Friday, May 9, 2008

Western conglomerates, Culpeper Basin

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:

ZPz = Neoproterozoic and Paleozoic metamorphic and igneous rocks.
TJs = Triassic and Jurassic sedimentary rocks. Jd = Jurassic diabase

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:

We were scouting out instructional locations to visit with students, and we found some good ones. One of them was an outcrop of another, different western conglomerate, part of the Waterfall Formation. Here's a shot of it:


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.

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Anonymous BrianR said...

That was a great post!

One question/comment - having preserved basalt clasts in a conglomerate is really cool and not that common. That's a rock type that usually doesn't produce clasts or sand grains. My guess is that this formation is very close to the basalt source ... are there any stratigraphic relationships preserved in the exposed geology? (e.g., onlapping of conglomerate onto basaltic terrane?)

May 9, 2008 12:43 PM  
Blogger Callan Bentley said...


You're right -- the conglomerate is VERY close to outcrops of the Catoctin, although the outcrops located about 1 mile away are metamorphosed to greenstone, and not as unaltered as the basalt in the clasts. I think it's a fair characterization to say the the eastward-dipping east limb of the Blue Ridge anticlinorium is at this location also serving as the western boundary fault of the Culpeper Basin. Which is to say: yes, the conglomerate is "lapping on" the basaltic rocks, though there are a few intervening layers in the subsurface (Chilhowee Group), the basalt is exposed all along the Bull Run Mountains at the surface.

May 9, 2008 2:47 PM  
Anonymous BrianR said...

cool ... I did my undergrad back East but haven't looked at or thought about the great geology in a long time

A great place to see a similar relationship is the Franklin Mts in the city of El Paso. You can take a short hike and see the Bliss Ss lap onto granite in the mountainside. There's even some paleotopography preserved ... the Bliss thickens and thins across an undulating contact.

May 10, 2008 10:40 AM  
Anonymous GeorgeH said...

Wow. This is great. I have had a chunk of this conglomerate on my back porch for several years, having 'rescued' it from Leesburg, where I work... I had a theory that the smashed nature of the tiny unweathered bits might be part of the outfall from the Chesapeake asteroid, but am gratified that it is actually 240 million years old and a result of the fragmenting of Pangea... Very cool.

August 25, 2009 10:41 PM  
Blogger Callan Bentley said...

Thanks George!

As a local, you may be interested in a one-day field course that my colleague Ken Rasmussen runs to the Culpeper Basin -- the class explores the variety of rocks and their interpretations in great depth. But it's introductory level, so perfect for an enthusiast like yourself. It'll be listed in the NOVA catalog under GOL 135 - Triassic rift valley.

Good luck!


August 26, 2009 6:46 AM  
Anonymous Anonymous said...

nice post. I would love to follow you on twitter.

February 15, 2010 8:13 AM  

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