The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.
A sinkhole is an area of ground that has no external surface drainage. Water that enters a sinkhole exits by draining into the subsurface. Many people are leery of sinkholes because of the damage they sometimes cause. Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake.
Not every kind of sinkhole is the dangerous kind though. The picture above shows a solution sinkhole. Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression. Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground.
In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone. These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.
If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features. Some stand tall like towers while others appear to be irregular to rounded masses. It is common to see only the tops of these features when the lake level is low to normal.
Top of sandstone mass in Beaver Lake. Photo taken in October, 2016.
Sandstone mass along Beaver Lake. Photo taken in October, 2016.
These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties. He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape. Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst. Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand. Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell. One very large sandstone mass was seen in the White River (Arrington, 1962). It is approximately 45 feet tall! Unfortunately, it is now covered with water.
Sandstone mass in Carroll County. From Owen, 1858
Sandstone mass in the White River near Hwy 12 access to Beaver Lake. From Arrington, 1962.
So how did these features form? First, let’s define paleokarst. Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record. Karst features include sinkholes, springs, and caves. These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).
The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation. The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya). It is likely that this formation was exposed to weathering at that time. Depressions of various size, called sinkholes, developed on the exposed land surface. Later, sand filled the depressions and eventually became rock called sandstone. The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya). Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.
Sandstone mass (center) surrounded by Powell dolostone along Beaver Lake. Photo taken in September, 2016.
Why is paleokarst important, other than being interesting features to observe? Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011). We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas. But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.
Many sandstone paleokarst features were located while mapping the War Eagle quadrangle. Fifty-two sandstone masses were located around Beaver Lake. This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.
Sandstone masses (yellow) located from recent geologic mapping around Beaver Lake.
The War Eagle quadrangle was mapped in preparation for State Park Series 4 – Geology of Hobbs State Park. Follow the link below to see the geologic map of the War Eagle quadrangle: http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/War%20Eagle.pdf.
Until next time,
Arrington, J., 1962, The geology of the Rogers quadrangle: University of Arkansas M.S. Thesis, 61 p.
Palmer, A.N., and Palmer, M.V., 2011, Paleokarst of the USA: A brief review: in U.S. Geological Survey Karst Interest Group Proceedings, Fayetteville, Arkansas: U.S. Geological Survey Scientific Investigations Report 2011-5031, p. 7-16.
Owen, D.D, 1858, First report of a geological reconnaissance of the northern counties of Arkansas made during the years 1857 and 1858: Little Rock, 256 p.
Purdue, A.H., 1907, Cave-sandstone deposits of the southern Ozarks: Geological Society of America Bulletin, vol. 17, pp. 251-256.
Purdue, A.H., and Miser, H.D., 1916, Geologic Atlas of the United States, Eureka Spring-Harrison Folio, Arkansas-Missouri: U.S. Geological Survey Folio No. 202, 82 p.
Staley, G.G., 1962, The geology of the War Eagle quadrangle, Benton County, Arkansas: University of Arkansas M.T. Thesis, 56 p.
According to the American Geosciences institute, a castle, in the geologic sense, is a natural rock formation bearing a fancied resemblance to a castle – sophisticated science, I know! The limestone boulder pictured above, which is from north central Arkansas, is one such castle. Rocks like this one owe their appearance to their solubility in weak acid.
Most rain water is actually slightly acidic, due to the CO2 it absorbs from the atmosphere and soil it passes through. Over time, this acidic water is capable of dissolving limestone bedrock into features such as caves, sinkholes, and, in this case, castles. The boulder pictured here has been flipped over by the creek’s current; they typically form with the castle side down.
below is an example of a castle that is still forming. The base of the rock dissolves faster than the upper part, because it is under the water more often. This differential weathering is what gives the boulder its characteristic castle shape.
Photos by Richard Hutto
To see the original post on travertine falls click here
Pictured above is a travertine falls. It looks like a waterfall except that, rather than being water, it’s composed of solid rock.
Travertine is made of calcite which also forms stalactites and stalagmites. Like those familiar cave features, travertine falls form by precipitation from water; the water is flowing in a creek, over a ledge instead of dripping from a cave ceiling. As the travertine precipitates in layer upon layer, it begins to take on the appearance of flowing rock.
Dripstone features like these only form in areas where the groundwater carries a high load of dissolved carbonate minerals. This one was photographed in Searcy County, Arkansas, not far from the Buffalo National River, near the contact between the St. Peter and Plattin Formations.
For another view of this travertine falls click here
The above picture shows what can happen when a sinkhole forms in the channel of a creek. This photo is taken facing in what use to be the upstream direction. If you look carefully, you can tell that the water is actually flowing into the fracture; the sinkhole has captured the flow of the creek and caused a strech of it to flow backwards.
Sinkholes form in a variety of ways, but this one evidently was the result of sudden collapse of bedrock into a subterranean cave that was forming below the land surface. You can’t see the sinkhole itself because it’s just inside of the dark opening in the rock.
If the photographer were to turn around and snap a picture in the downstream direction, you would see that the stream bed is dry just downstream. In geologic terms this is called a disappearing stream.