Today, some shots from my time in Yellowstone National Park last summer. Here's Mammoth Hot Springs:

Close-up of the travertine deposits at Mammoth:

Me advertising my brother's company at Mammoth:

Norris Geyser Basin, slime:

Norris Geyser Basin's loneliest tree:

More slime, this time two colors:

Nasty patch of slime. Looks like snot:

Bison herd:

Columnar jointing in basalt:

Me showing you where the columnar jointing is. (I'm pointing at it...)

Strata exposed in the Tower area:

And here they are again, labelled:

Lastly, heading north out of Yellowstone back to I-90 and Bozeman, here's a weathered-out Eocene dike in the Paradise Valley. The dike is more resistant to weathering than the rock it cuts through, so it stands up as a "wall"-looking feature.

Labels: basalt, field trips, hot springs, limestone, montana, primary structures, stratigraphy, travel, yellowstone

The Culpeper Basin is a Mesozoic (Triassic/Jurassic) rift valley in northern Virginia.

As Pangea was breaking apart, a series of normal-fault-bound basins stretched open in an NW-SE direction (giving them long axes that run NE-SW). Some of them connected together in a NE-SW direction, and kept spreading further and further open. Through continued seafloor spreading, these became the Atlantic Ocean basin. Some did not keep opening, and essentially filled in with dirt. Those are the ones that are still preserved up on the North American continent today, including the Culpeper Basin. These basins vary in size, but they run up and down the coast of eastern North America, from Newfoundland down at least into the Carolinas (presumably there are more buried beneath Coastal Plain layers even further south than that). Collectively, these basins are referred to as the Newark Supergroup. They are characterized by immature sedimentary rocks and mafic igneous rocks.

Here's an E-W cross section through the Culpeper Basin, by Chuck Bailey at W&M:

Structurally, then, the basin is a graben, bounded east and west by normal faults.

The igneous rocks in the Culpeper Basin are mostly diabase, but there are some basalt flows too. The sedimentary rocks are a motley mix, including arkose, red siltstones, and lake deposits including siltstones and anoxic black shales. Along the eastern and western boundary faults, we also find coarser sediments that have been lithified into conglomerates. Sediments flowed into the basin from source areas both to the east and west, so you would expect the conglomerates along each edge to look a little different. Indeed, they do!

A modern analogue for the Culpeper Basin is the Afar Triangle region of northeastern Africa (Ethiopia, Eritrea, and Djibouti). Note the sedimentary influx from both the east and the west. Note the lakes, and note the mafic extrusions:

Back to the Old Dominion: I've mentioned the Culpeper Basin's eastern boundary fault before, back in March, when I posted this picture of the conglomerate that outcrops in Clifton, Virgina. It is characterized by lots of clasts of highly-foliated metamorphic rocks (derived from the neighboring Piedmont).

...But I haven't talked about the western boundary fault much. And since I visited it yesterday, today's the day to talk about it.

One of these western Culpeper Basin conglomerates is kind of famous. It's the Leesburg Conglomerate, and it outcrops near Leesburg. It's mostly limestone cobbles and gravel, with some quartzite, too, set in a red matrix. It's a beautiful rock. Here's a couple of field photos taken on Route 15, a mile or two north of Leesburg proper:

The Leesburg Conglomerate was used in the awesome columns in the U.S. Capitol's Hall of Statuary (topped by the much less interesting Carrara Marble of Italy).

Yesterday, NOVA adjunct geology instructor Chris Khourey headed out to Thoroughfare Gap (see map below) to check on a couple of field sites. Thoroughfare Gap is a water gap in the eastern limb of the Blue Ridge Anticlinorium, and it's also the western boundary of the Culpeper Basin. Both Interstate 66 and Route 55 pass through this striking landscape feature:

Note how different this looks as compared to the Leesburg Conglomerate. One thing that immediately jumps out at you when you see an outcrop of it is the large proportion of the cobbles that are pieces of the Catoctin Formation basalt (see more photos of the Catoctin in Monday's post on rocks of Shenandoah National Park). Here's a couple of close-up shots of such cobbles, bearing distinctive amygdules (filled-in vesicles):

But there's also plenty of limestone cobbles and gravel in there too, as this photo shows:

As with the Leesburg Conglomerate, the Waterfall Conglomerate's limestone inclusions are likely coming from the Cambrian & Ordovician carbonates exposed today in the Shenandoah Valley and other valleys of the Valley and Ridge province. More on that later this weekend, when I'll post some shots from the Massanutten Synclinorium.

Labels: basalt, culpeper basin, limestone, mesozoic, sediment, virginia

A year or so ago, I picked up this nice sample of limestone in the Shenandoah Valley of Virginia (easternmost valley of the Valley and Ridge physiographic province). It was a cobble in a stream, not in situ, but it can't have come very far (by natural means anyhow) since it's quite angular. I liked it because of the alternating colors of its layers. I was not totally sure why they are different colors, but I strongly suspected it had something to do with different reactions to weathering (perhaps different calcite / aragonite ratios, or an increased silica / clay content in some layers?). I also liked the patterns of sedimentary layering, thinking back to undergraduate discussions of Flaser bedding and the like, but not remembering the details clear enough to interpret this one. Perhaps one of the sedimentary geologists can help clue me in? Still, I suspected it had something to teach me, so I brought it back to my lab at NOVA. Side view:

There, I sawed it in half. Top view:

To my chagrin, but not my surprise, the interior showed the layering less clearly. In the sawn section, I could clearly see where the weathering "front" had penetrated a short distance into the rock along the lighter-colored layers. While they were yellow-tan on the face of the sample, they were merely light gray in the interior. I decided to try and create a little weathering of my own, and reached for one of the students' acid bottles. I dropped about ten drops of acid on the sawn face, let it fizz for a bit, rinsed it off, and repeated the acid application. Almost instantly, the different layers jumped out into high contrast. The light-colored one was much more reactive than the dark colored one. Here's a view from the scanner which offers a comparison between the un-acid-treated sample (left) and the one I gave the brief acid bath to (right):

Not only does the layering jump out at you, you can see some micro-faulting too. Here's another view, from the camera, of the two samples, one stacked atop the other. I'm astonished at how 30 seconds of acid produced such a remarkable difference in their appearances:

As soon as I had documented the efficacy of the technique, I treated the second sample the same way as the first. One is now in the NOVA teaching collection, and the other is a proud new member of the CB office deskcrop collection.

Labels: limestone, nova, teaching, valley and ridge