Tag Archives: Everton

Notes from the Field

 

On a recent fieldtrip I realized how many great geologic features exist in the Everton Formation of northern Arkansas. Here’s a little background on the Everton Formation. The Everton Formation is named for exposures near the town of Everton in Boone County, Arkansas. All geologic formations are named for nearby geographic locations. This formation was deposited during the Middle Ordovician Period which means it formed around 470 million years ago. It crops out across northern Arkansas from Beaver Lake in Benton County to Sharp County. Depending on where you are in that portion of the state you might see sandstone, limestone, dolostone, or all three rock types.

Now let’s look at some neat features in the Everton Formation. We’ll start with stromatolites. Stromatolites are laminated structures built by blue-green algae, also called cyanobacteria, one of the simplest and earliest known life forms. Notice the mounded laminations in the photo below. These are stromatolites. The rock is a fine-grained limestone. Also notice the bumpy, weathered surface mid-photo. This is where individual stromatolites are weathering out of the rock.

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The next photo shows a better look at the top of this weathered surface. Finding these fossilized accretionary structures in outcrop helps geologists determine the environment in which this rock formed – in this case, a tidal flat.

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The next photo shows that modern stromatalites are still forming in similar environments today.

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Next, let’s look at travertine. Travertine is a chemically-precipitated, continental limestone composed of calcite or aragonite that forms around seepages, springs, and along rivers and streams (Pentecost, 2010). Precipitation results primarily through the transfer of carbon dioxide to or from a groundwater source, which leads to supersaturation and crystal growth on surfaces. Travertine cascades and dams are present on many of the small streams that are sourced by springs issuing from the limestone and dolostone of the Everton Formation.

The first photo shows a travertine cascade over a dolostone ledge.

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The second photo shows a geologist standing beside a tall travertine dam across a small creek.

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Finally, have a look at these fossilized mud cracks. These formed in a similar way to modern mud cracks. These rocks were originally mud that dried out and formed polygonal cracks. These were later filled with additional mud and over time all of it lithified into dolostone. Mud cracks preserved in this manner are another clue that helps geologists determine the environment in which the sediment was deposited. Again, this would indicate a tidal flat.

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Till next time. Get out in the field!!

Angela Chandler

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.

 

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.

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.