Latest gage information from the Potomac River near Little Falls. Looking out my window at Rock Creek this morning, I can see the water is way up, moving fast, and the color of teh tarik.

DC (and many other surrounding municipalities) are under flood warnings this morning as a result. The Washington Post's Capital Weather Gang is impressed by it all.

Labels: dc, floods, rain

Thanks to Andrew Alden for addressing the issue of the tidal influence on recent Cyclone Nargis' flooding of southern Burma. NASA recently published this before/after image online:

Thought I would pass it on.

Labels: asia, floods, hurricanes, news

Morning, folks. I awake to a challenge from Chris at GoodSchist, to show where my local bedrock was at the time of Pangea's incipient breakup. (I think Chris chose the late Triassic, 220 Ma, since Ron Blakely's map of that time shows New Zealand clearly in the south.) It's an interesting time for the rock beneath Washington, DC. After have just experienced ~50 million years of crunching between North America and Africa, DC's tortured bedrock is now being stretched as Africa begins to pull away again. A series of rift valleys mark the stretching of the crust, shown clearly in the map as a series of NE-SW oriented lakes along the axis of the Appalachian orogen.

DC's future location is between two of those rift valley lakes: one to the east, one to the west. If I owned DC real estate during the Triassic, I'd be very interested in this process, because one of those rift valleys is going to become a new ocean basin, and one isn't. The one that isn't is destined to stop opening and fill in with dirt. It will be a failed rift valley, an aulacogen of sorts.

The question is: which one is the weakest link? If the one to the west breaks open, that will be the new Atlantic Ocean basin, and DC will stay hitched to Africa. If the one to the east breaks open, that will be the site of the Atlantic, and DC will stay hitched to North America.

As it turned out, the eastern rift was the one that connected up with other rifts to the northeast and southwest, and became the young Atlantic. The western rift, known as the Culpeper Basin, stopped stretching open, and got filled in with sediment. DC stayed attached to North America, and that's the way it is.

Labels: culpeper basin, dc, geology, mesozoic, piedmont, plate tectonics

On Monday, my Physical Geology students and I experienced a magnitude-2.0 earthquake during our final exam. The hypocenter was 1 km south and 6km down.

I pulled out the geologic map of the Annandale quadrangle (1986) by Avery Drake and A. J. Froelich to check for faults in that area. Here's a scan of the map:

You can see our campus in the northwest, and I've highlighted the epicenter of the quake with the red and green concentric circle. Interstate 495, the Capitol Beltway runs north-south through the center of this area. To the south and east of NOVA's campus, you can see that there's a mappable thrust fault (the Red Fox Thrust) which dips to the northwest, presumably under the epicenter and under the campus. However, the map provides no information on the angle of dip of this thrust. Is it steep enough to get to 6km depth a mere ~3km north of its surface trace? (The map's cross section shows it dipping at about 52 degrees, but that's pretty speculative.) Or if the dip is shallow, is there a deeper (perhaps parallel) thrust underneath it? (There is none shown within the map area, though there is one to the north of campus that dips to the north -- making it unlikely to be the culprit.) Alternatively, was Monday's quake caused by a new fault? Perhaps a normal fault which cross-cuts these Paleozoic thrusts?

No new answers -- only more questions...

Labels: earthquakes, news, nova

While toodling along the web on some other business this week, I stumbled across this publication by the Virginia Department of Mines, Minerals, and Energy.

I had no idea that there were any diamond finds in Virginia. But apparently there are, scattered across three different physiographic provinces!

On Thursday's excursion, Chris and I tried to find the "Front Royal Peridotite," one of seven locations mentioned in the DMME publication. It's a single dike which crosses State Road 626 southeast of Waterlick, Virginia. But to no avail! There were no outcrops visible on either side of the road, and there was a dense little cluster of houses bearing manicured lawns. Bummer. That would have been cool.

I'll try and visit a couple other localities mentioned in the report over the next year or so, and hopefully I'll find some of these igneous source rocks, though I don't hold out much hope of actual diamonds.

Labels: blue ridge, diamonds, minerals, piedmont, valley and ridge, virginia

This one's a good one to assign to Historical Geology students who don't have time to read The Map That Changed The World. It's part of the series "On the Shoulders of Giants" by NASA's Earth Observatory: William Smith.

I love the way these pages are laid out: a single column of text with illustrations of different sizes and dimensions interspersed with the content. It's like a Dorling Kindersley book. NASA must have some good web designers on the payroll.

Labels: books, fossils, geologists, history, stratigraphy

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