Monthly Archives: June 2016

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

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Geopic of the week: Coprolites

Poop_coprolite

Ok, I know what you’re probably thinking, and well….. you’re right!  Coprolites are fossilized feces.  The sample on the right is from out of state, but the fine specimen on the left came from good ole Arkansas, near the town of Saratoga.

Because poop has no hard parts (such as a skeleton or shell) it’s rarely fossilized, but under the right circumstances it can get preserved.  Often, it isn’t certain what animal was responsible for the coprolite, unless it’s discovered in the abdominal area of a fossilized animal.  In cases where it can be identified, it tells us something about the animal, that um…made it.  By closely examining  coprolite, it’s possible to recognize fragments of animals or plants, which give us information about the diet of that species.

Geopic of the week: Ancient delta deposits

Deltaic deposits edited

The above picture shows rocks that were deposited in a delta.  A delta is a place where a river or stream empties into a standing body of water, typically the ocean or a lake.  As it enters the larger body of water, the stream loses velocity and starts dumping the sediment it’s carrying.  Coarse sediment settles out first and finer sediment is carried further from the shore before being deposited. 

As the delta develops, it builds a lobe of sediment out into the ocean.  The stream advances into the sea atop its own lengthening delta, and new sediment is carried ever further seaward.  Eventually, today’s coarse sediment is being deposited on yesterdays fine sediment.  Coarse-grained rock at the top, and fine-grained rock at the bottom, is characteristic of delta deposits.

The picture above shows two lobes of deltaic sediment, each with the characteristic coarsening upward architecture.  The coarsest, top-most bed of each lobe juts out, creating  a waterfall.  This picture is of the Atoka Formation and was taken near Lake Fort Smith, Crawford County, Arkansas.

 

Below is a land satellite image of recent Mississippi River deltas in the Atchafalaya Bay.

AtachfalaDelta