24 March 2014

Monday migmatite

Here’s a sweet little sample of migmatite (~470 Ma late Ordovician Taconian Orogeny, U/Pb date from zircon), that my students and I spotted last week on the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province:

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Note the white translucent quartz, the orangey (partially kaolinitized and rusty stained) opaque potassium feldspar, and the shreds of biotite torn and tangled between them:

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I love the fact that I have such easy access to gorgeous migmatite here in my local stomping grounds. It’s a profoundly satisfying experience to watch the comprehension of “birth of a granite” dawn on my students’ faces when we visit these terrific rocks.

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21 March 2014

Friday fold: a random sample from the campus of UTEP

I know nothing about this sample, other than the fact that it was a thrown-away sample found in the rock pile next to the old geology building at the University of Texas at El Paso.

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Happy Friday!

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20 March 2014

Montoya Group fossils and faults on the Crazy Cat landslide

Today, I initiate a series of posts based on some of the geology I saw over spring break, in west Texas and southern New Mexico, on the field exchange between Northern Virginia Community College and El Paso Community College that I helped facilitate.

We spent our first morning in the field in the Franklin Mountains, due north of El Paso (and, for that matter, Ciudad Juarez).

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It was unseasonably cold as we climbed up through the fine-bedded strata of the El Paso Group (Cambrian) to check out the more massive, heavily bioturbated Montoya Group (Ordovician):

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We started on the southern tip, on the west side, where several of the layers (tilted to the west due to Rio Grande rifting) had slumped down the dip-slope along weaknesses along the bedding plane.

Here, Sarah, Daniel, and Marcelo examine the rubbley mass of fractured rock where a sheet of the Montoya Group slid down atop younger layers. You can see the coherent dark gray limestones below them, the relatively coherent stratum above their heads, and also the whitish crumbly looking mass that extends 1-2 meters above their heads (it includes several large chunks of gray limestone):

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Overall, these bedding-parallel landslide blocks are known as the Crazy Cat Landslide.

The Montoya Group strata were originally deposited in passive margin conditions of the Sauk Sea, a major epeiric sea that flooded the ancestral North American continent during early Paleozoic time. It’s chock full of fossils.

Here is a great example of Halysites, a “chain coral”:

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Zooming in for a closer (though not entirely in-focus) look at the structure of the colony:

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It wasn’t all sessile critters. Here, side by side, are a straight nautiloid (an “orthocone”) and a big snail:

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Another enormous gastropod / snail:

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And here is another mighty orthocone:

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Lastly, consider this specimen of Receptaculites, the odd “sunflower coral” which was likely not a coral at all:

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We also spotted some faults that were not parallel to bedding, but instead cut clean across it. Here’s an example, filled with calcite-cemented fault breccia:

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Here’s another, pretty much crushed beyond breccia into a proper “gouge”:

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But what’s the offset there? I didn’t see a really distinctive marker bed that I felt I could hang my hat on. I walked out the trace of the fault.

I found a good layer some distance uphill that showed the offset, though it was in a small parallel fault and not the main zone of white gouge:

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Offset was apparently left lateral, with the north side of the subvertical fault moving ever-so-slightly westward.

As we hiked along, the sun rose, and we warmed up both physically and mentally. These were great rocks to start the trip with!

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18 March 2014

On ignorance, bias, data, and the tentative nature of (scientific) interpretations

Yesterday’s post on the angle of repose in shale scraps falling off the “Wallbridge Unconformity” (?) outcrop I visited with Alan Pitts concludes the series of blog posts about new observations made along Corridor H. Several people wrote to me or commented on the blog posts how much they enjoyed reading the “virtual exploration” of these new sites. Apparently I also have a reader at the Institute for Creation Research (ICR). This young-Earth creationist group is not merely anti-biological evolution; They also advocate for “Flood geology,” the idea that the world’s geologic strata were laid down in the Noachian flood of the Bible. I’m grateful to Peter L., who keeps an eye on the young-Earth creationists at the ICR at his blog Eye on the ICR, for alerting me to a creationist critique of my blog post a few weeks back on the possibility of a marine incursion into the red beds of the Hampshire Formation.

So – it appears we are not done with Corridor H yet. Let’s review the story, and seek what lessons we may from the creationist’s critique.

As you may recall, in exploring the new roadcuts along Corridor H, Alan Pitts and I were surprised to see a unequivocally marine layer sandwiched between red beds. We had tentatively identified the red beds (oxidized river + floodplain deposits) as Hampshire Formation, but the Hampshire Formation was supposed to be terrestrial only (there are no rivers and no floodplains below sea level in fluvial-estuarine-marine connected depositional systems). The marine incursion began sharply, and was 5 meters thick. Here’s the base of it:

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This observation left us with a conundrum of interpretation: As I said then,

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?

These questions are hypotheses – possible explanations for the novel observation. Geologists are comfortable with several possible explanations swimming around in their head at once. We don’t just pick one. We work with all of them, and attempt to pare them down to the least possible number by seeking additional evidence (data) to confirm one or rule out another. As an indication of the process of thinking about geology, I posted my photos here on this blog along with my list of two possible interpretations, and then eagerly awaited feedback from my readers, many of whom know more about sedimentary rocks and sequence stratigraphy than I do. Though I was confident in my observations, I wasn’t sure what I was missing that would allow this sequence to make sense. So I put my preliminary data out there and asked “What do you think?”

Sure enough, I soon got some great feedback. Let’s go ahead and file it under “peer review.”

To start with, the fossils I photographed were partly useless (snails, which can be freshwater or marine), and partly diagnostic (articulate brachiopods).

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Several of my readers, including Howard Allen and Mike Huggins, noted that this appears to be a productid brachiopod, which is an index fossil for the late Paleozoic. Howard noted it was a ‘dead ringer’ for a species which indicates Mississippian aged sedimentary deposits in Alberta.

So there’s one line of evidence to suggest that hypothesis A (“these strata aren’t Hampshire Formation”) was correct.

Mike Huggins also pointed me to a reference that suggested there are in fact brackish/estuarine strata within the Hampshire (evidence consistent with Hypothesis B).

Then there is the location: this stretch of Corridor H climbs the Alleghany escarpment, the lip of the Appalachian Plateaus province. The strata there are younger than what I’m used to seeing the Valley & Ridge province. Though we didn’t have a good geologic map with us during the field trip, if I use the online geologic map of West Virginia, it suggests an upper Mississippian age for these beds, though it’s hard to pin down the exact location because the road is so new it’s not on Google Earth. The outcrop in question was a few hundred feet below the lip of the plateau (lined with the wind turbines you can see in some of the post’s photos. So my simplistic “redbeds=Devonian” initial interpretation is likely invalidated by this non-Valley-&-Ridge position. That’s Mike Huggins’ interpretation, too.

A USGS sedimentary geologist and mapper whom I consulted concurs with that assessment. He got to briefly ogle the beds on a trip out to Canaan Valley the weekend before the blog post was published, and wrote (in an email) to me that he thought they were, “upper Mississippian Mauch Chunk Group or lower Pennsylvanian Pottsville Group.”

I’m very grateful to these three geoscientists for offering me their feedback. Based on their “peer review,” I think the situation was this: I was simply ignorant of the wealth of strata that feature red beds in our area. I was projecting the bias of my own limited experience onto rocks which were outside my experience. Based on the arguments my colleagues made in response, I now think these strata are likely of Carboniferous age, rather than Devonian. The strata aren’t the Hampshire, but are instead more recent, probably Pottsville Group. And thus my conundrum dissolves like smoke in a breeze of fresh understanding. With this more refined version of Hypothesis A in my head, I’m now eager to return to the roadcuts and evaluate them anew. I’ll form new interpretations (perhaps several, perhaps competing, all subject to denial) and seek new data.

The point of this blog post is to emphasize that my conclusions were (a) tentative, and (b) subject to dismissal based on new evidence. I was ignorant of key facts, and limited by my lack of experience. New evidence was presented, and that evidence changed my mind about my hypothesis. In other words, the conclusion is tentative, and must hew to the data.

I would encourage my readers to consider what evidence Tim Clarey, author of the ICR blog post, would find to be inconsistent with his hypothesis. Can any data (fossil content, sediment type, location on Earth’s surface) convince a Flood ‘geologist’ that the global flood hypothesis is incorrect?

Clarey writes,

Something is definitely wrong with the uniformitarian story—why else would scientists be so surprised by the black rock and marine fossils? Could it be that all these strata—the red and black rocks—are deposits from the great Flood? This interpretation eliminates the mystery of how marine fossils are found sandwiched in between red sands and shale. It also solves the mystery of the black, organic-rich shale.

Rapid deposition during the Flood would have preserved ample organic material to give a black coloration to the rocks. There is no need to call on special, restricted, low-oxygen conditions to explain the dark color. Organic debris was merely buried within the Flood sediments along with the fossil shelled animals.

A Flood origin for the sediments accounts for the rock types we observe much better than secular models. Creationists don’t have to fabricate tales of the sea level rising and then draining off the land suddenly, over and over. We just recognize it happened once, in a catastrophic way, about 4,500 years ago.

Clarey wants it both ways – the black is marine (and therefore indicative of not just any marine transgression, but a specific global flood of divine origin, inexplicable by a modern understanding of physics and Earth processes). But not only the black indicates Noah’s flood, according to Clarey, but does the red. I guess it follows that any sandstone you’d care to reference is from the Flood, and so is all the world’s conglomerate, and so is the limestone, including ooids and mudcracked strata, and so is the diamictite, and so is the shale full of plant fossils, and — what the hell — so is the granite and the schist and the basalt. Clarey holds a PhD in geology from Western Michigan University. You know they can’t be too psyched about that, when all his observations lead to the same conclusion. There’s only one conclusion to a young Earth creationist, and no data can ever dissuade him or her.

4,500 years ago, there were bristlecone pine trees growing in the White Mountains of eastern California. One wonders how Clarey reconciles that fact (a matter of counting tree rings) with his silly idea that the whole planet was under seawater.

I am amazed anew by the young-Earth perspective. It is a blinkered, resolute, evidence-free piece of sacrosanct silliness immune to any rational line of argument.

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17 March 2014

Angle of repose in Devonian shale, Corridor H

Where the Needmore Formation is falling apart on Corridor H, sloughed-off shale is piling up in a tiny talus slope:

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You could slap a protractor on that photo and get a pretty good measurement of the angle of repose of this chippy, flaky granular material.

Eyeballing it, that looks like about 45° to me.

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14 March 2014

Friday fold: rolled boudin

Howard Allen returns with a Friday fold contribution for this week. He says:

Not exactly a fold, but I thought you’d like this rolled boudin(?) (quartz) in a muscovite garnet schist (note garnets above the lens cap). This specimen is also from the Shuswap Metamorphic Complex, north of Sicamous, British Columbia.

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Lovely. Thanks for sharing, Howard!

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9 March 2014

Off to Texas

LoloI just wanted to let readers know that posting will be light this week, as I’m down in Texas running my field exchange course with Joshua Villalobos of El Paso Community College. Joshua’s supplying 12 students, and so am I. Together with talented colleagues, we’re aiming to give these students, many of whom hail from traditionally-underrepresented groups within the geosciences, a world-class field experience that will take them from the Border to the Beltway. We have a week in West Texas and New Mexico this week, and then we are reuniting in May in Virginia to examine mid-Atlantic geology. The program is funded by a generous grant from the National Science Foundation as an experiment to see if we can recruit a more diverse suite of geoscientists by facilitating a top-notch field experience early in their educations.

Anyhow – I’ve got a Friday fold scheduled for the end of the week, but don’t expect anything before then. Bye!

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7 March 2014

Friday fold: Obsidian on display at ASU

Happy Friday! Here’s a beautiful folded obsidian sample, replete with conchoidal fractures, on display outside the geology department at Arizona State University in Tempe, where I was last week:

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6 March 2014

Puzzling over the Wallbridge Unconformity along Corridor H

The Wallbridge Unconformity is a surface of stratigraphic hiatus or erosion between the depositional influence of the Tippecanoe and Kaskaskia epeiric seas. After Alan Pitts and I located ourselves in the Oriskany Sandstone (terminal Tippecanoe stratum), we looked stratigraphically above the quartz sandstone for the overlying unit, which should be the Needmore Formation shale (beginning of the Kaskaskia sequence). Indeed, the quartz sandstone was overlain by a black shale at the site:

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This road cut is on the south side of the highway, facing north. Hence, it was in deep shade when we visited in late afternoon. Uncharacteristically, I see I failed to include a sense of scale in that photo, but the peaklike sandstone outcrop at left is probably about 7 meters tall.

A better exposure can be seen on the north side of the road, where it’s not only sunny, but the contact is more discernible.

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I remembered to include a sense of scale for this shot. Thanks, Alan!

Here’s what strikes me about this exposure: the contact between quartz arenite below and black shale above does not appear to be abrupt or erosional, like I would expect a disconformity to appear. Instead, there are gradually thinner and thinner sandstone beds, and thicker and thicker shale beds. Let’s zoom in:

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Fundamentally, this looks transitional (conformable). It doesn’t look like an unconformity. But it’s supposed to be one. So what gives?

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5 March 2014

Oriskany Formation quartz arenite and its fossils, Corridor H

Today, a few more photos from the field trip last month to Corridor H, the fine new superhighway with so little traffic out in eastern West Virginia.

Our antepenultimate stop of the day was at an outcrop we inferred should hold the Oriskany Sandstone, a Devonian quartz arenite that lies stratigraphically above the Helderberg Group limestones and below the Needmore Shale. We were using Lynn Fichter’s indispensible stratigraphic column for our explorations, and while some of the Oriskany looked like what I expected it to, there was also a greater diversity of the unit than we expected: dark gray color in places (rather than the sugary white I recalled as classic Oriskany), conglomerates, etc.

We did find a bunch of brachiopod fossils, though, and that is something I associate with outcrops of the Oriskany such as those on Sandy Mile Road near Sideling Hill. Let’s see some…

Articulate brachiopod shells in cross-section:

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Here’s another brachipod, a huge one with a relatively flat morphology, a strophomenid, I reckon:

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Zoom in on the fine radiating costae on the shell:

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I pried off some of the overlying sandstone to reveal more of this lovely beast:

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And, just for variety’s sake, here’s a gastropod (snail) we found at the site, too:

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