15 April 2014
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.
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:
…But what’s that red rock, so much finer grained, in the background?
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:
It crops out along a stream my family and I crossed on a hike last weekend. Here’s a four-shot panorama, looking upstream:
That’s in this little tributary valley:
And downstream, a similarly- lichen-encrusted and moss-draped exposure:
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).
Elsewhere, I found a piece of float on the trail, showing Bloomsburg with manganese oxide deposits on/in it:
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.
14 April 2014
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:
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:
I infer that shadblow is named for the fact that its flowers “blow” (bloom) when the shad swim upstream to spawn.
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.
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.”
Baxter wanted to smell one:
…Smells like spring’s here!
11 April 2014
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.
A lovely little structure, don’t you think?
Happy Friday, all.
10 April 2014
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.
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.
9 April 2014
Hark! What gleams on yonder contact?
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…
Sure enough! There they are!
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:
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.
8 April 2014
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:
Do you notice a pattern to the feldspar phenocrysts on the left side? They’re aligned!
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…
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.
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.
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:
We asked the students to sketch this extraordinary display of deformation…
… and in each of their field notebooks, something like this unfurled:
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.
7 April 2014
There are two rock units in this photo. One is igneous, one is sedimentary. Can you find the contact between them?
It’s somewhere along this dashed line…
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:
…but that one is in the shade. It’s bolder in the direct line of the Texas sun:
NOVA and EPCC students got to check out this contact in person on our field course over spring break:
In the next photo, your perspective is orthogonal to the contact. You’re looking “through” a screen of shale into the andesite beneath:
We’ll be returning to these rocks again this week, to look at some additional details of the contact.
6 April 2014
Longtime reader and frequent contributor Howard Allen has three images to share with us today. Let’s see what he’s got…
Oh my. What is that?
West ridge of Mount Evan-Thomas, Opal Range, Peter Lougheed Provincial Park, Alberta. View looking south across Grizzly Creek at structural features (yellow boxes outline detail images to highlight key features, shown in subsequent images). Rocks are Carboniferous (Mississippian) carbonates. Location is 50.7660, -115.1259. Photo taken in 1981.
Oooh! Exciting! Let’s zoom in.
Detail 1: Folds in subvertical bedding, carbonate rocks:
Detail 2: High-angle reverse fault with drag folds:
Great stuff. Thanks so much for sharing.
Thanks again for your contributions, Howard. I’m hoping we can see this site for ourselves this summer!
4 April 2014
We have a guest Friday fold today, from reader Ben Mackay-Scollay of the Monash University School of Geosciences in Melbourne:
Hey there Professor Bentley, been a fan of your blog for a while and I thought you might be able to use this for your Friday Fold series. It’s an upright fold at Bermagui in New South Wales, where I visited recently as part of my post-grad coursework for a structural geology course. The scale is around 3 metres across in this photo, and the fold is defined by the contrast in weathering between sandstone and mudstone layers. The mudstone layer at the top of the fold has formed into a chevron as a result of flexural slip, with the mudstone being forced into the low-pressure area.
The whole area has undergone something like 4-5 deformation events, but hasn’t ever been deep enough or hot enough to grow metamorphic minerals – this can make determining the relationships difficult. Also, it’s a national park, so we’re not allowed to take samples for thin sections either. It’s great to have something to debate back at camp every night though!
I check your blog most weeks, as do most of my fellow Honours students who are serious about their geological knowledge. It’s great to read about your work.
You’ll also note some bonus tafoni in there! Cool.
Thanks, Ben. Hopefully you’ll be sending us more great images like this in the future. I think your goal is to get up to “Ben Folds Five“!
3 April 2014
These four slabs, collected at “Fossil Hill,” north of the Cristo Rey laccolith at the Chihuahua (Mexico) / Texas / New Mexico triple point, bear positively-weathering fossils of the benthic foraminiferid called Cribratina, an index fossil for the Albian age / stage:
My field notebook serves as an imperfect sense of scale.
Here’s the Albian in the context of the middle of the Cretaceous period of geologic time, from the International Commission on Stratigraphy’s geologic time scale:
Zooming in closer, you can see they are elongate, with multiple “bubble”-like body chambers:
I plan to image these specimens with our macro GigaPan rig soon, so that you can explore them on your own.