Tag Archives: Septarian concretion

Cannon Creek Waterfall at Parthenon/Brentwood Contact

Notes from the Field-Durham Quadrangle


Geologic Map of the Durham Quadrangle, Madison and Washington Counties, Arkansas

Geologic mapping of the Durham 7.5-minute quadrangle in northwest Arkansas was recently completed by the STATEMAP field team.  STATEMAP in Arkansas is currently focused on detailed 1:24,000-scale mapping in the Ozark Plateaus Region in north Arkansas.  It is accomplished through a cooperative matching-funds grant program administered by the US Geological Survey.   Field work was performed between July and February, and included hiking/wading/swimming the entire 12-mile stretch of the upper White River located on the quad.  Previous mapping delineated five stratigraphic units for the 1:500,000-scale Geologic Map of Arkansas, but at the 1:24,000 scale, we were able to draw ten. Further division is possible, but several units were considered too thin to map on the available 40-foot contour interval.

You can download your own copy of the map at this link:

Click to access Durham.pdf



Generalized Stratigraphic Column of Durham Quadrangle

The Drakes Creek Fault, which runs diagonally from the southwest corner to the northeast corner, is the most striking feature on the map.  It is part of a major structural feature in northwest Arkansas, forming a lineament that can be traced at the surface for over 45 miles.  The Drakes Creek displays normal movement, is downthrown to the southeast, and offsets strata an average of 230 feet.  Associated with the fault on the northwest side is a large drag fold. There, rocks parallel to the fault are deformed such that units typically present at higher elevations away from the fault bend down to a much lower elevation next to the fault.  Erosion along this side of the fault has exposed the core of the fold along Fritts Creek, Cannon Creek, and other places.


Detail of Cross-section of Durham Quadrangle

The Durham quad is far-removed from areas of previous STATEMAP projects in north Arkansas.  We completed work on the Mountain View 1:100,000-scale quad last year, ending on the Brownsville quad near Heber Springs.  Focus has now turned to the Fly Gap Mountain 1:100K quad as the next high-priority area.  When completed, we will have continuous 1:24K coverage for a large portion of the central Ozark Plateaus Region.  The Durham quad was an appropriate choice to begin mapping in this area due to its proximity to designated type sections for many of the formations in north Arkansas.  This facilitated easy comparisons between our field observations on Durham with the classic outcrops where these formations were first described.  Initial field investigations included locating, describing, and sampling these historic outcrops near Fayetteville. We visited many places the names from which the stratigraphic nomenclature we still employ was derived.  These places have such names as: Bloyd Mountain, Kessler Mountain, Lake Wedington, Cane Hill, Prairie Grove, Brentwood, Winslow, and Woolsey.  Having seen the stratigraphy in these areas firsthand better prepares us to track changes in lithology and sedimentation as we continue to map to the east and south of Durham in the coming years.

The following images were taken during this year’s field season and are arranged in stratigraphic order from youngest to oldest:


Liesegang boxworks–Greenland Sandstone.  Mapped into the Atoka Formation


Asterosoma trace fossils–Trace Creek Shale of the Atoka Formation


Kessler Limestone just below the Morrowan/Atokan Boundary–mapped into the Dye Shale of the Bloyd Formation


Parthenon sandstone resting on the Brentwood Limestone, both of the Bloyd Formation.  The Parthenon was also mapped into the Dye


Mounded bioherms in the Brentwood Limestone


Tabulate coral colony in the Brentwood Limestone


Herringbone cross-bedding in calcareous sandstone–Prairie Grove Member of the Hale Formation


Goniatitic Ammonoids in calcareous sandstone–Prairie Grove


South-dipping sandstone in the White River south of the Drakes Creek Fault–Cane Hill Member of the Hale Formation


Soft-sediment deformation–Cane Hill


Pitkin Limestone, below the Cane Hill near West Fork—Mississippian/Pennsylvanian Boundary


A cluster of solitary Rugose corals–Pitkin Limestone


Wedington Sandstone of the Fayetteville Shale at West Fork


Base of the Wedington–mapped into the upper Fayetteville Shale


Large septarian concretion–lower Fayetteville Shale


Pyritized Holcospermum (seed fern seed-left) and goniatitic ammonoid (right)–lower Fayetteville Shale


Boone Formation, along the White River in the northwest corner of the Durham quadrangle

This year, we’re moving east to map the Japton and Witter quads. Wish us luck as we begin a new field season.  We’ll try to keep you apprised, so until next time, we’ll see you in the field!


Richard Hutto and Garry Hatzell

Geopic of the week: Ironstone concretions

I’ve posted a little about concretions before, but on a visit to the Capitol building in DC this week our tour guide pointed out small iron concretions weathering from the pillars which he said have been mistaken for British bullets that are lodged in the sandstone columns. 

Concretions, if you recall, are mineral masses that grow in rocks by precipitation, usually from mineral-rich groundwater.  They are often visually interesting and probably the most misunderstood rock or mineral on earth.  Like billowing cumulus clouds, they take on fantastic shapes that  lead to colorful interpretations from the uninformed observer. 

Here is a collection of odd or interesting concretions gathered from various parts of Arkansas. 

empty egg closeupempty egg shellempty egg

This one in the above pictures is just begging to be called a fossilized egg.

two disks 2two disks

These are a couple of disk-shaped concretions that grew between beds of shale.  If we were to cut them open they would look something like these next ones

horn of plenty 2horn of plenty1spiral diskTaco1

The iron minerals in the above examples have arranged themselves in near-concentric bands. 


This same concentric precipitation often leads to so-called “pipes” that are stuck in rocks like these here

wicked smile 2wicked smile

Finally you have my personal favorite – Ancient Native American dentures!


Hope you enjoyed these and next time you find an interesting concretion you’ll know what you’re looking at.

Geopic of the week: Septarian concretions



The “fossilized turtle shell” pictured above is actually one of the more commonly misidentified pseudo-fossils in the rock record, a septarian concretion.  Concretions aren’t the remains of animals or their shells at all:  They are inorganic nodules that grow in rocks by precipitation from groundwater.  This kind of concretion is called septarian – a Latin word for partition – because of the cracks that divide it into polygons.

Though these are not uncommon, how they form isn’t well understood.  Explanations for the cracks range from dehydration and shrinkage (similar to mud cracks) to fracturing by either crushing or violent shaking (such as in an earthquake).  Once fractured, circulating ground water deposits more minerals into the cracks.

This septarian concretion was collected from the Fayetteville Shale near Leslie, Arkansas in the Ozarks.


To see more views of septarian concretions, click here