18 December 2014

One for the ichnologists

For those inclined toward trace fossils…

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…This is from Banff National Park, Alberta, Canada. I saw it on the trail to Consolation Lakes from Moraine Lake. I do not recall rock type – could be dolostone, could be Gog quartzite. It’s float (loose; not in situ), but I infer the photographed surface is the underside of the bedding plane; I’d be fine being totally wrong about that, though. There are two things I would like explained/identified here: (1) the prominent, arcuate ridges (which I infer to be trace fossils of some kind) and (2) the finer features, packed in sets, sometimes apparently emanating perpendicularly from the larger arcs, but also present where there are no “arcs.” They look like rodent “gnawing” marks to my untrained eye.

Anyone want to take a stab at identifying these features, and interpreting them for the sake of a poor geoblogger’s education?

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17 December 2014

Three ridges and three valleys

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That’s the view from Woodstock Tower, on the crest of Three Top Mountain, looking east/northeast across the Little Fort Valley and through Mine Gap (a water gap), across the main Fort Valley and then Massanutten Mountain itself, with the Page Valley separating Massanutten’s ridge line from the horizon-forming Blue Ridge.

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16 December 2014

Buzzard Rocks

Back in 2011, when we were still living in D.C., Lily and I made a hiking trip out to Buzzard Rocks. It was a destination. Now that I live out here in the Fort Valley, I see Buzzard Rocks all the time, and I love it. It’s such a cool feature – a spot on the crest of the hill where you can see the slabby expression of steeply-dipping beds of Massanutten Sandstone.

Here’s the view from the road:

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(Were you able to find the rock climber in that shot?)

From atop the crest of Buzzard Rocks, you can walk along this “mohawk of quartz arenite”…

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Here’s a look at a sample of the Massanutten Sandstone:

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At Buzzard Rocks, it is internally cross-bedded, as seen here:

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Buzzard Rocks needs three things in order to exist: (1) deposition of a lot of quartz sand during a time of passive margin sedimentation on the east coast [this was during the Silurian, between the dirty clastic influence of the Taconian and Acadian Orogenies], (2) deformation (tilting/folding) associated with the final phase of Appalachian mountain-building, and (3) differential erosion of the sedimentary strata. Those layers stratigraphically below the Massanutten Formation (graywacke, shale, limestone) have been etched away to make the Shenandoah / Page valleys, and those above it (limestone, shale) have been etched away to make the Fort Valley. In between, the hard, chemically-stable Massanutten stands proud of the adjacent recessive rocks, and makes the crest of the ridge.

Nowhere is this story more plainly stated than at Buzzard Rocks.

Yesterday, I hiked 1.5 miles up the Signal Knob trail to a higher vantage point, and shot this GigaPan from the trail:

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You can see the Blue Ridge in the distance there, across the development of Front Royal (see if you can find the Randolph Macon Military Academy’s campus in the shot). You can also see the incised meanders of Passage Creek exposed in the lower foreground, and the slopes of talus leading toward them from Buzzard Rocks.

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12 December 2014

Friday fold: Miette slate, Alberta

Previously I’ve featured a road cut at the intersection of the Trans-Canada Highway and the Icefields Parkway as the Friday fold.

Here are some other look at cool features to be seen at this outcrop, including folds, but also including ripple marks, cross-bedding, graded beds, cleavage, and boudinage.

First, the general setting, showing bedding (both horizontal and moderately dipping) and cleavage (upright, subvertical):

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Here’s a fold!

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Zooming in on the most folded part:

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A close look at one bed, exposed in cross-section…

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…at the top are pyrite crystals:

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Here’s a striking example of a graded bed:

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Another primary structure that can be seen here is ripple marks, exposed on the bedding plane where it coincides with cleavage:

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Boudinage deforms the stiffer, sandier-protolith layers (now quartzite transected by quartz veins):

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Finally, here are a suite of GigaPan images that Aaron Barth and I shot at the site for additional exploration by you. Check out these features in dynamic context, and revel at the ensuing rush of insight:

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Happy Friday, everyone.

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8 December 2014

Fluvial geomorphology of Mather Gorge

Mather Gorge is the name given to the narrow, deeply incised section of the Potomac River Gorge downstream from Great Falls. It’s named for Stephen Mather, the first superintendent of the U.S. National Park Service. It’s very straight, probably because it’s etched out along the trace of a fault. You can get a sense of the linear nature of Mather Gorge with this photo (paddle-boarding human for scale):

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The Gorge is a nested series of bedrock terraces or straths. Looking across Mather Gorge, these flat “steps” are obvious features to the landscape. Here, you see the Matildaville strath (upper, older) and the Bear Island strath (lower, younger). Each of these levels used to be the bottom of the Potomac River, prior to incision to deeper, newer levels.

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As you walk along the Bear Island strath (on the Billy Goat Trail, say), you encounter a great many features that indicate you’re walking on the abandoned bottom of the Potomac River. Exotic boulders (paleo-bedload, sourced to the Blue Ridge and Culpeper Basin, sites upstream of the Billy Goat Trail) are one such signature of the strath’s former incarnation, and so are the numerous potholes, each of which was drilled into the solid rock by a sustained vortex of sediment-laden river water:

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I love taking students to the Billy Goat Trail primarily for the stunning story woven into the bedrock geology, but these landforms are a welcome set of bonus features: easy on the eyes and enlightening for the mind.

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5 December 2014

Friday folds from the Shenandoah River

My student Josh B. found this beautiful map view of a plunging fold in the bed of the Shenandoah River, as viewed in Google Earth:

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Josh posted his results on Facebook, and then the other Josh (Joshua Villalobos of El Paso Community College) poked around the area and found some others downstream (north):

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Here’s a Google Maps link to the site. Check it out!

 

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24 November 2014

Non-bedding-parallel stylolites in Helderberg limestones, Corridor H

Saturday I posted some images of bedding-parallel stylolites from one member of the Devonian-aged Helderberg Formation (or one formation in the Helderberg Group; I’m not sure whose stratigraphy is preferable in this case). Here we are, further up-section, and you can see both bedding-parallel and non-bedding-parallel stylolites overprinting the limestone:

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Bedding-parallel stylolites can be understood readily in terms of sedimentary loading (compression from above), but non-bedding-parallel stylolites imply a maximum pressure direction that came not from above — but instead, from the sides. In other words, it’s a tectonic stress. These are likely the signatures of Alleghanian mountain-building in the late Paleozoic.

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Interestingly, this same outcrop includes extensional veins filled with white calcite – these rocks have been both squeezed and stretched in more or less the same direction. I should assign an Honors student to go measure their orientations in detail, and check for overprinting relationships (cross-cutting relationships) between the veins and the stylolites.

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Happy Monday – back at work!

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22 November 2014

Stylolites in Helderberg crinoidal grainstones, Corridor H

Long week, no blog.

But, hey – it’s Saturday, and I have a couple of hours of breathing room – so here are some stylolites in a crinoidal grainstrone in the New Creek member of the Helderberg Formation, exposed on Corridor H in West Virginia.

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Stylolites are pressure solution features, which overall form perpendicular to the maximum squeezing direction (maximum principal stress direction, σ1), and have little wiggle peaks that point in the maximum stress direction. So when a geologist sees this…

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…they interpret it this way:

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14 November 2014

Friday fold: GSW fall field trip

Last Saturday was the Geological Society of Washington’s fall field trip. Dan Doctor, Alan Pitts, and I led a team of ~20 geologists out to the great new exposures along Corridor H in West Virginia.

Here’s the team in front of some of the parasitic anticlines and synclines that decorate the larger structure of the Patterson Creek Mountain Anticline:

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The strata here are Silurian-aged tidal flat carbonates of the Tonoloway Formation. We collected salt casts, ostracod fossils, and mudcracks here while marveling at the overall structure…

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13 November 2014

Gypsum casts? You be the judge — UPDATE: Syneresis cracks!

I’m curious to hear what you sedimentologically-inclined readers think of these features:

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I collected this sample in May on Corridor H in West Virginia, at an outcrop of the Silurian-aged tidal flat carbonates of the Tonoloway Formation. It’s got a nice mud crack (dessication crack) triple-junction (yellow in annotation below). It has a calcite vein that cross-cuts the mud crack (pink). And it’s got lots of little lenses (in the geometric sense of the word) a few mm long, all over the “plates” of the mud cracks (blue). What are they? What do they tell us?

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My colleague Joshua Villalobos (El Paso Community College) snagged one then, too. Josh imaged his the other day with a jerry-rigged “macro” GigaPan set up. Here’s the result: It’s not as high-resolution or focused as a MAGIC macro GigaPan, but it does impart an additional perspective on these structures.

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I posted a link to Josh’s GigaPan on Twitter the other day, and asked what these things were. I got reactions that ranged from the ichnological to structural, but mainly clustered around the idea of gypsum casts.

Gypsum casts would be something that we would expect to find in tidal flat carbonates – an arid, dessicating environment would encourage the evaporation of seawater and the precipitation of evaporite minerals in the resulting brine. Halite casts are well known from this same outcrop of the Tonoloway.

The key question is then: Why are they all aligned? Any insights?

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UPDATE: Matt Kuchta has provided a compelling explanation: Syneresis cracks – dewatering structures, essentially. Click the link to the Wikipedia article and compare. Looks like it to me!

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