Geopic of the week: Salt casts

Salt casts are sedimentary structures that indicate a very saline environment of deposition.  This example was collected from the DeQueen Limestone in Howard County, Arkansas (peppermint for scale). About 146 million years ago, much of Arkansas south of the Ouachita Mountains was under near-shore marine conditions.

The cubes that stand in relief on the underside of this rock bed were once crystals of the mineral Halite – or common table salt.  These crystals grew in the mud of a tidal flat that was periodically flooded with very salty sea water.  The salt crystals were later dissolved, leaving cube-shaped molds that filled with mud, preserving the casts of the Halite crystals.

Ho Ho Ho, Merry Christmas! from the Arkansas Geological Survey

Geopic of the week: Starfish resting traces

Also known as Asteriacites (just call them starfish traces), these impressions were left by marine invertebrate animals on the sandy floor of a sea that covered northern Arkansas during the late Mississippian period: about 320 million years ago.  These trace fossils were collected from the Batesville Formation, a mostly sandstone unit that outcrops in a thin east-west oriented belt across the Ozark Plateaus.  The sample above was found near the town of Leslie, Arkansas in Searcy County.  I can’t reveal the exact location, as that’s the only place I know of starfish resting traces in Arkansas.  These largely predatory animals, which move using a number of small arms on their points and have a mouth at the center of their bodies, are rarely preserved in the rock record.