18 April 2014

Friday fold: Sideling Hill

Here’s what the Sideling Hill road cut looked like last month:

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It’s a terrific example of a syncline. Usually I show folds in profile view, but here, the view is essentially perpendicular (not parallel) to the axis of the fold:

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Sideling Hill’s rocks are early Mississippian in age, made of debris shed off the late Devonian Acadian Orogeny, and they were folded during Alleghanian deformation in the Pennsylvanian-Permian.

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17 April 2014

Pseudopictographs

I found this interesting looking slab of gray limestone last summer in the Bridger Range of Montana, in one of the talus slopes on the north side of Sacagawea Cirque.

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The high-contrast pattern reminded me of something, but I couldn’t say quite what. Then I realized: it looks like one of those indigenous pictographs, where the artist puts their hand up to the rock and spits paint all over it, coloring the non-hand space and leaving the space covered by the hand free of color. The color effect is sharp-edged and darkest right at the contact with the margin of the hand, and fades off in all directions away from that edge.

This rock shows the same pattern, except nature made it. It’s strikingly similar in form and boldness to writing or hieroglyphics. Lovely, but I’m not sure how exactly it formed. Let me speculate…

I think what was happening here is that there were certain preferred fluid-flow pathways in the rock. Some were cylindrical / linear, piercing the bedding plane as a point: burrows, perhaps?. Others were tabular / planar, intersecting the bedding plane in a line: fractures? bedding-parallel feeding/crawling traces? Either way, fluids were percolated through, and these fluids interacted with local consituents to produce a reddish staining (oxidation of iron, presumably). This effect falls off with distance away from the “plumbing.” Later, perhaps, a second batch of fluids, with a different composition, pumped through the same system. This time, the local effect was to bleach the rock back to its original color (so the fluids would have been “reducing” in their chemical activity). And then, a third batch, to make the deep red “cores” to the lines and points??

Anyone want to offer a different take on this? I’m sure I can learn from your insights…

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16 April 2014

Turitella in Buda Formation limestone

Back to Texas, today. Here’s a cross-sectioned Turitella snail from the Buda Formation limestone:

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It’s exposed in a block of rock on the north side of Mt. Cristo Rey.

You can explore these GigaPanned blocks of the Buda in search of your own Turitella… How many can you find?

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15 April 2014

Bloomsburg Formation exposed near Elizabeth Furnace

As noted previously, I live in a regional scale fold: the differential erosion of the Massanutten Synclinorium has produced the ridge of Massanutten Mountain, which separates the Fort Valley from the Shenandoah and Page valleys on either side.

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The Fort is “fort” like because the strata which underlie it are relatively friable, soluble, or otherwise erode-able.

The ridge-forming layer is the Massanutten Sandstone, a Silurian-aged quartz arenite. Here’s a boulder of it:

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…But what’s that red rock, so much finer grained, in the background?

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That’s the Bloomsburg Formation: red very fine sands, silts, and shale. It overlies the Massanutten. Here’s a look at a detail from Lynn Fichter’s masterful stratigraphic column for the Valley & Ridge:

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It crops out along a stream my family and I crossed on a hike last weekend. Here’s a four-shot panorama, looking upstream:

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That’s in this little tributary valley:

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And downstream, a similarly- lichen-encrusted and moss-draped exposure:

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You can see some pronounced differential weathering even there. Certain strata are more susceptible to erosion than others. I looked underneath a shady overhang at this site, and saw red siltstone with jointing orthogonal to bedding (that’s the orientation of the shortest fracture to break a tabular layer).

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Elsewhere, I found a piece of float on the trail, showing Bloomsburg with manganese oxide deposits on/in it:

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It’s a treat to find another piece of the puzzle, another layer in the stack. I’ve only seen redbeds like this cropping out in one other place in the Fort Valley. I’ll be keeping my eyes peeled for more of the Bloomsburg on future peregrinations around the Fort.

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14 April 2014

Shadblow (serviceberry)

A sure sign of the advent of spring in Fort Valley is the blooming of the shadblow, an understory tree species with clusters of white flowers:

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My wife and I took our son for a hike yesterday, and the shadblow was pretty much the only tree with anything on its branches:

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I infer that shadblow is named for the fact that its flowers “blow” (bloom) when the shad swim upstream to spawn.

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The shadblow goes by other names, too. Its scientific binomial is Amelanchier canadensis. One of the other colloquial names is “serviceberry,” so named (I’m told) because it signals that the ground has thawed – an important consideration if you had anyone die in your family over the winter. Now you can dig a grave and bury them, with a nice service.

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Certainly it would serve as a glorious addition to any burial service. A variant on this name is influenced when the Appalachian hill country accent attempts “serviceberry:” Instead, the name emerges as “sarvisberry.”

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Baxter wanted to smell one:

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…Smells like spring’s here!

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11 April 2014

Friday fold: kink from the Billy Goat Trail

My student James O’Brien took this image of a kink band along the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province.

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A lovely little structure, don’t you think?

Thanks, James!

Happy Friday, all.

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10 April 2014

Fusulinids and stylolites, Hueco Formation

My colleague Joshua Villalobos shared this image with me the other day – it’s a thin section of fusulinid-bearing limestone of the (Permian aged) Hueco Formation, from the Tom Mays Unit of Franklin Mountains State Park, Texas.

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Note the scale bar at lower left. The big fusulinid in the middle is 3mm in diameter! And that’s not even it’s longest axis!

Fusulinids were big honking burrito-shaped protists (foraminiferids) that lived on the bottom of the sea. (They were benthic, like Cribratina). They each had one single cell. That’s it. Huge unicellular organisms – the wonders of a rigid skeleton!

The other beautiful thing about this image are the lovely stylolites (wiggly dissolution surfaces) that run from lower left to upper right. Notice that with some of the fusulinids on the top and bottom, significant portions of their bodies are missing along these dissolution surfaces.

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9 April 2014

Faults disrupting the contact between the Muleros Andesite and Mesilla Valley Formation shale

Hark! What gleams on yonder contact?

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Well, there’s no glaciers to polish anything ’round these here parts (southernmost New Mexico + westernmost Texas), so I reckon it must be fault polish. Let’s test that hypothesis by looking for slickensides…

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Sure enough! There they are!

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Unlike the deformation we saw yesterday, this faulting of the contact between the Muleros Andesite (Eocene) and the Mesilla Valley Formation shale (Cretaceous) into which it intrudes must post-date the intrusion (since you cannot break a magma, but you can break the igneous rock the magma cools down to become). The slickenlines and fault polish would have to be imparted to a solid rock, not a gooshy melt. So therefore, faulting is post-Eocene.

There are other faults to be observed along the contact, too. For instance, the site with the lineated andesite we observed yesterday, the outcrop surface bears both steeply and shallowly dipping faults with approximately perpendicular orientations:

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Sweet – within a few minutes, we were able to tease out a sequence of events that must have influenced the rocks at this site, a little glimpse into their long history.

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8 April 2014

Deformation associated with the intrusion of the Muleros Andesite

Yesterday, I showed off a few views of the contact between the Cretaceous aged Mesilla Valley Formation shale and the hypabyssal Muleros Andesite which intruded into it during the Eocene at Mt. Cristo Rey (on the US/Mexico border where Texas meets New Mexico).

Today, I’d like to look at some of the structure associated with the contact zone. First off, take a look at this image, which is looking orthogonal to the contact, glimpsing bits of the vast andesite laccolith through scraps of a “screen” of the immediately adjacent shale:

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Do you notice a pattern to the feldspar phenocrysts on the left side? They’re aligned!

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This lineation may well be a magmatic feature, showing how the magma flowed when it intruded, with cooling and crystallization “locking in” the magmatic flow pattern for us to gaze upon >40 million years hence…

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Here’s my model for how this would have worked. As magma intruded, the overlying strata bowed upward, increasing their angle of dip along the flanks of the intrusion.

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Strata of the Mesilla Valley Formation slumped off the sides, wrinkling and piling up along the margins of the laccolith. This bolstered the edges of the pluton, making the least-stress-direction straight up. So the laccolith swelled and thickened, leading to magmatic lineations along its margins.

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As for the deformation in the shale, we saw that too. Here’s a nice contorted section (sandstone layers weather out as blocky, high-relief layers amid the shale) of the Mesilla Valley Formation:

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We asked the students to sketch this extraordinary display of deformation…

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… and in each of their field notebooks, something like this unfurled:

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This deformation isn’t Laramide. It’s syn-magmatic, I think.

…But wait, there’s more. Tomorrow, we’ll take a look at brittle structures which are probably post-intrusion.

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7 April 2014

Contact between Muleros Andesite and Mesilla Valley Formation shale at Mt. Cristo Rey

There are two rock units in this photo. One is igneous, one is sedimentary. Can you find the contact between them?

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It’s somewhere along this dashed line…

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The Mesilla Valley Formation is Cretaceous shale with some sandstone. The Muleros Andesite (pretty much identical to the Campus Andesite you find at UTEP) is Eocene.

Here’s a closer, more precisely-constrained, look at it:

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…but that one is in the shade. It’s bolder in the direct line of the Texas sun:

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NOVA and EPCC students got to check out this contact in person on our field course over spring break:

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In the next photo, your perspective is orthogonal to the contact. You’re looking “through” a screen of shale into the andesite beneath:

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…Same here:

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We’ll be returning to these rocks again this week, to look at some additional details of the contact.

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