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.