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

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.