Summertime and the Living is… Soily!

Guest post by Mike Salisbury, Grad Student at Auburn University and the Auburn Geomorphology Lab

 What comes to mind when you think of hot summer weather and being done with your spring classes? Digging trenches and augering old river terraces in the Ozarks, of course! On my recent trip to the Buffalo National Scenic River, AR with the AU Geomorphology Crew (Dr. Stephanie Shepherd, Mark Simon, and Samantha Eckes) and Kathleen Rodrigues from the Desert Research Institute (DRI), I did just that.

THE TEAM AUGERING AWAY TO GATHER SOIL FOR ANALYSIS. FROM LEFT TO RIGHT, SAM, MARK, MIKE (ME), AND KATHLEEN

THE TEAM AUGERING AWAY TO GATHER SOIL FOR ANALYSIS. FROM LEFT TO RIGHT, SAM, MARK, MIKE (ME), AND KATHLEEN

Fluvial terraces are remnants of former river floodplains. Over the course of a week, our crew augered, dug, analyzed soils, laughed, cried, and collected samples at a handful of Buffalo River terrace sites. Some of the soil samples will be dated via optically-stimulated luminescence (OSL) techniques at DRI by Kathleen. Dating these terraces can contribute to the overall understanding of local fluvial activity, among other things.

SANDY CLAY LOAM OR SILTY CLAY LOAM? DIRTY WORK, BUT SOMEONE HAS TO DO IT. SOIL ANALYSIS IN THE FIELD WITH THE CREW. FROM LEFT TO RIGHT, MARK, SAM, KATHLEEN, AND MIKE.

SANDY CLAY LOAM OR SILTY CLAY LOAM? DIRTY WORK, BUT SOMEONE HAS TO DO IT. SOIL ANALYSIS IN THE FIELD WITH THE CREW. FROM LEFT TO RIGHT, MARK, SAM, KATHLEEN, AND MIKE.

Having taken soil morphology last semester, I was able to put my new pedology skills to good use. With each auger, the soil was placed on a tarp (creating a soil profile of sorts), the auger hole depth was measured, and soil characteristics—color, texture, and consistence—were recorded. During this analysis, it was fascinating to observe the changes in sand and clay as we progressed; typically, there was very little change in soil color. OSL requires a soil that is a bit sandy, so if Kathleen felt the texture was appropriate, a sample for OSL was taken. This included attaching a black PVC pipe to the end of the auger to sample the soil and doing the, as I like to call it, “auger dance” without cursing too much. We also collected samples for the AU Geomorphology Lab--using a machine called the Mastersizer (great name, right?), which assesses particle size using laser diffraction, the samples will be examined in the lab and these results will be compared to field data.

Overall, the trip was a success. I am grateful for the experience and appreciate the new knowledge gained. 

-Mike

10 frosty days of field work on the Buffalo River!

Guest post by Hannah Heinke-Green, Geology Undergrad at Oberlin College

My name is Hannah, and I am currently a senior at Oberlin College, a small liberal arts college located in northeast Ohio. During the month of January, we do not have classes, instead students are given the opportunity to further explore an area of interest. As a geology and environmental studies major, who is particularly interested in geomorphology, I was glad to be a part of this research trip to Arkansas. I became connected with Dr. Amanda Keen-Zebert through a professor at Oberlin, Dr. Amanda Schmidt. I am so glad I was given the opportunity to go on this research trip. While some aspects of the work were labor intensive, such as digging trenches in order to obtain the OSL samples, it was completely worth it because I was able to learn so much throughout the entire trip.

Hiking into Boxley Valley to collect OSL samples.

Hiking into Boxley Valley to collect OSL samples.

Previously I had very little knowledge of OSL, however Kathleen walked us through the basics. It was fun to be able to help out with pounding the metal tubes into the soil, taking care to expose the cores to as little light as possible. At the first site I did have a minor rock hammer incident, however I am pleased to report my thumb is healing well!

Dr. AKZ using a rock hammer to obtain a horizontal core sample.

Dr. AKZ using a rock hammer to obtain a horizontal core sample.

At some locations in order to take samples we were required to dig trenches. Digging trenches was a little exhausting, but after the trenches were complete I was able to learn about soil horizons from the experts. This was great because while I had learned briefly about soil basics at school, this was my first time viewing the various horizons in the field.

Kathleen digging the lower trench at the Margaret White Bluff site.

Kathleen digging the lower trench at the Margaret White Bluff site.

On days when I wasn’t digging trenches, I worked with Malcom making 3D images of the bluffs, both at Steel Creek and Margaret White. Terrestrial laser scanning (TLS) is a surveying technique. It works by measuring the distance to a feature using a laser. In order to connect the various scans from different locations down the river we set up targets. The targets had to be visible from the scanner in both images, so that they could be pieced together later to make one consecutive data set. Therefore, the scanning process required that we set up three targets on either side of the scanner. The scanner then would run for approximately 30 minutes, would take pictures (in order to acquire color to the scans) for five more minutes, and then we would individually shoot all of the targets. In total each scan would take approximately 45 minutes. By the end of the trip I was confident in my ability to set up the required equipment in order to scan the surroundings.

TLS scanner at the Margaret White Bluff site.

TLS scanner at the Margaret White Bluff site.

Throughout the trip we were constantly being shown different aspects of the local geology. It was so nice to be exposed to these features, because seeing real life examples helped cement what I had previously learned at Oberlin. It was also great to be able to talk with the three graduate students from Auburn. Oberlin is not affiliated with a graduate school, so for the first time I was able to talk with graduate students studying geology about their journeys. Overall, I am so grateful for the opportunity to participate in this research trip. --Hannah Heinke-Green

 

We're not in (Ar)Kansas Anymore!

Guest post by Marcus Hill (Geology Undergrad at Oberlin College)

Hello everybody,

For the first part of my winter term project this year I was able to help take samples at a few sites within Northern Arkansas. The project began as way for Dr. Amanda Keen-Zebert and Dr. Stephanie Shepherd to return to home and do some investigating in their backyard and developed into a hugely collaborative endeavor. Professors, grad students and undergrads (hey, thats me!) from different institutions across the US have expressed interest and offered to help in whatever way possible. The goals of this trip in particular were to obtain OSL samples from several terraces within some federally recognized park sites and also to perform Terrestrial Lidar Scanning (TLS) on bluffs that were of interest.

While we flew in on Wednesday night, we didn't make it back to the cabin until around 1AM, so there wasn't anytime for introductions or anything like that. So the next morning after breakfast we had a more formal sit down chat where everyone had the chance to introduce themselves and learn about the project. After that was done we loaded up the cars with supplies and headed out to the first site, Steel Creek. This location was only a 15-20 minute drive from our cabin, which was really nice as the day became brutally cold very quickly. Dr. Keen-Zebert had intended to spend the day both digging and scanning but as it was such a easily accessible and popular location for campers, we didn't know if we had permission to dig. So onto scanning then. The scanner is a machine that looks like a larger than usual lunch box with a rotating camera embedded on the top, which has to be mounted and leveled on a tripod. This machine also costs upwards of $40,000, so you want to make sure that tripod is firmly planted into the ground. Scanning went well enough for most of the morning, but as snow and the temperature started falling, the batteries started failing. After running back and forth between charging stations and the scanner for nearly an hour we had to call it quits.

Here I was able to level the machine with the assistance of Malcolm Williamson (Red Hat) and Mike, and Auburn University graduate student.

Here I was able to level the machine with the assistance of Malcolm Williamson (Red Hat) and Mike, and Auburn University graduate student.

Friday and Saturday the weather was still cold but we were able to keep warm by digging! After making the two hour journey to the Margret White site, Dr. Shepherd and Keen-Zebert showed us the locations for the two trenches. There isn't a whole lot to say about these days, they just included a lot of physical labor. On Saturday, before we started digging we did get to do a small field trip and explore the surrounding region, learning about the different rock types that are present within the local environment.

The open field before we started trenching

The open field before we started trenching

On Sunday we had a little bit of a reprieve after digging for two days and were able to learn about and see how Optically Stimulated Luminescence (OSL) sampling works. Its a process which takes advantage of impurities in the structures of quartz rich samples. Specifically Amanda was looking for a sample which doesn't have a uniform composition and holes were allowed to develop at the molecular level. These holes allow electrons to get stored when the sample is not in the presence of light. The process is somewhat analogous to charging a battery. 

The team finding a location to sample in a cut bank.

The team finding a location to sample in a cut bank.

Monday we went back to the Margret-White trenching site, this time accompanied by Amanda and Stephanie's professor from grad school who specialized in soils. The weather had also made a complete 180 and turned out to be around 60-70 degrees.

Sam, a grad student from Auburn University, explaining soil horozons to Hannah, Kathleen, and Me.

Sam, a grad student from Auburn University, explaining soil horozons to Hannah, Kathleen, and Me.

Tuesday we had the fun job of refilling and erasing all of our hardwork on the trenches.

It's totally not a grave, I swear!

It's totally not a grave, I swear!

We then closed off the week by going back to the Steel creek site and helping finish off the TLS work that we could not do because of the weather earlier. The process involved setting up the scanner with the right parameters and letting it do its thing for anywhere from 30-40minutes. In that time you could do anything you want, Malcolm and I skipped a lot of rocks, but you just had to make sure to not be in the field of view while the scanner was going. 

Malcolm being a brave, brave soul and carrying the scanner into the water across very slipery rocks.

Malcolm being a brave, brave soul and carrying the scanner into the water across very slipery rocks.

Well, that was pretty much the extent of the trip, I had an absolute blast and if this at all interested you, there is a facebook page, Buffalo River Geoscience, set up which posts periodic updates about the project. Until next time, Marcus

Downstream Fining

The gravel bar near Boxley (left) has a median size of 35 mm although here it looks even larger than that! The gravel bar at Buffalo Point (right) is much smaller than upstream with a median grain size of about 10 mm and that's not counting the sand fraction that also increases downstream.

The gravel bar near Boxley (left) has a median size of 35 mm although here it looks even larger than that! The gravel bar at Buffalo Point (right) is much smaller than upstream with a median grain size of about 10 mm and that's not counting the sand fraction that also increases downstream.

If you’ve ever looked at the rocks in the channel at different sites in the Buffalo River, you may have noticed that the size of rocks in the channel at upstream sites like at Boxley and Steel Creek are larger than at sites farther downstream like South Maumee and Buffalo Point. In most rivers, the average size of clasts (loose rocks) decreases downstream. Geomorphologists call this downstream fining. It happens for 2 main reasons.  Smaller grains get winnowed or washed away by fairly frequent flows that have enough energy to carry small grains but not larger ones. Clasts also break apart during larger flows because of smashing into bedrock channel beds and against each other. This also causes rocks to become rounded. It’s like the river is a big rock tumbler. In some really steep rivers, with the right set of instruments, you can even hear the rocks smashing together. Some scientists are now using seismic acoustic technology to measure discharge of large floods from the impacts of rocks in the river rock tumbler.

When the flow is low and it is safe to enter the river, we measure average surface grain size of bars or of the channel using the ‘Wolman Pebble Count’ method, published by ‘Reds’ Wolman, one of the forefathers of fluvial geomorphology, in 1954. His summary of the method was to ‘walk like a drunk and point with a stick’.  Taking a wandering walk, the person making the measurements stops randomly and points a stick at the gravel surface. Pointing with a stick reduces the bias we humans inherently have to pick up rocks that are easy to pick up and easily fit in the hand. We measure across the ‘b-axis’. If your cell phone was a clast, the b-axis would be the width across the face if held upright. The a-axis would be the longest width from earpiece to the speaker at the bottom; the c-axis would be the thickness.  

We make at least 100 measurements to get a good statistical population, but of course, larger bars require more measurements.  Some studies indicate that at least 400 measurements are needed to get accurate results.  Instead of using the average, scientists typically report the median grain size. When 100 measurements are ordered from smallest to largest, the measurement of the 50th grain is the median or D50. The term 'D' is typically used to refer to the diameter of a particular clast size in sediment transport equations. Other measurements are also used in some assessments of grain size, for example, when considering the larger portion of the grain size distribution, the D84, or size of the 84th grain, is often used. 

In the Buffalo River, the median grain size decreases from 35 mm at Boxley to 10 mm at South Maumee.  Of course, the downstream trend isn’t exactly linear.  Inputs of sediment from tributaries can increase the grain size locally, particularly if the tributary is steep.  Rivers in watersheds that have lots of tributaries (what we call a high level of dissection) have lots of inputs of sediment along their length and may not have downstream fining at all. It may be easiest to conceptualize downstream fining on a continental scale.  Rivers in steep high mountain streams often have huge boulders the size of cars while at the coast, rivers supply sand to beaches. So, when you’re sitting on the beach on the Gulf of Mexico, you can think about downstream fining and imagine that you are enjoying sand that may have started  its journey within the large sandstone blocks in the Buffalo River watershed.    

 

How resistant are the rocks in the Buffalo River watershed?

Stephanie Shepherd getting measurements of compressional strength on Boone Formation outcrop with a Schmidt Hammer (it doesn't look like a hammer at all!)

Stephanie Shepherd getting measurements of compressional strength on Boone Formation outcrop with a Schmidt Hammer (it doesn't look like a hammer at all!)

Because one of our main research questions on the Buffalo River is: how does rock type affect river processes, we need to know in what ways rock types in the watershed vary.  In bedrock channels like the Buffalo River, a main factor in determining the rates of channel incision and the formation of valleys is how resistant rocks are to erosion.  When we think about rock resistance, we usually think about how hard a rock is, how easy it is to break apart.  That mechanical resistance involves several factors like joint and bed spacing, compressive strength (how resistant a material is to a force pressing against it--smashing) and tensional strength (how resistant a material is to pulling or stretching). If you have been caving in Arkansas or have tromped around in any karst landscape where there are sinkholes and streams that disappear and run underground, then you know that dissolution is also a factor in rock resistance to erosion. Chemical processes, including dissolution, are particularly important in carbonates like limestone and dolomite.

On the Buffalo River, we measured compressional strength and chemical strength of several rock types in the watershed. For compressional strength, we used a Schmidt hammer, and for chemical strength, we submerged rock samples in hydrochloric acid to determine how much of each rock dissolved. We also did statistical analyses on the results to find out if differences in our datasets were part of natural random variability or were quantitatively legitimate.  

We found that although the Boone and Everton Formations have statistically equivalent compressional strength the Boone Formation has significantly lower chemical resistance. Both the main body and the St. Joe Member of the Boone Formation have very high solubility (98% and 100% respectively) that is significantly higher than the solubility of the Upper Everton Formation (63%). Other lithologies we tested in the watershed have very low mean solubility ranging from ~ 3-33 %. The main lithologies in the Buffalo River watershed have similar mechanical resistance with the Boone Formation limestone being slightly more resistant. However, the Everton Formation is more resistant to chemical processes than the Boone Formation owing to both its higher content of insoluble material and the slower dissolution rate of dolomite versus limestone.

 With respect to river processes, we consider the Boone Formation to be the less resistant, "weaker" lithology owing to its high solubility. The relatively low resistance of the Boone Formation limestone is demonstrated by its near-complete experimental dissolution, the underrepresentation of limestone clasts in modern river sediment load (more on this soon), the wider valley (see previous blog post on this topic), and recognition that it is the predominate host of karst features within the catchment.

So which rocks are the hardest?  It depends on whether you are smashing them with a hammer or whittling them away with water. Stay posted for more info from our research team soon!

 

Why is Boxley Valley so Wide?

Have you noticed how wide the valley is at Boxley Valley? In 'How Rivers Work 101' (intro fluvial geomorphology), we learn that channel and valley width increase downstream along with discharge. A look at a topo map, like the clip from the Boxley map below, or a drive along Boxley Valley to the low water bridge at Ponca where the valley starts to pinch down shows that the Buffalo River is an exception to the rule. For our research team, one of the central scientific research questions is: how does rock type affect river processes? To learn more about valley width, we made measurements of valley width at regular intervals using geographic information systems (GIS) along the river and then partitioned the measurements by lithology to test whether our observation of valley width changing with rock type was statistically relevant. We found that it is. In the limestone reaches (like the reach from Boxley Valley where the Buffalo River begins to the the low water bridge at Ponca) the valley is 70% wider on average than in reaches that are more sandstone dominated (like the reach from Ponca to just upstream of Carver). While it is evident that lithology and valley width are related in the Buffalo River, we're still working to discover how rock type effects the erosion processes that control and produce the variations in valley width.  You can download the full version of the maps shown below from our Maps Page and check back for more research results from our team!   

The geology map at Boxley Valley shows the wide valley where the channel is incised into the Boone Formation limestone. Excerpt from Hudson, M.R., and Turner, K.J., 2007, Geologic map of the Boxley Quadrangle, Newton and Madison Counties, Arkansas: U.S. Geological Survey Scientific Investigations Map 2991, 1:24,000 scale.

The geology map at Boxley Valley shows the wide valley where the channel is incised into the Boone Formation limestone. Excerpt from Hudson, M.R., and Turner, K.J., 2007, Geologic map of the Boxley Quadrangle, Newton and Madison Counties, Arkansas: U.S. Geological Survey Scientific Investigations Map 2991, 1:24,000 scale.

The geology map from Ponca low water bridge to Steel Creek shows that where the channel cuts into the Everton Formation, the valley becomes narrower. Excerpt from Hudson, M.R., and Murray, K.E., 2003, Geologic map of the Ponca Quadrangle, Newton, Boone, and Carroll Counties, Arkansas: U.S. Geological Survey Miscellaneous Field Studies Map 2412, 1:24,000 scale.

The geology map from Ponca low water bridge to Steel Creek shows that where the channel cuts into the Everton Formation, the valley becomes narrower. Excerpt from Hudson, M.R., and Murray, K.E., 2003, Geologic map of the Ponca Quadrangle, Newton, Boone, and Carroll Counties, Arkansas: U.S. Geological Survey Miscellaneous Field Studies Map 2412, 1:24,000 scale.

The Middle Bloyd Sandstone

Middle Bloyd Sandstone

The middle Bloyd sandstone is not yet a formally recognized unit, but these are some of the most distinctive rock units in the Buffalo River region. The middle Bloyd is considered part of the larger Upper Bloyd Formation that is a sequence of sandstone with some interbedded siltstone, shale, and limestone. The middle Bloyd. It is a Lower Pennsylvanian (~299-307 Ma), Morrowan sequence of sandstone that contains quartz pebbles, and lycopod fossils (Hudson, et al. 2001) that is 80-120 ft (~24-37 m) thick. You can see it outcropping in many of the bluffs in the highest parts of the watershed. The middle Bloyd makes up some well-known bluff formations including Buzzards Roost in northwestern Pope County and the outcrop at Horseshoe Canyon Ranch where you can rock climb on and get a close-up look at these rocks. This photo is taken from AR state Hwy 43 at Mt. Gaither between Ponca and Harrison (see our Geosites page for location). The middle Bloyd is present in the Boston Mountains Plateau sub-province of the larger Ozarks Plateaus physiographic province. The middle Bloyd is recognizable because of the distinctive cross-bedding (e.g. Unrein 2007; Hudson et al. 2011). The figure below from Kevin Unrein's master's thesis describes some of the cross-bedding of the middle Bloyd at Mt. Gaither. The middle Bloyd was first differentiated from the rest of the unit by Zachry 1977. There is still a lot of scientific research interest in the middle Bloyd. The current explanation is that the formation was deposited as during a transition from a fluvial to estuarine system in the Morrowan stage of the early Pennsylvanian subperiod of the Carboniferous Period (e.g. Unrein, 2007). You can find more information in Kevin Unrein's thesis (downloadable on our Scientific Papers page), Mark Hudson's maps (on our Maps page), and in the Geological Society of America Karst Interest Field Trip Guide (downloadable on the Guides page).  You can find the location of this photo on our Geosites page

Description of cross-bedding of the middle Bloyd sandstone at Mt. Gaither from Unrein, 2007. (See full citation on our Scientific Papers page.

Description of cross-bedding of the middle Bloyd sandstone at Mt. Gaither from Unrein, 2007. (See full citation on our Scientific Papers page.

       

Roark Bluff geology

Roark Bluff upstream end

This is the upstream end of Roark Bluff at the beautiful deep swimming hole across from the campground. The dashed line indicating the boundary between the Newton Member of the Everton Formation and the lower part of the Everton Formation is approximate. Mark Hudson, one of the geologists on our team, has sampled tufa at the base of this cliff, and has verified that it is the lower Everton Formation (Oel on the geology maps). At the base of the Newton Sandstone Member (Oen), there is a change to more massive, rounded bedding higher up on the cliff. The exact contact is an estimate in the graphic. The differences visible in the photo are more massive beds and rounded texture in the Newton. In the lower Everton the beds are less massive, thinner, and are more angular.

For most, the overhanging ledge that marks the lower quarter of the bluff stands out. That is an erosional feature and its location is more likely related to hydrology instead of stratigraphy. During floods, the rocks below are eroded, undercutting the overhanging rocks which then become unstable and under the forces of gravity detach via rockfall.

Check out our Geosites Map for the location of this site.

Geology of the bluffs at Steel Creek

Steel Creek from Buffalo River Trail

There are a couple of bluffs at Steel Creek. The prominent one across from the campground on the left side of this photo upstream of the boat launch on river left is called Roark Bluff (there is a closer photo is in the previous post). The bluff labelled Bee Bluff on USGS maps is downstream of the boat launch on river right and the right side of this photo. (This was a mistake in labeling the bluffs.  Historically, this bluff is unnamed.) Roark Bluff is made up of the Everton sandstone, a Middle Ordovician interbedded dolostone, limestone, and sandstone. At Roark Bluff, the Lower Everton is present at the base, but the majority of the bluff is made up of the Newton Member of the Everton Formation. The Newton Member is composed of fine well-sorted, very rounded quartz grains and is really thick in the western end of the Buffalo River. This rock has a sugary texture and the sand grains within it are round like little marbles. The channel may incise down to the Powell Dolomite at the base of Rushing Pool (the deep pool across from the campground) but, it doesn't really show up in Roark Bluff and it is unclear if the channel really reaches down to the Powell in this reach. The Powell does appear at river level just downstream of downstream of Bee Bluff on river left where Cliff Hollow incises into it making a nice exposure. The Powell appears at channel level for just under 2 mi from Cliff Hollow to Beech Creek which enters river right. The Boone Formation, including the St. Joe Member do cap the bluffs in this reach but I don't think they are actually visible in the vertical bluff faces at Steel Creek.

I'm working on labeling photos of the prominent bluffs with the geologic units, so stay tuned for more!

Check out the Geosites Map for the location of this view.

What is that round thing in the rock?

Kevin Middleton sent us a message with this photo and asked about the round feature in the top right of the cliff. Here's what our expert research geologist, Mark Hudson (USGS), had to say about it:

From the overhang and the knobby nature of the outcrop I would guess that this is a middle Bloyd sandstone cliff. In that context (i.e., porous sandstone) it is most likely that circular patterns are iron concentrations (Liesegang bands) formed from groundwater seepage that make the bands more resistant to weathering and stick out. I don't commonly see them quite this circular, so this is an interesting example. Of course one would like to get a closer look to verify the guess.

Send us your geoscience questions we'll do our best to answer! Thanks for sharing this with us, Kevin!

Welch Bluff

Lat/Long: 36.064716, -93.126730

This is a view looking east across the Buffalo River at Welch Bluff showing the normal fault zone that forms the southern margin of the Braden Mountain graben. On the hanging wall (on the left side of the image to the north), the contact between the Boone and Everton Formations is dropped below the river level. On the footwall (on the right side of the image to the south), the contact rises above river level. The graben system formed during the late Paleozoic (~250 Million years ago) in association with the Ouchita uplift. The fault is fairly quiet now. If it was active, we might see a change in the gradient of the Buffalo River here, but it is constant here.  The info about the fault was adapted from Hudson et al., 2011 and Hudson 2000. So check out those papers for more scientific detail!  For more information on how faults work, see the USGS page on faults.

Photo of Welch Bluff showing the offset of the geologic stratigraphic formations.

Photo of Welch Bluff showing the offset of the geologic stratigraphic formations.