25 February 2014
While on Corridor H 2 weeks ago with Alan Pitts, we stopped astride the Patterson Creek Mountain Anticline, with extensive road cuts displaying Tonoloway Formation overlying Wills Creek Formation. We love this spot for its lovely folds and halite casts. See what I mean?
This time, however, my eye was drawn to the prodigious quantities of mudcracks to be seen in side-view (that is, in cross-section). Take a gander:
Zooming in on the crack betwixt the concave-up “dishes: of finely laminated carbonate mud:
Here’s another example:
And one more, from the other (sunny) side of the road…
What do these features look like when viewed from “above,” that is, viewed on the bedding plane itself (rather than in cross-section)? We’ve proudly displayed them before:
That last GigaPan is at a quarry on old Route 55, on the west flank of the Hanging Rock Anticline.
Students: What can we infer about conditions in West Virginia during the Silurian period of geologic time, when these rocks were deposited as sediment?
24 February 2014
While we’re out on Corridor H, let me show you some new halite casts I found (either the Wills Creek Formation or the Tonoloway Formation):
Salt casts are among my favorite primary structures. They speak very specifically to hypersaline depositional conditions.
21 February 2014
Howard Allen, a blog reader from Canada, digitized a bunch of folds for us from his old Kodachrome slides. You’ll be seeing selections from these images over the weeks to come. Get psyched! There are some great folds in this batch. Here’s the first:
3D folded marble, Shuswap Metamorphic Complex, north of Sicamous, British Columbia. Specimen was bulldozed out of the way for a logging road. Photographed in 1980 when I had a summer job working for a mineral exploration company. I *so* wished I could have this one in my front garden.
Thanks, Howard! Everyone else: enjoy your Friday, and send me your fold photos!
20 February 2014
While out on the Corridor H field trip last week before the heavy snow, I found this squeal-inducingly-lovely example of a stylolite in Helderberg Group limestones (Devonian passive margin carbonates):
The stylolite is a pressure-solution surface, made especially apparent in this example because of the starkly different grain sizes and colors on either side of the dissolved-away rock:
This is a gorgeous sample to explore in macro GigaPan view. Enjoy:
19 February 2014
Today, we continue with the story of the field trip I took last week out to Corridor H, the new superhighway in West Virginia that is practically unused, and decorated with multistory roadcuts of spectacular Valley and Ridge sedimentary sequences, and their attendant structures.
From the putative Hampshire Formation exposures with their uncharacteristic marine incursion strata, we moved downhill (and, down-section) to examine a road-cut of what we inferred to be Foreknobs Formation. The left (uphill/youngest) part of this roadcut had some interesting stuff going on:
Whereas most of the strata in the Valley & Ridge are relatively coherent, the red blobby layer caught our eye as being odd. That clued us in to the layer overlying it, which we think might be a big shaley mass transport deposit: a weak pile of sand and mud that slumped, pre-lithification.
Note how the layer in question varies in thickness from the lower right to the upper left.
You’ll note there’s a lot of massive quartz sandstone in this outcrop, although there are occasional breaks in that monotony, such as this black shale layer seen at the far left of the outcrop (penultimate stratum in this sequence):
But while shale is all well and good, I’m more interested in the structural story in the gray shale / sandstone layer:
Here’s Alan pointing at some key features in the unit, which appear to be dismembered (boudinaged?), folded competent sandstone layers, surrounded by incompetent shale.
Let’s examine a few of these chunks in more detail… 1, 2, and 3…
First, let’s look at fold #1, just to the right of Alan’s pointing finger:
Then, there’s fold #2:
…And fold #3:
What do you think – is this a “broken formation” sort of scenario (i.e., the precursor to a tectonic mélange), where the deformation is tectonic (related, presumably, the Alleghanian thrusting and faulting), or is this a primary sedimentary feature (a submarine slump, a mass transport deposit)? I’m curious to hear your thoughts.
18 February 2014
See the size of those road cuts? Count all those cars? Exactly. Why this place isn’t swarming with geologists, I cannot for the life of me comprehend.
17 February 2014
I went out last Tuesday to Corridor H, the exemplary new highway cutting through the Valley and Ridge province of eastern West Virginia. Joining me was former student Alan Pitts, a devotee of Corridor H from way back in the early days when we just called it “New Route 55.”
The boondoggle highway is now open all the way west to the Allegheny Front, practically into the Canaan Valley. On Tuesday, we went all the way to physiographic boundary, right near the big line of windmills that marks the transition from the Appalachian Plateaus to the first valley of the Valley & Ridge. This is the first of several posts of the new stuff we saw on that field trip.
The steam plume you see in the photo below is the coal-fired power plant at Mount Storm. Note the red beds (Hampshire Formation, presumably) and the snow cover:
The westernmost outcrop we examined in detail is this one, which we inferred to be the Hampshire Formation (Devonian red beds / molasse), a 2000-foot-thick stack of fluvial and floodplain sediments deposited on a massive delta complex that extended westward into the Kaskaskia epeiric sea from the Acadian Orogen. You can see that the exposure is pretty great on these Corridor H roadcuts, but that we had a moderate amount of snow and ice to cope with that day…
We saw some interesting “jellyroll” looking structures there… potential features indicating the “rolling up” of sedimentary layers as they destabilized, slumped and tumbled downslope.
Unfortunately, these “jellyrolls” were only delineated by the alternation between oxidized (red) and reduced (gray-green) layering. I would have preferred some more robust criterion, such as grain size differences. Redox reaction fronts can be an indication of primary sedimentary layering, but they can also be diagenetic, or controlled by structures like joints. Just the same, these ones are pretty cool, even if I’m not 100% positive that they represent soft-sediment deformation.
There was a thick sandstone unit there, too. Can you spot the cross-bedding in this exposure?
Here’s some more cross-bedding. Note the truncated tops to the cross-beds (indicating these beds are oriented with “up” being stratigraphic “up”; that is to say: they have not been tectonically inverted). Also, note the gray mud rip-up clasts right at the boundary between one massive sandstone bed and the next one overlying it:
Here’s a slightly more complicated scene: What do you see?
Surely you noted the coarsening of the grain size. Part of this outcrop is conglomerate: a sedimentary rock made of pebbles. Here’s what I see:
And here’s another conglomeratic bed, full of small carbonate rip-up clasts. Where did the carbonate come from? This is surprising to me: the Hampshire Formation is supposed to be terrestrial. Are these limestone clasts derived from the Acadian Highlands themselves? Or is this contemporaneous carbonate from an adjacent depositional environment?
You noted the white weathering rind on the big one at the bottom of that last picture, right?
Here’s a look at another outcrop, further down the road. More red beds. Note the rhythmic alternation between siltstone and shale:
Note how they both get thinner as you work your way up in the sequence.
There are reduced layers in here, too.
Note the coarse grain size in the middle of this gray-green reduced stratum:
If you trace this layer out to the right, you will find that it abruptly truncates, in contact with the fine-grained red beds:
Note the cross-cutting relationship here: the gray-green conglomerate must be younger than the red siltstone and shale, since it cuts across them.
A last look at this outcrop as we prepare to leave:
Note the low-angle cross-beds in the lower red bed unit (bounded above and below by thin greenish reduced layers).
Our third stop on the Allegheny Front was this roadcut… Click through for a full sized panorama of the outcrop.
This caught our eye because that’s not a whitish sandstone layer sandwiched between red beds. Instead, it’s a black siltstone / shale unit.
The black layer, about 5 m thick, was a limestone / limy siltstone / shale, with alternating beds 2-5 cm thick. It gets more massive toward the top:
In it, we found marine fossils, like these articulate brachiopods…
… and these gastropods (snails):
Wow – this surprised us. Neither of us thought the Hampshire Formation had any marine strata within it. Was this black, limy, fossiliferous layer representative of a small transgression (sea level rise) over our deltaic floodplains (red beds)? Here’s a closer look at the base of the black unit:
It seems to have come on pretty suddenly. That gray zone could reflect primary coloration, or its coloring could be a secondary diagenetic reaction between the oxidized zone below and the carbon-rich limestone above.
So, in summary: We interpret the black layer as resulting from low-oxygen marine deposition. But that doesn’t sound like the Hampshire Formation. Two possibilities occur to me: Does this suddenly black limy interval indicate that this isn’t the Hampshire Formation? Or did we just ‘discover’ a new marine portion of a previously-thought-to-be-terrestrial-only geologic unit?
14 February 2014
Scott Johnson contributed this special “Valentine’s Day” edition of the Friday fold: a lovely primary igneous structure that evokes a heart:
That’s a close-up view (lens cap for scale) that Scott took when he GigaPanned this feature:
… the big obsidian flow at Newberry Crater in Oregon. That’s a really dry lava flow that oozed out of the Earth sometime in the past thousand years or so.
It’s even moderately heart-shaped in map view. Enjoy – I hope you have a lovely Friday!
12 February 2014
At the eroded gully known as Devil’s Coulee in Alberta, you can find armored mudballs, dinosaur fossils (including eggshell), and even marine clams at higher levels in the sequence. Check out these lovely beasts:
They lived and died on the western shore of the Western Interior Seaway during the Cretaceous period of geologic time. My Canadian Rockies field geology students visited this site in 2012. I’m taking students back again this coming summer, in July. Let me know if you’re interested in joining us!
10 February 2014
When Michael Collier came to visit last year, he recommended a couple of books to me. I finally got around to reading the first of them – Refuge, a memoir mixed with natural history of Utah by Terry Tempest Williams.
The arc of the story is essentially twofold: the women in Williams’ family get cancer, and get treated for cancer (mastectomies, chemo, nutritional supplements to negate the nausea of the disease’s final stages), and they die. At the same time, the Great Salt Lake fills up to overflowing, drowning Williams’ favorite wildlife refuge, a place where she (and her family) appreciate going and seeing the birds. The homeostasis of the women’s bodies is out of whack, and so is the ratio of rain and evaporation that keeps the lake level constant. That analogy – nature and family out of balance – is the double-threaded core of the book.
Each of the chapters is titled with the name of a bird, rather than a number. Some of these connections are explicitly made – with the poor burrowing owls near the beginning, for instance, but others seem pretty tenuous – a brief mention of no real narrative use to the particular chapter. The lake level’s elevation is also noted prominently, although it’s hard to connect with these numbers (4204′ to 4211′ is the range). I suppose that’s meant to quantify the mounting sense of crisis, but from my perspective, it’s hard to tell what real difference 4206 has over 4205. Much more effective are her thoughtfully constructed anecdotes of what the lake does as it swamps the wildlife refuge: stories of people in nature having personal realizations about the world. It is in those moments of insight that the book is at its strongest.
Williams is a feminist and a Mormon. If you think that’s an oxymoron, you should realize she sees herself as culturally Mormon, but not a fundamentalist or a bright-eyed zealot. She was raised in Utah (and, as it turns out, is a Romney descendant – a cousin of the former presidential candidate). And her tough mother, the most intriguing character in the book, models independent thought for her children. Williams thinks for herself, and this allows her to both explore her connection with the natural world and the fact that she is a person in her own right, capable of embracing parts of Mormon culture, and leaving other ideas aside as invalid, destined only for the dustbin of history.
Some of my favorite moments and bits: Staring up at the stars, she quotes a friend who says “City lights are a conspiracy against higher thought.” That rang true for me – it’s a profound thing to stare out into the night sky, to ponder the age of the stars. Seeing the stars again at night is one of the huge benefits I’ve realized from my move a year and a half ago from DC to the Fort Valley. I look up at night and receive photons on my retina – photons that have just expired inside my eyeball after traipsing across the universe for billions of years. It’s an astonishing thing, which brings out in me a sense of my own insignificance in the grand scheme of things. I find that liberating, though others find their unimportance terrifying. Some see God in those vast reaches, others a total absence of anything godlike. Either way: Wow.
Here’s another moment where Terry Tempest Williams taps into a universal truth (p. 160):
Peeling an orange is a good thing to do in the mountains. It slows you down. You bite into the tart rind, pull it back with your teeth and then let your fingers undress the citrus. Nothing else exists beyond or before this task. The naked fruit is in your hands waiting for the sections to be separated. Halves. Quarters. And then the delicacy of breaking the orange down to its smallest smile.
Isn’t that beautiful? Isn’t that true? Boy, if a writer gives me a few transcendent moments like that in the course of a book, they’ve got my attention all the way through.
Terry Tempest Williams reminds me of one of my favorite authors, Edward Abbey. But where Abbey is rough, Williams is smooth. Where Abbey is cantankerous, Williams is considerate. Both relish life amid the wonder of the desert. Both celebrated Utah’s wild places. And both seem to tap into the same vein of natural philosophy (in the modern sense of the phrase). While there are topics that each write about that don’t speak to me, the visceral authenticity of key passages will keep me reading, looking for more.
I was impressed with Refuge, and I know I’ll be adding the rest of Williams’ canon to my list of books to read.