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.

2 thoughts on “Geo-pic of the week: Tempestite

  1. Paul Lowrey

    I have similar samples I’ve always attributed to the Cane Hill (based on not much other than gross stratigraphic position and clasts likely from the Pitkin and Fayetteville). Interesting to hear it’s Prairie Grove instead; can you share your rationale? Not arguing, just genuinely curious.

    1. argeology Post author

      Tempestites are not unique to the Prairie Grove, but are more a product of stormy conditions in a shallow marine environment than a particular stratigraphic horizon. In fact, these conditions seem to have been common in northern Arkansas in the late Mississippian as many examples of tempestites have been noted in the lower Imo interval–a unit commonly mapped as part of the Cane Hill on older maps.


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