Tag Archives: karst features

Buttress on Little Buffalo River near Parthenon

Notes From the Field: Boone Buttresses

Buttress on Little Buffalo River near Parthenon

Buttress on Little Buffalo River near Parthenon, Newton County

The photo above shows an unusual rock column located near Parthenon in Newton County.  Judging from the man standing at the base, it is probably over 100 feet tall.  Recently, I was asked what to call these impressive features.  The term we’ve used at the Survey is buttress, which is defined by the Glossary of Geology as a protruding rock mass on, or a projecting part of, a mountain or hill resembling the buttress of a building; a spur running down from a steep slope.  Example: a prominent salient produced in the wall of a gorge by differential weathering.  We’ve used the term buttress, instead of other terms like pinnacle or rock pillar, because these terms refer to a free-standing column of rock, whereas a buttress is, at least nominally, attached to the bluff line.  The term also differentiates these particular features from others that are similar in shape, such as pedestals or hoodoos, which typically form in clastic rocks like sandstone and siltstone.  Their development is controlled by joints, which are planar fractures with no displacement, and by the presence of a resistant caprock, which acts to protect the underlying, less-resistant rock from weathering as quickly.  This process leads to a characteristic shape that is wide at the top and narrower below.

Sandstone pedestal at Pedestal Rocks, Pope County

Sandstone pedestal at Pedestal Rocks, Pope County

A buttress, on the other hand, is typically either uniform in diameter or may taper slightly towards the top, probably because they develop in fairly homogeneous rock.

Buttresses are known to be present in two locations in Arkansas: along the Little Buffalo River near Parthenon in Newton County and along Bear Creek near Silver Hill in Searcy County.

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Buttresses on Bear Creek, Searcy County

They are all developed in the Mississippian Boone Formation which averages about 320 feet in thickness, and is composed of interbedded limestone and chert.  Limestone is dissolved by slightly acidic surface and groundwater, and over time this process leads to many unusual surface and subsurface features known as karst.  Buttresses are one such feature.

The exact mechanism for their development is poorly understood, but some of the factors that contribute to their formation are known.  First, dissolution of limestone can produce similar shapes on a small scale, as seen in this photo of coarsely crystalline Fernvale Limestone in a creek bed.

Dissolutioned limestone in creek bed, Stone County

Dissolutioned limestone in creek bed, Stone County

This process may be all that is needed to produce the buttresses at a larger scale.  Second, all rock units have planes of weakness due to the regional history of tectonic stress.  This stress is usually expressed as a joint system, and is one of the most commonly observed structural features in an outcrop.  Observations at these two sites have shown that jointing is poorly exposed, but as you can see from the aerial photograph on Bear Creek, weathering of the buttresses roughly aligns with the most prominent joint trends in the area (N/S and NE/SW) as indicated by the joint diagram from the Geologic Map of the Marshall Quadrangle.

Aerial view of buttresses on Bear Creek showing prominent regional joint orientationsMarshall Rose

Aerial photo of buttresses on Bear Creek showing prominent regional joint orientations

So even though the joints are poorly developed, one can interpret that pathways for water preferred these orientations, enlarged them over time, and left the buttresses as erosional remnants. 

However they occur, they are certainly beautiful rock formations and worthy of further study.

Buttresses on Bear Creek, Searcy County

Buttresses on Bear Creek, Searcy County

Many thanks to Angela Chandler for the featured image!

Richard Hutto

Archimedes in Pitkin Limestone

Notes from the Field: Pitkin Limestone

 

The Pitkin Limestone

One of the most fossiliferous formations in the state is the Pitkin Limestone. It was referred to as the Archimedes Limestone in the late 1890s because it contains an abundance of the screw-shaped bryozoan fossil Archimedes. It was formally named the Pitkin Limestone in 1904 for exposures near Pitkin Post Office in Washington County, Arkansas. If you can’t find the town of Pitkin on a map, don’t worry–it’s now known as Woolsey.

The Pitkin began as carbonate sediments deposited in the Mississippian Period around 320 million years ago.  At that time, northern Arkansas was covered by a shallow sea that was fairly close to the equator.  Warm, shallow seawater is a prime environment for the build-up of carbonates.  Marine organisms extracted calcium carbonate out of the seawater to form shells or other hard parts.  This material accumulated and eventually turned into limestone.  Some of those secreted structures are preserved as fossils in the rock and are clues to the environmental conditions that existed at the time.

The Mississippian in Arkansas

The area of what is now Arkansas during the Mississippian

The Pitkin Limestone is a bluff-former that crops out in the southern portion of the Ozark Plateaus from just south of Fayetteville eastward to Batesville, typically along the Boston Mountains Plateau Escarpment.  It is mostly limestone, however, there is some nodular black chert present locally.  Black shale intervals are common in the eastern portion.  Because limestone is a soluble rock, karst features such as caves, sinkholes, springs, and disappearing streams are common in this Formation.  About 9% of the known caves in Arkansas are in the Pitkin.  Its thickness varies from an average of about 50 feet on the west side of the state to about 200 feet in the eastern part with a maximum of about 400 feet in the central portion.  It typically rests on the Fayetteville Shale and is overlain by the Cane Hill Member of the Hale Formation in western Arkansas and by the Imo interval from the area of western Searcy County eastward.

Geologic Map of Arkansas-detail

The Pitkin outcrop belt is within the light-brown area in this Ozark Plateaus detail of the Geologic Map of Arkansas

To download the entire Geologic Map of Arkansas, click here: http://www.geology.ar.gov/ark_state_maps/geologic.htm

Cane Hill/Pitkin Contact near West Fork

The Cane Hill overlying the Pitkin near West Fork, Washington County

Pitkin/Fayetteville Contact at Hwy 65 Roadcut

The Fayetteville underlying the Pitkin near Marshall, Searcy County

Pitkin top in Little Red Creek

Top of Pitkin in Little Red Creek near Canaan, Searcy County

Now, let’s look at fossils commonly found in the Pitkin.

Archimedes in Pitkin-Batesville Archimedes in Pitkin-Fayetteville

The photos above contain fossils of Archimedes.  The fossil is named for the ancient Greek engineer who invented a device that incorporated a large screw to lift water for irrigation.  The left photo was taken south of Batesville and the right photo was taken south of Fayetteville.  It’s remarkable that these fossils are so persistent along this great extent.  Although this fossil is characteristic of the Pitkin, it can also be present in adjacent formations.  The illustration below is a sketch of a fenestrate Bryzoan of which Archimedes is a type.

Fenestrate Bryzoan

Archimedes as it may have appeared in life

Crinoid stems and Columnals-Batesville Crinoid Stems-Batesville

Pieces of fossilized Crinoids are also abundant in the Pitkin.  Most commonly, small button-shaped pieces of the stem and arms, known as columnals, are preserved in the limestone.  That is a columnal in the center of the left photo.  The larger crinoid fossils above were preserved in shale and were most probably washed onto a mud flat during a storm event.  These photos were taken south of Batesville, but crinoid detritus is abundant throughout the Pitkin and most other limestone in Arkansas.

Crinoid

Crinoid as it may have appeared in life

A great location to see the Pitkin is along Richland Creek at its confluence with Falling Water Creek.  When the creek level is low, you can hike upstream from the campground and see many fine exposures of Pitkin Limestone in the creekbed.  Locally, colonies of tabulate and rugose coral were preserved in the Pitkin and can be discovered upon close inspection of the outcrop.

Moore Quadrangle-detail

Detail of Geologic Map of the Moore Quadrangle showing Pitkin along Richland Creek (Mp=Pitkin)

To download the entire Geologic Map of the Moore Quadrangle, click here: https://ngmdb.usgs.gov/Prodesc/proddesc_76560.htm

Tabulate Coral in Pitkin Limestone

Tabulate or colonial coral in the Pitkin Limestone along Richland Creek.

Rugose Coral Colony in Pitkin Limestone

Rugose coral in Pitkin

Locally, the Pitkin consists of oolite, a type of sedimentary rock composed of ooliths.  Ooliths are small, spherical structures (<2 mm) that form by accretion of numerous concentric layers of calcite on a central nucleus such as a shell fragment or sand grain.  The environment of deposition would have been areas where strong bottom currents or wave action rolled the fragment around in carbonate-rich sea water.  This would include environments like beaches and tidal flats.

Oncolites and stromatolites are also preserved in the Pitkin.  They have a similar structure to ooliths, but are much larger (up to 10 cm), can be round or irregular-shaped, and are formed by a different mechanism.  Like ooliths, they nucleate on a shell or other fragment, but are built up by encrusting layers of blue-green algae or cyanobacteria.  Stromatolites form in much the same way,  but create columns, mats, or large heads.  Stromatolites and oncolites typically indicate a paleoenvironment of warm, shallow water in a calm sea, lagoon, or bay.

Oolitic Pitkin

Oolitic Pitkin

Oncolitic Pitkin

Oncolitic Pitkin

Stromatolitic Pitkin

Stromatolitic Pitkin

During fieldwork for our geologic mapping, finding Pitkin Limestone is always exciting because there is something new and interesting to discover every time.  We hope this brief introduction to one of Arkansas’ most intriguing formations has convinced you to seek out the Pitkin and have a closer look.

Until next time, we’ll see you on the outcrop!

Richard Hutto, Angela Chandler