The Central Virginia Piedmont Province geology is a fascinating story of geologic wonder told as we travel back in time to see the amazing earth processes that took place and helped to sculpt the land we see today. In Virginia, geologists search for understanding of these events by finding clues that help unravel the scrolls of time. Much of what once was, has been eroded away yet some evidence is still found today in the rocks; evidence that once gathered and pieced together, tell us of the many events that took place so long ago and help us to seek understanding of the geology we see today.
Chesapeake and Ohio Canal is a treasure trove of geologic clues in the evolution of Maryland. On Sat, Feb 24th, the George Mason University structure class took a trip out to see what we could piece together. By examining the rock age and type and by understanding a basic history of plate tectonics, our journey began. Here is what we found...
Monday, March 5, 2012
Gathering clues
Location: N38°59.542’ W077°14.790
Our first stop on the Billy Goat trail was at an outcrop with abundant jointing. Here I took several measurements of strike and dip, trends and plunge, foliation and bedding. I am gathering this information to compile a steronet which should shed light on what stresses and changes this area has undergone. I will also use the collected data to try and solve the mystery that have plagued geologists which is why are the Lamprophyre Dikes offset and whether Mather Gorge (an stretch along the river that is abnormally straight compared to the rest of the river) was cut by a dike.
In order to get a large amount of data trials, my classmates helped to take measurements and later we combined our data.
The primary rock type here is a greywacke. Greywacke is an intersesting rock because its presence tells us that it came from a low-oxygen environment in the deep sea. One interesting feature about greywacke is that in many cases, one can find graded bedding in the rock. This is because as sediment is weathered from its source and carried out to the ocean, it can be deposited in alluvial fans. The largest grains drop out first followed by finer and finer sediments. Turbidity currents are what drive the sediment onto the alluvial fans as its deposited. Notice the presence of a large quartz vein in the picture above. I will address how this came to be injected into the greywacke later on. So my mission was to find evidence of graded bedding. Depicted below are some images of just what I found.
I have to say I got a little too excited when I found my first fold...the first of many. I think how silly it is now as I called the professor over with so much pride inside when I triumphantly announced "Look at this fold I found!!" Little did I know at the time how pathetic this little guy was. Professor Bentley was so gracious to play along with a quick pat on the back and a small grin "yeah...look at that little guy. Very good." Thanks professor.
And we found some bedding!
What a nice bedrock sample this is. Just look at the grading on it. We can see several successions of deposition. Love It! So now we know that this was probably deposited in an alluvial fan in the deep ocean. Oh...and lets not forget about composition...this greywacke is a dark grey mafic rock lending itself even more support as to the deep depths at which deposition most likely occurred. So how did all of this greywacke get to be 132ft above sea level? Lets go find more clues of the geologic past that might help better explain how it all went down...or should I say up.
In order to get a large amount of data trials, my classmates helped to take measurements and later we combined our data.
Look at the dip on that. These beds are dipping at 150°, 41°
The primary rock type here is a greywacke. Greywacke is an intersesting rock because its presence tells us that it came from a low-oxygen environment in the deep sea. One interesting feature about greywacke is that in many cases, one can find graded bedding in the rock. This is because as sediment is weathered from its source and carried out to the ocean, it can be deposited in alluvial fans. The largest grains drop out first followed by finer and finer sediments. Turbidity currents are what drive the sediment onto the alluvial fans as its deposited. Notice the presence of a large quartz vein in the picture above. I will address how this came to be injected into the greywacke later on. So my mission was to find evidence of graded bedding. Depicted below are some images of just what I found.
And we found some bedding!
What a nice bedrock sample this is. Just look at the grading on it. We can see several successions of deposition. Love It! So now we know that this was probably deposited in an alluvial fan in the deep ocean. Oh...and lets not forget about composition...this greywacke is a dark grey mafic rock lending itself even more support as to the deep depths at which deposition most likely occurred. So how did all of this greywacke get to be 132ft above sea level? Lets go find more clues of the geologic past that might help better explain how it all went down...or should I say up.
Joint Sets
Location: N38°59.530’ W077°14.822'
What a wonderful example of jointing in meta-greywacke. This jointing occurred when the tensile strength of this rock was exceeded. Slip along these joints resulted in a domino geometry aided by the steepness of its plunge.
Yes, I said "meta" referring to the metamorphic processes that helped change the greywacke we saw before to this new rock. We know that heat and/or pressure must have been present here in order to metamorphose or "change" our greywacke. So what kind of event must have taken place and what would the conditions have been like to bring on such a change? Lets see what the rocks tell us. On observing a fresh hand sample, I see layers in the rock to which the rocks could and do in fact readily break along. I observed Muscovite, which would help explain why this could be happening being that Muscovite is a sheet silicate and has platy cleavage. Muscovite must have come from metamorphosed mud/shale. The precence of mud would indicate calm water deposition..yet another piece of the overall puzzle. The Muscovite is quite abundant along the fractured zones and is in a parallel orientation to the other grains. This had to be the work of an orogenic tectonic event due to the metamorphosis and realignment of minerals.
We know that the Taconian Orogeny took place aprox 450-435 mya, where mountain building, metamorphism and deformation took place as the North American Plate collided with the Taconian Island Arc (Chopawamsic Terrane.) Sediments that were scraped off as the the plates collided formed an accretionary wedge piling up a bunch of mud clasts, sand, and gravel with some pieces of oceanic crust. These are found abundantly throughout the rocks of Great Falls National Park.
As the Teconic Island Arc overode the North American plate, metamorphic processes were at work. Partial melting of the rock led to the separation of the more felsic minerals as they worked their way up through the molten mass. This process was caught in the act as seen in the photo below producing a rock we call migmatite. Here is where igneous rock says hello to meta-graywacky.
Yes, I said "meta" referring to the metamorphic processes that helped change the greywacke we saw before to this new rock. We know that heat and/or pressure must have been present here in order to metamorphose or "change" our greywacke. So what kind of event must have taken place and what would the conditions have been like to bring on such a change? Lets see what the rocks tell us. On observing a fresh hand sample, I see layers in the rock to which the rocks could and do in fact readily break along. I observed Muscovite, which would help explain why this could be happening being that Muscovite is a sheet silicate and has platy cleavage. Muscovite must have come from metamorphosed mud/shale. The precence of mud would indicate calm water deposition..yet another piece of the overall puzzle. The Muscovite is quite abundant along the fractured zones and is in a parallel orientation to the other grains. This had to be the work of an orogenic tectonic event due to the metamorphosis and realignment of minerals.
We know that the Taconian Orogeny took place aprox 450-435 mya, where mountain building, metamorphism and deformation took place as the North American Plate collided with the Taconian Island Arc (Chopawamsic Terrane.) Sediments that were scraped off as the the plates collided formed an accretionary wedge piling up a bunch of mud clasts, sand, and gravel with some pieces of oceanic crust. These are found abundantly throughout the rocks of Great Falls National Park.
As the Teconic Island Arc overode the North American plate, metamorphic processes were at work. Partial melting of the rock led to the separation of the more felsic minerals as they worked their way up through the molten mass. This process was caught in the act as seen in the photo below producing a rock we call migmatite. Here is where igneous rock says hello to meta-graywacky.
Folds and More Folds
She's A BEAUT!! Look at the Flanks on that one...
More folds...
Lamprophyre Dikes: Late Devonian-abt 360 mya
Following the Taconic orogeny was the Acadian orogeny. The Acadian orogeny took place about 360 million years ago when a landmass called Avalonia smacked into the North American continent and began to subduct. During this time of subduction, magma plutons were generated deep within the earth’s crust. Some really neat structures were formed from this event, one being the lamprophyre dikes which are still seen today cutting across the Potomac River at Great Falls National Park. Here is a picture of what the dikes on the Virginia side looks like today. Notice how deep the grooves are? This is because the mafic minerals that make up the lamprophyre are relatively unstable compared to its surrounding rock. Potassium and Argon dating has dated these to be about 360 million years old. Hey…that appears to be around the same age as the Acadian orogeny…could there be a connection?
Here are the dikes on the Maryland side of the Potomac River. Standing on the edge of the river, it’s not hard to see that these two have a pronounced lack of alignment. Controversy on the reasoning behind the offset is still in question. We will input all of the data we have collected and come up with a hypothesis later….check back.
Amphibolite Sills
Walking along, looking down at my feet, I see that the rocks have made an almost sudden change into Amphibolite. Amphibolite is an mafic igneous rock that contains a large amount of meta-greywacke.
The rocks then tell us that hot magma must have been present deep within the earth to form here. Boudinage (meaning sausage shaped) are meta-greywacke in this photo.
These boudins were formed by extension of a bed being stretched and defromed amidst less compitent surroundings. The copitent bed begins to break up and results in the formation of sausage shaped boudins. Here sigma 1 and sigma 3 are shown.
Here is a picture of migmatite along the Billy Goat Trail. In the boudinage above, migmatite is a little hard to see. But, in the cracks between the necks of the boudins there appears to be migmatite present. This happens because magma was the fluid available to fill the empty space.
The rocks then tell us that hot magma must have been present deep within the earth to form here. Boudinage (meaning sausage shaped) are meta-greywacke in this photo.
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| Migmatite. Photo coutesy of Callan Bentley. |
Here is a picture of migmatite along the Billy Goat Trail. In the boudinage above, migmatite is a little hard to see. But, in the cracks between the necks of the boudins there appears to be migmatite present. This happens because magma was the fluid available to fill the empty space.
Sunday, March 4, 2012
Putting It All Together
Stereonets can be be a useful tool to geologist as they can help visualize large amounts of data and plot them to find trends. Remember when we talked about the Lamprophyre Dikes and how they were off set. There are two hypothesis as to why they are off set. The first is that there is a fault at Mather Gorge. The second is that the dikes were naturally crooked and when the Potomac River incised down over time, the truth of the offset was eroded leaving the two limbs on either side of the river offset.
If there is a fault under Mather Gorge then we should expect to see no apparent trend line that lines up with Mather Gorge on the stereonets.
I constructed a stereonet from the class data. After plotting the poles, I compared that to the plot of Mather Gorge and found no trend from the class' data that lined up with Mather Gorge--or even came close. In fact, the class' data was so random that a clear trend was not possible, or even a murky trend. For this reason, I was not able to conclude whether, or not, there was a fault under Mather Gorge or a crooked dike. It does give evidence for the presence of a fault. I wasn't able to disprove the presence of a crooked path, however, because in order to do this I would have to physically look at the dike. I can't do that nor can anyone because the river is too deep and treacherous with large boulders in the base of the channel. I guess the mystery of Mather Gorge will have to be solved on another occasion.
If there is a fault under Mather Gorge then we should expect to see no apparent trend line that lines up with Mather Gorge on the stereonets.
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| Mather Gorge (green line) compared to poles to jointing |
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| Mather Gorge (green) poles to foliation |
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| Mather Gorge (green) poles to bedding |
I constructed a stereonet from the class data. After plotting the poles, I compared that to the plot of Mather Gorge and found no trend from the class' data that lined up with Mather Gorge--or even came close. In fact, the class' data was so random that a clear trend was not possible, or even a murky trend. For this reason, I was not able to conclude whether, or not, there was a fault under Mather Gorge or a crooked dike. It does give evidence for the presence of a fault. I wasn't able to disprove the presence of a crooked path, however, because in order to do this I would have to physically look at the dike. I can't do that nor can anyone because the river is too deep and treacherous with large boulders in the base of the channel. I guess the mystery of Mather Gorge will have to be solved on another occasion.
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