Monthly Archives: May 2018

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.

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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.