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!