18 May 2015

Spheroidal weathering in sandstone

Here’s a nice example of spheroidal weathering in a sandstone, developed using orthogonal jointing and bedding:

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This is one of many sites I visited Saturday near Bolt, West Virginia, on a field trip with NAGT’s Eastern Section.

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15 May 2015

Friday fold: Crumpled Carboniferous cyclothems

Twitter follower Bob J. submitted this week’s Friday fold: Carboniferous cyclothems at Scremerston, Northumberland.

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Thanks Bob!

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8 May 2015

Friday fold: Foreknobs formation

Last one in the triumverate sent in by reader Eric Fulmer:

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Annotated:

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Happy Friday and (for many of you) happy end of the semester!!

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1 May 2015

Friday fold: Tight syncline/anticline pair in Brallier Formation

Reader Eric Fulmer provided last week’s Friday fold, and this week’s too:

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This is also along the banks of the C&O Canal (mile marker 159.5), showing turbiditic strata of the Devonian Brallier Formation, crunched up into a tight fold pair. The Brallier is made of sand, silt, and clay that was shed off the Acadian Orogeny into a neighboring sedimentary basin.

Annotated version:

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This deformation, entirely typically of rheologically weak units in the Valley and Ridge, is inferred to have accumulated during late Paleozoic mountain building, when Gondwana (Africa) rammed into North America in an event called the Alleghanian Orogeny.

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30 April 2015

Early Paleozoic fossils in Nealmont Formation, Germany Valley, West Virginia

Looking at Ordovician carbonates in Germany Valley, West Virginia, a few weeks ago on Rick Diecchio’s GMU sedimentology and stratigraphy course field trip:

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Lots and lots of brachiopods…

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Crinoid columnals mized with brachiopods:

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A set of coarsely-infilled trace fossils:

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Crinoid stem:

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Nice strophomenid brachiopod:

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Bryozoan? Receptaculid?

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28 April 2015

Epidotized tuff, Tucson Mountains, Arizona

I was in Tucson this past weekend for a book project meeting, and my editor and coauthor and I took a hike on Sunday morning in the Tucson Mountains to Wasson Peak. Not far from the summit, we saw an epidotized tuff, where the fiamme and pumice blobs had undergone reactions to produce pods of epidote, giving the rock a look like a sick dalmatian:

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This is a cool rock (in my mind) because it tells a story with two chapters –

1) explosive eruption of a volcano, and

2) later wet metamorphism/alteration.

There was an adit (mine opening) in heavily epidotized rocks a few meters away – probably a fault or other vein of mineralization that had been tapped and explored a bit, but was clearly never a ‘big time’ operation. We did see a little bit of malachite, too – so presumably it would have been copper the miners were after.

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24 April 2015

Friday fold: Bloomsburg Formation at lime kilns on C&O Canal

Milepost 127.4 (High, 2001) on the C&O Canal:

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…Cool if you’re into history.

…Cool if you’re into economic geology.

…Cool if you’re into Friday folds!

 

Reader Eric Fulmer sent me this photo (along w/ two others you’ll see in weeks to come). Thanks, Eric!

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17 April 2015

Fault breccia in Beekmantown limestone

Here’s another sight at the Eocene dikes site in Bluegrass Valley, Virginia, mentioned yesterday:

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That’s a gorgeous fault breccia, emplaced parallel to bedding, and parallel to the felsic dike (which can be found a few feet to the west / right of these photos):

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It was very poorly lithified, shockingly crumbly to the touch, considering the big slab of rock downhill (to the right) of it.

Here’s a link to the site on Google Maps if you want to visit it yourself.

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16 April 2015

Eocene dike and sill in Ordovician limestone

Yesterday I introduced you to coincident

  1. warm/hot springs (indicative of higher-than-normal geothermal gradients) in western Virginia and eastern West Virginia, and
  2. Eocene aged igneous intrusions in the same region, the youngest volcanic rocks anywhere east of the Mississippi.

The two may have a common genetic relationship, though this has yet to be decisively linked in the scientific mind.

Today, let’s examine a new outcrop of the latter phenomenon: Eocene intrusives in much older rocks.

This quarry (site on Google Maps) in the Bluegrass Valley shows a felsic sill and a mafic dike that intrude into the early-Ordovician-aged Beekmantown formation carbonates. Bedding dips at a high angle (to the east) here, and thus so does the sill. These photos are looking along strike to the north:

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Closer…

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Closer…

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Annotated:

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GMU student for scale:

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Western contact of sill with host limestone strata (taken looking along strike):

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Eastern contact of sill with host limestone strata (taken looking across strike):

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Close-up views of porphyritic texture of the felsite:

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Rounded xenolith included in felsite sill:

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Several xenoliths (intrusion breccia) along eastern side of sill:

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Looking back along strike to the south, from the middle of the sill toward the rounded outcrop of sill in the middle of the quarry:

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Closer to the middle of the quarry is the mafic dike. Here it is: The mafic dike is almost perfectly perpendicular to the sill (and, thus, to bedding). It is vertical:

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Close-up view:

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What’s the deal with this suite of bimodal intrusive rocks? Like Mole Hill in Harrisonburg, they were intruded during the Eocene, around 45 million years ago. Did the lithospheric mantle beneath this region delaminate then? Did a little hotspot punch up? We’re not sure. These rocks are a bit of a mystery for the time being…

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15 April 2015

Spring time

This past weekend, I had a chance to visit Bath, Highland, and Alleghany Counties, Virginia, three amazingly beautiful places I had never before seen. I was tagging along on my colleague Rick Diecchio’s annual sedimentology & stratigraphy field trip for George Mason University. I was eager to learn from some awesome field sites from him in the year before he retires. We saw some terrific sedimentary rocks, of which more later.

More to the point of this post, the area is host to both some weird Eocene-aged igneous intrusions (unique in the Appalachian Valley & Ridge) as well as some unusual hot and warm springs, also unique along the length of the Appalachian mountain belt.

One of the warm springs in Warm Springs, Virginia, was developed into a couple of bath houses, one for men and one for women. They are charmingly derelict old UFO-shaped structures, preserved in roughly the form they were in when Thomas Jefferson soaked here (1819):

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I’ve been meaning to see this place for myself for years, ever since I had an Honors student use the site as the basis for her final project.

We didn’t take time to take the waters ourselves, but moved on to a point of geological interest further to the south:

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That’s Falling Springs Falls (no joke), and the impressive thing about it is not merely that it’s a six-storey waterfall, but that the entire waterfall edifice is travertine, actively being deposited by the calcite-saturated water, which derives its CaCO3 from its time percolating through Cambrian and Ordovician limestone strata below the surface. All the rock you see there is not the remnants of what is otherwise being eroded away (such as at places like Great Falls or Niagara Falls) but instead the opposite – new rock being laid down by the agitated water. This falls progrades over time – inverting the traditional conception of a waterfall’s relationship to its host landscape.

Thomas Jefferson described this site in his Notes on the State of Virginia (1781). The waterfall was mined for lime (for agricultural purposes) for many years, but is now a tourist site. Jefferson claimed the height of the falls as “200 feet” but today it’s more like 80 feet. This discrepancy may by explained due to the mining, perhaps. Above the falls, there are extensive travertine deposits rising to higher elevations – perhaps these were added into Jefferson’s calculations?

I’m not usually impressed by waterfalls (I mean, what else do you expect where water is pulled by gravity without any ground beneath it?) but this was striking. I’m glad to have seen it.

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