Geo-pic of the week: Fossiliferous mudstone

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Above is a picture of loosely-consolidated, fossiliferous mudstone from the Midway Group: a group of near-shore marine rocks exposed in southwestern Arkansas.  The Midway Group was deposited in a variety of near-shore marine environments that were common in what is now Arkansas around 60 million years ago. The majority of the fossils pictured here are casts of a variety of sea snails (gastropods), but there are also oysters and tiny bryozoans (marine filter feeders).

 

To see more views of fossiliferous mudstone click here

Geo-pic of the week: Spring boxes

 

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Before the invention of electric refrigerators, blocks of ice in insulated wooden cabinets called “iceboxes” kept food from spoiling quickly in warm climates.  This required access to ice, which had to be hauled in from cold climates by boat, and wasn’t always available, especially in remote places.   The picture above shows a cool water spring that was modified long ago into a primitive kind of refrigerator. The structure is made of concrete.  When it was in use, it would have had a door to keep cool in and keep animals and insects out, as much as possible. 

Just like caves, cool water springs in Arkansas stay close to 56 degrees in the summer – the ambient ground temperature.  Anyone that’s spent a summer in Arkansas knows it gets oppressively hot.  Having a place you could store milk, eggs, and other perishables would certainly have come in handy.  You still come across these old structures if you spend a lot of time out in the woods around the state.  This one was photographed near Hot Springs, Arkansas, in the Ouachita Mountains.

Geo-pic of the week: Ripple marks

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Ripple marks are sedimentary features formed by water flowing over sediment.  They form in different environments including river channels, beaches, and just about anywhere water flows.  You probably saw some the last time you walked along a creek.

As water flows over sediment, it sweeps some of the loose sediment along in the direction of the current.  As the sediment migrates, it forms miniature dune-like structures.  These features are short on one side and tall on the other. The tall sides point in the direction the water was flowing.  In this case, it flowed into the picture, or away from us.

Ripples like these often get destroyed before they ever harden into a rock, but occasionally they get preserved.  Ancient ripples serve as clues to the environmental conditions that existed in a place in the distant past.  These ripples formed in what was once a deep ocean trough near Hot Springs, Arkansas .

Geopic of the week: Salt

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Everyone is familiar with salt, but have you ever seen it up close?  The picture above is a magnified image of table salt.  Notice that each piece is a near-perfect cube.  This is no coincidence.  Salt, which has the chemical formula NaCl (sodium chloride), is a mineral and as such, it has a regular internal structure (a crystalline structure).  The arrangement of the atoms that make up salt form cube-shaped molecules; when a piece of salt is broken down into smaller fragments, it tends to break into cubes.

Salt is not currently produced commercially in Arkansas, but there are abundant salt deposits in the state both at the surface and underground.  Salt from shallow wells in Saline County was produced locally by settlers to the area in the early 1800’s, and by Native Americans prior to that.  The majority of Arkansas salt is extracted with petroleum from deep wells in southern Arkansas.  

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

Geo-pic of the week: Amber

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Pictured above are several polished samples of the fossil amber; the sample in the top left is unpolished. 

Amber is the fossilized resin of a tree.  As you can see, some of the wood of the trees got preserved with the resin, which gives this amber a wood-grain appearance .  These pieces were collected near Malvern, Arkansas, where amber is found in beds of lignite: a soft, low-grade coal.

People have prized amber since pre-historic times for its beauty and its scent.  It continues to be popular today in the production of jewelry and perfume.