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. 

Geo-pic of the week: Freshly exposed anticline

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This is an anticline exposed on Mc Leod Street, southwest of Hot Springs, Garland County, Arkansas.   It’s not unique as, anticlines are common in the Ouachita’s and other mountain ranges throughout the world.  Most often though, these structures are large scale and cover expanses of land that can’t be viewed from a human vantage point.  When they do form on a scale that’s small enough for human observation, we typically don’t have the benefit of a freshly blasted exposure like this one. 

In fact, many times geologists must infer that folds like this exist in places deep underground that no one has or will ever see.  That’s why, if you see a geologist on the side of the road, taking something like this in, as in the picture above, just let him have his little moment.  The exposure is of deep marine sedimentary deposits of the Stanley Formation.

Geo-pic of the week: Flute casts

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Fig. A – Many flute casts that truncate one another.  Black arrow points downstream.

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Fig. B – Flute casts with characteristic round heads pointing in upstream direction.  Black arrow points downstream.

 

Pictured above are beds of sandstone displaying flute casts.  Flute casts are common in channel environments (for instance river channels) where water is carrying sediment and debris (rocks, shells, sticks etc..). 

As debris is carried along, it randomly grazes the mud in the channel bottom, scouring divots.  The divots are typically deeper and narrower in the upstream direction, with a round head pointing upstream.  Flute casts form subsequently when sand in-fills these divots and later becomes a rock, preserving casts of the divots on the bottom of the sand bed. 

Flow direction indicators, such as flute casts, are one of many clues geologists use to reconstruct the history of the earth.

Geo-pic of the week: Lodestone

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Above is a picture of lodestone, which was collected from Magnet cove, Arkansas.  Lodestone is composed of iron and oxygen and is just like the mineral magnetite except, lodestone is naturally magnetic. 

Magnetic minerals are rare on earth but, they have been a crucial part of human evolution.  Discovery of magnetic lodestone by ancient people led to the invention of the compass.  The compass, in turn, revolutionized navigation, and lead to the spread of technologically advanced cultures around the world.

Scientists are still uncertain how lodestone became magnetized but, the most accepted theory holds that it forms when magnetite is struck by lightning, which has a strong magnetic field.  The fact that lodestone is primarily found at the earth’s surface supports this theory and scientists have been able to produce rock identical to lodestone by exposing magnetite to lightning.