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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14 April 2015

Compare and contrast: Two Chesapectens

Two new GIGAmacro images of fossil scallops from Virginia’s Coastal Plain –

Chesapecten nefrens:
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Chesapecten jeffersonius:
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My vision is to get the opposite side of each of these samples as well as a half-dozen other species in this genus, perhaps even multiple individual specimens of each species, to allow students to do a lab where they plot morphological changes over geologic time as an example of what the fossil record shows about evolution.

(Credit for making these images goes to my student Robin Rohrback-Schiavone, as usual.)

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6 April 2015

Exploring Mahantango Zoophycos traces in GigaPan

Recently, I posted about an excellent road cut in Fort Valley showing well-developed 10 cm+ Zoophycos trace fossils.

Presented here are three new GigaPan images (two outcrop; one macro) of Zoophycos from the Devonian-aged Mahantango Formation:

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These images are part of a new “virtual field trip” that I organized to supplement my historical geology field trip to examine the geologic history of the Massanutten Synclinorium. The link there will take you to a webpage with dozens of embedded GigaPans of the region (including one annotated comparative view), showing outcrops and hand samples that tell the tale of this region through sedimentary layers recording the Cambrian through Devonian (including the waxing and waning of early-stage Appalachian mountain building recorded as clastic input), as well as deformation associated with late Paleozoic mountain-building.

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27 March 2015

Friday fold: Isla Escarpada, Chile

I’m very nearly delinquent on posting the Friday fold… Here you go – a Google Earth view of a differentially-weathered fold partly above and partly below sea level in Chilean Patagonia, south of Puerto Natales:

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They call it Isla Escarpada. Awesome.

Here’s a Google Maps link if you want to explore it yourself.

Happy Friday!

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24 March 2015

Martinsburg Formation outcrops in Edinburg Gap

Last Friday’s fold was one I spotted when out geologizing last Thursday in Edinburg Gap, a wind gap in the western Massanutten Range, one of the main paths of egress for those coming into or leaving the Fort Valley. There, massive outcrops of Martinsburg Formation grace each side of the twisty road, and reveal a totally different ancient world.

In the late Ordovician period of geologic time, this site was deep underwater, a trough-like basin receiving copious quantities of sediment from the active Taconian Orogen to the east. The sediment was brought out in pulses — underwater avalanches called turbidity currents; that settled out to produce graded beds of shale and graywacke.

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Classic graded bed – sudden, crisp base, and gradually fining away from it…

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Many such:

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A relatively clean outcrop showing pervasive planar-laminated sands:

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Bedding exhibits a variety of attitudes in Edinburg Gap – symptomatic of parasitic folding on the larger Massanutten Synclinorium, no doubt.

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Beauties:

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Here’s another example, in float, seen to be up-side-down on consideration of its sedimentary structures’ orientations:

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Here, the base of a graded bed shows some meager fossil content, mainly preserved as external molds:

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Zooming into the right side – crinoid columnals mainly:

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Here’s a lone skinny crinoid stem lying elsewhere in the fine sandstone:

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Here are some trace fossils – dark mud-filled burrows which cross-cut the lighter-colored sandy turbiditic layers:

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Example #2 of trace fossils transecting graded beds:

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Example #3:

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Of course, there isn’t merely a sedimentary / biological story recorded here; structural geology plays an important role in our interpretation, too. These turbidites (rocks deposited by turbidity currents) were involved in Appalachian mountain-building. When ancestral Africa collided with ancestral North America in the orogeny that assembled Pangaea, these strata were squeezed. Pressure solution on a microscopic level imparted a cleavage that’s most pronounced in the shalier layers:

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The rock breaks along the cleavage more easily than the bedding, resulting in rectangular scree, each a miniature cross-section of the turbidite layers:

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I’ve been driving past these outcrops for almost three years now; but things in my life are so busy that I didn’t get a chance to stop until just last week. That’s nuts! Parenthood has really cut into my geology habit. I have a dozen other examples of this – places close to my home that I just can’t seem to find the time to visit for the purposes of geology.

That said, I’ll note that the Martinsburg Formation is my new favorite formation in Virginia. I love the geopetal structures, the evocative abyssal fan imagery, the imagined violence of the ancient mass wasting, the connection to orogenic processes, the fossils (both body and trace), the later Alleghanian structural overprint. It’s a great formation, and I look forward to learning more about it as the years go by…

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20 March 2015

Friday fold: flexed turbidites

Spring is almost here! As you get ready for the equinox, enjoy this gentle fold on a Friday:

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These are turbidites (graywacke and shale) of the late Ordovician Martinsburg Formation, seen in Edinburg Gap, western Massanutten Range, greater Shenandoah Valley, Virginia.

Bedding is flexed very slightly here, from moderately-dipping to more steep, and then back to moderate again. Slickensides on the top of some exposed layers indicate the beds shifted slightly relative to their upstairs and downstairs neighbors: an example of folding through flexural slip. Cleavage here dips to the left, at a steeper angle than bedding.

Happy Friday, and happy Vernal Equinox!

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19 March 2015

Zoophycos trace fossils in Mahantango Formation, Fort Valley

Spotted these lovely ichnofossils this morning in the southern Fort Valley, on Saint David’s Church Road.

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These are all wet because I washed and scrubbed the outcrop in preparation for GigaPanning it.

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Unwashed heathens follow…

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Zoophycos is a helical feeding trace fossil – the signature of some critter systematically mining the mud in its vicinity for digestible organic content. This outcrop is really great, and I hope to share a GigaPan version of it with you soon.

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16 March 2015

Mechum River Formation, near Batesville, VA

Previously on this blog, I’ve mentioned the diamictites of the Konnarock Formation, the Rockfish Conglomerate, and the Fauquier Formation.

There is another probable glaciogenic unit in the Virginia Blue Ridge: the Mechum (pronounced “Mitchums”) River Formation, which crops out in a narrow belt in the central part of the Blue Ridge Anticlinorium, overlying basement rocks (gneiss, more or less). I visited it last week with Chuck Bailey from the College of William and Mary, my student Josh Benton, and three of Chuck’s students.

It starts out with a clast-supported diamictite at its base:

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There’s some nice deformation to be seen in this clast:

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Zooming in for a good look at small offsets along small faults, changing the clast’s shape and aspect ratio:

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Note how foliation wraps around it at the base:

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Higher in the sequence, you can see a much finer general aspect to the deposit, including continuous strata of laminated sands.

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But the big clasts haven’t disappeared. They are now isolated and less common in general, though when they occur, they tend to occur in the same horizon:

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Bailey and Peters (1998) interpret these sediments as glaciogenic — that is, deposited under conditions that were influenced by glaciers, perhaps those pesky Snowball Earth glaciers we keep hearing about. Specifically, they think there might be discernible annual seasonal rhythyms in these sediments coarse vs. fine grain size.

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During the summer warm season (or perhaps during longer-duration interglacial periods) the ice melted, the water flowed, the icebergs calved, and all of this is recorded with a switch from finer sediment to coarser, in particular the presence of outsized clasts (putative dropstones):

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Note the cross-bedding at the bottom of this one:

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Taken together, these sedimentary deposits of the Mechum River Formation give me a proverbial shiver – proximity to glacial ice tends to do that to me, even if at a temporal remove of ~600 million years…

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