Monthly Archives: March 2017

Geo-pic of the week: Shark Tooth

Shark tooth westfork

You wouldn’t know it to look around now, but Arkansas, at times in the distant past, was teaming with sharks (and other marine fish).   Indeed,  Arkansas was in part or wholly covered by ocean many times in the past.  One such time was 250 million years ago, during the Carboniferous Period.  The fossilized Cladodus tooth pictured above belonged to a primitive shark that had sharp teeth with multiple points of varying size that it used to gig fish before gulping them down.  The long point at the middle of the tooth is broken off and displaced to the right in this picture.  

This particular specimen was found near West Fork, Arkansas.  It was collected from the Prairie Grove Member of the Hale Formation, a limey sandstone.  However, shark teeth can be found locally, throughout other parts of the state, in marine rock layers spanning hundreds of millions of years. 

Geo-pic of the week: Dardanelle Rock

dardanelle rock from river

Pictured above is Dardanelle Rock located on the south side of the Arkansas River between the towns of Dardanelle and Russellville. The white truck in the lower right corner shows the scale of this outcrop. It was designated a Natural Area by the Arkansas Natural Heritage Commission in 1976.

The Arkansas River Valley is north of the Ouachita Mountains and is characterized by gently folded sedimentary rock that was subject, to a lesser extent, to the stress that folded the Ouachita Mountains.  The rock pictured here is the south limb of a broad syncline, or down-warped fold.  The north limb is about two miles to the northeast.  The bedrock dips to the north (toward the white truck), goes sub-surface beneath the Arkansas River, then reverses dip direction and rises back to the surface just southwest of Russellville.  If you could see a cross-section of the folded rock, it would look like giant a smiley face with the middle of the smile underground and the corners sticking up in opposite directions, two miles apart.

This picture gives perspective to the colossal size of geologic features geologists study.  Folds like this one, which can trap upward-migrating fluid, are sometimes rich oil and gas reservoirs.

Geo-pic of the week: Pebble Molds

pebbles-great(photo courtesy of Angela Chandler)

The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.

Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?

This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.