Tag Archives: Geology

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:

http://www.geology.arkansas.gov/maps_pdf/geologic/24k_maps/Durham.pdf

 

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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.

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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:

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Liesegang boxworks–Greenland Sandstone.  Mapped into the Atoka Formation

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Asterosoma trace fossils–Trace Creek Shale of the Atoka Formation

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

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

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Mounded bioherms in the Brentwood Limestone

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Tabulate coral colony in the Brentwood Limestone

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

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Goniatitic Ammonoids in calcareous sandstone–Prairie Grove

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

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Soft-sediment deformation–Cane Hill

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

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A cluster of solitary Rugose corals–Pitkin Limestone

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Wedington Sandstone of the Fayetteville Shale at West Fork

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Base of the Wedington–mapped into the upper Fayetteville Shale

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Large septarian concretion–lower Fayetteville Shale

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

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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!

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Richard Hutto and Garry Hatzell

Geo-pic of the week: Iridescent Iron Oxide on Novaculite

iridescent iron edited(photo courtesy of John David McFarland)

The photo above illustrates surface iridescence.  Iridescence is a play of colors caused by the interference of light waves.  This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens.  Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors.  Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves.  A color is seen when the waves interfere constructively.  The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.

Geo-pic of the week: Slickensides

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The grooved surface pictured above is a slickenside.  Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).  

Slickensides form when fault blocks move against each other.  The natural irregularities on each scratches grooves into the other.  The grooves are parallel to movement;  for instance in this example, movement was either to the right or the left.  To tell whether it was right or left, you can rub your hand along the slickensides.  They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head.  Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.

The shale above was photographed in Big Rock Quarry, North Little Rock, AR.  It’s a part of the Jackfork Formation (Pennsylvanian).

Geo-pic of the week: Syn-depositional Faulting

Fault with lables

 

Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas.  The fault pictured extends from the upper right to the lower left and is highlighted.  This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited.  It results in disturbed-looking outcrops like this one.

Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited.  The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress. 

Some of the deformed features of the outcrop are labeled above.  The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone.  The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone.  The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.

Geo-pic of the week: Pebble Molds

pebbles-great(photo courtesy of Angela Chandler)

The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.

Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?

This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.

Geo-pic of the week: Zinc Ore, Rush Creek Mining District

sphalerite and dolomite (1)

Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas.  The brown mineral is sphalerite: an ore of zinc.  The pink mineral is dolomite – it’s pretty, but not economically valuable.   They were both deposited on the gray dolostone; you can just make it out on the right, in back. 

Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena.  They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together.  That area was mined for lead and zinc in the late 1800s and early 1900s.

It’s typical to find rich ore deposits in rock that’s been fractured by faulting.  The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures.  It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.

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