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Posts Tagged ‘basalt’

This final post of my Columns “Week” comes from an outcrop on I-84 along the Columbia River in northeastern Oregon. Technically this outcrop is in Washington, but since I took this picture from the Oregon side, and I am now a resident of Oregon, I am going to claim it as ours. This stretch of interstate is full of excellent examples of columnar jointing from the basalt flows that cover most of the state, and is a beautiful drive.

Becoming a bad habit for me is that this picture was taken from a moving car, and suffers from the slightest bit of blurriness and lack of good scale.

Even with the photographic flaws, I think it is apparent that there is some interesting distortion going on in these columns. Below, I highlighted the edges of most of the columns.

What stands out most to me about this outcrop is the change in direction of the narrow columns. Admittedly there is a high amount of fracturing going on, but the most apparent columns seem to diverge and converge in places.

Another interesting aspect of this place is the presence of what could be more, larger columns above the distorted, narrow ones. Here they are highlighted in yellow.

This gives me the sense that there are two separate flows existing here. A quick amount of research leads me to believe the lower flow could be the Middle Miocene Wanapum basalt (15.0 Ma), and the upper flow could be the  slightly younger Saddle Mountains basalt (13.5 – 6.0 Ma). Of course, to get a good grasp on the difference I would need to get my nose to the outcrop and take a better look.

If these are two separate flows, here is another annotated picture showing the contact between the two in blue.

Maybe I will find time to head back and get a closer look.

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The next post in my Columns “Week” series is an outcrop I saw along I-5 just north of Eugene back in March, and came away with an immediate impression. I was sick at the time, but still knew exactly what it was I was looking at as we drove past at 65 mph. Since coming back, this outcrop has become a bit more confusing to me. Not because I disagree with my initial impression, but because it has been explained to me as something else and I still agree with my first thought. Hopefully today I can get some input that will shine light on the situation to me.

Enough talk! Here is the outcrop in question:

This outcrop is located on the eastern side of I-5 with no safe place to park and walk up to, so these pictures are taken from across the interstate just off of Coburn Road near the McKenzie River.

Clearly there are some distorted basalt columns here, that is not in question. But, my initial impression was that these columns were distorted due to a lobate cooling front, and it has been explained to me (by very reputable sources) as a dike rolling into a sill. To help visualize my thought process, here is an annotated picture of the outcrop with the progressive cooling front in orange, and the direction of cooling shown with a red arrow. Also, a second picture with a passing tractor trailer to give a relative sense of scale.

Here is another angle of the outcrop where in my hypothesis the cooling front would be moving from the upper left of the picture towards the lower right.

Another interesting aspect of this outcrop is that the basalt can be seen overlying Oligocene aged marine sediments of siltstone and sandstone. Here is a picture of the contact between the two with another less than desirable “passing truck” for sense of scale.

Does anyone have a counter hypothesis? I would love to have a discussion on what I may not be seeing at this outcrop.

For good measure, here is a picture of the entire outcrop taken from a northern angle (again with cars for scale).

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After spending an evening around a campfire spinning some good ole’ Ben Roethlisberger yarns, and good night sleep was needed. Unfortunately, we were in for a night of hurricane force winds. Not really, but it was pretty windy and the tent kept slapping me in the face all night. It must not be a fan of my low-brow satirical sense of humor.

Today we hopped back in the van and headed out towards Skyline Drive; which if you don’t live in Virginia, is where city folk bring traffic to rural folk. It is also a great place for some pretty spectacular views of the Commonwealth and West Virginia, and some pretty decent hiking trails.

After making a few stops along the road at some beautiful, if not incredible windy, overlooks, we parked and set out down the Limberlost Trail (Google Earth file). What the Limberlost trail lacks in strenuous hiking it makes up in a two geologically exciting outcrops of columnar jointing in basalt flows. This basalt is part of the Catoctin Formation which flowed to the surface during the rifting of Rodinia. The Catoctin is younger slightly as the Swift Run in found beneath it. As the basalt cooled, it contracted and cracked, leaving (for the most part) hexagonal columns with arrest lines running perpendicular to the joint surface, for every period of jointing.

The columns would have sat there looking like the Giant’s Causeway of Ireland for a few hundred million years until the Alleghenian Orogeny (AO) knocked them around. Taking the strike and dip of the columns, we were able to determine another instance of East over West deformation; further evidence of the AO coming from the east.

The typical case of columnar jointing exhibits six sides with 120° angles between. After deformation the “tops” (since we were unable to determine paleo-up) of the columns were sheared and stretched to create larger angles or in some cases shortened (100° and 90°). The arrest lines which were originally perpendicular to the edges are now at angles of ~106°.

After getting our gentile sufficiency of columnar jointing we headed back down the fire road to our vehicles, and were off to our campsite and then one final stop for the day (possibly my favorite of the trip).

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