Tag Archives: limestone

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: 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: Archimedes

archimedes pitkin 3-2 for geopic trip

Pictured is the remains of an animal, called an Archimedes, that flourished in Arkansas about 350 million years ago.  At that time, Arkansas was covered by a warm tropical sea, and invertebrate organisms such as Archimedes lived by straining tiny food particles from the sea water.   Though similar filter-feeders exist today, this particular one is extinct.

In this photo, we see a cross section of the animal; the screw-like pillar supported it from the center and delicate, barely visible lattices whorled around the screw – in cross section they look like ribs of a fish bone (see picture).  Those were the filters the animal used to trap food.  Because the lattices were brittle, they were rarely preserved with the fossil, which makes this specimen exceptional.

The limestone pictured is from the Pitkin Formation in northern Arkansas.

For more views of Archimedes click here

Geopic of the week: Orthogonal joint set

orthogonal joints

Here is a photograph of Pitkin Limestone from the Ozark Mountains near Fox, Arkansas.  This exposure displays a classic orthogonal joint set.  The joints are the easy-to-see fractures that divide the bedrock into square blocks.  Orthogonal means the joints formed at roughly 90 degree angles to each other, hence the resultant square blocks.

Joints are common features in sedimentary and crystalline bedrock, and they form in a variety of patterns in response to the stresses the rock has been subjected to.  Essentially, bedrock is being compressed, and the joints form to relieve that pressure.  The squeezing and resultant fracturing result from natural processes such as burial, erosion, and plate tectonics.

Joints are important because they convey information about stress-fields that have acted on the rocks in the past.  They can also be useful for understanding the flow of fluids through a petroleum reservoir or aquifer when trying to maximize production from an oil or water well.

To see more joint set pictures click here