Tag Archives: Ozark Mountains

Geo-pic of the week: Calcite-filled Tabulate Coral

tabulate coral

Continuing with our previous theme “Sharkansas”,  this week’s geo-pic is on Arkansas corals.  Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.

The picture above is of a tabulate coral: a now-extinct variety of colonial coral.  Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water.  The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock.  It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.

Other varieties of coral are found in the rocks of Arkansas.  For more views of Arkansas corals click here

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

Statemap 2015-16 Update

 

Hello all!

Well, another year, another map!  The Brownsville quad is now published (see map below), and a link to it will be posted on our website soon.  This year marks the 22nd anniversary of Statemap, aka the National Cooperative Geologic Mapping Program, in Arkansas.  Statemap is partially funded by a USGS grant, and was established to encourage the states to map their surface geology at the 1:24,000 scale.  To date, our mapping teams have completed thirty-three quadrangles in the West Gulf Coastal Plain and, with the recent publication of the Brownsville quad, forty quads in the Ozark Plateaus.

Geologic map Brownsville, AR

The geology of the area around Greers Ferry Lake has never been mapped in great detail until now.  Previous work had been to produce the 1:500,000-scale Geologic Map of Arkansas.  Because we mapped the Brownsville quad at the 1:24,000 scale, we were able to make some observations new to science.  A fault was discovered that had never been mapped previously.  We named it the Shiloh Fault for the old town, now inundated by the lake, that lies along its trace.  Meanders of the Little Red River channel approached this fault but didn’t cross it, probably due to encountering more resistant rock on the north side of the fault.  The Witts Springs Formation had not been mapped south of the Choctaw Creek Fault before, but we were able to draw in its upper contact with the Bloyd Formation along the Devil’s Fork and several other drainages.

Overturned cross beds in massive sandstone of the undifferentiated Bloyd Formation

As on other quads around Greers Ferry Lake, we continued to find terrace deposits left behind as the Little Red River carved the valley down to its present elevation.  Some of these are stranded as much as 260 feet above the current channel bottom (now located on the bottom of the lake).

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For many years now, our mapping program has focused on completing the Mountain View 1:100,000-scale quad.  This area encompasses thirty-two 1:24,000-scale quads and stretches from Richland Creek to Sylamore Creek on the north side and from the Illinois Bayou to Greers Ferry Lake on the south side.  Now that this area is finished, our Statemap Advisory Committee has decided we should jump over to northwest Arkansas to complete work on the Fly Gap Mountain quad, just west of the Mountain View quad (see map below).

STATEMAP index for blog

So for next year, the Statemap team is going to start work on the Durham quad in the northwest corner of the Fly Gap Mountain quad near Fayetteville.  We’ll have to spend a few weeks getting our feet on the ground, so to speak, because we won’t have the benefit of already mapped quads adjacent.  Fortunately, we will be very close to the type-sections for most of the formations we’ll be mapping, so hopefully, we can study the classic outcrops and trace them into our new field area without too much difficulty.

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A type-section is an area, or even just an outcrop, where a particular formation was first described.  They are named after a local geographic feature.  Formations first described in northwest Arkansas include: the Fayetteville Shale, the Pitkin Limestone, and the Hale Formation which has the Cane Hill and Prairie Grove as members.  Members are smaller, discernable units within a formation.  The type-section for the Bloyd Formation, including the Brentwood, Woolsey, Dye, and Kessler Members, and the Trace Creek, which is the basal member of the Atoka Formation (named for its type locality in Oklahoma), is on Bloyd Mountain near West Fork.

I would like to take this opportunity to thank my field partners that accompanied me this past year.

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I started the year with Ty Johnson, who has since moved into a permanent staff position at the Survey, so congratulations to him!  He was with me for just a year, but we covered a lot of ground together.  He’s now mapping the geology of the Lake Ft. Smith area with an emphasis on landslide mitigation.

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The writer and also principle investigator of the Statemap grant, Angela Chandler, went out a few weeks in the late fall before we could fill the vacancy Ty left behind.  No matter how much I learn, she always manages to teach me something new.

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We hired Garry Hatzell, a recent U of A grad, who started fieldwork in January.  He brings an enthusiastic knowledge of paleontology to the mix, and I look forward to his continued insight into the biostratigraphy of our field areas.

Without the help of these fine folks, we couldn’t have gathered the data or produced the map.  Also, I would have been stuck in the office—a torture for the unrepentant field geologist.

Wish us luck on the Durham quad!  And if you’re in northwest Arkansas during the next twelve months and happen to drive by a Jeep Cherokee with the AGS seal on it, be sure to stop and introduce yourself.

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Until then, I’ll see you on the outcrop!

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Richard Hutto

Geopic of the week: Paleokarst on the Buffalo River

Big Plug Paleosinkhole Gimped 20 Apr 01

This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas.  Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.

In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock.  When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand.  Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).

All of this happened about 450 million years ago.  Paleokarst features are one more clue geologists use to decipher earth’s history.  If you didn’t know better, you might float right by and never give it a second thought.

Geopic of the week: frost flowers

frost flower (1)

This week’s geopic is a bit of a stretch, but since ice is technically a mineral, a frost flower is a crystal form of the mineral ice.  Plus they are just cool.

Frost flowers form when the air is below freezing but the ground isn’t frozen.  In Arkansas, these conditions typically occur early in the morning in Fall or Spring.  They begin due to freezing of moisture in the stem of a plant which expands and makes small cracks in the stem.  Water in the plant freezes as it is extruded from the cracks, and more moisture is continually wicked up from the soil.  The resultant ice forms delicate ribbons that curl around the stem resembling flower petals.

Because they are so fragile and rarely last more than a few hours, most frost flowers are never seen.  If you happen to be out on a crisp morning, come across one, and you have a camera handy, be sure to take some pictures.

For more views of frost flowers click here