Oncolite is a limestone made of oncoids, the roundish, tan things in the picture (average size less than an inch). Oncoids are made by microbes called cyanobacteria. Cyanobacteria, which also form larger mounds called stromatolites, are thought by many scientists to be one of the earliest forms of life to evolve on Earth.
The microbes attach to a nucleus – in this case fossil fragments – and encrust it in layers of calcium carbonate. The bacteria gather energy by photosynthesis and, thus, require access to the sun. Because they are easy to recognize and mostly limited to shallow marine environments, oncolites are useful to geologists, both as a stratigraphic marker and as an indicator of the depositional environment of the rock they are preserved in.
These were photographed in the Kessler Limestone Member of the Bloyd Formation, northwest Arkansas.
Photos courtesy of Garry Hatzell
To see the original blog on Arkansas coral click here
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.
Pictured above is the internal mold of an ammonoid fossil – a group of invertebrate marine animals abundant in the world’s oceans from 416 – 66 million years ago. They died during the same mass extinction that killed the dinosaurs.
Ammonoids were not stationary bottom dwellers, but had an interesting way of getting around in the water. Their shells were partitioned into chambers, which are evident in the picture above. The squid-like ammonite only occupied the final chamber of the shell. The rest were empty so that the animal could control its buoyancy, and swim by taking in and expelling water.
Because ammonoids were abundant, widespread, and evolved new species quickly, geologists use their fossils to correlate rock units of similar age worldwide. This one was collected from the Fayetteville Shale in northwest Arkansas. Its gold color is due to the original organic material having been replaced by pyrite – also known as fool’s gold.
This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas. The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.
There were many types of Graptolites. Some were attached to the sea floor, like corals, while others floated in the water, like plankton. The feather-shaped fossils pictured here are actually the nests in which the animals lived. Each tooth-like tube, on the edges of the nests, housed a tiny animal. Several of these nests would be linked together into a larger colony.
At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons. Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks.
To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf
Above is a picture of loosely-consolidated, fossiliferous mudstone from the Midway Group: a group of near-shore marine rocks exposed in southwestern Arkansas. The Midway Group was deposited in a variety of near-shore marine environments that were common in what is now Arkansas around 60 million years ago. The majority of the fossils pictured here are casts of a variety of sea snails (gastropods), but there are also oysters and tiny bryozoans (marine filter feeders).
To see more views of fossiliferous mudstone click here