Happy National Fossil Day – Earth Science Week 2023

To celebrate, we are highlighting one of the more rare fossils in the state! This ammonoid fossil measures about three inches across and is beautifully preserved in Pennsylvanian aged sandstone in northwest Arkansas. Ammonoids are coiled cephalopods that have been extinct since the end of the Cretaceous Period but are present in the Mississippian and Pennsylvanian-age rocks of the Ozark Plateaus and in the Cretaceous-aged rocks of the West Gulf Coastal Plain.  These animals were swimmers and floaters in shallow to deep water. Because particular species only existed for a relatively brief time in the geologic record, paleontologists can use their fossilized remains to determine the age of the rocks in which they are found and correlate them to similar-aged rocks worldwide.

The left photo shows an ammonoid from the Mississippian Fayetteville Shale and the right photo an ammonoid from the Prairie Grove Member of the Hale Formation. Notice the suture pattern or, wavy lines, on the fossil.

Ammonoids are classified according to their suture pattern: goniatitic (fairly simple), ceratitic (more decorative), or ammonitic (highly decorative).  Ammonoids with a goniatitic suture are called goniatites, those with a ceratitic suture, ceratites, and those with an ammonitic suture, ammonites. Mississippian and Pennsylvanian ammonoids are classified as goniatites because they have a goniatitic suture pattern.

Ammonoids in Cretaceous rocks in the southwest part of the state have highly decorative or ammonitic suture pattern, therefore, they are called ammonites. The photo below shows the complex suture of an ammonite found in southwest Arkansas.

For more information about fossils visit our website at https://www.geology.arkansas.gov/geology/fossils.html

Happy Earth Observation Day!

Earth Science Week 2023

Today we are observing landslides using LiDAR technology! Landslides are common in north central Arkansas in Newton, Madison, Washington, and Crawford Counties. They can be identified using Light Detection and Ranging (LiDAR) data that is processed to create digital slope models within ESRI ArcGIS mapping software.

On the above digital slope model, the darker areas indicate steep slopes, whereas lighter areas indicate more gradual slopes. Note the translational landslide located in the center of the image outlined in red. Landslide dimensions are 607 feet in length and 236 feet at its widest point with the toe and flanks clearly visible.

Mass movement of material occurs through a wide variety of processes that result in the downward and outward movement of rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing. Mass movement in Arkansas can be attributed to multiple causes, some of which include weak, weathered, and highly jointed, or faulted bedrock, permeability of the rock material, amount of water, and human interaction.

The photo above shows an example of a landslide south of Jasper, Arkansas between Highway 7 and County Road NC6020 near Newton County Road Department & Fairgrounds.