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

Last week, I visited the Taconian Unconformity in the Catskills region of New York. I found out about the outcrop via the informative website the USGS put together in 2003 to explain southeastern New York's varied and interesting geology (Click here for a map).

Here's me at the angular unconformity, demonstrating the layering with my forearms:


Here's the same outcrop, sans goofball, avec annotations:


This is a classic angular unconformity. It even graced the cover of the (excellent) GSA publication Excursions in Geology and History: Field Trips in the Middle Atlantic States (Frank Pazzaglia, editor; cover photo by Marli Miller). Why should we care? Because like the "original" angular unconformity at Siccar Point in Scotland (described by James Hutton), this outcrop represents a lot of geologic time. First, during the Ordovician period, the Austin Glen formation had to be deposited as layers of clastic sediment in an ocean basin. Then, during the late Ordovician Taconian Orogeny, those layers had to be deformed: folded and buckled so they stood up on end, and then eroded down to their nubs. Then, on that newly-formed erosional surface, a fresh layer of sediment had to be laid down, in this case, the Rondout Formation was deposited as a layer of carbonate mud during the late Silurian period. Then, that too was deformed, during the Devonian period's Acadian Orogeny. Finally, the whole package had to be uplifted to the surface and exposed (in this case, when a highway roadcut was completed). That's a lot of time!

I'm delighted to have had the opportunity to visit it first-hand!

Labels: devonian, maps, mountains, new york, ordovician, silurian, structure, travel, unconformities

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

Just got a batch of images from the NOVA photographer, Kevin Mattingly. I particularly like this image of last spring's Field Studies class at the Billy Goat Trail:

Here, we're overlooking the upstream end of Mather Gorge, checking out some ~360 Ma lamprophyre dikes exposed there -- but offset on either side of the river!

Labels: devonian, field trips, national parks, nova, piedmont, teaching

I've discussed the phenomenon of jointing on this blog before, and how when rocks fracture, sometimes they leave behind structures we can see that tell us something about the jointing process. Where did it start? Where did it stop? To answer these questions, we turn to structures like plumose structure, arrest lines (concentric ribs), and hackle fringes.

On this past Sunday's field excursion out to the Massanutten Synclinorium (Shenandoah Valley), MSSE John Graves and I saw some more nice examples of these phenomena, and as usual, I took some photos of them.

Let's start with this one, which shows plumose structure (and thus joint propagation) starting at the right and heading to the left.



A closer-up shot of this same fracture surface (in the Ordovician Martinsburg Formation):



Here's another one (in the Devonian Needmore Formation):



Sorry -- no sense of scale in that (above) one -- it was a few feet above my head. Total width of the photo is about two feet (call it half a meter).

This one (also in the Needmore) shows some really wavy plumes:



At the end of joint surfaces, we find hackle fringes, these "rough edges" where the little ridges and valleys of the plumose "topography" flare up and out in a spiralling kind of shape. When you slice through this spiral shape, it appears as a series of little itty-bitty joints at an angle to the main joint. Here's some hackle fringes on a joint surface from the Martinsburg Formation:



Each of these represents the edge of the fracture at one point. But then stresses built up again past the rock's strength, and it cracked anew, extending the fracture and producing a new hackle fringe. A closer-up shot (rotated) of the above fringes:



And back to the Needmore again, for a lovely series of hackle fringes that I've shown you before, but I couldn't resist photographing again. But to mix it up a bit, this time I used a penny instead of a quarter for scale...



Contrastified version of the above, with annotations:



Lastly, remember that I showed you this photo on Monday, from the Billy Goat Trail?



Well, I think you can see some hackles there, too. Take a closer look...

Below, I've zoomed in on the far upper right of the previous photo, and rotated it 90 degrees. I've also transplanted the penny from another part of the photo to maintain a sense of scale, and drawn a quick sketch of the fractures:



I think the little itty-bitty fractures (again, infused with quartz, making them weather out in high relief) traversing the main left-right joint trace are hackle fringes associated with that joint. Anyone care to differ?

Labels: devonian, geology, joints, ordovician, quartz, structure, virginia

Hat tip to Michelle A. for the link!

Labels: canada, devonian, fossils, humor, music