Concerning Relative Sea Level and Isostatic Uplift

Not so recently an article was sent to me that discusses how in areas of Sweden the relative sea level is falling and land bridges that did not exist within the last century are starting to appear. For some this could seem as counterintuitive with what is known about rising eustatic sea level caused by climate change. But, the process behind this relative falling sea level is simply isostatic rebound from the Last Glacial Maximum.

During the LGM, the massive Eurasian Ice Sheet depressed the land, and since deglaciation it has been recovering from that depression. What is happening specifically is that the rate of Swedish rebound is higher than the rate of sea level rise.

The process of isostatic rebound is becoming a concept of large importance for me to work with in a couple of ways. One of the ways we can reconstruct the glacial history of a region is by looking into the relative sea level changes experienced there. Using those changes along with an understanding of the global sea level can suggest the presence or lack of overlying ice sheets.

In a 2007 paper, Marshall McCabe et al discuss the relative sea level changes in northeast Ireland experienced after the LGM and up to the Younger Dryas. By using stratigraphic relationships between dated beach deposits and glacial diamictites, McCabe reconstructed a relative sea level curve for the region.

From McCabe et al 2007

At a Kilkeel outcrop (point 2 in the figure), beach notches are found 30 m above current sea level and are infilled with glaciomarine muds. This suggests that at the time of formation of the notches the coastline of Ireland was isostatically depressed 30 meters. Considering that eustatic sea level was 130 m below current during the LGM means the coastline was depressed ~160 m below present. Deglaciation is suggested by a subsequent fall in relative sea level as the unburdened coastline experienced uplift.

Another aspect of isostatic uplift that I have to consider has to do with the surface exposure dating I will use, and is a topic I have discussed at length with fellow students. Glossing over a whole bunch of details, to get as accurate of a date as possible we have to consider not just how long a particular sample was exposed to cosmogenic rays, but also at what altitude the sample was at while being exposed. Objects at higher altitudes will be exposed to more cosmogenic rays than those at lower altitudes. This comes into play where a glacial erratic may be deposited on a moraine that was 160 m below its current altitude during the LGM. Depending on what altitude the erratic was at would effect the calculated age of exposure. Do we use the current altitude in our calculations or the depressed altitude? Or do we compensate for the uplift by trying to adjust the increasing exposure with uplift? If so, do we assume a linear uplift, or the rebound curve that shows high initial rates followed by lower rates of uplift? Also, how significant of a difference does it make on the final calculation?

I understand that these are a lot of questions to consider, and ones I am sure I will have to tackle in the coming months. The truth is I welcome the discussion that will come, and if anyone reading this has thoughts or suggestions please feel free to chime in on the comments section.

 

McCabe, a. M., Cooper, J. a. G., & Kelley, J. T. (2007). Relative sea-level changes from NE Ireland during the last glacial termination. Journal of the Geological Society, 164(5), 1059–1063.

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It Begins…

Tomorrow will mark the official start of my graduate school career at Oregon State. Though I have been  present on campus throughout the summer, and been able to get my feet wet in research procedures, tomorrow when I sit down for my first lecture at 9 a.m. I will be official. 

Much like a lot of this experience so far, I am a combination of nerves and excitement. Clearly this is where I want to be and what I want to be doing, but I also recognize that this is on a higher level than what I have encountered so far. Just by simply reading over the syllabi gave me a moment of…”whoa”. I’m sure this is also how I felt with the beginning of every other level of education, and hopefully without sounding too confident, I have succeeded there as well. Though, there is an awful lot of reading ahead of me. 

All in all, I am very eager to begin. 

Here now is my obligatory mention of how I plan to post more often to this blog, especially with all the new information I will be gathering through my experiences.

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Columbia River Columns

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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McKenzie Crossing

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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Skinner’s Butte

I have recently transplanted myself in the Pacific Northwest, specifically the city of Eugene, OR. It has only been a couple of weeks, but I am already making myself right at home with the food, people and most importantly the geology. While I still have a lot to learn in terms of the local geology, what I have seen so far has prompted me to declare my own personal “Columns Week” on this blog. I have set aside three Oregon outcrops to discuss throughout the week, starting off with one suggested to me by Lockwood DeWitt at Skinner’s Butte.

Located on the northern side of town west of the campus, and just a short bike ride from my place, this outcrop shows some of the most spectacular basalt columns I have come across. I have been to Giant’s Causeway in Northern Ireland, and I think these ones belong in the same category.

The columns to the right are so well preserved!

Photo by V. Malinay

Photo by V. Malinay

If you click on the image for the larger version, you can see the arrest lines.

Special thanks to Lockwood DeWitt for bringing these to my attention, and also the guy climbing for the sense of scale.

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TFG Field Trip Guide

The following is a field trip guide for a Structural Geology class’ trip to Thoroughfare Gap in northern Virginia.

Where:

Thoroughfare Gap (TFG) is the break in Bull Run Mountain where Broad Run, I-66 and some train tracks pass through the range. TFG is located along the western boundary of the Culpeper Basin and represents the end of the Piedmont and the beginning of the Blue Ridge provinces of Virginia.

As geologists, there are multiple reasons to travel to this particular area; the transgressive sequence of Rodinian rifting, well displayed joint sets and deformation from Appalachian mountain building, and conglomerates collected within the basin from the rifting of Pangaea.

Bull Run Mountain is composed primarily of the Weverton quartzite, a meta-quartz arenite with clasts ranging in size from fine sand to pebbles; including conglomeritic sections. The resistant Weverton is the lowest formation of the Chilhowee Group which is overlain by the Harpers and Antietam Formations . These sediments were deposited during the break up the supercontinent, Rodinia, at the end of the Ediacaran and beginning of the Cambrian periods. Underlain by the Catoctin, a metabasalt characterized by a green color from both epidote and chlorite minerals, the trangressive Chilhowee represents sea level rise as the depositional environment progressed from a beach, to a lagoonal setting, followed by a point bar; though the Antietam Formation is not found exposed at TFG. Eventually the Chilhowee was overlain by limestone deposits which can be seen west of TFG in the Valley and Ridge province of Virginia and West Virginia.

What:

A major part of the assignment associated with this trip will be to use measurements to determine the orientation of the stress that was applied.

The next large scale tectonic activity to occur in the area of TFG was the Appalachian Orogenies that created the Blue Ridge and Bull Run Mountains. We will start out the field trip by hiking the trail to the western overlook atop Bull Run Mountain, making sure to take notice of a few easterly dipping outcrops of Weverton along the way. Once at the overlook we can correlate those outcrops with the Blue Ridge Mountains seen miles away. Here you will be standing on the eastern limb and looking at the western limb of the Blue Ridge Anticlinorium.

The next chapter of this tectonic trilogy involves the rifting of another supercontinent. Once the final phase of Appalachian mountain building was complete, Pangaea had been formed. Just like Rodinia before it, excess heat beneath the continent built up eventually leading to rifting and the subsequent creation of the Atlantic Ocean.  While multiple faults were created only one would become the Atlantic, while the others, like the Culpeper Basin, would be left high and dry to collect the freshly weathered sediments coming off the Appalachian Mountains. East of Bull Run Mountain is the western boundary of the C. Basin where an outcrop of the Waterfall Conglomerate is located. Contained within the muddy matrix of the conglomerate are clasts of recognizable members of the Chilhowee Group among others.

How:

Back down at the beginning of the trail we will walk west along the train track being mindful of any trains heading our way. From the parking lot heading west we will pass outcrops of the phyllitic Harpers Formation with good exposures of fissile cleavage. Eventually we will come to a large outcrop of Weverton where most of the measurements will be taken. At this location, a few joint sets are readily visible, and are perfect for measuring and plotting on stereonets.  Some questions to consider would be: what direction did the continental collision come from? What happens to rocks as they are folded? How come the Broad Run water gap is located where it is?

This will be our first time running this field trip so some fine tuning will be expected while out on the trail. Remember to be observant of all structurally significant features as small features can relate to the regional picture.

 

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Fun with Stereonets: Finding the Axial Hinge

Since I graduated at the end of the Fall semester I have had the opportunity to be the TA/Lab Instructor for George Mason’s Structural Geology class. While I could spend a whole post discussing the rewards of being on the other side of the desk I will instead go over one of the prominent topics discussed in the class: stereonets.

For the sake of time I am going to make a large assumption here and proceed like anyone reading this has a basic understanding on what stereonets are and how to use them.

What I am hoping to accomplish with this blog entry is giving step by step instructions on how to find the axial hinge of a fold by using stereonets. Since we create them manually in the lab using Equal Area stereonets and tracing paper, I will explain the steps the same way.

Let’s say we have a fold with the all the necessary parts: two limbs, an axial plane, and the axial hinge. Let’s also say we already have the strike and dip measurements for each limb with Limb A oriented at 225, 66 and Limb B oriented at 016, 50. Here is a picture of just such a fold.

With these two measurements we can determine the orientation of the axial hinge without having to climb inside and attempt to measure that thing upside down.

First we need to plot the two limbs, and knowing that fold limbs are planar features we’ll mark them down as great circles on our stereonet. We start by marking 220 degrees for Limb A.

Next we need to rotate our “tracing paper” so that our 220 mark is oriented to the North. From here we can find our dip measurement of 56 degrees on the horizontal axis, and then draw in the associated great circle.

Super. Now we rotate the tracing paper back to its original position, and our Limb A is plotted.

Now we need to plot Limb B of the fold by following the same process: mark 016 degrees, rotate to North, mark the dip and draw the circle.

Understanding that the axial hinge is the point of maximum curvature or bending in a fold, it is going to be located at the intersection of the two limbs or great circles in this case. Since the intersection of two planes forms a line, we will plot the hinge as a dot on the stereonet.

Here are the two limbs plotted together.

Let’s go ahead and mark the point of intersection between our two circles.

From here we can check the the direction of the hinge by looking at its location on the stereonet. Looks to me like it is trending at 032 degrees. To find out how much the hinge is plunging, if at all, we can rotate our tracing paper one last time to the horizontal axis and measure it there.

According to our stereonet the hinge is about 18 degrees from horizontal. Therefore our antiform is trending 32 degrees from North and plunging 18 degrees. (T&P:  18 –> 032)

I hope this was helpful. If so, maybe I’ll explore some other uses of stereonets later.

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Aerial Geomorphology (with snow)

Last week Callan Bentley decided to go digging through some old images and his action inspired me to do the same. Fortunately for me, I knew exactly the purpose of the images I had set aside. I took this picture with my iPhone while flying back from the AGU Meeting in San Francisco last December, and was struck by the saw-tooth appearance seen in the landscape.

What I am describing as “saw-tooth” appearance are the eroded V-shaped notches in the bedrock and the snow. Using what a professor of mine described as the “Power of Geomorphology” we can infer a few things about this landscape. The orange annotated notches below all point to the left side of the picture indicating that the homoclinal ridges are dipping the same direction. The fact that the same phenomena is shown in the snow covering is just a nice added bonus.

Highlighted in blue is the dendritic drainage pattern indicating that the melt channels are working their way into a homogeneous material, in this case snow, and heading down slope to the east (since I was facing south when I took this picture…I think).

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Glacial Feature: Cryoconite Holes

The other day while reading a paper on the Dry Valleys of Antarctica I came across a new (to me) geologic feature: cryoconite holes. These vertical holes are often found in the ablation zone of glaciers and form in an interesting way. Aeolian sediments collect in little melt pools which then increase the absorbency of solar radiation relative to the surrounding glacier. The increased rate of melting created from the solar radiation forms these long, cylindrical features that are filled with water. In areas like the Dry Valleys the cold air temperatures will freeze the surface giving the hole an ice cap. It’s a self formed bottle of water.

Below is a sketch I made of the formation process while reading about them.

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Optical Illusion Offset

The other day while measuring joint orientation in the Weverton formation at Thoroughfare Gap, I stumbled upon this felsic vein down section in the Catoctin. We had already seen some minor faulting in the outcrops leading up to this one so it wasn’t a big surprise to find another one. From this angle it looks like the vein was offset laterally to the right, but when it is viewed from another angle the offset disappears. Just a case of tricky jointing.

Here is the vein looking offset…

…and here the offset is gone.

This could be a good lesson in seeing everything in 3D space, and “apparent” vs. “true”. Enjoy the video below as another example.

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