Tag Archives: sedimentary structure

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

Geopic of the week: Bioturbation

Knabioturbation

Pictured above is clay and sand of the Nacatoch Formation of southern Arkansas.  The clay beds, which stand slightly in relief, have been churned, and the sand has numerous cylindrical structures of various sizes.  These are hallmarks of bioturbation, or reworking of sediment by living things.  The cylindrical structures are the preserved casts of roots.  The disrupted clay beds are evidence of the burrowing of mud-loving critters.

Bioturbation structures are commonly preserved in rock and offer glimpses into the environment where sediment was deposited.  Specifically, it tells us that this sediment was near the surface in a relatively calm near-shore marine environment long enough after deposition for living organisms to move in and set up shop.  Clues like these are one of many tools geologists use to piece together the early history of the earth from the rock record.

For more views of bioturbation click here

Geopic of the week: Sandstone pipe

sandstone pipe

This is a “sandstone pipe” in the Witt Springs Formation of north central Arkansas.  At first glance, it seems that someone has managed to insert a pipe into the outcrop (either that, or someone had fairly sophisticated plumbing 320 million years ago).  Actually, these naturally occurring features result from iron minerals precipitating out of ground water as it moves through rock.  When minerals precipitate from a solution, they do so in concentric bands known as liesegang bands.  They were named for Raphael Liesegang: the chemist that first produced them in the laboratory. Once the band of minerals has formed, it makes that part of the rock harder, and, as the rock erodes, the iron-fortified band stands out in relief.  Typically liesegang bands form in organic shapes like the ones that surround the pipe above.  When they form a cylindrical band, however, they look almost identical to iron pipes.

Photo taken by Richard Hutto

For more views of sandstone pipes click here

Geopic of the week: injection feature

 

Sandstone injection feature hollywood quarry

Pictured above is what geologists term an injection feature or sand dike.  It formed when sand was violently forced upward into overlying clay before the sediment was cemented to form rock.  In environments where sediment is accumulating very quickly, water can get trapped and buried in a sand body;  as more sediment is deposited on top of the sand, the pressure causes the sand body to compress.  When water erupts upward to relieve the pressure, it carries sand with it which fills the fissure created by the escaping water. 

Geologists look for clues like injection features when trying to unravel the mystery of what conditions were like when a rock was deposited.  This particular rock is part of the Jackfork Formation which is exposed at the surface around Little Rock Arkansas and surrounding areas; it was deposited when the area was at the bottom of a deep ocean basin more than 300 million years ago.  Ink pen is for scale.

Statemap Field Blog, April 7-9, 2014

2014-04-07 006

Hello all,

Well, this is the last week of field work for the 2013-14 season.  Of course, there’s always more one would like to have a look at, but we have to stop sometime.  On Monday, we started down by the M&NA railroad bridge at Shirley.  The big fault that makes the SW/NE lineation goes through here somewhere, but it’s difficult to say where exactly.  There are lots of non-vertical joints and deformation bands in the area, which are all good fault signs, but nothing very definitive.  The area north of the bridge is about as thick as it could possibly be with greenbriers –only passable with much effort and many scratches.  We saw very thick-bedded sandstone there which we took for Witts Springs that day, but when we came back on Wednesday, we decided it may be north of the fault, and therefore would be Imo.  We have Imo across the valley, so it’s not out of the question to have it here, but it may be just a relatively thin slice.  There are many cut and fill channel beds there, some of them with very nice soft-sediment deformation at the margins.

On Tuesday we finished up some loose ends in the northwest corner of the Shirley quad.  After we climbed way down in a hollow that had an old tornado track going through it, Danny realized he had lost his camera somewhere.  We hiked back up to the Jeep to see if it was there (it wasn’t), then retraced our steps from earlier that morning.  Still nothing.  He remembered the last time he had used it was in that horrible briar patch the day before, so after we climbed out again, we headed back there.  Sure enough, in the thickest part of the patch, where he had been practically crawling to get through, a briar had reached in his carrying case and pulled it out.  It was still dangling there about a foot off the ground right on the river bank.  At least we got it back!

Deformation bands in massive sandstone near Middle Fork north of Shirley2014-04-07 017

2014-04-09 026 2014-04-09 011On Tuesday afternoon, we went down a drainage on the west side of Middle Fork looking for more signs of a fault we have traced from the Old Lexington quad.  We definitely found a lot of deformation bands in the Witts Springs massives down there and figure there might be as much as 80 feet of throw on the fault.

2014-04-08 0192014-04-08 027Wednesday was our last day in the field this year, and we spent most of our time on the Middle Fork just north of Shirley where we had left off on Monday.  Did look like the fault goes through there because we found very-thick bedded massives on the north side (Imo) and shale interbedded with very thin-bedded sandstone on the south side (Cane Hill).  Our last couple of hours we spent getting points in several road cuts in and around Shirley.  We took a final photo in front of the town sign.

2014-04-08 048 (2)2014-04-09 047This will be Danny’s last year out in the field with me, so I’d like to take this opportunity to thank him for putting up with me and the sometimes horrendous field conditions we’ve faced together the last five years.   Looks like I’ll have to break in a new field partner next year, so should be interesting.  Now comes the time of year when we have to sit in the office and draw the maps, create the layouts, and finish the database, all to be turned in to the USGS by June 30.  It seems like a long time, but we’re always editing down to the last minute.  By the time we make it back out in the field, it will be mid-July, so the ticks and snakes will be out in full force, it will be nice and hot, and all the vegetation will be full grown.  At least that gives us something to look forward to.  Until then, I’ll see you in the office.  After that, I’ll see you on the outcrop!

 

 

 

GeoPic Of The Week: Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple marks are sedimentary structures preserved in sandstone and limestone. They may be asymmetrical in shape, with the steep side pointing downstream in the direction of current flow.   In this picture the steep side is toward the viewer and so is the current direction.  Ripples form naturally by the movement of water currents in rivers and streams, on beaches of tidal and long-shore currents, and in deep-ocean basins.  This picture was taken of Ordovician age sandstone in the Everton Formation along Beaver Lake.

 

Statemap Field Blog, Sept. 23-25, 2013

Hello all!

Another good week in the field!  Temperatures are getting low in the morning, but still warming up in the afternoon.  We started out by finishing up the rest of Tick Creek at the north edge of the map.  Got the lower Cane Hill sandstone up Files Hollow and again on the east side of Tick.2013-09-23 001  2013-09-23 014Still have good channel-bedded Imo below in the creek bed with a shale unit between the two.

2013-09-23 021

Tuesday and Wednesday we finished up the lower end of Indian Creek.  There were several massive-bedded sandstone units that had cut down into lower units of thin- to medium-bedded sandstone interbedded with shale.

This is probably all in the Witts Springs Formation, but we won’t know for sure until we see more of the rock in the area.  One of the indications that it is Witts Springs sandstone is the characteristic curved reentrant at creek level.  This is caused by exfoliation or spalling of curved sheets of rock due to more rapid dissolution of the calcareous cement near the creek.  The Witts Springs sandstone is typically more calcareous than overlying formations.    2013-09-24 0452013-09-24 060

You never know when you might see a classic example of a sedimentary structure while hiking around.  This week we saw some good examples of load casts in the creek float.  A sand deposit formed an irregular bulge as it pressed down (loaded) into the mud below.  Later lithification preserved the cast of that shape on the bottom of the sandstone bed.

2013-09-24 040

As we approached the southwest/northeast topographic lineation along the Middle Fork at the mouth of Indian Creek, we began to see more and more signs of a major structure.  Aside from a 5 to 7 degree southeast dip, there was an increasing abundance of deformation bands in the massive sandstone.   Also, we began to see a lot of non-vertical joints and small faults.  Took photos of a couple of fault planes with the slickensides still evident.2013-09-25 013  Slickensides are the parallel grooves or scratches left behind on the fault plane caused by the abrasion of one rock surface against another.  They are typically smooth in the direction of movement and rough in the opposite direction, so can indicate which way the fault moved.  Unfortunately there is no way to estimate throw, or the amount of offset on the fault, without knowing which formations are on either side.  That’s why we’ll have to be extra thorough in that area.  There’s probably a big fault somewhere along that lineation, but we’ve yet to find it so far.

By the way, Indian Creek must have gotten its name from the number of moccasins lying around.

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2013-09-24 0382013-09-25 004

We’re taking a little hiatus next week so will be at least two weeks ’til the next installment.

See you on the outcrop!

Snake count: at least 4

Tick attacks: still severe