Geo-pic of the week: Trilobite Romance

The photo above shows trace fossils that record the travels of two trilobites.  Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya).   The tracks the animal left are known as the trace fossil, Cruziana.  It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while.  At the center of the photo are resting traces known as Rusophycus.  Perhaps they became friends or maybe they were even more than friends?  It is Valentine’s Day.  Their traces are preserved in the Atoka Formation of west-central Arkansas.  

Geo-pic of the week: Tempestite

A tempestite, like the one pictured, is a rock composed of debris deposited by a storm.  It’s mostly a sandstone but also contains various fossils, pebbles, and other clasts that were picked up and tossed about by the waves.

Waves are generated as wind energy is transferred to water.  Naturally, during a storm, waves are bigger and more energetic.  This increased energy allows the waves to pick up, and in some cases rip up, various relatively large clasts and fossils and transport them.  The large elongate fossil above is an extinct squid-like creature known as a conical nautiloid.  Other marine fossils in this sample include gastropods, and crinoids.  It also contains plant material.

The presence of tempestites in a rock outcrop indicate the area was a shallow marine environment when those rocks were being deposited.  This sample was collected in Northwest Arkansas from the Pennsylvanian Prairie Grove Member of the Hale Formation.