Geological excursion in Silver Spring, Maryland
Yesterday morning, I took a jaunt with a local amateur geologist, Owen P., to go look at some outcrops in streambeds in and adjacent to Silver Spring, Maryland.
Owen wanted me to look at these surfaces, our local unconformity between foliated metamorphic rocks of the Piedmont below, and unconsolidated sediments of the basal Coastal Plain above (cell phone for scale):
The lower rocks are metagraywacke schist of the Sykesville/Laurel Formation (different aspects of the same thing, as far as I am concerned, and not worthy of two different formation names). They were metamorphosed during the Taconian ("Taconic") Orogeny, ~460 million years ago. These rocks were then eroded, and new sediments deposited on top of that eroded surface -- this is an unconformity like the ones I posted about over the past couple of days out in Wyoming and Arizona.
My host thought the layer above the unconformity might be tsunami deposits associated with the Chesapeake Bay bolide impact at 35.5 million years ago. However, that's not what I saw. Instead, the high proportion of angular quartz, and the fact that it was clast-supported rather than matrix supported, suggested to me that the upper layer was a gravel deposit from this very stream. It was good for me to see such a collection of angular clasts atop the unconformity -- on hilltops in DC, I'm used to seeing the Potomac Formation in this position. It's a Cretaceous-aged river deposit, with a real mix of sand, clay, and well-rounded (mainly quartzite) cobbles.
Another look (with cell phone for scale):
After I explained why I didn't buy the tsunamite hypothesis, but encouraged him to keep looking, Owen took me to another cool location, on Northwest Branch (a creek) just outside the Beltway at Burnt Mills Park. Here's a location map:
There, we found an outcrop of migmatitic metagraywacke very reminiscent of the one I visited on Four Mile Run in Arlington, VA, in March of this year. Cutting down, Northwest Branch has exposed a complex of clearly metasedimentary, clearly granitic, and not-so-clearly transitional migmatitic rocks. It's pretty cool, and not only because some of the potholes went all the way through the rock, making wormhole tunnels that a geologist can (and will) crawl through...
I found a couple of cool igneous contacts. Here's a dike of granite cutting through metagraywacke. I like this outcrop because it shows that these things are in fact filled-in cracks, and cracks have a propagating edge, a tip. Most granite dike exposures don't show this fracture edge, but this one does. In spite of the graffiti, it's a good look at that process caught in the act.
And here's a nice example of cross-cutting relationships. Host metagraywacke (notice the pebble-sized clasts of various lithologies in the upper left) is cut by two granite dikes: first a finer-grained, darker-colored one, and then by a coarser-grained, lighter-colored one. Beauty!
Thanks to Owen for showing me these outcrops -- I appreciate the interest and the invitation!
Owen wanted me to look at these surfaces, our local unconformity between foliated metamorphic rocks of the Piedmont below, and unconsolidated sediments of the basal Coastal Plain above (cell phone for scale):
The lower rocks are metagraywacke schist of the Sykesville/Laurel Formation (different aspects of the same thing, as far as I am concerned, and not worthy of two different formation names). They were metamorphosed during the Taconian ("Taconic") Orogeny, ~460 million years ago. These rocks were then eroded, and new sediments deposited on top of that eroded surface -- this is an unconformity like the ones I posted about over the past couple of days out in Wyoming and Arizona.
My host thought the layer above the unconformity might be tsunami deposits associated with the Chesapeake Bay bolide impact at 35.5 million years ago. However, that's not what I saw. Instead, the high proportion of angular quartz, and the fact that it was clast-supported rather than matrix supported, suggested to me that the upper layer was a gravel deposit from this very stream. It was good for me to see such a collection of angular clasts atop the unconformity -- on hilltops in DC, I'm used to seeing the Potomac Formation in this position. It's a Cretaceous-aged river deposit, with a real mix of sand, clay, and well-rounded (mainly quartzite) cobbles.
Another look (with cell phone for scale):
After I explained why I didn't buy the tsunamite hypothesis, but encouraged him to keep looking, Owen took me to another cool location, on Northwest Branch (a creek) just outside the Beltway at Burnt Mills Park. Here's a location map:
There, we found an outcrop of migmatitic metagraywacke very reminiscent of the one I visited on Four Mile Run in Arlington, VA, in March of this year. Cutting down, Northwest Branch has exposed a complex of clearly metasedimentary, clearly granitic, and not-so-clearly transitional migmatitic rocks. It's pretty cool, and not only because some of the potholes went all the way through the rock, making wormhole tunnels that a geologist can (and will) crawl through...
I found a couple of cool igneous contacts. Here's a dike of granite cutting through metagraywacke. I like this outcrop because it shows that these things are in fact filled-in cracks, and cracks have a propagating edge, a tip. Most granite dike exposures don't show this fracture edge, but this one does. In spite of the graffiti, it's a good look at that process caught in the act.
And here's a nice example of cross-cutting relationships. Host metagraywacke (notice the pebble-sized clasts of various lithologies in the upper left) is cut by two granite dikes: first a finer-grained, darker-colored one, and then by a coarser-grained, lighter-colored one. Beauty!
Thanks to Owen for showing me these outcrops -- I appreciate the interest and the invitation!
Labels: granite, maryland, metamorphism, migmatite, primary structures, sediment
5 Comments:
That last picture also looks like a good example for thinking about how rocks move during intrusion. (The older dike looks as if it has been offset, bottom to the right and top to the left. But if you draw lines straight across the dike, perpendicular to its walls, the edges of the older dike connect, or almost connect. So I think the younger dike could have simply opened, not followed a fault.)
That could be a good picture for a structural geology class.
Hey Kim,
You're welcome to it. Want a bigger copy? I can e-mail it to you at your gmail account...
C
Way cool!! If you have a chance, and if the sites (unconformity and/or the intrusions) are in publicaly-accessible spots, could you send me info about finding them? Merck & I are revising our field trips for the Earth, Life & Time program, and the department is in the midst of revamping GEOL 110 lab, so having some spots that are short distance from UMCP would be most useful!
Callan -
Yes, please!
(Now I need to start cataloging the images I've sponged off of people this summer, so I can actually use them in classes...)
The two other sites mentioned are
1) North Portal Drive & Birch Dr (N38 59 22 W77 02 23) access by parking on Birch Dr and walking SE to the creek.
2) Between West Beach Drive and East Beach Drive (N38 59 39 W77 02 35) Easier access by going north on West Beach Drive to the Dead End and park. Walk SE to the creek and proceed north.
Note the light gray clay layer on top of the basement rock 20 feet north of the WSSC pipe and on the west bank.
For the record according to the USGS 1994 Map, Flemming et.al. the lower schist is the Northwest Branch Laurel motif. I think the creeks cut through that rock and the quartz and other rocks collected in the creek. Some quartz coming from veins in the basement rock and other probably transported. Prior to the early 1990’s most stratigraphers were unaware of the 35 Ma Chesapeake Bay Impact and the Late Eocene cataclysm that must have had major consequences up and down the Atlantic seaboard. There is a lot of opportunity to reevaluate previous conclusions concerning the lithography of the greater DC area. Remnants of the Tsunami probably mixed with the underlying coastal plain deposits of the Potomac and Aquia formation that rest on the basement rock.
... Owen
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