Chapter 9, The Cotter and Jefferson City Dolomite

The Cotter Dolomite and Jefferson City Dolomite are recognized as separate rock units, separated by an unconformity, in Missouri. But in Arkansas, no boundary or distinction between these units has been discovered.

Rocks and Minerals of the Cotter and Jefferson City Dolomite

Pyrite

Pyrite is also commonly known as iron pyrite, or fool’s gold. It is an iron sulfide mineral that is heavy and brassy gold in color. It occurs as very small dispersed crystals in dolomite in the valley floor, and occasionally as fossil or trace fossil replacing material.

Around Eureka Springs, it can be found as tiny (<5 mm) crystals or occasionally as trace fossil replacing structures in the undifferentiated Cotter-Jefferson City dolomite, meaning the Ordovician age rocks of the lower parts of the valleys, or as sparse but well-formed nodules within the Chattanooga Shale. Tiny crystals of the mineral can be found in the rocks in many of the stone buildings around town, since these were quarried form the stone that makes up the valley floor, but the pyrite in Eureka Springs is unstable, and much of it that was quarried a hundred years ago or more has turned mostly or completely to rust.

Pyrite has many historical and modern uses as a mineral resource, but it does not occur in mineable quantities or concentrations in the Eureka Springs area. Iron pyrite was, however, mined in small quantities from 3 deposits in exposures of the Cotter Dolomite, near Berryville, up to the the 1930s to 1940s as a source of sulfur. Pyrite drove some prospecting activity in the region in the late 1800s to early 1900s because the mineral commonly co-occurs with copper, lead, zinc, and precious metals. Precious metals are absent from pyrite deposits in this region, but most of the lead and zinc mines in southern Missouri and northern Arkansas co-occur with pyrite.

To learn more about pyrite, you can find an excellent article at Wikipedia: https://en.wikipedia.org/wiki/Pyrite

Iron pyrite replacing trace fossil burrows in the Ordovician undifferentiated Cotter-Jefferson City Dolomite in Eureka Springs.

Fossils of the Cotter and Jefferson City Dolomite

Stromatolites

Ancient mounds of rock formed when layer upon layer of cyanobacteria, or blue-green algae, trapped sediments on the floor of a shallow inland sea. These are one of the most frequent fossils types found around Eureka Springs, but they are often difficult to recognize. They can take the form of slightly domed layered mats, well formed domes meters across, or small stone hummocks that you can hold in both hands, like the one show below. All of the stromatolite fossils in the Eureka Springs area are found in the older rocks in the lower parts of the valleys. These rocks are the Lower Ordovician undifferentiated Cotter-Jefferson City Dolomite Formation, and they date form about 480 million years ago or so.

Ordovician Stromatolite fossil mound from the Undifferentiated Cotter-Jefferson City Dolomite near Eureka Springs, Arkansas. About 480 million years old.

When stromatolites are formed as low-domed or nearly flat layers, they are sometimes called cyanobacterial mats, and when they are very large and complex, they are sometimes referred to as bioherms. The ones around Eureka Springs are extremely varied in shape and size, but are pervasive at certain elevations in the stratigraphic rock record, with sequences of nearly continual occurrences extending for tens to hundreds of meters horizontally in the stratigraphy.

Large Ordovician stromatolite (blue-green algae fossil) mat or mound, several meters across, from Eureka Springs, Arkansas. Large stromatolites like this are often a bit more complex, incorporating thicker zones of sediment or other fossils between the cyanobacterial laminations (layers) and are sometimes referred to as bioherms in geological literature for this region.

An eroded stromatolite found in the trail along the bank of Lake Leatherwood, just outside of Eureka Springs.

When stromatolites are exposed in vertical cross section in a bluff or road cut, they look like bumps or columns made up of thin layers. But when they are cut across by weathering and erosion of a flat surface, they look like concentric circles. Cyanobacterial algae were photosynthesizing organisms. They used sunlight, like modern plants, so they occur where abundant sunlight reached the floor of the shallow inland sea that covered the area. So when you walk across a surface that exposes these structures in the Eureka Springs area, you are walking on an ancient sea floor that is nearly half a billion years old, and that was shallow enough that you could see the bottom clearly from a boat, or that may have even been exposed by tides.