The photo above shows trace fossils that record the travels of two trilobites. Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya). The tracks the animal left are known as the trace fossil, Cruziana. It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while. At the center of the photo are resting traces known as Rusophycus. Perhaps they became friends or maybe they were even more than friends? It is Valentine’s Day. Their traces are preserved in the Atoka Formation of west-central Arkansas.
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.
The Pitkin Limestone
One of the most fossiliferous formations in the state is the Pitkin Limestone. It was referred to as the Archimedes Limestone in the late 1890s because it contains an abundance of the screw-shaped bryozoan fossil Archimedes. It was formally named the Pitkin Limestone in 1904 for exposures near Pitkin Post Office in Washington County, Arkansas. If you can’t find the town of Pitkin on a map, don’t worry–it’s now known as Woolsey.
The Pitkin began as carbonate sediments deposited in the Mississippian Period around 320 million years ago. At that time, northern Arkansas was covered by a shallow sea that was fairly close to the equator. Warm, shallow seawater is a prime environment for the build-up of carbonates. Marine organisms extracted calcium carbonate out of the seawater to form shells or other hard parts. This material accumulated and eventually turned into limestone. Some of those secreted structures are preserved as fossils in the rock and are clues to the environmental conditions that existed at the time.
The area of what is now Arkansas during the Mississippian
The Pitkin Limestone is a bluff-former that crops out in the southern portion of the Ozark Plateaus from just south of Fayetteville eastward to Batesville, typically along the Boston Mountains Plateau Escarpment. It is mostly limestone, however, there is some nodular black chert present locally. Black shale intervals are common in the eastern portion. Because limestone is a soluble rock, karst features such as caves, sinkholes, springs, and disappearing streams are common in this Formation. About 9% of the known caves in Arkansas are in the Pitkin. Its thickness varies from an average of about 50 feet on the west side of the state to about 200 feet in the eastern part with a maximum of about 400 feet in the central portion. It typically rests on the Fayetteville Shale and is overlain by the Cane Hill Member of the Hale Formation in western Arkansas and by the Imo interval from the area of western Searcy County eastward.
The Pitkin outcrop belt is within the light-brown area in this Ozark Plateaus detail of the Geologic Map of Arkansas
To download the entire Geologic Map of Arkansas, click here: http://www.geology.ar.gov/ark_state_maps/geologic.htm
The Cane Hill overlying the Pitkin near West Fork, Washington County
The Fayetteville underlying the Pitkin near Marshall, Searcy County
Top of Pitkin in Little Red Creek near Canaan, Searcy County
Now, let’s look at fossils commonly found in the Pitkin.
The photos above contain fossils of Archimedes. The fossil is named for the ancient Greek engineer who invented a device that incorporated a large screw to lift water for irrigation. The left photo was taken south of Batesville and the right photo was taken south of Fayetteville. It’s remarkable that these fossils are so persistent along this great extent. Although this fossil is characteristic of the Pitkin, it can also be present in adjacent formations. The illustration below is a sketch of a fenestrate Bryzoan of which Archimedes is a type.
Archimedes as it may have appeared in life
Pieces of fossilized Crinoids are also abundant in the Pitkin. Most commonly, small button-shaped pieces of the stem and arms, known as columnals, are preserved in the limestone. That is a columnal in the center of the left photo. The larger crinoid fossils above were preserved in shale and were most probably washed onto a mud flat during a storm event. These photos were taken south of Batesville, but crinoid detritus is abundant throughout the Pitkin and most other limestone in Arkansas.
Crinoid as it may have appeared in life
A great location to see the Pitkin is along Richland Creek at its confluence with Falling Water Creek. When the creek level is low, you can hike upstream from the campground and see many fine exposures of Pitkin Limestone in the creekbed. Locally, colonies of tabulate and rugose coral were preserved in the Pitkin and can be discovered upon close inspection of the outcrop.
Detail of Geologic Map of the Moore Quadrangle showing Pitkin along Richland Creek (Mp=Pitkin)
To download the entire Geologic Map of the Moore Quadrangle, click here: https://ngmdb.usgs.gov/Prodesc/proddesc_76560.htm
Tabulate or colonial coral in the Pitkin Limestone along Richland Creek.
Rugose coral in Pitkin
Locally, the Pitkin consists of oolite, a type of sedimentary rock composed of ooliths. Ooliths are small, spherical structures (<2 mm) that form by accretion of numerous concentric layers of calcite on a central nucleus such as a shell fragment or sand grain. The environment of deposition would have been areas where strong bottom currents or wave action rolled the fragment around in carbonate-rich sea water. This would include environments like beaches and tidal flats.
Oncolites and stromatolites are also preserved in the Pitkin. They have a similar structure to ooliths, but are much larger (up to 10 cm), can be round or irregular-shaped, and are formed by a different mechanism. Like ooliths, they nucleate on a shell or other fragment, but are built up by encrusting layers of blue-green algae or cyanobacteria. Stromatolites form in much the same way, but create columns, mats, or large heads. Stromatolites and oncolites typically indicate a paleoenvironment of warm, shallow water in a calm sea, lagoon, or bay.
During fieldwork for our geologic mapping, finding Pitkin Limestone is always exciting because there is something new and interesting to discover every time. We hope this brief introduction to one of Arkansas’ most intriguing formations has convinced you to seek out the Pitkin and have a closer look.
Until next time, we’ll see you on the outcrop!
Richard Hutto, Angela Chandler
Pictured here are the fossil remains of the first living organisms known to ever flourish on earth: the stromatolites. They were not giant, ugly mushrooms, as you might expect from this picture. They were in fact, structures built by microbes, and made of sediment they trapped in their secretions.
Stromatolites are formed by colonies of marine bacteria or algae. They build the stromatolites up into mounds, like the one above, by secreting a layer at a time. The algae and bacteria are shallow water critters that absorb their energy from the sun and don’t require oxygen, thus they were able to thrive in the oxygen free atmosphere of early earth. Though they lived more than 2.7 billion years ago, they still exist today, but they aren’t abundant anymore.
This stromatolite fossil was collected in northern Arkansas from the Ordovician Everton Formation, and is about 450 million years old.
For more views of stromatolites, click here