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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12 November 2014

New GigaPans from Team M.A.G.I.C. – hours of fun exploration

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It’s been a long time since I’ve shared some of the work of our GigaPan making team. Here are some of the highlights from the last five months of work…

In the images below, see if you can find (a) ten thousand fusilinid forams excavated by Texas ants, (b) Devonian trace fossils in black shale, (c) resistant beds of graywacke in a vertical orientation, (d) gastropod-rich limestone sandwiched between redbeds, (e) Ediacaran fossils, (f) a Canadian “loonie,” (g) shear sense indicators in a metadiamictite, and (i) a vesicular sand grain from a Grecian beach.

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In addition to the three students I supervise (Alan Pitts, Robin Rohrback, and Joshua Benton), images were also contributed by Aaron Barth (Oregon State University), Joshua Villalobos (El Paso Community College), and Jay Kaufman (University of Maryland).

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11 November 2014

Santorini panorama

Here is the view north across the central caldera of Santorini, Greece:

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This caldera formed during the Bronze Age, maybe as early as 1628 BCE or so, maybe as late as 1500 BCE. There are new volcanic islands rising in the center.

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10 November 2014

Root wedging: a recent example

What is Kenny pointing at here?

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Why, it’s a boulder. Where did it come from? Look uphill:

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This is as perfect an example of root wedging as I’ve seen!

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Spotted it last Friday along the C&O Canal towpath.

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9 November 2014

Silurian mudcracks in cross-section

Spotted these cross-sectioned mudcracks yesterday on Corridor H, on the GSW fall field trip:

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They are in the Tonoloway Formation, a batch of tidal flat carbonates with lots of evidence of shallow arid conditions.

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6 November 2014

Santorini ashfall and Akrotiri

Santorini is an island with nice exposures of the Tethyan subduction complex, yes. But did you know there’s also a volcano there? :)

Here’s a shot of some snorkelers, with a lovely stack of pyroclastics rising up behind them. Ash, lapilli, more ash — Santorini’s volcano has been very active over the years.

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This is a prodigious quantity of volcanic material.

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In the year 1627 BCE, the eruption of Santorini’s volcano burped up a nice bolus of lapilli and ash, and smothered the town of Akrotiri, on the south coast of the island. Today, you can visit the ruins of this town in one of the best-preserved Minoan archeological sites anywhere. Like Pompeii, it’s a lovely blend of history and volcanology:

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It is also plausible that the sudden “wiping Akrotiri from the face of the Earth” may have been inspiration for the mythology of Atlantis. Certainly it marked the demise of the Minoan civilization.

The development/preservation of the site is extraordinarily well done, from my perspective. A roof has been raised over the whole town, spanning many acres of ruins.

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3500 years ago, during the Bronze Age, there were people like you and me living in these rooms, walking on these steps…

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And then, one bad day, it all ended…

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Stream gravel deposits (much coarser and better sorted) followed by colluvium, overlie the volcanic sequence at Akrotiri, and this is preserved in the stratigraphic record of the nearby hillsides:

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Nature keeps working, doing her thing, and we can get out of the way or be buried and forgotten. The sense of a lost civilization was palpable to me at Akrotiri. I’m grateful the site was discovered and excavated – it was a fascinating insight into the ancient clash of volcano vs. humanity.

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4 November 2014

Joints highlighted with hematite, Anapra Sandstone, Cristo Rey

Good morning. Here are two images from last March’s “Border to Beltway” field trip to West Texas, on the north flanks of the Cristo Rey laccolith. Specifically, these are Cretaceous strata of the Anapra Sandstone, looking at the bedding plane of the rocks. Cutting across bedding are a series of fractures (joints) that have been highlighted by the oxidation of iron (rusting) along their edges.

In the first photo, the highlighted joints are all sub-parallel, and are cross-cut by later joints that formed after the rusting fluids had already passed through the rock.

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In the second photo, there is a more complicated picture. On the left, there is are two joint sets that are more or less orthogonal to each other, and both sets of fractures existed at the time the orange-staining fluids moved through this body of rock. In the middle, a radial pattern emerges from a depression in the sediment -

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This depression is actually a dinosaur footprint, and it contorted the strata sufficiently that it changed the stress field when these rocks broke – and the joints formed in different orientations than they would have if the sandstone was un-stepped-on.

Just for fun, here’s another dinosaur trace fossil from this same site:

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Your perspective has changed in this photo – rather than looking down on the top of a bed of sandstone, you’re now looking “up” at the bottom of a bed, and these sinuous lines are casts of the hollow furrows that must have once existed in the underlying bed (now removed). These were interpreted by our guide as toes dragging in the wet sand, but since so little weight was imparted to the foot which left them, perhaps the dinosaur that made this track was swimming, and its weight was partially supported by the water column.

There’s no structural information to be gleaned from this trace, but who doesn’t love the idea of a dinosaur swimming a little out of its depth, toes scrabbling for a hard purchase on a bottom that’s almost out of reach?

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