Category Archives: GeoPic of the Week

Geo-pic of the week: Chalk

 

Annona Fm (2)

Like the White Cliffs of Dover, England, the “White Cliffs of Arkansas” (pictured above) are composed of chalk.  Chalk is a marine sedimentary rock that forms of calcite-rich mud that accumulates in semi-deep marine environments.  The mud is composed of the accumulated skeletal remains of algal microorganisms called coccolithophores.  These algae grow and shed skeletal parts called coccoliths which they arrange around them, in life, in a structure called a coccosphere.  Below is a scanning electron microscopic image of some coccospheres (borrowed from news.algaeworld.org).

coccolithophores

Chalk in Arkansas is found in the Annona Formation, which formed in the late Cretaceous Period, and crops out in southwest Arkansas as well as parts of Texas.  In addition to being mined to make blackboard chalk, this resource is also used in brick, and cement manufacture. 

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Geo-pic of the week: Igneous Dike

igneous dike

 

100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas.  In fact, all of the igneous rocks discovered in the state were emplaced around that time.  Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro.    There are also lots of smaller igneous intrusions like the one shown in the picture above. 

Small igneous intrusions are found throughout the Ouachita Mountains.  There are so many small intrusions that new ones are regularly discovered.  Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions. 

If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.

Geo-pic of the week: Syringodendron

Syringodendron

The above picture is of a syringodendron, the fossilized trunk of a giant tree-like fern that lived in Arkansas in the Pennsylvanian Period (318 to 299 Million years ago).  The Pennsylvanian Period is a sub-division of the Carboniferous Period of geologic history which was so named due to the preponderance of lush vegetation that existed at the time.  Because there was so much vegetation , the geologic record for the Pennsylvanian period is coal-rich and also preserved a lot of plant fossils.  This syringodendron was rescued from an abandoned coal mine in Scott County Arkansas by the land’s owner prior to the land being reclaimed.

The syringodendron is a somewhat rare as a fossil.  It is actually from the same plant that produces another more common plant fossil called a Sigillaria.  Here is an artists representation of what that plant may have looked like:

sigillaria_by_unlobogris

Syringodendron is a Sigillaria trunk that has lost its bark prior to fossilization.  The double impressions that run vertically at regular intervals are called Parichnos scars, or “hare’s trails” colloquially.  Between the vertical impressions are slight linear protrusions called ribs.  If you examine the top edge of the fossil, the ribs give a wavy or undulating shape to the margin of the trunk.

Geo-pic of the week: Siliceous Oolite

Silicified oolite

Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite.  They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions. 

Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite.  The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas.  The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed.  The SiO2 then replaced the CaCO3 the rock was initially composed of.   The polished surface provides an ideal view of the internal structure of the spherical ooids.

Geo-pic of the week: Fluvial Erosion

Buffalo River Valley

Recently, we posted a blog explaining that the Ozark Mountains are actually incised plateaus and that the hills are remnants standing between the incised river valleys. If you missed that one you can see it here.  Now, we will talk about how a river is able to erode solid rock.

The picture above is of the Buffalo National River in its valley. As you can see, an impressive volume of rock has been excavated by this little river. A common misconception is that the water is carving the rock. Water is soft and softer things generally do not abrade harder things. Slightly acidic water can dissolve rock very slowly, particularly carbonate rock like limestone, However, the majority of the erosion in a river is due to the sediment suspended in the flowing water. As the sediment – which can range from tiny grains of silt to boulders– is carried downstream by the current, it skips along the channel, colliding with the bedrock. The repeated collisions break down the sediment, chipping off edges and rounding it. By the same process, new sediment is ground away from the bedrock and the valley is slowly enlarged.

The same thing is true of wind erosion such as in a desert setting.  The wind itself really can’t erode the rock.   The erosion is due to strong winds lifting loose sand and blasting it against the solid rock, slowly wearing it away.

Geo-pic of the week: Sigmoidal Veins

Sigmoidal vein in sandstoneedited

The picture above shows a boulder of Hot Springs Sandstone with well-developed sigmoidal veins.  Sigmoidal veins – sometimes called tension gashes – form in rock by shear stress.  That’s stress that causes adjacent parts of a rock to slide past one another.  In the above picture the yellow arrows indicate the approximate orientation of the stresses that were applied to this boulder to create the sigmoidal veins.

Sigmoidal veins, at their inception, are shaped like parallel lines that bulge toward the center and taper at the ends.  They originate due to tension created between the two opposing forces acting on the rock.  Essentially the rock tears to alleviate this tension.  If the shearing continues long enough, these openings in the rock begin to rotate.  The eventual shape, seen above, is like the letter S.  The ends of each S point opposite of the direction of the force that created them.  Therefore, sigmoidal veins can indicate the forces at work on bedrock when it was buried underground.

The veins pictured here are at the edge of a parking lot next to the Arlington Hotel in Hot Springs Arkansas.  After they developed the veins were in filled with quartz.  The Hot Springs Sandstone is a member of the Mississippian Stanley Formation.

Geo-pic of the week: Ozark Plateaus

Ozark Plateau

If you live in Arkansas, chances are you’ve heard of the Ozark Mountains.  Actually, the correct geologic term is Ozark Plateaus.  Unlike typical mountains in which the bedrock has been squashed and folded, the Ozarks are one broad dome-like structure made up of flat-lying sedimentary bedrock.  The hills and valleys of the Ozark topography are the result of rivers carving into this dome, rather than compression or deformation.  

The picture above was taken overlooking the Buffalo River.  The various hills, from the foreground to the distance, are roughly the same height.  Of course they are!  If not for this and other rivers, the landscape pictured here would be one solid flat surface, as tall as the highest peaks in the picture, stretching to the horizon.