Tag Archives: Buffalo River

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

Geopic of the week: Travertine on the Buffalo River

Travertine at the barns

Travertine is a common feature in the northern Ozarks and along the Buffalo River due to the abundance of soluble limestone there.  Common in caves (stalactites, stalagmites), travertine forms by the precipitation of minerals from ground water.  In the example above, it formed on the face of a bluff, giving the bluff a melted appearance.

Like limestone, travertine is composed of the mineral calcite which dissolves if exposed to acid.  When rain falls, it picks up CO2 from the atmosphere and soil, and becomes slightly acidic.  It then flows underground through the bedrock dissolving some limestone along the way.  When the groundwater re-surfaces at a spring or seep, The pressure drops, forcing the CO2 out of the water.  The loss of CO2 lowers the waters acidity; It can no longer hold the calcite in solution, and calcite precipitates as the sedimentary rock travertine.

For more views of travertine click here

Geopic of the week: Angular Unconformity

angular unconformity

The above picture at first glance doesn’t look like much but from a geological perspective these rocks convey a lot of information about the history of the earth.  This is what geologists call an angular unconformity.  An unconformity is simply a gap in the rock record; it represents a period of time during which either erosion was taking place or there was no sediment being deposited. 

We call this kind of unconformity angular because the lower rock beds are tilted at a different angle than the upper beds.  We know from the differences between the fossils in each of these rocks that there was about an eighty million year gap between deposition of the lower and upper rock sequences.  During that 80 million years the lower formation was exposed above sea level, eroded, and tilted by tectonic forces in the earths crust.  Eventually deposition resumed and the upper unit was deposited on the eroded and deformed lower unit.

It’s amazing what you can learn from a few rocks!  If you’d like to see these rocks in person, float the Buffalo National River from Tyler Bend to Gilbert.

GeoPic of the Week: Dolomite (pink) and Sphalerite (brown) In Dolostone

Dolomite and Sphalerite in DolostoneDolomite and Sphalerite in Dolostone

Dolomite (pink) and Sphalerite (brown) In Dolostone

Dolomite and sphalerite are two minerals present in limestone and dolostone in the lead and zinc districts of north Arkansas.  Dolomite commonly occurs with the sphalerite, however it is not an ore mineral and is considered worthless.  Sphalerite is the primary ore of zinc.  Zinc was mined in the lower end of the Buffalo National River in the late 1800s and early 1900s.  One of the largest mining communities was located at Rush, Arkansas.  Zinc is used as a coating of iron or steel to protect it from corrosion.  It is also used in batteries, small non-structural castings, and alloys, such as brass.  This mineralization is present in the Everton Formation.  It is thought that migration of warm mineral-rich fluids expelled by the pressure of the mountain building event that caused the Ouachita Mountains is responsible for the mineralization in northern Arkansas.  Note the brecciated texture (angular fragments) of the rock.  Open spaces, called cavities, in the rock caused the overlying rock to collapse, and break into angular pieces.  Mineralized water then flowed around the broken pieces and the dolomite and sphalerite precipitated in the open spaces.

Statemap Field Blog, Sept. 8-13, 2013

Hello all!

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Silence of the cicadas

Signs that summer is almost over despite the continued warm temperatures include the silencing of cicada calls one at a time.  A welcome turn of events!

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More “crazy-bedding” in the upper Imo

This week was a long one because we worked in two different field areas.  On Monday and Tuesday we were back on Tick Creek looking at the last two major eastern drainages.  Found good Cane Hill/Imo contacts in each and a fairly consistent
irregular-, channel-bedded sandstone unit with abundant soft-sediment deformation near the top of the Imo.  It’s so irregular that I was calling it “crazy-bedded” before long. 

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“Crazy-bedding” in the upper Imo

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Yet more “crazy-bedding” in the upper Imo

There was also a section in the middle of the Cane Hill that was so perfectly cut by its east/west joint that it formed a smooth wall on the north side of the creek.  There was a parallel joint face on the south side, but it was not nearly as well exposed.

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Danny views joint face “wall” in Cane Hill

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Buttresses on Bear Creek

On Tuesday evening we joined Angela Chandler and Lea Nondorf, also of the Survey, on Bear Creek in Marshall to work on several other projects the rest of the week.  One ongoing project is to try to resolve edge-matching issues whenever we get a chance.  This involves gathering new data in boundary areas between quads that were mapped by different people or who used a different stratigraphy.  This week we were also looking for localities in the Imo interval for an upcoming field trip that Angela, Erin Smart and I are leading this spring for the GSA (Geological Society of America) conference in Fayetteville.  While looking at various road cuts, we also took new points on an area in the corner of four quads.  

Angela in front of lower Imo sandstone at the type section

Angela and Danny at the foot of the basal Witts Springs sandstone near Bryan Mountain

Wednesday and part of Thursday we looked at several possible field trip stops in the Imo, none of which seemed particularly suitable for one reason or another.  Mostly this is because the Imo doesn’t tend display good outcrops in this area due to its shaley composition and its typically being covered by the flaggy sandstone of the Cane Hill above.  We did visit the type section which is in Sulphur Springs Hollow to see if that could be used as a stop, but deemed the area too rugged unless a very small, sturdy group of geologists sign up.   A type section is an area where a formation or rock unit is first described and studied in detail.  In this case the type section was proposed then summarily  abandoned, but there are those working hard to see it reinstated eventually.

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Coalified wood prints in Imo sandstone at the type section

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Crinoids in red, fossiliferous Imo limestone at the type section

Another project that we are working on for the National Park Service is a compilation of all the quad maps along the Buffalo National River. Our agency and the US Geological Survey have each done about half of the quads in that area.  Most of the quads mapped by our agency did not include the higher terrace levels above the river–some as high as 200 feet!  This week we were able to get points on a few more of these terraces on the Snowball quad. 

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Smart Bluff above Arnold Bend on the Buffalo River

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Calcite fracture-fillings used as building stones in St. Joe

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Quartz crystal encrusted Boone chert used as building stones in St. Joe

We looked in Arnold Bend Thursday afternoon, and differentiated several terrace levels there, then on the way back to town, we stopped at a roadcut on Hwy. 65 that Angela knew about where there are quartz crystals growing in fractured Boone limestone and chert.  Danny had stopped at St. Joe on a previous field trip and so directed us there to see the quartz and calcite crystals encrusted on some of the old building stones in town.  These stones were no doubt found nearby along the several mineralized fault zones in the area.

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Early morning fog on Bear Creek

The next morning there was fog on Bear Creek and we headed up to Jamison Bluff to look for Plattin in the riverbed where it had been mapped previously.  This is part of the 6 mile section of the Buffalo between Woolum and Margaret White Bluff that dries up during the summer.  All we could find was St. Joe, so that part of the map remains as is. 

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Jamison Bluff along the Buffalo River

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River crossing at Woolum on the Buffalo River

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Looking down on Skull Bluff on the Buffalo River

 We crossed the Buffalo at Woolum and walked along the top of Skull Bluff to the Nars Cemetery most of which was covered with a terrace deposit (along with almost impenetrable black locust, cedar and briars).  

And of course, we couldn’t leave the area without a quick trip to “the Nars” itself for Danny and Lea to see for the first time.

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“The Nars”. Buffalo River left, Richland valley right.

“The Nars” is an almost sheer rock wall in the Boone formed as an erosional remnant between the valleys of the Buffalo River and Richland Creek.  Quite impressive as usual.  After that we had to get back to Little Rock and “the real world”.  See you next week.     

Tick attacks: severe

GeoPic of The Week: Painted Bluff at Buffalo Point along the Buffalo National River

Painted Rock at Buffalo Point along the Buffalo National River.

Painted Bluff at Buffalo Point along the Buffalo National River

Painted Bluff gets its name from water seeping over the top portion of the bluff.  This darkens the rock giving it a painted look.  The rock formation that is painted is the St. Peter Sandstone.  The rock formation below the painted portion of the bluff is the Everton Formation.  Thin bedded limestone and dolostone layers make up the lower portion of the bluff.  The rock formations are both Ordovician (485-444 million years ago) in age, however there is an unconformity between the two formations.  An unconformity is a rock surface that represents a gap in the geologic record either due to a period of erosion or non-deposition.  Notice the wavy line halfway down the bluff.  This wavy line separates the sandstone from the limestone and is the unconformity surface.  The top of the limestone was at one time the rock exposed at the earth’s surface in this area.  The limestone was eroded, and then the sandstone was deposited upon it.