rhythmic alternations between sand and mud, now preserved as alternating layers of sandstone and mudstone:

Then, some nice "ball and pillow" structures, as heavy sand sank downward into squishy mud. In places, the mud skooshes upward in "flames":

And lastly and most amazingly (for me), some awesome exposures of flute casts. These are erosional scours into a layer of sediment by a current, which then fills in the scours (called "flutes") with sand, making these flute casts on the underside of the overlying layer of sand:

The flutes "point" upstream, and open up (and shallow) in the downstream direction. More later!

Labels: appalachians, meetings, nova, pennsylvania, primary structures, sediment, teaching

Cool folds (in metamorphic foliation) in this sample:

Here's the real prize: a big chunk of peridotite (upper right) that's partly surrounded in a crinkly foliated matrix of chlorite schist (lower left):

I'm off to Buffalo, NY today with four Honors students to attend the northeastern section meeting of the Geological Society of America. If anyone from the geoblogosphere happens to be up there, I hope you'll say "howdy." Posting may be sporadic over the next few days... we'll see what the Internet connectivity issue is like up there.

Labels: appalachians, iapetus, meetings, nova, piedmont

I've already introduced you to two of my Honors students' field projects. Now for the last of the three -- Jason's project on the strained metaconglomerate of Klingle Road. Klingle Road is a "road" in D.C. that was damaged by a storm some years back, and never repaired. Some people have started using it as a park, while others clamor for the road to be fixed. Geologically, it's interesting because it exposes a rock unlike any other nearby: a distinctly foliated metaconglomerate. Because I am so clever, I call it the Klingle Road Metaconglomerate. It's part of the "Laurel Formation," which is one of many flavors of metagraywacke / accretionary wedge complex that make up the bulk of the Piedmont in this area. Here's some of the squished clasts that Jason is interested in:

We know these rocks got heated up a fair bit. How do we know this? Well, they flowed out into elongated shapes all oriented in the same direction for one (see the additional photos here). The outcrop is peppered with clusters of little plus-shaped protuberances: they are clusters of sericite (cryptocrystalline muscovite) in the shape of staurolite porphyroblasts. Staurolite is a reasonably high grade metamorphic mineral, and when we see the three-dimensional shape of staurolite, but it's been turned into relatively-low-grade sericite, it's an indication of "retrograde metamorphism." Basically, after hitting the peak of its particular metamorphic conditions (high temperature and pressure, growing staurolite), the rock is readjusting to lower temperatures and pressures, and those staurolite crystals are reacting to a mineral that's more stable at those lower temperatures and pressures: sericite.

But anyhow -- back to the metaconglomerate. It's made of clasts, and those clasts have been stretched. The question is: how much have they been stretched. Sometimes when strain estimates are made, we assume an initial sphere shape, and then measure the lengths of the various axes of the resulting ellipsoidal shape (the "strain ellipsoid"). But is the assumption of initial sphericity valid? Jason is testing this issue by measuring the axes of cobbles and pebbles from the metaconglomerate as well as loose cobbles and pebbles found in nearby Rock Creek. We want to get a sense of how ellipsoidal cobbles are before they experience orogenic shortening/stretching. Here's a shot of Jason, Spencer, and Victoria measuring cobble axis lengths on a gravel bar near the National Zoo:

And a shot of the crew close-up:

And, just for fun, here's one more shot from Victoria's field area on Broad Branch. We hiked up to the contact with the Kensington Tonalite (a ~464 Ma felsic intrusive rock -- essentially a granite) and found a series of small waterfalls over this resistant rock unit. In the sequence of cascades were a series of deep pools. I submerged myself in one of them:

Labels: appalachians, dc, field trips, nova, piedmont, sediment, structure, teaching

Lastly, in this final picture, you can see (on the left and in the foreground) what a lot of the large bodies of migmatite looks like: mostly granite with wisps of mafic residue strung out as thoroughly-foliated xenoliths. Their common alignment is oriented in the same direction as regional foliation. This granite yields U/Pb ages of ~460 Ma, which is Taconian in age.

Labels: appalachians, migmatite, piedmont, virginia

Last Friday, I had a fun local field trip, in search of ultramafic rocks included in the Piedmont's metamorphosed accretionary wedge complex. My companions on the trip were David and John, both of whom are retired gentlemen pursuing geology as a hobby. Because they're doing geology for fun, they are among the most dedicated and interested students I've met at NOVA. Friday's trip was something I've been meaning to do for a while, and both of them thought it sounded like an eye-opener, so they came along too.

Our goal was to find some new outcrops that we hadn't seen before. Of primary interest were several mafic and ultramafic bodies included in the larger metasedimentary complex of rocks that we know today as the Piedmont. As I've mentioned before, these Piedmont rocks are interpreted as being the rocks of an ancient (Neoproterozoic - Paleozoic) ocean basin. When the ocean basin closed during Appalachian mountain-building, the sediments of the ocean got squished and squeezed between North America and Africa. Mixed in with them were chunks of the ancient Iapetus Ocean crust, which would probably be recognizable as ophiolites if it weren't for that pesky regional metamorphism they endured as a result of the collision. Up and down the east coast, there are outcrops of these mafics and ultramafics along the presumed "suture" zone between ancestral North America and terranes (blocks of crust) that were once a volcanic island arc in the Iapetus Ocean. As with most geology field trips, we also found some other stuff worth noting, even though it wasn't our primary objective.

Our first stop (located thanks to Diecchio & Gottfried (2004) in USGS Circular 1264) was in Clifton, Virginia, where we went to see the unconformity between the Piedmont metamorphic rocks and the Triassic sedimentary rocks which overlie them in an ancient rift valley called the Culpeper Basin. Tragically, instead of a beautiful outcrop, we found freshly graded surfaces and several new McMansions. There was only a small strip of undeveloped land, about 20 feet wide and 50 feet long which had any rock left. But in that area, we found an outcrop of soapstone. Here, John scratches the soapstone (talc) with his fingernail. It's soft!

In this case, the soapstone is interpreted as being metamorphosed ultramafic rock. Close to it, we found this piece of conglomerate:

The conglomerate is the base of the sedimentary sequence in the Culpeper Basin: it's the Reston Member of the Manassas Sandstone Formation. Notice that it contains clasts of foliated metamorphic rocks -- these were derived from the older Piedmont rocks it unconformably overlies. The Piedmont rocks got metamorphosed during Appalachian mountain-building, and then when Pangea broke up, the Culpeper Basin (one of the Newark Supergroup basins) opened up and got filled in. The source for the infilling sediment was the neighboring area, not surprisingly including pieces of the Piedmont. Up-sequence, the conglomerate is overlain by the regular Manassas Sandstone, which is a rich brick red in color (classic Triassic red beds), and contains a wealth of primary sedimentary structures. I found this one piece, which unfortunately broke into chunks when I picked it up:

It displays ripple marks, raindrop impressions, and a few horizontal branching trace fossils. Anyhow, that was about it for the Clifton stop. We were bummed about the development destroying the outcrop. On to the next location, Indian Run, on the east side of Annandale. There, using the geologic map that accompanied Drake & Lyttle (1981), we walked along the creek bed looking for exposures of rock. We didn't have to go far before seeing some heavily-rusted green rocks:

The above photo is dominantly chlorite, but check this out:

Pyroxene-rich inclusions (xenoliths? olistoliths?) were observable in the heavily-weathered exposures. The outcrops here were saprolitic, meaning they were essentially "rotten rock." David was struck by how soft they were. He said "It feels like velvet!" We turned our attention to the more coherent specimens which were weathered out and deposited as cobbles in the streambed. I got a watermelon-sized specimen that's about 40% massive peridotite and 60% greenschist. (I showcased this leprechaun-colored specimen last night in Historical Geology lecture, when we were discussing the Taconian Orogeny.) We also found intriguing hints of mountain-building in clasts like this:

That's a couple of beautiful folds in gneissic metamorphic foliation. As above, the bright green minerals are chlorite. We also found some cobbles of sedimentary rocks mixed in with the locally-derived metamorphic rocks. For instance, here's a nice semispherical cobble of flint, likely derived from the flint-bearing limestones of the Shenandoah Valley:

How did this flint nodule travel ~50 miles from its source area to its current resting place in Indian Run? Likely, it was transported by an ancestral version of the Potomac River, which brought many westward-derived cobbles eastward during the Cretaceous. About 100 million years ago, this river deposited a layer of cobbles all over our local area, preserved today as the Potomac Formation. It unconformably overlies the Piedmont rocks, and can be found today as the basal layer of the Coastal Plain. It's even found as a layer topping our highest local hills. The exposures in Indian Run actually offered a nice view of the unconformity surface, with foliated metamorphic rocks below, and unlithified Cretaceous gravel deposits on top:

Just to close out this post, I'll show a few other cobbles found in the streams. Here's a gneiss containing big, beautiful porphyroblasts of garnet:

And here's a Skolithos-bearing boulder of the Antietam Formation (quartz sandstone / quartzite), which I originally posted a few days ago, but is so gorgeous it should be shown again if I'm talking about boulders.

Finally, as a preview of tomorrow's post, I'll show a boulder which hints at the complex relationship between the foliated metamorphic rocks (gneisses) of the Piedmont and felsic igneous rocks (granites) which were derived from the partial melting of the gneiss. In other words, this is a boulder of migmatite: rock that has experienced partial melting. We'll explore this in more depth with some in situ photographs tomorrow.

Labels: appalachians, culpeper basin, field trips, flint, geology, nova, piedmont, unconformities, virginia

DC Metro area residents, you're hereby invited to join me (NOVA) or Phil Justus (NRC) or Michelle Arsenault (NSF) on a geology hike along the Billy Goat Trail, a popular and rugged hiking trail upstream from DC on the Potomac River, downstream from Great Falls. Michelle and Phil and I take turns leading this excellent hike. You'll learn about the Iapetus Ocean, Appalachian mountain-building, and the incision history of the Potomac River. You'll see potholes, amphibolites, metagraywacke, migmatite, and the mysteriously-straight Mather Gorge. The Park Service has just posted the spring schedule online here. Reserve your space today!

Labels: appalachians, dc, geologists, piedmont

Picking up from yesterday's post about my hike along Windy Run in Arlington, Virginia...

Just downstream from the waterfall (and crossing the trail) is a recent rockslide. Between D.C. and Great Falls (12 miles upstream), the Potomac River flows through a canyon called the Potomac Gorge. It's hundreds of feet deep overall, and consists of a series of nested straths (bedrock "terraces"), each shaped roughly like (half) a canoe. (At the tip of each canoe is a waterfall leading up to the next strath). Where the vertical distance between straths is great, as it is at Windy Run, mass wasting events serve to break down the cliffs and reduce the crisp profile of the straths.


This rockslide happened in 2005, and the area of "raw" rock up at the top of the cliff reveals the source area for the rock debris below. I wish I had taken a photo of this three years ago when it was really fresh -- it would be an excellent place to do repeat photography to show how the talus pile and cliff face change over time. Upstream are several examples of older talus aprons that have been overgrown by plants and buried in soil. Already, you can see that a few Ailanthus trees (single, upright pole-looking things) have taken root in this fresh landscape.


Once you get down from the Windy Run trail to the Potomac Heritage trail, here's the view of the river, looking upstream. Virginia's on the left; D.C. on the right. A slight "shelf" can be seen on the Virginia side where a notch has been cut to host the George Washington Parkway.


As I hiked along, I found this dead mole. It's a big fat sucker, and it must be quite fresh: probably a casualty from the previous 24 hours. Lens cap is 5 cm in diameter.


More critter evidence: here's a couple of small tree trunks that were decapitated by a beaver. Again, this is recent -- note the fresh curls of wood shavings at the base of the trunk.


But enough with these living entities: let's look at some rocks. This is the metagraywacke rock that makes up most of the Piedmont in our area. This rock is metamorphosed to various degrees up and down the Potomac River, in some places all the way to gneiss and migmatite. In some places, it's schisty, but in others primary sedimentary structures are still preserved. Upstream by Great Falls, for instance, we find graded bedding in isolated less-metamorphosed, less-deformed areas. Down along this stretch of the river, it preserves a diversity of sedimentary clasts, as shown in this image:

Here, you're seeing the graywacke matrix mixed in with a bunch of dark chunks. Today, these dark chunks are mostly biotite, but that's metamorphic. Originally, they were probably mud clasts. Little pebbles of granite and vein quartz are mixed in too. It's worth noting that not only are they metamorphosed, but they're also stretched out in the same direction: foliated and lineated. Many are squashed into X>Y>Z ellipsoidal shapes (where the letters refer to the lengths of the different axes of the ellipsoid), like a mango seed. Lens cap is 5 cm in diameter.

Let's pause for a moment and bring people up to speed if you haven't previously spent any time thinking about Appalachian geology. These rocks are part of the Appalachian mountain belt, which runs from Newfoundland to Georgia (by one definition) or from Texas to Scandanavia (by a more inclusive definition). The Appalachian mountain belt consists of three provinces: from west to east: the Valley and Ridge, the Blue Ridge, and the Piedmont. Two of these are topographically mountainous today: the Valley and Ridge and the Blue Ridge, as their ridgey names imply. But the Piedmont certainly counts as part of the ensemble, and if you compare it to the other two, you'll find that it experienced the most metamorphism, the most deformation, and is intruded in many places with syn-orogenic granites (which neither of "the Ridges" can claim, at least not for Paleozoic orogenies). The Blue Ridge and the Valley and Ridge are deformed, yes, and even lightly metamorphosed, but the Piedmont is really where the action is: this is the center of the ancient Appalachian mountain range. These rocks experienced some serious continental convergence.

So what was the Piedmont before it was the Piedmont? An ocean basin. Before the Atlantic, before Pangea, there was an ocean basin off the "east" coast (it was really the south coast at that point, but no matter...). We call this dead ocean the Iapetus Ocean. The Iapetus was closed via subduction throughout the Paleozoic, and it closed for good when Africa rammed into North America, metamorphosing these rocks and raising the Appalachians. As subduction narrowed the Iapetus, sediments atop the oceanic crust were scraped off in a big jumbled pile called an accretionary wedge. (It is for this mixed-up melange that the infamous geo-blog carnival is named.) You want to see an accretionary wedge being scraped up today? Dive down to the Peru-Chile Trench, off the west coast of South America. You want to see a fresh one at the surface? Visit California's coast ranges, which are a Mesozoic accretionary wedge, raised above sea level. You want to see what an accretionary wedge looks like after it's been tectonically squeezed between two continents? Come to the Piedmont!

Our metamorphosed accretionary wedge consists of a bunch of the sediments that were deposited in the Iapetus Ocean, including what was originally graywacke (a mix of sand & mud). Occasionally, you find a sedimentary clast that's a bit more intriguing, like this one (white arrow):

What intrigues me about this little sedimentary cobble is the fact that it's foliated, which indicates metamorphism and differential pressure, but its foliation does not line up with Appalachian foliation. This cobble was foliated before it was deposited in the accretionary wedge. Therefore, it was derived from some area that had previously experienced mountain building & regional metamorphism (presumably a continent). That ancestral orogenic episode produced a source rock from which this cobble was derived. Then that cobble was deposited by sedimentary processes somewhere and (possibly later) incorporated into the accretionary wedge, which then was metamorphosed (& foliated) itself. Lens cap is 5 cm in diameter.

Here's another one, which shows its foliation a bit better:

When I see something like this, I start to wonder, where did this cobble come from? What was its sedimentary provenance? Is this a North American cobble that attained its foliation in the Grenville Orogeny (~1 Ga)? Is this an African cobble that got squeezed in some pre-Pangea Gondwanan orogeny? Is it derived from a nameless microcontinent that was formerly marooned in Iapetus oceanic crust (a la Madagascar) and is now accreted to some continent as an exotic terrane? Do the answers to these questions change how we think about the (1) closure of the Iapetus, (b) Appalachian Orogeny, (c) assembly of Pangea?

Elsewhere in the Potomac Gorge, there are other clasts in the accretionary wedge complex that encourage similar thoughts (for instance, you can check out the photos at the top of this page). Another question raised by these clasts is this: Does their position amidst such relatively fine grained sediments (the mud and sand of the graywacke) represent original deposition? Or is that simply tectonically-induced "shuffling" in the blender-like environment of the accretionary wedge? The rocks in an accretionary wedge are not stratigraphically coherent, but sometimes they have little areas that are. If these clasts are in their original depositional position relative to the graywacke matrix, what does that tell us? Are these landslide deposits? Or are these "Snowball Earth"-related glacial dropstones? Without the original sedimentary bedding (destroyed via orogenic metamorphosis & deformation), it's impossible to answer these questions, but it sure would be nice to know.

Lastly, I'll note that everything I've talked about so far (metagraywacke, mysterious clasts, quartz veins, granite intrusions, and regional foliation) are all cut by a series of joints, brittle fractures in the rock. These joints are arranged in a series of joint sets which intersect one another, resulting in the "blocky" nature to bedrock exposures in the Potomac Gorge (example). Here, along one Gorge-bounding cliff, I saw that the joints had begun to accomodate some sliding of the blocks of rock on either side. Technically, they aren't joints any longer, but faults, instead. Total offset is only a few inches, but it shows up well in a photo like this. Note the similar sense of motion on the more distant fault "scarp." A housekey (with pink ribbon attached!) is jammed into the closer fault to give a sense of scale.


All in all, an hour strolling along Windy Run provides some terrific opportunities for reflection on the checkered geologic past of the Piedmont and the Appalachians, and the continuing geomorphic evolution of the Potomac Gorge landscape. I enjoyed my little stroll. It was with reluctance that I turned around and headed back to the house to grade exams...

Labels: appalachians, faults, geology, mass wasting, plate tectonics, virginia

Last week, I put up some pictures of the folded strata at "the Whaleback" outside of Shamokin, Pennsylvania. Today, I'll augment those with some images of the fossils found at that site and at another outcrop of the Llewellyn Formation near St. Clair, Pennsylvania. Here's a fern impression to start with:

Labels: appalachians, fossils, pennsylvania

It's funny how one thing leads to another. In promoting our Climate Change Symposium on Friday, I wrote to Cerphe (pronounced "Surf"), probably the best DJ in the world, who's on air in the afternoons on 94.7 The Globe, our DC-area "world-class rock" station that also features a green message. Cerphe wrote back, saying he'd get some mentions on the air this week, and also mentioned that his wife has a small business building green homes. I noticed that the business is headquartered in Bentonville, Virginia, out in the Shenandoah Valley between Massanutten Mountain and the Blue Ridge. And it occurred to me that I've looked at a bentonite layer out there in the Valley (see photo), not too far away from Bentonville. Bentonite is a common clay mineral that in stratigraphic layers is usually interpreted as weathered volcanic ash. (The one pictured above is possibly the "Big Bentonite" that accompanied the onset of the Ordovician Taconian Orogeny in eastern North America.) Could it be that bentonite is named for Bentonville, Virginia? Well, Wikipedia tells me that "The absorbent clay was given the name bentonite by an American geologist sometime after its discovery in about 1890 ...after the Benton Formation in Montana's Rock Creek area." So that took me to the entry on Fort Benton, Montana, which was named for the first 5-term U.S. Senator, Thomas Hart Benton. He was an advocate of westward expansion by the United States, the idea that later was dubbed "Manifest Destiny." So: as near as I can follow, bentonite is a mineral named for a place, which is in turn named for a man. What this has to do with world-class rock and climate change is anybody's guess.

Labels: appalachians, history, montana, volcano

Outside of Shamokin, Pennsylvania, is a coal strip mine that has had the coal stripped away. Under the coal was a Pennsylvanian (in the time sense of the word) carbonaceous shale (the Llewellyn Formation), which is now preserved in lovely undulating Appalachian folds. Thanks to the removal of the coal, these fold surfaces appear in three dimensions -- a rarity for structural geologists like myself. The area is known as "The Whaleback" because of one anticline (center) with a shape that evokes a surfacing cetacean:

I went to the Whaleback last fall on a fossil-hunting trip with the The Calvert Marine Museum Fossil Club. In today's post, I'll take a look at the structure, and in a later post, I'll show you some photos of the fossils themselves. Here's some of the guys on the trip:

At the north end of the excavation, a cross-sectional view of the absent upper levels is preserved, showing this syncline. It once continued towards the camera's perspective in the air, a downflung fold between the Whaleback anticline and the neighboring anticline which made up the background "wall" in the first photo.
Okay, that's it for today. Tune in soon for the fossiliferous sequel.

Labels: appalachians, concretions, pennsylvania, structure

In last week's issue of Science, Paul Silver (of DC's own Department of Terrestrial Magnetism at the Carnegie Institution) and Mark Behn (formerly a post-doc at Carnegie, and now at Woods Hole in Massachusetts) published a paper putting forward an intriguing idea: maybe plate tectonics proceeds in fits and spurts.

Silver and Behn note that most of the world's subduction zones are located in the circum-Pacific belt, and that the Pacific is getting smaller over time. (Subduction destroys oceanic crust, and since the Earth presumably isn't increasing or decreasing in volume, subduction in the Pacific is balanced by seafloor spreading elsewhere, like the Atlantic.)

The Pacific is "predicted" to close in about 350 million years, assuming that the tectonic plates continue to move at about the same rate they're moving now. The death of the Pacific would come as the Americas smash into eastern Asia and Australia, raising up a Himalayan/Appalachian style mountain belt. Silver and Behn posit that this would basically end subduction on planet Earth for a time. This was a startling idea to me at first, but then I thought, "Why not?" Then I thought, "I wish I'd thought of that."

My understanding of mountain belts comes from the Appalachians, which built up in three successive episodes called orogenies. Check out the diagram below (from the excellent textbook Essentials of Geology by Steve Marshak, that I use in my Physical Geology course at NOVA) and follow along so you can see why this new concept startles me a bit (but in a good way):

There used to be a big ocean basin off the "east" coast of North America that closed via subduction over the course of the Paleozoic Era. This extinct ocean goes by the name of Iapetus. This was not a simple event: it was more like a pile-up on the highway than a simple head-on collision. This ancient ocean basin was not just empty ocean. It also included islands and small chunks of continental crust ("microcontinents" like modern-day Madagascar). First a subduction zone developed out there in the ocean, closing a portion of it. This brought a chain of volcanic islands closer & closer to North America. The islands hit North America (around 460 million years ago), in a mountain-building event called the Taconian ("Taconic") Orogeny. Once that had happened, a new subduction zone developed on the ocean side ("outboard") of the islands/accreted terranes. That began to close another part of the Iapetus Ocean. Around 360 million years ago, that episode of subduction ended when a microcontinent (dubbed Avalonia) smacked into North America. This collision caused more mountains to rise: the Acadian Orogeny. Then yet another subduction zone, outboard of the newly accreted Acadian terrane, kept the closure of the Iapetus Ocean going, until finally the continent on the other side of the ocean (Africa) smashed into North America, raising more mountains. This is the Alleghenian Orogeny (sometimes spelled "Alleghany"), which really crumpled up the landscape, starting around 300 million years ago. The moment the Iapetus died was the moment Pangea was born.

I go into all this because the model of plate tectonic convergence the Appalachians display is one that says collisions between plates don't stop the overall convergent forces. As soon as one subduction zone is snuffed out, a new one develops outboard of the continent, where the weaker, denser oceanic crust gets shoved downward.

But does it actually work that way all of the time? Silver and Behn suggest maybe not. Maybe it's an "on-again, off-again" affair. They cite among their evidence an earlier orogeny, the Grenville Orogeny, which sutured together many continents at a much earlier time (about a billion years ago). When that collision had ended, the supercontinent Rodinia was born. Silver and Behn note a lack of volcanic activity around the world for hundreds of millions of years after the Grenville Orogeny (most volcanoes are caused by subduction). Rodinia did eventually break up amid much volcanic activity (including the eruption of the mid-Atlantic's infamous Catoctin Formation), and giving birth to the Iapetus Ocean basin in the process -- but that didn't happen for a long time after Rodinia got assembled. What gives? Does that mean subduction was inactive during that period?

They also offer a modern example: India and the Himalayas. 20 million years ago, India was a microcontinent out in the Indian Ocean, with a pavement of oceanic crust separating it from Eurasia. India moved north, the oceanic crust got subducted, and eventually India plowed into Eurasia, raising the Himalayas. But why hasn't a new subduction zone developed south of India? That would be what would happen if India's orogeny were following the Appalachian example.

Maybe plate tectonics has periods of intense activity (lots of subduction), but then has periods where it's "clogged up," and the movement of the plates slows. Eventually heat builds up in the underlying mantle (the source of plate movement) to the point where the mantle begins to convect more vigorously, and the plates start getting dragged around again. It's kind of a cool notion. I'd be interested to hear what you think about it. Please post any thoughts you have in the comments section below.

The whole idea reminds me of the concept of punctuated equilibrium, a model of biological evolution which bucked the long-standing notion (originated by Darwin himself) that evolution proceeded slowly and methodically over time. Thanks in part to an eye-opening appreciation of the Earth's immense age, the prevailing wisdom was that evolution was gradual, smooth.

Then (in 1972) Niles Eldridge and Steve Gould published a landmark paper that suggested otherwise. Instead of "gradualism," they argued, changes in populations of living organisms may have happened suddenly, experiencing a lot of change in a short period of time. Once equilibrium was achieved, the new status quo was preserved as a non-dynamic scene for a long time. (See image at left, which came from Wikipedia).

They cited the fossil record as their primary evidence: most of the change seen in fossils is a sudden switch of biological "regimes," with new fossils showing up, lasting a while, and then abruptly vanishing. I'm oversimplifying here, but I hope the analogy is clear: if evolution can do it, why not plate tectonics? Is there any reason to think plate tectonic motion couldn't happen in spurts of more activity followed by periods of quiescence? Ponder it...

Reference: Silver, Paul G., and Behn, Mark D., 2008, Intermittent plate tectonics?: Science, v. 319, p. 85-88, doi: 10.1126/science.1148397.

For those without a subscription to Science, you can read the press release about Silver and Behn's work that Carnegie put out by visiting their website.

Labels: appalachians, evolution, iapetus, plate tectonics