Tag Archives: Ordovician

Geo-pic of the week: Graptolites

 

Graptolites

This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas.  The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.

There were many types of Graptolites.  Some were attached to the sea floor, like corals, while others floated in the water, like plankton.  The feather-shaped fossils pictured here are actually the nests in which the animals lived.  Each tooth-like tube, on the edges of the nests, housed a tiny animal.  Several of these nests would be linked together into a larger colony.

At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons.  Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks. 

To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf

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: Paleokarst on the Buffalo River

Big Plug Paleosinkhole Gimped 20 Apr 01

This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas.  Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.

In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock.  When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand.  Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).

All of this happened about 450 million years ago.  Paleokarst features are one more clue geologists use to decipher earth’s history.  If you didn’t know better, you might float right by and never give it a second thought.

Geopic of the week: Stromatolites

Stromy1

Pictured here are the fossil remains of the first living organisms known to ever flourish on earth: the stromatolites.  They were not giant, ugly mushrooms, as you might expect from this picture.  They were in fact, structures built by microbes, and made of sediment they trapped in their secretions.

Stromatolites are formed by colonies of marine bacteria or algae.  They build the stromatolites up into mounds, like the one above, by secreting a layer at a time.  The algae and bacteria are shallow water critters that absorb their energy from the sun and don’t require oxygen, thus they were able to thrive in the oxygen free atmosphere of early earth.  Though they lived more than 2.7 billion years ago, they still exist today, but they aren’t abundant anymore.

This stromatolite fossil was collected in northern Arkansas from the Ordovician Everton Formation, and is about 450 million years old.

For more views of stromatolites, click here

Geopic of the week: Breccia

Dolostone breccia (1) corrected

A Breccia is a rock made up of angular rock fragments mixed with finer sediment.  The one above was deposited about 450 million years ago (Ordovician) in a shallow sea in what is now the Ozark highlands, north of Mountain View, Arkansas.

Breccias can form by a variety of processes such as landslides, volcanic eruptions, storm events, cave and sinkhole collapses and others.  This one likely formed during a storm.  Turbulent ocean waves and currents washed fragments of marine organisms (dark gray) into calmer water where marine mud (light gray) had recently accumulated.  The turbulence ripped some of the mud up and mixed it with the organic material.

Breccias are just one of many clues, recorded in rocks, that help geologists understand how the earth has changed through time: a story that itself constantly evolves as new rock is unearthed and studied.

For more views of breccias click here